General Electric HM-171 #2 Restoration
The Cabinet. (pictures) The cabinet is in poor condition and is being restored by Old World Furniture in Columbus. The two extra holes, added when the audio output circuits were added, will be plugged.
Though our set had an original escutcheon for the channel selector, two other collectors had GE sets that were missing theirs. After quite a bit of research, we were finally able to find a company that could make replicas. Here is the replica, along with the original plate:
The Chassis. Here are pictures, schematic diagram and technical information. The chassis 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. New electrolytics will be installed inside the old ones.
This chassis has been modified to add an audio output stage, using a design similar to that used in the HM-185. In the HM-185, the 6H6 detector was replaced with a 6SQ7 detector/audio amplifier, and a 6V6 was added. In our set, the 6H6 is retained, and a 6SF5 audio amplifier and 6V6 audio output tube have been added. The HM-171 chassis already had one extra hole for a tube socket, and in our set a second hole was installed. The chassis has two additional holes on the front for a volume control and a tone control. We suspect that the audio circuit was added in the 50s, based on the components used.
This set appears to have most of its original tubes, and very few components appear to have been replaced.
The first task was to rebuild the 4 high voltage sweep coupling capacitors and the HV (EHT) filter capacitors. These capacitors are soldered together, and require a special procedure for rebuilding them.
The electrolytic capacitors in the set had been replaced with more modern ones, probably from the early 40s. I replaced them with the original type
The one in the set
After rebuilding all the high voltage capacitors, but before rebuilding the paper capacitors I decided to apply AC to check the power transformer. After operating the set on reduced AC for a while, I applied full power. Amazingly, the set displayed a raster. However, I could hear a slight popping noise coming from the power transformer, and the screen flashed with each pop. I removed the power transformer and removed the side covers to look for any damage. There was none, but I put heat shrink tubing over all the leads that have HV (EHT) on them. I then replaced the transformer, and still had the popping.
I noticed that when I removed the 2X2 HV rectifier tube the popping stopped. It seems that the transformer is breaking down internally when under load. Having the transformer rewound is expensive, and over half of the transformers I've had rewound failed within a year. So, I wondered what would happen if I reduced the load on the transformer. There is a 39k resistor from the HV AC output of the transformer to the plate of the 2X2. I tried replacing it with a 270k, and the popping stopped. The HV output of the 2X2 dropped only a few volts, and the raster no longer flashed. I will run the set for several hours to see if this holds up.
The breakdown in the transformer was in the 2X2 filament winding, since it disappeared when the tube was removed. After running the set for a couple of hours, I replaced the 39k resistor, and the popping and flashing stopped. Apparently the heat from the transformer baked out some internal moisture that had built up over the years. The high voltage has worked perfectly for many hours of operation now.
All of the electrolytic capacitors in this set are fine. None of the cans overheat, a sure sign of leakage, and each tests normal. I rebuilt all of the paper capacitors. A normal raster appeared, with the horizontal and vertical sweep on frequency. Next I cleaned the contacts of the three position tuner switch, and lubricated the mechanism. Then I applied a channel 3 signal and adjusted the local oscillator and the two RF trimmer capacitors. The set displayed a very nice picture.
The tuner. RF trimmers are on the front and top
The actual picture looks better. The CRT is missing the magnetic shield, resulting in hum from the power transformer
Next I turned to the audio section. I have limited test equipment where I am restoring the set (Colorado), but I do have a frequency counter. I set the fine tuning for a local oscillator frequency of 74.0 mHz (65.75 mHz, the channel 3 aural carrier + 8.25 mHz, the aural IF frequency) and put a scope on the 6H6 video detector plate. I then adjusted the 3 audio IF transformers for maximum deflection on the scope. Then, I moved the scope to the audio output of the set and detuned the fine tuning to put the audio carrier on the slope of the IF bandpass. I could see the audio on the scope, so the audio section is working now.