Early Electronic Television
HMV 905 Restoration
This table model. It has a 7 inch CRT and a AM/SW radio. Here is technical information.
Cabinet. (pictures) The cabinet is in excellent condition.
Chassis. It has a single chassis, which is in good condition. This chassis is much more complicated and crowded than most pre-1945 sets. The chassis is identical to the HMV 904 chassis we restored previously.
All paper capacitors will be rebuilt (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 has to be rebuilt.
All of the can electrolytics had been replaced with postwar models. I removed them and installed pre-1945 cans with new guts.
I then connected the set to the power line (mains) through a varaic and measured the AC voltage coming out of the two power transformers. The high voltage transformer (EHT) overheated after a few minutes. I could see arcing inside, and attempted to remove the outer insulation to stop the problem. Unfortunately, I was unsuccessful, so a new transformer will have be be made. Ed Dinning has told me he can make a replacement that is slightly smaller than the original.
I will put this set aside until the new transformer arrives.
Since the 904 and 905 use the same chassis, I installed the 904 chassis in this cabinet to test the CRT. The image is bright and clear.
The new transformer arrived, and I "antiqued" it like the last one.
I discovered a couple of bad resistors, one in the horizontal oscillator and one in the local oscillator. After replacing them the set had a good raster and decent picture. There was no sound, however. I did an alignment of the audio IF sections, which were way off. Now the sound is fine.
This set has a factory modification that is not shown on the schematic. An autotransformer was installed between the audio detector/amplifier and the audio output stage. This results in about twice the audio output of the original design, which produced marginal audio level.
When I installed the set at the museum and connected the coaxial cable RF feed, there was a large spark and the circuit breaker tripped. American sets have bypass capacitors from both sides of the power (mains) input to the chassis, but British pre-1945 sets typically do not, so I was puzzled as to what could cause the problem. I discovered that the metal shield that is installed over the audio output tube (valve) was too close to the power (mains) input, and shorted it to the chassis.
Restoration of this set is now complete.