[Collins] re: newbie question

[Carl]

We are at the point where previously "unbreakable" capacitors are indeed 
breaking.

I would say over the past 5 years Ive experienced a very slow but steady 
increase in bad disc ceramics, dogbones. plus regular and silver micas.

Since I restore radios of all makes, ham, military, home and auto and see no 
particular pattern, I'll leave the reasons up to the chemical engineers.

Carl
KM1H


----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at storm.weather.net>
To: <collins at mailman.qth.net>
Sent: Sunday, June 15, 2008 7:29 PM
Subject: RE: [Collins] re: newbie question


> 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
>
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