[NJARC] Power Supply Question

[njarc@mailman.qth.net]

Dear Jim and John and everyone else following this saga,
       Thanks for your suggestions.  
       I checked the power supply and it is definitely not running hot.  I 
rechecked the electrolytics that I replaced and they tested OK.  Took another 
look over the wiring and the caps that I replaced and all seems OK.  No stray 
pieces of wire or solder.  The tubes are also OK.  So I thought I'd try a 
different value of electrolytic(s) at the power supply end and at the audio 
output end.  I thought that maybe it needed to be filtered more.
       The schematic calls for a 8mfd on the power supply end and a 16mfd on 
the audio end.  I had originally replaced the 8mfd with a 10 and a 22mfd in 
place of the 16. (Schematic called for the 8 and 16)  I thought I'd see what 
would happen if I increased the value at the power supply end with a 22mfd.  
The hum decreased.  Next I doubled the 22 and found that the hum became even 
less.  While trying this at the power supply end, I didn't ignore the audio 
end.  What I found was that if I disconnected the electrolytic at the audio 
end, the hum decreased.  
       So after all of this, my question is, what have I accomplished by 
doing this and why did it work?   Is this just hiding the underlying problem? 
 There is still a hum, not unexpected with an AM radio, but not nearly as 
loud.  Reception and sensitivity remained the same along with audio output.  
As much as I'd like to check the voltages, I'm in over my head on this one.  
What the schematic calls for and what I'm seeing are two very different 
things.  

Thanks again.
Joe




In a message dated 1/2/03 10:26:25 PM Eastern Standard Time, 
oldradio@worldnet.att.net writes:


> Hi Joe,
> 
> I think the transformer is OK, because the radio works.  Re check the 
> filter capacitors to make sure you put the correct value in there (there 
> may be more than one, check them all).  Make sure each one's not defective 
> (substitute).  If you have access to an oscilloscope, you can look at the 
> B+ line for hum on top of the DC.  Follow it to it's source.
> 
> Also look for a tube with a cathode-filament short.
> 
> Look for a capacitor you might have missed when replacing them - also wired 
> 
> in, incorrectly.
> 
> Good Luck, it's going great so far.
> John Dilks
> 
> At 10:04 PM 1/2/03 -0500, you wrote:
> >Hello and Happy New Year to all.
> >
> >        Thank you to everyone who helped me a few months ago with the 
> Sparton
> >Console Model 15 that I am working on.  Well, so far, I've recapped it,
> >replaced the power resistor and have a working radio.  It has very 
> impressive
> >reception without an antenna and excellent with an antenna.
> >        The only problem I have with the radio is the constant 120 cycle 
> > hum.
> >I've tried to localize the problem by trying another speaker and 
> substituting
> >capicators.  No change there.  So what I've narrowed it down to is the 
> power
> >transformer.  I'd like to know if someone can help me make the final
> >diagnosis and if I do need a new transformer, which one to get.   Is it
> >possible to get the original rewound?   Should I be checking it without 
> tubes
> >and powered?  With tubes and powered?  I'll bring the radio to the next
> >meeting so it can be tested.
> >        For a quick reference the transformer has outputs for 2.5, 300, 
> and 5
> >volts.  It has 5 taps on each side.
> >
> >Thanks in advance for your help.
> >Joe




In a message dated 1/3/03 12:02:35 AM Eastern Standard Time, 
antqradio@juno.com writes:


> Greetings Joe
> Interesting radio problem.  Here are some other checks you can preform
> until the calvary arrives.
> 
> Power transformer checks:
> Warm or hot after 30 minutes?  Warm = OK, Hot = troubles.
> 
> Source of 120 cycle hum:
> Ground wiper of volume control, does hum go away?
> If yes then source of hum is before audio amplifier.
> If not then source of hum is in audio amplifier.
> 
> In any event, substitute all vacuum tubes one at a time with tubes you
> trust to eliminate heater to cathode leakage (tube checker short test is
> helpful here).
> 
> Check B+ regulation.
> Measure DC voltage on B+ line.
> Set meter to AC volts to read ripple voltage.  Divide this reading by the
> first and multiply by 100.  This should be % regulation.  As a guess,
> less then 5% should be OK.  This is a crude approximation, John suggested
> an o'scope which is preferred (if you have access to one).
> 
> Let us know what you find,
> Jim
> 

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