[R-390] R-390A IF deck caps in T501 or T502

[Larry H]

Well, after numerous interruptions, I finally finished changing the 200 pf
caps in IF xformers T501 to T503.  Yes, all 3 have the same cap
configuration, a 200 pf silver mica and a 39 pf across each coil.  It was
easy to change all the caps with out removing the IF xformers.  The
resonant point on each coil is still quite stable, even during warm-up.  As
far as I can tell, only one was bad and 2 had drifted slightly.

I'm not an expert on SM caps, so does it mean that the 2 caps that drifted
were on their way to being bad?

Regards, Larry



On Sat, Jan 26, 2019 at 1:54 PM Larry H <larry41gm at gmail.com> wrote:

> Hi Dave,  Yes, I changed the caps with the T501 still installed.  It was
> easy since my IF xformer does not have the thick coating Jacques was
> referring to.  I replaced both caps on the one failing coil because I
> wanted to be sure it was a cap problem.  I temporarily installed a 240 pf
> without  trimming the leads as a test.  Once determined, I was going on the
> assumption the bad cap was the 200 pf mica and installed a new one of
> those, and reinstalled the original 39 pf.  As it turned out, I guessed
> right.  A person could go the other route and leave the original 39 pf in
> and replace the 200 pf as a test.  Good luck.
>
> Regards, Larry
>
> On Sat, Jan 26, 2019 at 10:07 AM Dog T <agfa at hughes.net> wrote:
>
>>
>> I was working on this issue last spring/summer. It took me quite a while
>> and it was all described in my thread 'Gain Drift'. I used a Boonton
>> (really a Moto) millivolt meter to start measuring gains along the IF tubes
>> and found that between T502 the input was good, but the output changed.
>> Could find no problems with tubes or voltages, although I did not check all
>> the voltages, but I did narrow it down to T502. I just got sorta burned out
>> at that point and let it go for a while. Then I see Larry's post. So I
>> happen to have some 500V dipped SM 200pf Caps (4 of them) and I'll put them
>> in T502 and report back when I get motivated.
>> It was a bear to troubleshoot because it only happens when leaving the RX
>> off for quite a while and then the gain changes within a few minutes of
>> turn on. It's very noticeable though.
>> Larry, did you change the caps in situ? It would be a real pain to take
>> out T502, and maybe that's why you changed both of them. How would one know
>> which one was bad?
>> Some of my messages to the list are still not going through, I can't
>> figure it out.
>> Dave N3DT
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