[R-390] Recapping question

[Cecil Acuff]

Tim and Group,

My experience has been a bit different in that I have found several 
occasions where leaky paper caps were the cause of poor performance and 
after changing them ended up with a much more lively radio and sometimes 
correction of weird problems noted during testing or rough alignment work. 
Resistors have been the same but to a much lesser degree.  I'm convinced 
getting resistors back in spec. lengthens tube life in certain instances 
where major performance differences may not be noticeable.

If nothing else a load was taken off the power supply by removing the 
cumulative additional load placed there by leaky paper bypass caps....

I have had several cases where in the R1051 series a non-functioning radio 
was restored to operation by the replacement of shorted bypass caps....

Cecil....
----- Original Message ----- 
From: "Tim Shoppa" <tshoppa at wmata.com>
To: <n4buq at knology.net>; <r-390 at mailman.qth.net>
Sent: Thursday, September 07, 2006 7:17 AM
Subject: RE: [R-390] Recapping question


> N4BUQ wrote:
>> Tim wrote:
>>> Barry, am I too far off if I guess that the
>>> resistors which drifted worst were the 3.9K's?
>
>> Yes, the 3.9k plate resistors were both high by nearly the exact
>> same amount (don't remember exactly how much).  They aren't
>> charred nor are they 50% or  100% high, but are well out of spec.
>
> I don't have a lot of statistics to back this up (having only a couple
> of 390A's and a bunch of other mil-spec and consumer stuff from
> the era) but to overgeneralize:
>
> 1: Triode stages seem to be a lot more likely to char and burn plate
> and cathode resistors than pentode stages. In R-390A terms this means
> the ones around 6C4's and 12AU7's.
>
> 2. The carbon comps that drift up the most tend to be in the low K-ohm
> to 10's of K-ohm range. The others are not immune but the problem
> is not as endemic there.
>
> Finally:
>
> 3. Even though I have a vengance against parts that are 50% or 100%
> out of spec, I cannot say that replacing them has often repaired
> anything.
> Most oscillators were still oscillating, most mixers were still mixing,
> most
> amplifiers were still amplifying before and after!
>
> Same goes for most leaky wax/paper/electrolytic caps that were
> measurably
> or visibly bad: fixing the cap rarely fixes a problem that stopped the
> radio from
> working. The only exception is leaky coupling capacitors (some of
> which were responsible for charring the plate/cathode resistor of the
> next stage!)
> Now getting rid of those crackling filter lytics probably was a good
> idea anyway
> but despite massive leakage they usually didn't actually cause things to
> not
> work (although hum and crackling of course improve after replacement.)
>
> By far most repairs are effected by cleaning loose connections, fixing
> cold
> solder joints, and cleaning dirty switch contacts.
>
> In transmitters things are often more clear-cut, because at least some
> bad
> parts (not necessarily the one for the fault!) char, explode, etc.
> making
> it more obvious, and usually plate currents in the power stages are
> metered
> and will blow a fuse if they go too high.
>
> Tim.
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