[Collins] re: newbie question

[Dr. Gerald N. Johnson]

On Sun, 2008-06-15 at 11:15 -0700, Chris Kepus wrote:
> I had a similar problem with a different rig...undoubtedly simpler circuitry
> but nonetheless, very, very similar symptoms.  
> 
> The problem was a failed disk ceramic bypass capacitor that had not
> completely shorted.  I know, these things are not supposed to fail but in my
> case, one did.  Here's the whole story if you care to read.

I wouldn't say disc ceramics don't fail, but they are not chronic
leakers like the black beauties where all have excessive leakage. Where
its most productive to replace all black beauties on sight (shotgun)
without testing, the failure rate of discs is low enough to not make it
good for the radio or repair productivity to shotgun all the disc
ceramics. 
> 
> I was checking out the HV power supply with all the tubes out of the set.  I
> had already checked the power xfmr with no connections to the filter system
> and found it to be healthy… At 117VAC input, it was cranking out 736 VAC
> across the entire secondary.  When I plugged in my “diode 5U4” (which
> allowed me to bring up the B+ gradually in the remaining B+ distribution
> circuit minus tubes) and started bringing the AC voltage, my meter on the B+
> DC line wasn’t showing the VDC advancing as it should.  I knew I had
> problems.  I expected a minimum DC voltage of .9 Vin.  So at 100 VAC in, I
> expected *at least* 100 VDC into the cap input filter (C-L-C) (possibly a
> lot more since there was no load other than a 25K bleeder).  The actual VDC
> reading at the output side of the choke was around 27 VDC.  So I stopped the
> voltage tests and went into the troubleshooting mode.  I quickly eliminated
> the choke and the filter caps as suspects.  Then what?
> 
> The B+ distribution circuit showed only 560 ohms to ground resistance
> (rectifier pulled).  Not good.  When all the tubes were pulled and the
> bleeder lifted off ground, there was no direct connection from the B+ line
> to ground.  At worst, the resistance should have been 1 meg which was
> provided grid bias and it checked OK at 1 meg+.   The only "suspect"
> components left after triple checking all solder joints and the
> aforementioned resistor were five disc capacitors that were between the B+
> line and ground points.  But disc caps never fail, right?   Wrong. 

Disc ceramics don't fail often, but nothing is perfect.

> The
> second cap I disconnected to test was a .001 bypass on the modulation tranny
> secondary…and VOILA!  "Infinite" DC resistance was restored between my PSU
> and the remainder of the B+ distribution circuit.  I measured the resistance
> across the cap after removing it and it was…..yep….560 ohms.   I put a new
> .001 cap in and fired the rig up again.  This time all was well.

You did good. Reasonable troubleshooting technique. Sometimes you can
put the the various isolation resistors in a B+ distribution line to
work showing greater voltage drop towards a short. I did that once for a
12 short in a new radio where I could read millivolt rises and search
for the lowest voltage along the PC board traces while limiting the
current to the radio. Found a solder bridge.

Sometimes you can leave power on for a bit and find a hot capacitor and
know its leaking excessively. That may have worked for your .001 disc
ceramic. Trouble is a really bad capacitor can take the power supply
parts with it and they are harder to find and replace.
> 
> Good luck, YMMV.
> 73,
> Chris
> W7JPG
> 
> -
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer