Early Electronic Television
HMV 904 Restoration
Cabinet. The cabinet was in fair condition and we have had it refinished.
Chassis. (pictures) It has a single chassis, which has severe rust. We will remove all the components and have it replated. This chassis is much more complicated and crowded than the RGD audio/video chassis we had replated, and the tube sockets are riveted in place. First, high definition photos are taken of the top, bottom, front, and back. This will give us a reference for part locations, etc. when we re-assemble the chassis.
Then, the 4 large terminal strips under the chassis are removed. sketches are made of each, and each wire that is clipped is labeled with a sequential number. A log is made of the point to which each number attaches.
We will remove about half of the wiring at a time. The rivets holding the tube sockets will be drilled out, and parts on the top of the chassis will be marked and removed. Finally, the sockets are related wiring will be removed as one piece.
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.
All of the components have now been removed from the chassis. This required marking over 100 connections that were cut. The chassis and other metal components will now be taken to be replated. The replating is complete.
I have now reassembled about half of the chassis. So far, the documentation I did when I stripped the chassis has been adequate, and, with the schematic, I have been able to put things back as they were. However, I am just starting on the bandswitch area, which is the most complicated.
Most of the chassis is now re-assembled. Many of the potentiometers are defective, and I have been able to clean most of them. One, however, has a broken shaft and the carbon inside is broken. I can repair the shaft, but I don't know what to do about the carbon. I can replace the entire assembly (it is a dual pot), but the replacement would be postwar. Both of the toggle switches did not work. I took apart one and cleaned it, and that fixed it. The other had a broken part inside. Fortunately, I had another switch with the same part in it.
All of the high voltage (EHT) wiring was bad. I replaced it with test lead wire covered by the sheath of RG-58 coaxial cable to make it look like the original.
After rebuilding the electrolytic capacitors, I installed the rectifier tubes (valves) and applied AC. The power supplies both worked. I then installed the horizontal (line) and vertical (frame) tubes and the CRT. I got a nice, bright raster on the CRT. There is a large ion burn on the screen, but it is not very visible.
The chassis is completely re-assembled, but I have several questions about where connections need to be restored. I have bought another 904, which has arrived. I will use it find out how to finish the connections.
I have completed restoration of the chassis, and am now testing the RF sections. The video section now works. I am now working on the audio and radio section.
Restoration of this set is now complete.