Soooo...I need help (Dynaco FM-3 related)

[caseyjames]

Disclaimer: As with most things concerning vintage gear and electronics, I am essentially clueless. When purchasing things, I have relied on the good people of AK to help and guide me. For that, I am much appreciative.

So here is my latest problem/issue/question: I picked up a Dynaco FM-3 from here. Not really because I needed one but I thought it would be useful for the times when I just want to turn the radio on and not flip records. It arrived today, well packed.

I plugged in all the tubes, hooked it up to my PAS-3X, turned it on, and immediately started picking up stations. Lots of them. Without an antenna. Nice, I thought. I turned the dial over to WMPG which is a rad station and it came in well and the little eye indicated a strong signal and I let it play for a few minutes while I made a salad. Got back, sat down, started eating, and then the sound just cut out. Not like, absolute-no-power-silence, but like dead air silence, if that makes sense. I gave it a moment thinking it was on the station's end. When it didn't come back on, I turned the dial and all the stations I had been getting (minus a couple) were gone. The indicator light for WMPG still showed a strong signal.

So, I hooked up some rabbit ears. Nothing. However, I noticed that when I touched the left one with my palm, I'd get the station. If I just touched the right one, static. If I touched both, static.

What the what?! Am I missing something completely obvious? Any help would be greatly appreciated.

 

[Lavane]

Is it factory wired? Rivits. Or is it a kit? Screws. It could be a bad solder joint. Try gently moving the tubes a bit, might be a dirty tube pin. If they easily come out try removing them one by one and reseating them. Also download the manual and check the voltages.

 

[Lavane]

Bad tube maybe? The manual is pretty helpful to have on hand.

 

[Lavane]

If everything comes back after touching one leg of the rabbit ears I would start looking for bad looking solder joints and reflow them. I would look at the front end first. Facing the tuner from the front its the board on the far right. There is a round headed screw sticking out of the bottom of it that may look like someone soldered a nut to the underside of the board. thats factory. Dont turn it, its an adjustment. But just tap it and see if that makes the station come back in. With the kits the person who built it may have globbed a bunch of solder on it.

 

[caseyjames]

It is kit wired.

I just reseated all of the tubes and fired it back up and it was working like it did initially; lots of stations. However, after about 5 minutes, it reverted back to my current problem.

I played around with it some more and think I have narrowed it own to the tubes on the right side card, PC-7.

Bad solder joint maybe?

 

[Lavane]

Could be. After the board heats up something may be breaking contact. Hope you get it figured out. They do pickup stations pretty good. I rebuilt one a few months ago. Maybe check some of the resistors also. I replaced them all. A goodly amount of those carbon comp resistors were out of spec on mine. one or more could be drifting once heated up. Solder joints would be the first thing I would look at though.

 

[amptramp]

Tube sockets can get dirty and require a blast of contact cleaner and if the socket pins are visible for their length, they may have to be slightly compressed to regain contact. The tube pins can get dirty or corroded, so if resoldering doesn't work, this might.

 

[ConradH]

I've had tubes that would die after a few minutes of operation. Rare, but can happen.

 

[caseyjames]

I've had tubes that would die after a few minutes of operation. Rare, but can happen.

Like die die or die out and start back up after a rest? I don't think it is a tube specific problem on my case. My gut, based on what @Lavane has talked about, is that it is a faulty soldering path. But I won't really know until I can have someone look at it.

 

[ConradH]

I've had tubes that would fail and then come back. A really annoying one steadily increased its leakage until the circuit (scope) wouldn't work. Then as it cooled back down, the leakage went down and everything seemed OK. For a couple minutes anyway. Still, some other problem is more likely.

 

[Punker X]

The bad solder joint could be in a tube. I have seen many tubes go once warmed up. Either quit working or become noisy.