03-12-2016, 01:40 PM
Just finished up a 1936 RCA T7-5. Total recap, resistors checked & replaced as needed. Crumbling wire replaced. It was pretty straightforward, and started to sing immediately after the work was completed. An alignment brought in even weak stations across the dial, including across the short wave and police bands.
I had it playing out of the cabinet for 3 days on the bench while working on other projects, and waiting for a repro dial scale from Radio Daze. Yesterday, the dial scale arrived, which I installed, and put it all back in the cabinet. Again, it played great. I turned it off after about 45 minutes, went upstairs and had some dinner, and after I ate, I went back down with my camera to take some pictures of it. I turned it on, and had it playing for about 5 minutes. When I turned to pick up the camera, all of a sudden Diana Ross stopped singing in mid song. I thought the local station lost power....something that happens not too infrequently. So I tried tuning to a different station. NOTHING. Absolutely nothing across the entire AM band, save for a little bit of static at the very low end of the dial. So I flipped to the short wave band. It worked as it did before....had Mexico, England, Japan playing. I then flipped to the police band, and even got a few stations on it. So I determined the other two bands are working fine, but the AM band suddenly went dead.
I started at the beginning...the most obvious, although I didn't expect it would be that simple. But one by one I tried swapping with known good tubes. As I figured, it made no difference, but at least I can safely rule out that it's NOT a tube.
I lightly blew some compressed air into the variable capacitor, thinking maybe something fell between the fins, but that didn't help either. Nor did cleaning the band switch again. So now I'm to the tracing wires stage from the band switch, but I wonder if anyone else has experienced something like this and can help me zero in on what could be the culprit? What would suddenly cause the AM band to go dead? It almost acts like a coil opened. Here's the schematic:
http://oldtech.net/RCA/T7-5/35P390.gif
And here's the wiring diagram:
http://oldtech.net/RCA/T7-5/35P391.gif
Thanks,
I had it playing out of the cabinet for 3 days on the bench while working on other projects, and waiting for a repro dial scale from Radio Daze. Yesterday, the dial scale arrived, which I installed, and put it all back in the cabinet. Again, it played great. I turned it off after about 45 minutes, went upstairs and had some dinner, and after I ate, I went back down with my camera to take some pictures of it. I turned it on, and had it playing for about 5 minutes. When I turned to pick up the camera, all of a sudden Diana Ross stopped singing in mid song. I thought the local station lost power....something that happens not too infrequently. So I tried tuning to a different station. NOTHING. Absolutely nothing across the entire AM band, save for a little bit of static at the very low end of the dial. So I flipped to the short wave band. It worked as it did before....had Mexico, England, Japan playing. I then flipped to the police band, and even got a few stations on it. So I determined the other two bands are working fine, but the AM band suddenly went dead.
I started at the beginning...the most obvious, although I didn't expect it would be that simple. But one by one I tried swapping with known good tubes. As I figured, it made no difference, but at least I can safely rule out that it's NOT a tube.
I lightly blew some compressed air into the variable capacitor, thinking maybe something fell between the fins, but that didn't help either. Nor did cleaning the band switch again. So now I'm to the tracing wires stage from the band switch, but I wonder if anyone else has experienced something like this and can help me zero in on what could be the culprit? What would suddenly cause the AM band to go dead? It almost acts like a coil opened. Here's the schematic:
http://oldtech.net/RCA/T7-5/35P390.gif
And here's the wiring diagram:
http://oldtech.net/RCA/T7-5/35P391.gif
Thanks,
Greg V.
West Bend, WI
Member WARCI.org
