Early Electronic Television
DuMont 180 Restoration
The Cabinet: (pictures) The cabinet is in excellent condition. All that is needed is some touchup.
The Chassis: (pictures) 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. If bad, new electrolytics will be installed inside the old ones. The high voltage (EHT) capacitor also had to be rebuilt.
The CRT and sweep circuits are copies of the Cossor 137 design, using American rather than British tubes.
The RF amplifier, oscillator and first IF stage have been modified with postwar miniature tubes (valves), and one stage of IF amplification has been eliminated. I have re-installed the original tubes for the first and second IF (6AC7). Now I need to find out what the original tuner looked like. DuMont made two different chassis for their sets. Our 180 is an early model, and is different from our 183X. The tuner is completely different. I will attempt to find an early chassis and get photographs of the tuner so that I can reconstruct the original design.
I now have pictures of the original tuner. Most of the components are visible, but the coils in the RF amplifier stage are hidden under the switch. Here are photos of the original tuner and the replica I have built.
I had to experiment with the RF coils until I found the proper number of turns. I now have the RF amplifier and mixer stages working. Only the oscillator needs to be completed.
The tuner is now complete, and it works perfectly. The capacitors have been rebuilt. This set has 4 power supplies: +3500v, -3000v, +1500v and +300v. All of these supplies now work, and I have started debugging the sweep sections. The sweep sections now work, as do the video amplifier and sync circuits. I inserted a video signal at the input of the first video amplifer, and got a decent picture on the screen.
DuMont had four different CRTs in their pre-1945 sets. The first was the 144-9-T. These tubes were used in the 180, 181 and 183 sets of 1938 and early 1939. In mid 1939 an intensifier anode was added to the 144-9-T. Later that year the 14AP4 was introduced, with an intensifier anode. These tubes were used in the 180X, 181X and 183X sets. After the war the K1003P4 was introduced as a replacement for these tubes.
Our 180 has a 144-9-T with the intensifier anode. Unfortunately, it has an open filament. Our 183X has a K1003P4 tube, which I tested in the 180 chassis. The tube works perfectly. Actually, our K1003P4 is a developmental tube. The CV-1085 radar tube with a P7 phosphor should work in this set as a replacement.
While debugging the set the HV (EHT) transformer started smoking. It will have to be rewound. Apparently the transformer had moisture in it which has been driven out by the heat. It no longer overheats.
The set now has a good picture and sound.
When I took the chassis from my workbench to the museum and put it in its cabinet there was a periodic discharge of the high voltage (EHT), causing the picture to flash about once a second. I took it out of the cabinet and turned the lights out and brightness down, but couldn't see any flashing.
I took the set back to my workbench, and the problem disappeared. Took it back to the museum, and it started the discharge again.
Back at the museum I listened to see if I could localize a thumping noise that went with the discharge. It seemed to come from the area of the chassis where the CRT voltage divider for the focus, brightness, etc. was. I then started disconnecting parts. Finally, when I disconnected the coupling capacitor from the video output to the tube the discharge disappeared.
That capacitor is a .05/4000v inside a metal tube with cardboard on top. I had previously removed the cardboard, cut the metal in half, removed the old capacitor and installed a new one. I took it apart again, and noticed that a small piece of metal that was part of the old capacitor was still there, creating a gap of about 3/16 inch between the metal can and the lead coming out (which was connected to the grid of the CRT, at -3800v). This gap was causing the discharge.
My theory is that the humidity was different at the bench, and thus no discharge.