[Collins] re: newbie question

Sun Jun 15 23:09:47 EDT 2008

Hi Carl!!
    I've been at this for a while as well.  I've found that paper caps 
are usually not useable and I replace them out of hand.  I always check 
micas, and ceramics as I have found a number that are leaky or bad. 
Dogbone caps are nothing more than paper caps in a different wrapper. 
Moisture and environmental heat do more damage than anything else, but 
certain types or manufacturing techniques are more prone to failure than 
others.  I don't trust anything that's been there for a while. 
Resistors don't fair any better.  These components are cheap, compared 
to the time to troubleshoot one component in a set.  So, unless 
something is very difficult to get at, and can be tested easily, when 
I've been there, it's in the box of stuff that gets handed back to the 
customer as "not useable."
----- Original Message ----- 
From: "Carl" <km1h at jeremy.mv.com>
To: <geraldj at storm.weather.net>; <collins at mailman.qth.net>
Sent: Sunday, June 15, 2008 7:54 PM
Subject: Re: [Collins] re: newbie question


> 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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>
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