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Garbled output Stomberg Carlson 561 - Printable Version

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Garbled output Stomberg Carlson 561 - garyh1 - 01-04-2009

I purchased a Stromberg Carlson model 561 which is a six tube superheterodyne receiver designed for operation on either DC or AC currrent. It did power up and receive some stations but with a loud hum at all volume levels and after a few minutes of operation at higher volume levels output would become intermittant and garbled. I ordered schematic and engineering info as well as electrolytics and other capacitors. After recapping the hum was gone and the radio performed nicely until after a few minutes the output became garbled at any volume above around twenty five percent. This is my first attempt at working on a tube radio and don't know where to go from here. I understand that there may not be much you can tell me without engineering info in front of you, but I was hoping that this may be a common problem and you could suggest where to begin looking. Also there one other thing about this radio that I am not sure is normal. When first powered up the dail light turns very dim and then brightens as the radio warms up. Thanks Gary

Re: Garbled output Stomberg Carlson 561 - codefox1 - 01-04-2009

Did you clean the volume control ? How about the tube sockets and tube pins. Did you clean them and re-tention the tube sockets to assure good contact? Did you get all the old wax and dust out of the chassis and wiring? This could be it. Did you replace all the coupling capacitors, especially in the audio section? Go over all your solder joints again with the soldering iron just to make sure you don't have a cold joint. Did you measure the resistors when you had the set apart? Anything more than 20% off should be replaced. Any sign of cracking or discoloration on resistor indicates it should be replaced. There is also the possibility of a gassy output tube, or a weak 1st audio/detector. If you have spares, swap them out and see if it makes a difference. Check your voltages on the 1st audio and output tubes, if your schematic shows them. I assume the speaker is in good condition, but you can check this out by connecting a known good one to the voice coil winding. As this is probably a hot chassis type, be carefull not to touch anything when it is plugged in. Anyway that's where I would start.

The pilot light is behaving normally. As the tubes begin to conduct after warming up, the pilot light will become brighter. At high volumes, the pilot light may "pulse" slightly with the sound.

You can upload the schematic to this site, and then we can all have a look.

If you are receiving a few stations, and the hum is gone, you are 90% there.

Re: Garbled output Stomberg Carlson 561 - garyh1 - 01-04-2009

After taking the set apart and injecting some tuner cleaner into the volume control the radio seems to be playing much better. It has been operating at about half volume for about an hour now with no deteriation of sound. I think I will give it another hour or so before installing it back into the cabinet. The tube socket pins seem to be ok and I had replaced all the paper capacitors except one as it was not available from my supplier at this time. I was also wondering if it is ok to use the "Orange dip" style capacitor in place of the original "tubular axial" style. Thanks a ton for your great advice.. I have really enjoyed working on this old radio. Gary

Re: Garbled output Stomberg Carlson 561 - codefox1 - 01-05-2009

Sounds like you are OK. Od course, orange drops are OK to replace the tubular caps. The range of -20 to +50 percent is usually OK in values, if you cannot get an exact match. I always replicate the windings around the grounding caps (6 or 7 turns around the .15 or .25 capacitor to somehow null out possible problems with the I.F. There are two of 'em on shortwave sets. There were even "magic" capacitors in some sets in the late 40's which attached one lead of the capacitor to the center of the foil to do the same thing. I now think this is snake oil, as I can't tell the difference when I sub a new cap of about the same value for the old leaky greasy plug. Somebody is going to disagree with me, I'm sure, but I still wind a few turns of wire around a nice old (non leaking) oil capacitor from my stash to preserve the looks and perhaps intention of the design.

If you need a non standard value of capacitor, you can probably parallel two to make up the value in many cases. If you need an .03 capaciror, abd you have only .01 and 0.2, you can probably just put them together in parallel and be done with it. I would probably just go with the .02 alone and see what happens. Remember that old leaky moisturized capacitors drift up to 50% and keep on going for decades before causing serious problems. That's how fault tolerent tube circuits are.

I hope this is helpful.