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 Post subject: Weird problems with Silver Marshall 37
PostPosted: Apr Sun 08, 2012 7:55 pm 
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Location: Newark, California, USA
I’m fixing up a very nice Silver Marshall 37. When I got it it “played” to some extent (just a couple of strong stations), but not very well. Most of the wirewound resistors are open or out of spec, and it looks like, to “fix” it, someone many years ago rewired it so that, for example, the screen grid voltage comes from the plate B+ and one of the resistors is intentionally shorted out. My guess is this was a quick fix to get it to play at all. But now I need to try to rewire it as intended. This isn’t that easy because the Rider’s schematic, for example, doesn’t seem to show a connection between the power supply and the B+. It just goes to the speaker plug. The wiring in the set, all crowded and all of it now color coded gray, in a harness, is hard to follow too.

I’ve put it back to the original wiring as best I could, and it plays a little better, but there are some weird things about it. First of all, connecting the Ground terminal to a real ground (copper water pipe) makes the radio play considerably QUIETER. I’ve seen it the other way before, but this is just weird. Also, when I measure the plate voltage on the IF tubes, touching my meter probe to the plate connection makes it play a lot LOUDER. I’ve tried connecting a capacitor from plate to ground in case the existing one is open, but this doesn’t help.

Oh gurus of all things antique radio-ish, any suggestions? Hope you have a nice day.

Bob


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 Post subject: Re: Weird problems with Silver Marshall 37
PostPosted: Apr Sun 08, 2012 9:20 pm 
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Make sure that none of the coil windings are open. If you have a signal generator, try peaking the IF transformers at 175 khz.

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 Post subject: Re: Weird problems with Silver Marshall 37
PostPosted: Apr Sun 08, 2012 11:25 pm 
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Thanks, Tim. No open coils, which is pretty amazing considering how many of them there are. I'll fire up the signal generator and try feeding it 175 kHz.

Bob


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 Post subject: Re: Weird problems with Silver Marshall 37
PostPosted: Apr Mon 09, 2012 12:26 am 
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Actually, it's tuned pretty well for 175kHz IF. Nice, strong output from the push-pull 45s. I almost couldn't attenuate the signal enough to avoid going deaf.

One thing I've also noticed: it looks like whoever did the rewiring has taken the mica capacitor from the plate of the 2nd detector and, instead of going to ground as in the schematic, has connected the other end to the cathode of the same tube. At that socket it came with a 56 instead of the schematic's 27, which I've since replaced it with. Don't know if that has anything to do with the problems. Guess I should try connecting the capacitor back the way it was meant to be.

Bob


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 Post subject: Re: Weird problems with Silver Marshall 37
PostPosted: Apr Mon 09, 2012 2:28 am 
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The 56 can replace the 27, so that's nothing to worry about. However, you should rewire the set exactly as per the schematic; it sounds like someone was doing a little "backyard engineering". I would check all the wiring around the mixer, oscillator, and antenna coil.

As for the wiring to the speaker plug, they are using the speaker field as a filter choke, with the output transformer mounted on the speaker. That was a very common arrangement in the old days.

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 Post subject: Re: Weird problems with Silver Marshall 37
PostPosted: Apr Thu 12, 2012 12:56 am 
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Well, the set now works roughly as well as my Radiola 80, a superhet of the same vintage. One of the resistors (R7) runs hot, but I've put in a 25-watt US Navy special there, so it should be able to take it.

The volume increase on touching the IF tube plate terminals with my digital multimeter is really bizarre. Doesn't do that if I use a VTVM. Doesn't do it if I put in a capacitor or resistor instead. Doesn't do it if I touch the control grid. Just the plate to a DMM, and only when volts are selected on that. That and the reduced volume on connecting a ground are real and reproducible. Still, I shouldn't complain, considering that it works and I haven't managed to a) electrocute myself or, b) burn it (and the house) down.

That's pretty good for me.

Bob


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 Post subject: Re: Weird problems with Silver Marshall 37
PostPosted: Apr Thu 12, 2012 2:14 am 
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Here's the schematic:

http://www.nostalgiaair.org/PagesByMode ... 029670.pdf

Have you replaced C12? If it is leaky, excessive current will be pulled through R7. I would also check the value of R4; a previous repairman may have used an incorrect part.

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 Post subject: Re: Weird problems with Silver Marshall 37
PostPosted: Apr Thu 12, 2012 3:03 am 
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Tim Tress wrote:
Here's the schematic:

http://www.nostalgiaair.org/PagesByMode ... 029670.pdf

Have you replaced C12? If it is leaky, excessive current will be pulled through R7. I would also check the value of R4; a previous repairman may have used an incorrect part.


THAT'S IT!! Replacing C12 give a major improvement in sensitivity and R7 now only runs lukewarm. I had only checked C12, an original bathtub condenser, for shorting, and it didn't, so I had just left it in. R4 I had replaced already since the original was open.

The weirdness is still there, but the set now plays so well (like many old tubes sets, MUCH better than most radios today) that I don't care. It's now a pleasure to listen to.

I can't thank you enough for the advice! Great forum, great contributors.

Bob


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