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New guy here, scratching my head with radio problem.

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Post by bobwilson1977 on Sun Feb 08, 2009 4:57 pm

Hello there,
I've been messing around with radios off and on for about 10 years. I'll admit that I'm definitely more of a amateur, thus not as well informed as most of you all are. Anyhow, I have this mysterious problem with a radio I've been restoring. Its a Hoffman set from the 40's with typical all american 5 layout. The unit uses a transformer for primary power. When I brought the set home, all of the tubes lit but there was no audible sound. Only a very quiet hum, which sounded more like feedback from the transformer.

Since then I have replaced all capacitors and tested all the tubes. All tubes tested very good. Once done re-capping, I tested the set out. Again there is no sound. Everything lights up. All wiring appears intact. All connections to the speaker and speaker transformer appear intact as well. When I mean quite, I mean nothing. No static or anything. So I am scratching my head on this one and hoping some of you might have an idea of some things I could try to remedy the problem. My only guess- and this is a big guess, is that perhaps the volume control is faulty.

Thanks in advance!

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Post by mbird97x on Sun Feb 08, 2009 5:11 pm

Welcome Bob. AA5 and power transformer is contradicting. AA5's are series strung filaments using no power transformer. That aside, you might touch the center tab on the volume control and see if you get a loud noise or growl from the speaker. If not, you need to check the components in the audio amplifier stage. Speaker, Output transformer windings, resistors in that stage. If you do get the loud noise out, audio amp. stage is working. Do you have a model # so we can look up a schematic and see the circuit?
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Post by geno on Sun Feb 08, 2009 5:16 pm

This may sound too easy, but are you sure the tubes are in the correct sockets? I've seen it before, where someone had switched a tube around.

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Post by bobwilson1977 on Sun Feb 08, 2009 5:22 pm

Apologies, its actually a 6 tube unit. The model is a A300 Hoffman. Now, when I touch the center tab of the Vol. control, I assume I would also be touching the chassis? Thanks in advance.

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Post by bobwilson1977 on Sun Feb 08, 2009 5:22 pm

Yes, I actually did check that as well. They are all correct.

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Post by bobwilson1977 on Sun Feb 08, 2009 5:24 pm

I'm also going to check the resistors in the circuit as there was one that appeared to be slightly damaged- burned.

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Post by bobwilson1977 on Sun Feb 08, 2009 5:43 pm

New guy here, scratching my head with radio problem. Radio111

Here's a pic of the underside. I also realized that I missed one of the capacitors. Its a .001, 600 volt dry capacitor in a metal wrapping. It connects to several pins of the 6X5 tube as well as on of the transformer leads. I wonder if this is the culprit.

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Post by bobwilson1977 on Sun Feb 08, 2009 5:44 pm

New guy here, scratching my head with radio problem. Radio212

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Post by dorpmuller on Sun Feb 08, 2009 6:34 pm

You did a nice job though... looks a lot better than the RCA I just did.
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Post by sprman on Sun Feb 08, 2009 7:01 pm

Check and make sure its not the silver/mica caps bad in the IFs! Dont know if it has them or not.Sprman : <

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Post by bobwilson1977 on Sun Feb 08, 2009 7:02 pm

Well, just got back from soldering in the last cap. Still nothing. There is a very slight hum that is definitely coming from the speaker. The switch is also the tone control. Vol is separate. I think the next step might be to see if the schematic pasted to the bottom mentions the speaker transformer power output. I'm sort of guessing at this point.

As far as compliments on the job, well it was a heck of a mess soldering some of those in there. It was a real rat's nest. The new ones sure take up a lot less room so it looks nice and tidy!

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Post by bobwilson1977 on Sun Feb 08, 2009 7:04 pm

Me being an amateur and all, when you mention mica caps in IF's, what do you mean by IF? Sorry for my ignorance.

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Post by Tim Tress on Sun Feb 08, 2009 7:09 pm

With the volume control turned up, you should get a hum from the speaker when you touch the grid of the 6SQ7, or the center terminal of the volume control. Have you checked any DC voltages on the tubes?

Schematic is here:
http://www.nostalgiaair.org/PagesByModel/181/M0009181.pdf

Tim KA3JRT

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Post by bobwilson1977 on Sun Feb 08, 2009 7:20 pm

Haven't checked the DC of the tubes yet. Again, professing to my ignorance, when touching the center terminal of the volume knob, what is the procedure? Sorry for the dumb question.

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Post by mbird97x on Sun Feb 08, 2009 7:29 pm

Marcus and Levy's book on servicing radios suggests touching it with tip of soldering iron. Most just touch it with a finger.
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Post by bobwilson1977 on Sun Feb 08, 2009 7:30 pm

Thanks. I'll try that and see how it goes. I'll get back with results.

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Post by bobwilson1977 on Sun Feb 08, 2009 7:36 pm

Nope. no sound at all after touching center terminal. I suppose the next stage is to check the transformer and resistors. There's only a few resistors in the amplifier stage so I might just go ahead and replace these while I'm at it. If I get the radio working, there will be very little original electronics left!

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Post by exray on Sun Feb 08, 2009 7:52 pm

Do you have a voltmeter? That would confirm that there is actually B+ voltage available.
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Post by bobwilson1977 on Sun Feb 08, 2009 8:14 pm

My housemate has one and he is on vacation. Mine for some reason bit the dust.Had i for 15 years. I need more components anyway, so I might as well buy a new one.

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Post by sprman on Sun Feb 08, 2009 8:44 pm

Hi i mean the intermedate freq amps.Some have silver/mica caps in them and they migtate and cause set not to work.Most are set to 455kc or some other feqs.Sprman : )

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Post by MinnesotaHam on Sun Feb 08, 2009 9:20 pm

mbird97x wrote:Marcus and Levy's book on servicing radios suggests touching it with tip of soldering iron. Most just touch it with a finger.

The book recommends either method.
I'm too squeemish to use my finger on the bottom side of a powered chassis.
There shouldn't be much voltage on the volume pot unless a few things are wrong with the set but.....I don't do it.
I use an insulated screwdriver or the soldering iron.

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Post by bobwilson1977 on Sun Feb 08, 2009 9:35 pm

Ya, I too am a bit leery of touching it with my hand. Some of the caps I replaced are extremely high voltages. I'd kind of hate to accidentally touch something wrong in there. Thanks for all the advice so far!

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Post by bobwilson1977 on Sun Feb 08, 2009 9:40 pm

Thanks Sprman.intermediate freq amps... we're talking the big square boxes mounted topside? Are the mica caps replaceable in these? Never taken one apart which is why I ask.

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Post by mbird97x on Mon Feb 09, 2009 12:15 am

Looking at thr Riders on Nostalgia Air, They look like the type w/2 trimmer caps in them. Usually not a problem.
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Post by Terrible Terry on Mon Feb 09, 2009 2:35 pm

Bob, when you go to a doctor, the first thing they usually do is check your blood pressure, right? This is because it gives the Doc a good preliminary indication of your over-all health. It can tell him if you are about to drop dead from some pervasive system-wide malfunction or if you've just got some local trouble somewhere. Same thing with a radio. You want to check some voltages to see if all the stages are getting the proper power currents that are needed by them, just like your body needs proper blood flow in order to work.

The Rider's information gives a nice table of voltages at the tube socket pins. Connect the negative lead of your voltmeter to the chassis and the positive lead to the pins indicated. Count the pins clockwise (when viewed from the bottom) starting at the keyway on the black plastic center prong of each tube. Switch to "A.C." on the meter where indicated.
New guy here, scratching my head with radio problem. Voltag10
Once you have made these measurements, if you post the results, or at least any that are significatly different, many folks on this forum can give further help. Until you know that the voltages are more-or-less correct, you are just speculating and probably spinning your wheels.

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