Early Electronic Television
Andrea 8-F-12 Restoration
This set is a 12 inch mirror in lid model with a radio and phonograph.
Cabinet. The cabinet is in fair condition, with many scratches on the top. It will need to be refinished.
Chassis. There are four chassis: Low Voltage Power Supply, HV Power Supply/Time Base, Audio/Video, and AM/SW Radio. 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.
Low Voltage Power Supply Chassis: (pictures). This chassis is in excellent condition. It contains the low voltage supplies for the TV chassis. All of the electrolytic capacitors have been tested, and, amazingly, they are all good. This chassis now works.
High Voltage Power Supply/Sweep Chassis: (pictures). This chassis is in excellent condition. It contains the HV (EHT) supply for the CRT, and the sweep circuits. It was made by RCA for several other manufacturers, including Emerson and Andrea. I have completed rebuilding the paper capacitors. This chassis was modified after the war, and two of the tubes (valves) used as sync separators have been disabled. I have restored the circuit to the original design. I also tested the high voltage (EHT) transformer, and found that the primary is open. I will attempt to take it apart (it is potted in tar) and see if I can see a fault. Otherwise it will need to be rewound.
The transformer cannot be repaired. Since it is inside a metal housing, I will have a new one made that will fit inside.
The horizontal (line) and vertical (frame) sweep circuits work properly, and so does the sync separator and amplifier. I will now borrow the high voltage (EHT) transformer from another set awaiting restoration and try this chassis and the Audio/Video chassis in the cabinet with the CRT.
I have replaced the high voltage (EHT) transformer, and now have 7000 vdc. I installed the chassis in the cabinet, and the CRT is sharp and bright. There is a problem with the vertical (frame) output - insufficient height. Also, the focus is not quite right. I will check the resistors in the voltage divider.
Audio/Video Chassis: (pictures). This chassis is in excellent condition. It contains the RF, IF and sound and vision detectors. The original 5 channel tuner was replaced after the war with a 12 channel unit. In addition, the chassis was extensively modified after the war. The number of video IF stages was reduced from 5 to 3, the video detector was changed from a 6H6 to a 6AL5, and a second 6H6 used as automatic gain control was removed. Different video output and sync separator tubes (valves) are used. I have decided to return the video detector, sync separator, DC restorer and video output stages to their original design. This involves entirely removing the present circuitry and replacing it. The decision is based on the likelihood that the present modification was not done by Andrea, and was done by a postwar experimenter. Though the changes I am making are extensive, I feel it is better to put the set back to its original design.
For now, I will leave the IF and tuner as it is. It appears that most of the IF transformers have been replaced with very different designs, and finding the original transformers will be impossible. If I am able to get photographs of the original tuner, I will attempt to return the tuner to the original design.
I have now restored the original circuits for the video section and the three IF stages. I have tested the RF, IF, and video sections, and they work normally. The additional gain of the postwar tuner allow the set to work with 3 instead of the original 5 IF stages.
While checking the IF alignment I discovered that the IF has been changed from the original 8-12 mHz to 21-25 mHz. I also discovered that the schematic I have for the set includes sketches of the original IF transformers. They were apparently not inside cans. I will investigate building replicas and returning the set to the original IF frequency.
When installed in the cabinet, I discovered that the sync is not very stable. I will investigate the cause.
Finally, the entire sound section has been changed. In the original circuit, one sound IF amplifier was used, feeding 8.25 mHz to the radio chassis. In this set, two stages of sound IF are used, with a FM ratio detector and an audio amplifier stage. Audio instead of RF is fed to the radio chassis. I plan to return the audio section to its original design.
I finally got some photos of what the original chassis looked like. It uses an unusual trimmer capacitor which will be hard to find. I will have new sheet metal shields made, then attempt to wind the tuner and IF coils.
Several collectors told me that the Philco 38-7 radio used a similar trimmer. I was able to locate one, and it appears that I can make replicas with sheet metal and plastic. I will put a modern trimmer inside.
AM/SW Radio: This chassis is in excellent condition. It contains a high quality AM/Short Wave radio.