Pye 817 Restoration
This set just arrived. It is complete, with little rust on the chassis. The cabinet is in almost mint condition. There are four holes on the front where a magnifier was attached. The service manual says that most sets were supplied with Hivac T5M 5" tubes, and some with Mullard tubes. This set came with a Emiscope 3/1 tube, so the tube had been replaced.
The set is missing the rubber mask to mate the tube with the safety glass. A cardboard replica will be made.
Cabinet. (pictures) Because the cabinet is in good condition, we will have it touched up.
Chassis. (pictures) All of the components are installed on one chassis that wraps around the CRT. Restoration and service will be difficult, since it is hard to get to things. Each will be cleaned with water and a mild detergent using soft brushes to get into small places. Then, all paper capacitors will be replaced with modern ones (see the procedure for this). Each electrolytic capacitor will be tested for leakage and capacity. If bad, new electrolytics will be installed inside the old ones. The high voltage (EHT) capacitor also had to be rebuilt.
Since we have no technical information on this set, I first determined which tubes (valves) the set used. Here is technical information.
The 6J5 and 6V6 were later replacements for a Hivac AC/Z?, a triode-tetrode specially made for the set. The substitution looks like a factory one.
I have traced the sync separator, power supply, and sweep circuits. The electrolytics in the power supply have been replaced, and the high voltage (EHT) capacitor has been tested and found to be good. Both power supplies now work.
I have now rebuilt all the paper capacitors in the sweep and sync circuits. The set now displays a dim raster. When I first powered up the set, I had about 2500 volts on the CRT anode. Now I have only about 1000. I suspect that the .07 mfd/3500 volt capacitor in the high voltage (EHT) circuit has gone bad.
The problem with the high voltage (EHT) was that the smoothing capacitor had become unsoldered. Reconnecting it restored the proper voltage.
This set, like all British pre-1945 sets, has a terminal strip to select the AC (mains) input voltage. It was set to 250 volts. I changed it to 230, which is the voltage I have at my test bench. The result was a bright, fully scanned raster.
I connected a RF signal, and, without doing anything, I got an excellent picture. A check of the audio output showed that the sound section was working too.
While testing the set, the power transformer started arcing. I immediately shut the set off, and discovered a black mark on the outer winding of the transformer. That winding is for the filament of the high voltage (EHT) rectifier. It was arcing to the winding below.
I was able to remove the outer insulation, remove the winding, remove the next layer of insulation, install a new layer of insulating cambric, install a new winding (only 11 turns), and put a new outer insulation layer on.
The set now works well, with a bright, stable picture.
The remaining items to be done are to find a new elastic strip to hold the CRT in place, clean the cabinet, and reassemble the set.
After finding a way to hold the CRT in place (I used black friction tape), I reassembled the set. I left it working for about an hour while I disassembled my next project. Unfortunately, the power transformer began to overheat and smoke. I've removed it and sent it off to be rewound.
The transformer could not be rewound, but a new one was fabricated, which is slightly smaller than the original. It fits fine on the lower chassis, but spacers had to be made to attach the top of the transformer to the upper chassis.
The set worked perfectly for a short period of time, but then the CRT failed. I fitted a 5 inch 50 degree deflection test CRT (5AXP4), which required changing the filament supply to 6.3 volts, and adding some external magnets to center the picture.
In November of 2007 an original Hivac CRT became available, which I purchased and had shipped from England. The tube was salvaged from another 817 about 20 years ago, and I had no idea of its condition. Fortunately, it works like new. Here is the story of the tube and the set it came from.