Hugo Holden's RCA 621 Restoration
Hugo Holden has undertaken the restoration of another set, stripping all components from the chassis and rebuilding it like new. He purchased the RCA 621TS in the United States, and had it shipped to Australia. Hugo's earlier restorations included an Andrea KT-E-5, a Meissner 10-1153, and a HMV 904.
Here is a narrative describing his progress:
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I'm in the process of restoring the 621. All the metal parts
were stripped of the original cadmium, aluminium oxide blasted to
remove every last microscopic rust crystal, and treated with the
process of "Electroless Nickel" doesn't use an electric
field. This plates into corners, and hole edges with an even
thickness, in this case 20 Microns. It gives a great metallic
look. Most electroplaters won't do cadmium now(Toxic concerns) and
the fact is that the Electroless system is much better for complex
shaped formed objects. However your electroplater must have a good
knowledge of electrochemistry and be very patient to get this sort
of result. Here are before and after photos of the chassis and
other metal parts:
As you can see I'm also rebuilding the
tuner and also I've had to replace the power transformer with one
of the same sized stack (and similar windings) the Hammond 300BX,
still keep the original transformer covers so it looks the same.
I have attached some before and after photos of the tuner restoration, and also a diagram of the "Spectacular" converter or mixer coil. At first thought this outrageously large coil looked like is was just for show, but a closer look reveals that it is a very clever piece of engineering. Have a look at the diagram below.
The tuner in this set is "space age" sophisticated for 1946. Features include a neutralised push-pull RF amp with neutralisation based on a 6J6, a 6J6 Kalitron oscillator and the spectacular mixer coil driven by the combined anode signals from another 6J6. Also the use of both ferromagnetic materials and brass slugs is advanced.
The idea behind the mixer coil is to create a very high Q
loosely coupled selective sound take-off. The large sound IF coil
is spaced away from the former to avoid it being tuned by
distributed capacity, it is largely tuned by the "high Q" 62pF
dog-bone ceramic cap. The mixer anode coil for the video is broad
tuning and loaded by the 10 K resistor and the plate resistance of
the 6J6 mixer tube.
With regards to the tuner restoration, I have replaced most of
the bypass / coupling caps with silver mica ones, except the local
oscillator feedback caps, these were replaced with 500V 4.7pF mil spec
dogbone ceramic caps with the same tempco which match the
originals, the same goes for the 1.5pF neutralisation caps on the
6J6 RF amp. Also used mostly metal film resistors. helps keep the
noise down a little. The tube sockets are nos, including a the
original push on shield type for the L/O. I got a nos push on
shield, identical to the rusted up original from Angela
Instruments. The rivets and the original screws have been replaced
with 4-40 and 6-32 stainless steel screws (avoids future rusting).
These can be obtained from PSME (Precision Scale Model
Engineering). PSME is in Milford, MA, email psme@psmescale.com
and the fellow who runs it, Lawrence Milo is very helpful. Although there are locking washers, never hurts to
apply varnish to the threads, in this case my wife's Chanel No 5
nail varnish.
I'll send you a complete photo later. It is probably fair to say that a tuner restoration itself is about 1/3 the work of the main chassis and other componentry. As you can see from the before photos, the tuner body had unacceptable rust and bad sockets, so I gave it the "treatment".
Shown in the photo is the original
621 power transformer, I took the copper flux band and restored
covers off it and added them to the Hammond 300BX transformer,
and discarded the Hammond covers which are quite different. The
two transformers have very close to the same geometry stack,
with the holes placed a little differently.
The original brackets were blasted
free of rust and black powder coated and the finish looks very
similar to the original and is corrosion and scratch resistant.
The two wires for the 120V configured primary windings have to
exit via an additional hole in the top bracket.
The original 621 transformer is not
a safe proposition to run here in Australia with our 50Hz supply
frequency. The transformer has very aged and cracked insulation
and runs an excessive magnetisation current on 50Hz. For
example, off load, the RMS current on 115 to 120V, 50Hz is
1.5amps(compared to 47mA for the Hammond configured for 120V
which is designed for 50/60Hz). In general old 60Hz American
transformers run hot here on 50Hz, also there is significant
stray magnetic fields generated. So the Hammond transformer
solves the problem. The windings on the Hammond are close to
exact:
Two 1.2 A 5V, parallel to 2,4A to
run the 5V4G Damper diode,
One 5V 3A for the 5U4G
one 6 V 6A for the main heaters,
and one 800Vct for the HT.
There is only one winding "missing",
a small 6.3V one for the CRT heater, but I'm adding a small
auxiliary transformer for that, there is a convenient place to
mount it, and only one hole needs to be drilled to fit it. (on
Hammonds website, their data sheet says the 300BX has only
one 5v at 1.2A winding when in fact it has two of these).
In the background is the restored EHT (high voltage) cage. This had the rust and oxide blasted off. I could not get any electroplating done which would resemble the brassy bronze color that it once had, so to get a good look, and protect it, I had it powder coated with metallic bronze, it looks very good now.
I've also attached a picture of the EHT (flyback) transformer
that was disassembled to clean the rust from the core and varnish
the windings for improved protection. The core has a darkish blue
colour now due to the organic rust converter. I had to replace the
2 turn heater winding for the 1B3 as the original had degraded
insulation, I found some identical geometry wire inside the
red sheath of some modern 25Kv anode wire. All of the large Allen
Bradley resistors in the focus chain were open circuit! So I've
replaced them with 10Kv rated Philips focus grade resistors. Also
the doorknob cap has been replaced with a 1000pF 15 Kv (same
physical size as the original 500pF) and this help to allow for
the CRT if it has no external aquadag. I'm not 100% sure if the
original doorknob cap is ok, it only reads 375pF and I've read
reports of them failing in the 621.
The work on the set continues. The
underside of the chassis is now re-wired, all new resistors,
wiring and tube sockets. The resistors are now all 2 watt metal
film, but yet the same size as the original 1/4 or 1/2 watt
types. 500V Silver mica caps were used to replace the original
moulded micas, and the wax paper caps re-built with poly's with
twice the voltage rating. The cardboard shells had all the wax
cleaned off and were varnished with marine grade varnish, so
they look excellent but the surface is not tacky to touch and
won't adhere to so much dust as is the case with wax. The octal
sockets were replaced with American mil spec brown phenolic
sockets with wrap around pins and stainless steel saddles. The 7
pin sockets replaced with American phenolic sockets from AES.
Antique Electronic Supply (USA) have supplied all the new
capacitors, including the micas, electros and poly's and also a
good number of NOS tubes for the set as well as the 300BX power
transformer and new tag strips. They always send me excellent
tubes and parts at competitive prices I find.
The adjustable IF coils had very
rusty spring mounting clips, they were all replaced with new
ones as they are a common standard part on many NOS old
stock coils. The originals were soldered to the chassis on the
top, presumably to prevent capacitive effects varying and
altering the IF tuning. I this case I simply soldered them to
the nearest earth lugs under the chassis with a short link wire
to avoid soldering to the chassis on the top surface.
All of the potentiometers, except one, were restorable to a good standard, the on/off vol pot got
replaced.
The new wiring is medical
grade silicone rubber covered hook-up wire which looks exactly
like old fashioned covered rubber wire, but is extremely heat
resistant and this insulation never melts back near the solder joints (even if the iron is on 480 degrees C). The wire is pleasant to
handle and flexible, but stays where its put on the whole. It is
about 2.5mm outside diameter and has 16 strands. Once you have
used superior wire like this its very hard to go back to PVC
covered wire or anything else, including Teflon. This silicone
rubber wire is available from RS components.
I stuck to the color scheme on the
wiring diagram where possible. Fabric covered wire is available,
though I suspect it would meet the same fate as the original
wire over the next 60 years. The silicone rubber wire will
outlast it I'm sure. Ideally I want the restoration to look
about the same 50 years later, the chassis, as well as having
the 20 micron thick nickel, also has a good coat of protective
lacquer. This also avoids corrosion and finger marks.
In general, during a restoration,
its best to build all the lightweight components back in first
and add the heavier items
later. That way the chassis is easier to handle.
I've wound a small 1:1.17 ratio
isolating transformer which gives 6.3 to 6.3V rms at 0.6 amps
load to provide the heater supply to the CRT from the power
transformer's 6.3V winding. At turn on from cold, the heavy
loading of all of the TV's low resistance cold heaters results
in a good slow rise in initial heater currents due to the
limited current delivering ability of the power transformer, and
in that sense the larger tubes protect the smaller ones at turn
on. In series heater chains resistors or thermistors are needed
as the internal resistance of the mains is very low and the
initial surge current in the cold heater chain is very high. The
smaller tubes warm up first and more voltage is developed across
their heaters without current limiting. However it is
interesting to note that the same problem will occur "within" in
any indirectly heated tube, if you connect the heater pins
across a power supply of very low internal resistance. The part
of the heater, close to it's internal connections, warms up
first as there is less thermal inertia there than the part in
contact with the cathode, and you will get an initial bright
flash from that area of the heater at turn on. In fact you can
get this effect if you unplug nearly all of the tubes in your
TV, except for a small one, at turn on you'll also get a bright
flash, as the large power transformer is, under these
circumstances, able to maintain the 6.3 volts across the
single tube's pins nearly immediately.
All that needs to be done now is to
add the tuner, rebuild the EHT area, add the yoke and the power
resistor box and the 5u4 and it's socket and power transformer,
and also fit the black cardboard insulating covers over the
electrolytic cans, fit the CRT socket which has been cleaned up
with new flying leads, then fit a NOS set of tubes and test and
align the set. Hmm, then there's the cabinet!
As the 621TS restoration progresses
I'm awe inspired by the futuristic technology that it employs in
1946. The 621 is a special set, America's first post war "mass
production" set, however made in limited numbers. This set
represents the culmination of pre-war television technology and
concepts which were distilled by 1938 and 1939, before WW2
disrupted proceedings and diverted the attention of Electronic
Engineers to the war effort.
The photo shows the inside of the
EHT (high voltage) cage with the 6BG6 line (horizontal) output tube and 5V4 damper tube and
the 6SN7 Horizontal osc and discharge tube not present to
improve the view. At a glance it is obvious how the technology
was progressing, and in fact, was in this case, setting the
standard for all monochrome TVs to follow.
The first person to suggest the use
of a "damper" diode in magnetic deflection scanning, in 1936,
was Alan Dower Blumlein, the "inventor" of stereo audio. He
patented "binaural" audio recording in 1931. Of course people
thought he was "nuts" at the time, because this would require
twice the hardware for both recording and reproduction and it
didn't seem to be an economical prospect. Alan was killed in a
plane crash in 1942 while testing radar for the war effort. This
loss was described by Winston Churchill as a national tragedy.
Following this, damper diode
function was well examined by RCA laboratories presumably during
wartime and in the immediate two years thereafter . RCA produced
a series of review articles and in "Magnetic Deflection Circuits
for Cathode-Ray Tubes" Otto. H.Schade Volume V 1947-1948 Pg 105,
RCA Labs, reference is made to Blumlein's original 1936 patent,
and damper diode technology is thoroughly explained. I've
written an article describing the
development of the damper diode.
Over the following years it became
obvious that magnetic line scanning systems really needed to be
"Energy control and management systems". The energy required
to deflect the beam to the right, stored as the potential energy
of the magnetic field in the yoke and line output transformer,
could be recovered after flyback, and returned to the power
supply, by the damper diode, and the left side of the scan
generated by damper diode current. Therefore the line scan
energy required is just enough to overcome the losses, now much
less in the absence of energy wasting resistive damping. This
idea was a huge leap forward in efficient scanning, and over the
years, damper diodes became synonymous with "efficiency diode",
or "booster diode" (due to the availability of boosted B+
voltage). No doubt this also paved the way for the concepts
which have resulted in modern switchmode power supplies.
Also in the photo the new EHT (high voltage)
bleeder and focus chain resistors (10KV rated Philips
resistors) can be seen attached to the "framework" which mounts
the 1B3 EHT (high voltage) rectifier. This framework is composed of clever
interlocking pairs phenolic sheets, which all lock together when
the 1B3 socket is inserted.
Now for something "Australian" to
add to the project. The 1B3 shown in the EHT (high voltage) cage photo, and the
box it came in (the other photo), is Australian made, by
Amalgamated Wireless Australia (AWA) probably some time in the
1960's or early 1970's. AWA had a massive tube plant that made
versions of most of the common RCA tube types, for both
television and radio. These tubes were every bit as good as
their American counterparts. AWA also made radios and appliances
for Australia going back to 1913 when it was founded in Sydney.
Some of their wonderful Bakelite 1930's era radios, like the AWA
Fisk Radiolette, that look like an Empire State building,
attract high prices from American collectors. This tube
plant does now not exist. It was a terrible loss to
Australia. The company made its last products in the late 1970's
and ultimately after being acquired by a gambling and betting
agency (Casino's and Tab) evolved into an information and
communications technology company. The AWA brand is still
licensed out however. In respect to this great past tube maker,
I will also fit an Australian made 5U4G. Don't worry, all the
other tubes will be American!
Progress continues, the tuner is finished now. It has a very impressive tube shield arrangement with a push on metal shield, with spring flanges, which contact the bulb of the 6J6 local osc tube to the shield. Around that is a thick lead shield which is attached by a spring clip to a stabilising arm to the tuner top plate. Clearly very effective mechanical and electrical shielding which would also reduce microphonic effects probably. The photos show the restored tuner fitted with NOS tubes ready to install back in the set, and the other photo is of the top of the tuner the way it looked prior to restoration mounted in the chassis. Its good to see the tuner looking like it must have when it was new, 60 years ago.
I've installed the 300BX power transformer and
I've taken a couple of photos that might be of interest to others restoring
the 621. The small added transformer (to supply the picture tube
heater) started out life as a Hammond P-T166L6 115V/ 6.3V CT. I took all the
wire off the bobbin and re-wound it, primary 93 turns of 0.63mm dia wire and
secondary 110 turns same wire, eg a ratio of 1:1.18 so it yields 6.3V at 0.6A
from the main 6.3V supply. The original Hammond 6.3 volt winding was rated at
2A and had a total of 60 turns. Its better to run a higher turns per volt
within limits of the copper losses and regulation being acceptable, to reduce
the core's magnetization current and stray magnetic fields in a TV
application. As can be seen in the photo, the black powder coat on the transformer covers and the resistor box resembles the original paint, but is superior. The two additional black silicone rubber covered wires for the transformer primary are not too obvious as they exit the top transformer cover. The mounting holes were in a very slightly different position to the original, so you can see if you look closely at the empty chassis photo, I elongated the transformer mounting holes so it will fit either type. The bolts which secure it are high tensile steel, with stainless nuts and washers. it's a very heavy transformer! The Hammond 300BX is a far superior power transformer to the original. You can see I've used a ceramic socket for the 5U4G, this is a vintage American NOS socket with a plated brass saddle and wrap around pins, very good quality, and suited to a hot running tube. I've also included a small picture of the Australian made 5U4G, which AWA labeled as AWV. Notice how it looks exactly like an RCA 5U4G.
I've modified the connections to the ion trap magnet, each wire now has a small gold plated in line connector, so I can unplug this electromagnet. When the CRT is not fitted, the magnet has a tendency to swing around on its wires and this can mark the chassis. I've also included for interest, a photo of an AWA(AWV) 6SN7. These were a little taller than their RCA counterparts, same specs of course.
The chassis before restoration
The chassis after restoration
The 621 restoration continues well with only minor problems so far.
My philosophy of restoration is based on the appearance of the set and a
perfect working result, without any intermittent faults or degraded
performance. I did my best to preserve all the potentiometers and not
replace any unless absolutely necessary, but I came unstuck this time. I
usually like to use NOS Clarostat or Allen Bradley 2 watt pots with
stainless steel shafts. After powering up the set, I noticed right away
that the contrast and brightness pots were worn out and intermittent,
despite being immaculately cleaned up and lubricated and appearing to be
"reasonable" on a meter.
The horizontal hold and Vertical hold pots are ok luckily, they are a
bigger more robust design than the contrast and brightness pot dual pot.
I had already replaced the vol pot.
The intermittent pot is a dual shaft variety and not easy to replace,
so I have disassembled a dual Clarostat pot and I'm replacing
the resistive elements with those of the correct value from other NOS
clarostat pots, and I'm machining new shafts to match the geometry of
the original RCA dual shaft pot. I had similar problems with the HMV
904, and had to do the same thing to create pots of the correct values,
with the original geometry shafts. I'll send a photo of this later.
In the meantime I've attached a photo and sketch of a CRT support bracket for the set made of hardwood (In this case varnished Tasmanian Oak). The 621 relies on the CRT to be mounted properly only when the chassis is in the cabinet, very inconvenient when the chassis is out of the cabinet. So I designed the support as shown in the photo and diagram, it is attached by lengthening the two upper speaker screws and adding two spacers (not fitted yet), and it sits on the speaker brackets. The screw holes are best marked out after the bracket is sitting in place so they are not measured out on the diagram. The CRT sits on it and you can strap the CRT to the bracket with a large sized ( Industrial) nylon cable tie when the chassis is out of the cabinet. The added wooden bracket is such that it can stay put when the chassis is re-fitted to the cabinet, and the CRT mounted in the usual way. The radius of curvature of the cut-out in the wood is 93.5mm, and this reduces to 90.5 mm when the rubber cushion is added to hug the CRT curvature. The bracket geometry ensures the CRT neck is very close to level with the chassis surface. No extra holes need be drilled to fit it. ( Sorry all metric here so to convert millimetres to inches divide the mm by 25.4). The depth of the 14 mm grooves on the added CRT bracket are in the order of 3 to 3.3mm, or about 1/8".
I aligned the tuner RF and converter stages and the local oscillator
and the video and audio IF's. All very easy to align with the sweep
generator without any difficulties at all. I'll send a photo later of a
screen image. The performance is wonderful. The Hammond 300BX power
transformer runs as cool as a cucumber, and with 120 Vrms. 50 Hz input
supplies exactly 6.3 volts rms to the main heaters.
I've attached the photos of how I created a replacement brightness
and contrast pot for the 621. I started with a dual Clarostat pot,
then fitted a thinner longer central shaft(1) to the rear wiper
assembly, then made shaft adapters(2 & 3), which are press fit and
soldered for strength. I got the correct value ( 10K and 500K) carbon
tracks out of other single gang Clarosat pots of the same type. Took a
while to acquire everything needed. Dual shaft pots, especially with
odd shaft sizes like these, are a challenge to replace aren't they?
Here is one of the first screen images I
acquired on the 621TS. This is with a 7DP4, digital camera photo of
the screen, flash disabled. It is a 625 line image (our analog
system), the set easily locks unmodified to our H of 15,625KHz and
vertical field rate of 50Hz in this case with a small change in value of
the vertical blocking oscillator time constant capacitor. I'm still
looking for a good 625:525 line converter, preferably with an on
board VHF modulator.
The screen image you see on your monitor
may have curvilinear lines which are a form of "spatial
interference", or moire pattern, with the scanning lines on the
image and the monitor display. These vary with the magnification of
the image on your monitor. If you print this photo out these
disappear. Also the darkish wide horizontal band at the top of the
image is an artifact, related to the camera's exposure time, vs the
frame rate.
Here is a sweep of the 621, swept across channel 4, at the
antenna input. The basic response of the tuner is fairly flat, but
there are some contributions to the final shape so there is some
inter channel variation in the over all response, but it's small.
The swept response is recovered at the video detector. You can see
the absence of response at the sound channel frequency due to the
great trapping in this set.
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Click for Hugo's restoration of the cabinet