[NJARC] Power Supply Question

Sat, 4 Jan 2003 00:06:04 -0500

Hi Joe
Glad to hear that the power transformer is OK.  Tubes and caps are much
cheaper then iron!

I wouldn't go much higher then 1.8 times the original value of capacitor.
 The tolerance of the older electrolytics was +80%-20%.  So the 8mfd
could go up to 14mfd and the 16mfd could go up to 28mfd.  You wouldn't
want to go much higher because the rectifier tube will see a higher peak
current then it can safely handle.  The tube will work harder and have a
much shorter life.

By adding more mfds, you are probably hiding the real cause of the hum. 
Honestly, I would try tube substitution.  Those cathode to heater leakage
currents will modulate the cathode bias and the tube will amplify this as
just well as any desired signal on the control grid.  The tube that has
this problem could be anywhere, not just in the audio chain.  Try tapping
each tube with a pencil eraser.  Does the hum level change?

Another thought.  Beg, borrow or steal a copy of the Radiotron Designers
Handbook, 4th edition (red book) and check out the information around
page 1150.  It is a great radio book, full of all kinds of relevant
information (with regards to your problem) on power supply filters, audio
power amplifiers and why things are done the way they are (were).  I
never leave home without it!

Keep in touch,
Jim

On Fri, 3 Jan 2003 22:49:41 EST [email protected] writes:
> 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.

snip
The tubes are also OK.  
snip.  
>        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? 
snip
> 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, 
> [email protected] 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, 
> [email protected] 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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