Early Electronic Television
Brian Cuff's GEC BT-8161 Restoration
Brian Cuff recently purchased an empty GEC BT-8161 on Ebay. Here is the story of his reconstruction of the set, from posts on the V.R.A.T discussion group. Text in italics are from Brian, standard text is our comments:
September - October 2012
As many of you know, I purchased the empty GEC 8161 TV on ebay. There are no known examples of this set and so this cabinet is very important. I am going to attempt to re-create the innards as closely as possible to the originals but unfortunately, there seems to be a dearth of info on this set or on its smaller cousin, the 9122. I have been trawling through my RX data for a likely candidate for the radio chassis. GEC seemed to have gone mainly for rotary pointers and dials and the (poor) pictures of the 8161 we have indicate a linear dial. Looking at the knob positions and scale size, I think that the radio could be a modified "GEC Fidelity SW5". Does anyone have one of these and even better, does anyone have one of these which they are willing to part with as a possible donor? As soon as I get the cabinet, I will clean it up a little and post some pictures. The vendor lives in Sudbury but I find that there are two Sudburys! One near Ipswitch and the other near London . I am waiting to hear which - I bet it's the Suffolk one!!!
This is going to be a loooooonnnnng drawn out thread I imagine and any advice is welcome.
(Brochure courtesy of oldticktock)
When I get the cabinet, I'll see if I can make some sense of the fittings that are left. Hopefully, the chassis mounts will be either threaded bushes or some sort of counter-bored holes which took rubber washers rather like the HMV 800 chassis mounts. I will also look for witness marks where the brackets were. This cabinet was rescued from an Attic. The builders rescued the cabinet because they recognizable it as a piece of furniture. I wonder if they skipped the chassis because they were not recognizable as anything.
Till Eulenspiegel provided these pictures of his still not restored chassis of his BT9121. Note the compact timebase unit.
He also provided circuit diagrams of the GEC BT8161 and the GEC BT912. Although the two chassis are similar there a few noteworthy differences. Both are superhets operating with low IFs. In the TV only receiver 9121 the RF amplifiers and frequency changer are on the vision chassis section. In the more higher specced up 8161 it would appear that the television early stages are actually on the "A" chassis, the radio receiver section. more research will have to be done to confirm that. The vision demodulator in the 8161 is of the full wave type, which is desirable because of the very low intermediate frequency and the large 15" CRT. The much cheaper BT9121 has a simple half wave vision demodulator. So it would seem that the main differences are in the signal circuits. The timebases and vision power supply arrangements are similar between the two chassis.
Till commented further: More thoughts about the GEC BT8161 TV and radio receiver. The vision IF in the TV only BT9121 is 6Mc/s, the sound is 2.5Mc/s. However I am becoming of the opinion the the vision IF in the BT8161 is even lower, as low as 4Mc/s. That's very low indeed but is worth bearing in mind that early Murphy TVs used a very low vision IF, in the V58A it was 4.25Mc/s. The sound was 0.75Mc/s.
Till also supplied a schematic of the radio chassis:
Well, I have the cabinet at home at last. It's not in bad condition - no woodworm or large chunks of veneer missing. Badly scratched and the top especially will have to be re-finished some time.
It hasn't been too badly messed about with although there are some witness marks where blocks have been removed. There are 2 mounting blocks fixed to the outside of the two chassis on each side of the tube but no corresponding pairs on the inside of the apertures. The lid is counterbalanced by two pulley systems, attached to the stays, which lift counterweights as the lid is raised. However, the weights are missing.
One mystery is solved - the knob in front of the CRT screen is the magic eye for the radio (Y63 introduced in 1937). There are small threaded angle brackets which support the wooden escutcheons covering the two chassis so they will be easy to make once the layout of the knobs has been determined.
I have been thinking about the variable bandwidth mechanism and, assuming that the radio chassis is mounted vertically, there will have to be some sort of cam system operating either a system of levers or two cams acting directly on the IF transformers - how exciting!!!
There is a GEC Super 6 on ebay so I have set a bid for it. At least the magic eye mounting looks the same.
Till provided these pictures of the other GEC BT9121. The timebase module is placed to the right of the CRT rear support. The GT1B thyratrons are inside the screening cans.
A bit of progress in the last couple of days! I have, at last, located the owner of the "other" GEC set. It turns out that it is a BT9122 which is the smaller (12") brother/sister of the BT8161 (15"). He has agreed for me to visit him to see the set and take pictures, measurements and the like - that's a real step forward.
I have spoken with the 9122 owner who is very pleased to help the project along! One very important thing - he has a full manual which covers both the 9122 and the 8161. Evidently, they are identical except for the central CRT mounting (12" or 16"). The manual includes full chassis layout drawings and component values etc, which were missing from the data to hand at the moment. Hopefully, when I visit him, I will be able to scan the docs - he is not willing to trust the vagaries of the postal system, having had some bad experiences.
Meanwhile, I decided to give the cabinet a bit of a polish and take some pictures. Here are a few of them:
The speaker cloth is modern(ish) Tygan so not original - I will see what the original looked like when I visit the BT9122! There is also a hole for a 12" speaker cut in the baffle board (as well as the original 8" hole!) so I guess it was converted into a hi-fi cabinet, fortunately with not too much hacking around!
Perhaps it was not very clear, sorry, but I said in my previous post that there was a 12" cut in the baffle board - I meant cut by person or persons unknown. The hole has been either enlarged or has been cut out for the conversion job.
I can guarantee that the baffle is not a replacement. It has had an extra hole cut in it. The smaller hole to the bottom left hand corner is also original. The large hole placed more centrally has been crudely cut and not that many years ago. The edges are still very clean (but rough).
November - December 2012
A bit more progress.
I have sourced an 11.5" electrostatic CRT, a CV1085. Gerry at Crowethorne tubes had one he was willing to part with together with some of the valves required. The CRT is a really bulbous jobbie and really looks the part. I can only imagine what the 16" version looked like!
I have also got, in my spares stock, a 12" GEC mask which fits after a fashion as can be seen from the pictures. I am still waiting for my visit to the 9122 but unfortunately, the owner has had some bad family news and I don't want to trouble him at this difficult time for him.
It is a radar tube so the phosphor is not quite right. It should be P4 but is P7. A P7 is a P4 type phosphor and a yellow, long afterglow phosphor which is necessary for radar. However, hopefully, a blue filter will minimise the lag caused by the long afterglow.
It's going to be a long haul, Rich but at least I have now started but need the service info and the visit to go much further. I also need to commission the mains transformer for the TB chassis but need to measure the 9122's example to get the lam stack as close as possible. I have worked out the heater windings but can't do the same for the HT windings until I get the component values (I have the brightness pot's traveler range of 80 - 200V but there are resistors in the chain).
I now have all the valves for the TB (B) chassis except the 2 KT66s and an MS4B. I Haven't checked my stock yet for the Radio (A) Chassis but I know that I have some of them.
Just to put the record straight, the "other" GEC mirror-lid set is not a BT9121 as previously thought. It is a BT8121 which makes it identical to the BT8161 except for the CRT (12" not 16") and the cabinet. The brochure that Till posted shows both the BT8121 and the BT8161 - brother and sister!A bit more information on the set. The following are pictures of the 8121 taken some while ago. Note the simple CRT mounting. That will be easy to reproduce!
The set is in great condition. It came from a gentlemans' club in London so was never in a dirty environment (well, not in the dusty sense).
January - February 2013
I've received the info from John and have been able to do a bit more research and preliminary planning. You may have read about the sound and vision IF frequency discussions in the thread about the Cossor 54 pre-war TV. Well, it turns out that the IFs for the GEC 8161 are also very low at 4MHz and 465(500)kHz for vision and sound respectively. That means that the local oscillator will run at 41MHz giving the differences of 500kHz and 4MHz. It seems strange that the local oscillator frequency is so close to the sound carrier but I suppose it must have worked!
The info from John included layout drawings of both chassis so I have been able to draw them properly using Autocad. I had no absolute measurement as a reference so I have drawn everything referenced to a length I estimated one side to be. When I get to see the existing 9122 in the flesh, I can measure the reference sides and then scale up (or down) all the other measurements to arrive at full size drawings.
A long, long way to go but I'm on the way. Till's work on his 12" mask has inspired me to get on with some woodwork, hopefully this month. I now know that the radio or "A" chassis is on the left of the CRT and the TV or "B" chassis on the right and I will start on making and veneering the two plywood decks which cover the chassis.
Hmmm - not a lot to do.
I bought this GEC radio on Ebay.! So I now have the first main hunk of the "A" Chassis (front end and radio). I now have to decide whether I should try to modify its chassis by cutting the holes for the cams, extra coils and valve or to just use the components off it. I am tending towards the latter as the "B" Chassis (timebase and vision IF etc.) must be made from scratch so will be fabricated from steel. Maybe it's better to have them both the same!
The chassis layout looks identical to the A chassis for the 8161 except that it has no RF stage. the 5 valves are in their appropriate positions and there is a gap where the RF coils and the 6th valve would go! I would also imagine that the IF cans do not have variable coupling but the tuning cap is underneath the chassis unlike the BC3760 so a combination of the two would be perfect.
March - April 2013
I have at last got the GEC radio chassis for the 8161. It's turned out to be a BT3750 which is a 5 valve superhet but the chassis is identical to the BT8161 radio section except for the holes for the RF amp. bits. I was worried about 2 things with this chassis, the tuning cap, - must be 3 gang - no problems, the radio has a band-pass aerial section so the tunicg cap is 3 gang. The other was the variable coupling in the IF cans - also no problem. The facility is provided in this chassis too! In fact, the BC3750 is really a better chassis that the BC3760 because of its mechanical arrangement.
Soooohhhhh. What to do. Do I mess around with the set adding an RF stage by drilling holes and mounting the extra valve and coil can (their positions are free) or say bu**er it and just modify the short wave range to cover the correct frequency. Of course, the sensitivity will be down but I'm not short of signal strength as the Aurora pumps out loads!
Just a quick update on the BC3750 radio chassis: I am really pleased that I was able to buy this set. The chassis, I know now, is mechanically the same as the TV version except for the missing RF stage valve base and coil can. The valves have screening cans riveted onto the chassis over the valve holders so there are 4 fixing holes for the RF and low signal stages. In the position where the 6th valve is on the TV radio chassis, there are 4 holes with the correct positions in the radio chassis. This proves that it has been manufactured from the same basic drawing .
I thought it would be of interest if I uploaded some pictures of the variable IF coupling mechanics. It is mechanically a very simple method but I feel that it has been done very well by GEC in this chassis, even down to the little spiral connections to the moving coil.The radio is now working and the short wave is very lively so maybe I can get it up to 45MHz!
I'm glad I didn't need to make this!
Close-up of one cam
Very neat construction. Shown with maximum coupling.
Showing minimum coupling.
I thought that I would do a bit of experimenting for a change! I need to construct some 4MHz IF transformers for the new "B" chassis which carries all the TV only components. I had bought a scrap contemporary GEC chassis a couple of months ago for its large round coil cans. The IF cans are also of similar shape to the "A" chassis cans shown in the previous post and I needed one of these for the 4MHz pick-off transformer which is in the anode CCT of the frequency changer - the vision IF is 4MHz and the sound IF is 445kHz (500kHz).
The experimental 4MHz IF transformer
As to the pick-off transformer, only the secondary is tuned so I need to make another version. Has anyone any idea of the coupling required? I would have thought that it needs to be fairly tight to give a reasonable signal for the following stages. Should I bifilar wind it of just put two windings close together? Any ideas?
I have been a bit sidetracked so have not looked into Spice etc. (sounds a bit complicated to an old 1V in 75 ohm man like me)!
The visit was very interesting and so useful. I got pictures of how both chassis mount in the cabinet (all the cabinet fittings are in place in my cabinet ) and got a general idea of how it goes together. It confirms that the radio chassis I have (BC3750) is definitely the right chassis to have and it will be possible to fit it properly into the cabinet - even the knobs are the same with the four controls operated from two shafts using linkages, one of which can be seen in the pictures.
The mains power switching is taken care of by two cam-operated toggle switches which are mounted to the front of the chassis with complicated brackets so I thought that I would have a go at reproducing them today. The two pictures show the BT8121 switchery and my attempt at reproducing it. No too bad, I think. The toggle is for switching on the B chassis when TV is selected. I will use the SW position for TV as it will be nigh on impossible to add the 4th switch position. If I get another BC3750 I might have a go but I don't want to bu**er up the chassis I have.
A bit more mechanical progress in the last couple of days. I'm getting close to having an "A" (radio) chassis that is a good representation of the original. The dial of the BC3750 radio has been lowered and mounted vertically instead of sloping backwards and I have drawn the proper BT8161 dial in Photoshop. I will probably try and find a tame silk-screener to make a dial using the Photoshop files. Due to the mods., the control knobs and dial will now be in the same horizontal plane and I will manufacture the veneered escutcheon panel. The proof of the pudding will be the fitting of the chassis into the cabinet! Pictures show the progress.
This is how the radio chassis looked before modification
The finished control area of the radio chassis
A print of the new dial
One thing that this work has proved to me is that I have repeated exactly what the designers of the BT8161 TV did when they modified the BC3750 chassis for use in the TV!
May - June 2013
I'm not going to bother adding the RF stage, Till, as I might bu**er up the chassis. If I can get hold of another BC1750 chassis, then I may have a go after I have got the "B" chassis done - that's the real big job. I am a trifle worried about the scan output transformers. As you say, they have to have 6kV insulation primary-secondaries and I suppose even more secondary-secondary! If I am going to have them made, I will need to know the turns ratios so when your doing the checks to see if that secondary is faulty, you'll need to bung a sine wave through it so perhaps you could find out for me .
The dial on the BT8121 was as depply positioned as this one - it did look a bit strange. I might re-make the two brackets holding it onto the chassis to move it a bit closer to the panel.
The can on the extreme right is the 4MHz vision takeoff transformer with the phono output.
A full frontal with perspective distortion!
The template in position
Just a thought: It might be thought that using a phono (or RCA) connector for the vision IF connection was a little strange! The BT8161 used a modified 2 pin 5amp mains plug (the pins were turned down a bit) and I didn't want to do that so the best solution for me was a phono socket. Evidently, the phono plug was used a lot for RF in its early days (the ones with the extra long centre pin) so I feel it's OK to use. The pre-war Pye 815 TV uses a phono plug as the aerial input!
A little more progress, this time "theoretical"! I have drawn the B (TV) chassis using Autocad so that I can get a local engineering company to make it. I can't really handle 1.6mm steel plate of this size. The drawing includes the holes for the valves and RF transformers and 10mm locating holes for the capacitor boxes which I have to make. These are where the connection wires will exit the blocks. I have asked a couple of companies to quote and have heard back from one already - a bit expensive, so I hope that the other will be a bit cheaper!
I have also placed a post in the wanted section for the 4 knobs I require to complete the set (9 in total). I was very fortunate to get hold of an extra lever control for the B chassis, the two for the A chassis comming from the BC1750 radio.
I have noticed that there is another BC1750 on ebay at the moment and I have, of course, placed a bid so that if I get it, I can try the full mods to convert it to a real A chassis (adding an RF stage and further switching).
As you can see from the drawing, I will drill all the small assembly screw holes, 6BA, 4BA etc.
I am still waiting for prices.
However, I decided to start on the capacitor boxes with the hardest, the EHT smoother, first. This cap has rounded corners so I knew it would be a bit of a bu**er but I've started. The pics show progress so far together with a special joggle tool made to enable the two halves to be joined - the 130mm sides could have been formed out of one piece but at over 300mm long it would have been too wide for my guillotine. I have soldered one of the side pieces to the top but it was a real struggle and I just hope it won't pop when I solder the other side on! More later.
The 4 basic parts
The joggle produced by the tool
The joggle tool (150mm wide)
It's easy to see how the joggle tool works from the picture, the tool being squeezed together in the vice which deforms the brass sheet accordingly. I must buy some tinplate. It will be easier to work and doesn't "work harden" as brass does.
Well, the EHT cap is finished - hooray. It was quite a job and I shouldn't "waste" my time on such detail but I took it as a challenge. As can be seen on the original (in the BT8121), the capacitor can is swaged together like a bean tin which, of course, I couldn't do so I soldered some 16swg tinned copper round the rim to represent the seam. It works quite well. It should be 2 x 0.15mfd but I put in 2 x 0.1mfd. That's quite enough at 6.5kV .
The cap as it is in the BT8121
As can be seen from the picture, the chassis has been made and delivered (well, collected actually). The owner of the company was very obliging and made a good job of the chassis (although the drain marks on the plating are not too good!). He new most of the guys by and for whom I had mechanical work done when I was in business including some companies that no longer exist such as Braybury Electronics, IPK Broadcast Systems and Ampex Systems.
Anyway, I digress! I have placed the cap and xfmr boxes so far made onto the chassis in the relevant places and it's looking good! I have to make straps to hold most of the caps down so I have bought some 1/4" x 1/16" mild steel strip which I will bend into shape and drill.
I have ordered a set of pots from Allied Electronics and they are reasonably priced (all large format 2W components) but the shipping, at $45 is dreadful! Still, what can we do - they seem to have a much better range of components available ex-stock over the pond. Why is that? Is it just that our suppliers are too crippled by taxes to have large inventories or are we even more in thrall to accountants now?
The new chassis with cans placed in posotion (for show!!)
Hi guys. I'm a bit peed off at the moment as I had composed a long post showing the last few days' progress on the BT8161 and, instead of hitting the minimise button, I hit the red cross and lost everything. Surely, it's a great failing in the software not to give the normal window "are you sure" reminder when there has been something added to the page which is open!
The drawing is pretty self explanitary so I won't describe it in detail except to point out that the the pots, in green, are at 6kV (shifts and focus) so are mounted on the 6mm Tufnel sub-panel which in turn is mounted on Tufnel pillars, the isolation being completed by the Acetal couplers. A detail not shown in the drawing are the three 1/4" bushes mounted in the control panel where the pot shafts pass through. I got some threaded 6mm bushes from Farnell and reamed them out to 1/4" so the shafts are well supported
For those of you not familiar with metalworking, I thought I might describe how I mark out and centre punch hole positions. A hole position is normally defined by the intersection of two scribed lines (the blotches of black in the pictures are marker ink through which the lines are scribed - this makes them easily seen). I then use what is called an Optical Centre Punch. This comprises three components: a conical body with a non-slip base and a vertical hole through the centre, a special lens with cross-wires which fits into the hole and an accurately ground centre punch which also fits into the hole. In use, the body, complete with lens, is placed over the intersecting lines and positioned, using the cross-wires, exactly over the intersection. The lens is them carefully removed and the punch inserted and whacked! The dimple produced is always exactly on the intersection of the lines which means that, if careful when drilling, the resulting hole is in the correct position.
This picture shows the chassis in the BT8121.
Today, I finished smithying the mounting bars and finally tried the B chassis assembly in the cabinet - and it fits . The pictures show it in position.
Bracketry required to support the chassis
I have today cut a plywood escutcheon panel for the B Chassis (TV) side. I was amazed how out of square the cabinet is. I thought that I was the only one to - what's the polite word "compromise" on the meaning of accuracy .
Anyway, I've cut it to size and now have to place the control holes in the right place and to cut the aperture for the 6 pre-set controls. At least, if I can accurately place one control, probably the dual concentric one which is in the middle, I can use my acad drawing to position the other 8 holes. Once the board has been completed, I will endeavour to veneer it in a suitable veneer. Unfortunately, the veneer I have in stock, the leftovers from a Defiant TV job, are all too small and I don't want to make the job too difficult!
July - August 2013
I've just finished a tricky mechanical bit! The two hold controls are in a dual-concentric form but they drive separate pots so I had to put my thinking cap on. I saw from the BT8121 that the "inner" pot was fitted to the front of the chassis with a Z type bracket so assumed that the inner shaft passed though the "outer" pot. So I dismantled the outer pot and drilled a 3.4mm hole right through the shaft and a corresponding hole in the rear cover. This allowed me to pass a 3/32" shaft through the pot. An adapter bush joined this shaft to the 1/4" pot spindle. Then I could fit a control knob to the inner pot shaft after I had soft-soldered a 1/4" bush with a flat milled into it on the top end. The outer control uses a GEC lever-type "knob" which sits under the normal inner knob. I modified a bush onto which the lever knob fitted from the sacrificial radio so that it could be clamped (grub screw) to the 1/4" shaft of the outer pot - the one with the 3.2mm hole drilled through it. Thus, when the outer pot is mounted on the control panel and the inner pot on the chassis front, the 3/32" shaft passes through the outer pot and connects to the inner one. The pictures help to decipher my description.
A bit more progress today (too hot to be outside!). I have started the trial mechanical fit for the cap and transformer boxes I have made (and stuffed in the case of the capacitors). I came across one snag where the flanges from one of the capacitors fouled those of the line-scan output transformer so I had to cut away part of the transformer flange. No real hardship! I guess that the transformers are mounted in another fassion in the real thing but that's a minor detail which does not effect the overall appearance. Here are a few piccies:
One thing I had to do before this assembly was to make a couple of tag strips for the scan transformers. The secondaries of the Xfmrs are up at 6kV so the insulation needs to be very good and they have heavy Ebonite connection blocks. Fortunately, Mike the dismantler had some very useful blocks of Ebonite which I was able to use. I also had some eyelet tags which I bought for the resoration of a Pye 8151 when the mains transformer had to be re-made as a poor substitute had been made. The second two pictures show the tag strip both outside and inside the can - note that the Ebonite passes through the brass can to help to prevent arcing.
A bit of cabinet progress today. I have made the basic escutcheon for the TV side which carries all the TV controls, 6 through a rectangular window and the others via shafts through it
Unfortunately, I have been unable to source the other three + one large knob I need. The ones shown are the only two I have so far. They are used on some GEG radios (several models BT37** and BT38**). Please note the extra 6 "ventilation" holes. These are a result of thinking three times and cutting twice! Normally, I think twice and cut once - I got mixed up this time. The holes will be plugged before the veneer goes on! Then, I have to make a little rectangular lid which covers the pre-set control aperture. The original on the BT1812 shown below gives an idea of what I am trying to achieve.
I need to get hold of some Sapele Mahogany veneer and a small piece of solid mahogany. It will also be fun trying to veneer the finger-chamfer above and below the pre-set aperture! Slowly but reasonably surely.
Alas, there is still little progress on the mechanical/electronics side of things - in fact I have gone backwards! When I came to start fitting the valve bases to the chassis, I found that I had got many of the large hole sizes wrong! Some were too large and some too small (this is what happens when things are rushed. I should have waited until I got the actual components). Although the small hole could be enlarged, those that were too big cannot be made smaller (tidily). So I bit the bullet and modified the drawing and sent it in for re-manufacture! Expensive but why spoil the ship for a ha'pth of tar!
A bit more progress in the last couple of weeks. The cabinet work is progressing. I have filled the large speaker hole that the murderer cut in the baffle for his loudspeaker by cutting the hole square and inserting a piece of MDF. This repair was painted black and the new speaker cloth applied using spray adhesive. It looks so much better than the dark Tygan which had been fitted for the "conversion".
The new speaker cloth looks much better
The match isn't too bad from a distance
I will post a bit more progress in a couple of days, this time on the new chassis. All the valve holders are fitted and the mechanical work done on the first chassis has been transferred to the replacement. Pictures soon.
As mentioned, albeit a bit late, I have taken some pictures of the progress on the B chassis. Apart from "transferring" the work already completed on the first chassis, I have added all the valve holders, the sheet metal screen for the N43 high-level IF stages and, under the chassis, the EHT mains transformer and the three smoothing chokes which are mounted on a vertical plate.
Unfortunately, this picture shows bad linishing before the chassis was plated.
Just for fun, I decided to complete as much of the chassis as possible, plugging in the valves I have so far and placing the coil cans in position. The picture shows the result!
I hadn't made the tag block when I took this picture
Not too much more to do now before I begin the wiring, starting with the heaters. Unfortunately, the main power transformer had be returned for modification but should soon be back and I can start it then.
I know that this is supposed to be a replica but maybe it shouldn't be called that. I am very near to starting the wiring and, in the absence of the main power Xfmr, I have decided to start with the timebases which are fairly simple and use standard components (apart from the O/P transformers). Not having an original under-chassis picture, I have decided to build as much as possible on tagboards as being the best and neatest way to progress. I will document the layout for an extra page(s) in the manual and to get as logical a tagboard layout as possible. The pictures show some of the tagboards that I will use for the timebase circuitry together with the proposed layout - with the vertical tagboards laid out flat for clarity. What do you think? Should I take this route or will it be too drastic a change from the original (possibly)? Your comments will be appreciated.
Layout for the first 5 tagboards
I have started the wiring in earnest now by wiring the heaters. There are 7 heater windings on the mains transformer, 4 for the timebases & signal valves, 1 for the sync separator, 1 for the CRT and, of course, 1 for the HT rectifier. The EHT transformer has just the high voltage winding and the heater winding for the EHT rectifier. Once the heaters are finished, I will start with the timebases and perhaps power them with bench PSUs to start with.
Just a quick note to say that I've been looking at Till's layout sketches of his BT1921 which is a console direct-view TV of the same vintage. I seem to have made the right decision about using small tag/group boards as the sketches show that this is what GEC used in the BT9121. One of the really useful drawings is the layout of the EHT bleeder and CRT resistor chain boards! As the circuit is almost identical, by checking with the photos I have, I have been able to tweak my layout so that it must be a reasonable facsimile of the original BT8161.
Progress is steady on the 8161 however and the pictures show how the wiring is going. The long tagstrip will act as terminations for all the heater windings except the HT rectifier (I will make sure that the tags will not carry the full current for the two 6A circuits!) - otherwise, the heater wiring would be rather unwieldy.
The heater wiring is temporarily tacked onto the appropriate tags
The two timebases - Line above field Field.
I have been doing some mechanical work on the IF transformers for the B chassis. I have mentioned the electrical problems with this part of the circuit earlier in the thread but I would like to raise the question again. There are 7 tuned circuits (tuned by self and stray capacity only), and damped with reasonably low value resistors to form the 4MHz IF strip. The table below shows the winding resistances and the damping values. There are no capacitors except the one at the head of the list. This sits at the end of a long coaxial cable between the two chassis.
In order to achieve the bandwidth required, I assume that the transformers must be stagger tuned and be reasonably tightly coupled. The center dips caused by over-coupling perhaps overlap one another resulting in a fairly flat response! Is this possible? What do the experts think? All suggestions welcome.
A bit more mechanical progress this last week. I have fitted the transformers and powered up the HT side (no EHT yet!). The two timebases seem to work, both producing sawtooth waveforms although into a resistive load as I have no scan transformers yet.
The various stages plus the swaging tool.
The former mounted on its termination panel
The IF screening cans are mounted the same way as the valve screening cans
In order to run the timebases properly, I needed to have the hold and amplitude controls operational so I decided to fit the control panel and wire it permanently to the main chassis as the panel carries. The pictures of the BT8121 chassis shows a cable form passing down the chassis from the control panel to the rear where the timebase circuitry is so I connected long wires to the controls and have started lacing the form together. Before fitting the panel, I wired the EHT cabling from the panel to the EHT tagboards and from the mains transformer to the rectifier etc.
The panel cable form taking shape!
Close-up of the EHT sub-panel wiring
When the panel wiring is complete, I will be able to check both timbase oscillators properly as I have three GT1B thyratrons now (thanks again, Rich).
Today, I completed lacing the cableform from the control panel to the rear of the chassis where the timebase circuitry is, drilling holes for grommets where the cables pass through the chassis.
I did get the urge to lace properly from my days in the BBC. I remember CAR at TVC when it first opened. The box lacing was something to wonder at and, throughout my days as a TV systems engineer, I was very conscious of the way the wiring in the racks looked - woe betide any wireman that didn't do it properly
September - October 2013
A fair amount achieved today:
The the surface silvered mirror has been ordered from a company called Vacuum Coatings in Walthamstow. They undertake to cut and coat or to strip and re-coat surface-silvered mirrors so I'm having my 703 mirror rec-coated as someone tried to clean it and damaged the coating quite badly!
Are there enough components for 2 timebases?
The completed control panel
Now came the time to see if the scan generators worked so I plugged in the HT rectifier, the two thyratrons and the one KT66 I have and switched on, monitoring the HT line. Up came the HT with no sign of smoke or smell so on went the scope. The picture shows the two waveforms - I haven't checked their frequencies, but one is fast and the other slow. The phase difference is because the line scan is after the output valve and the frame at the grid of the output valveholder.
In a previous existance, I used to manufacture (from raw castings and stuffed PCBs) Link Electronics Vidicon cameras (types 108/109. IIRC). These consisted of four PCBs with a loom connecting them together. The looms were laced on a nail board so that the individual wires came out in exactly the right places to meet their appropriate pins on the PCBs. This cut wiring errors down to almost zero and save a huge amount of test time. As you say, Rich, the looms were absolutely stable and would keep their shape whilst being cajoled into position in the equipment.
I don't know if anyone has tried "Spirap" (or some such name). It was (is?) a coiled plastic extrusion which was wrapped around a cableform, with the "arms" passing between the turns, a bit like finger trunking. I found it hopeless as the cableform twisted under the stress of the Spirap and ended up with the arms coming out at all sorts of angles - hopeless. In my opinion, decent lacing cannot be beaten.
I've started on the RF section now as I'm waiting for the first iteration of the line output transformer without which I can go no further with the scanning and display side.
The resulting Acad drawing
And here it is in real life:
A bit like the drawing! The tinned copper wires are the HT feeds to the IF transformers.
The IF transformers (4MHz) present a problem. I have no idea what they were like in real life so it really has to be a bit of calculation and then empirical trial and error. I am thinking that the best way to proceed is to install B4 valve bases in the transformer positions and make some plug-in coil formers using old valve bases and the Acetyl rod which I intend to use in the finished article. It will certainly make life easier and would be a lot kinder to the wiring that is already done.
I have all the input and output capacitances for the IF valves and will add a bit for strays and start from there. As to the coupling between the windings, which will be very important, I will make a special coil former consisting of two bobbins, one of which can slide up and down a central rod. This will make it very easy to "adjust on test" whilst looking at the response curve.
I am off to the Isle of Wight for a few days soon so will be taking a rest from this most absorbing project.
Here's a picture of the first variable IF transformer attempt. The two pictures show the upper bobbin in two positions. I will fit 4 eyelet tags to the Tufnel plate (inside the base diameter), which will be wired to the valvebase pins. The "works" are held in position in the base by a plug of the appropriate diameter which will be different for each valvebase, I would imagine!
Do I need to make 6 of these or say three and work backwards - or maybe forwards. What do you guys/gals think?
There has been a bit of a pause in the project while Ed Dinning and I work out what is required of the scan output transformers. These are different to most other scan transformers for two reasons: they drive into what is effectively an open circuit which is the deflection plates and they have to have very high insulation as the plates are at about 5.5kV. We seem to have got somewhere and, after the second iteration, I have a 300V p-p linear line sawtooth which looks as though it could be OK. The frame transformer is being rewound as a result of very reasonable tests on the first iteration example and I should get the second version by the end of next week. Hopefully then, I should be able to display a raster with reasonable geometry!
That brings me to what I consider to be the most difficult bit of all – the IF strip. A centre frequency of 4MHz with a double sideband bandwidth of at least 2 MHz. I have no clue as to the construction of the originals except that they are in 2” diameter cans which are about 4” high and their primary and secondary DC resistances. I have absolutely no RF design experience and I must admit, I got a bit lost reading some of the treatises I found on the internet. I decided that what I needed was a spread-sheet which enabled me to vary a number of coil parameters such as diameter and wire gauge to see what effect they have on winding length, resistance, inductance, capacitance and turns ratio. I have uploaded a jpeg of an example of the spread sheet, the yellow boxes being the input cells. Embedded formulae automatically calculate the unknowns which are displayed in the appropriate cells. So far so good! Now I have to decide what to do with the information thus produced to define what I require for the IFTs. Both primaries and secondaries of all transformers are damped by resistors in the low kΩs which will obviously help to widen the bandwidth. However, I am not at all confident in my ability in this area and would appreciate any help offered.
November - December 2013
A bit of progress on the cabinet today - I sent the lid to be professionally finished as there is quite a large flat area to be polished and it's so easy to get that wrong without the proper gear - I know, I've done it! The guy delivered it today and did a little touch-up work on the cabinet and it now looks "the business".
Before fitting the lid back onto the cabinet (16 screws on one leaf of the piano hinge) I fitted the new surface-silvered mirror I had had made by Vacuum Coatings Ltd in East London (I also got them to re-coat the mirror in my 703 Mastergram as it had been cleaned incorrectly at some time and there was a large area of damaged silvering. It looks so much better now!).
In order to fit the mirror, I had to make some molding out of hardwood and stain it to suit the cabinet finish. I even managed to find some 3/4" x 4 brass raised head wood screws to fit the molding! The cabinet outcome is very pleasing and it has put even more pressure on me to get the electronics sorted but there is still a long way to go. However, I do feel that I have also come a long way forward from the original decision to remake a set of electronics so maybe half way there.
A Cossor 15" CRT hiding behind the GEC
Cheating a bit - no electronics behind the knobs!!
At last I've started to "design" the IF transformers with a little help from FJ Camm's Practical Wireless Encyclopedia (12th Ed.) - thanks for the tip, Cathy. There is a design for a TV receiver with a sound IF of 3Mc/s which is pretty close to the 4Mc/s of the BT vision IF amp which is pretty close. Their design uses 3/8" Aladin coil formers placed side by side inside a can, each winding having 50T of 34SWG wire. I just happen to have 12 suitable coil formers so I will use them. However, I need to be able to adjust the tightness of the coupling between the windings and the side by side construction would not be suitable so I will arrange for 2 vertical 6BA studs on which two formers, each carrying one winding, will be mounted using nuts to position them. In this way, I will be able to move them independently. The assemblies will be mounted on the chassis and the screening cans mounted separately. I have already done a test with two formers and it does show promise. The next step is to put the theoretical plans into practice and make a complete transformer - I have enough 6BA studding for about three!
The sound on the BT8161 comes from the "A" or radio chassis and as you know, I will be using a modified BC3750 for this chassis which also produces the 4Mc/s vision IF. If I do get sound first (450kc/s), I will automatically get an IF signal which I can use for the vision IF development!!! So I may just get on with that once I have finished the mechanical design of the vision IFs. It is just a re-design of the frequency changer to take it up to 45Mc/s. The TV band will be in place of the SW as the BC3750 has 3 bands, not 4 and will have bandspread tuning to overcome the inevitable oscillator drift.
Till Eulenspiegel wrote:
I can find no reference to whether the 8161 uses double, upper or lower sidebands in its IF stages and, as the transformers are described as band-pass transformers and have no variables, either capacitive or inductive, there are no alignment instructions. I guess, therefore, that I can use the USB which makes the centre frequency about 5.5MHz which could ease the design a little. This is quite close to the 6MHz of the 9121! Comparing the dc resistance of the windings between the two sets however, does suggest that the 8161 uses the LSB as the resistance of the windings is much higher suggesting more turns of thinner wire. I wonder if the reason for the differences between the two sets was the better frequency response of the KTZ series over the MSP/VMP series, the N43 being used only in the final two IF stages driving the full wave detector. Perhaps the MSP41 couldn't handle the voltage swing required! . Some investigation is required.
Till Eulenspiegel wrote:
I'll go for the USB as, as you say, it's easier to get the bandwidth higher up! The photos show the first prototype IF transformers assembled for testing! The windings on the third IFT were done before I had the idea of Aladdin formers and 6BA studding and it will be rewound later so that the coils can be closer together. Obviously I have to determine the right number of turns as well as the coupling between them. I feel that this is going to be a long slog but it will be worth it. The primary and secondary windings are terminated on the tags top and bottom so the primary will be on the top for valves which have an anode top cap and the secondary at the top for the grid top caps.