{Collins} Re: R388 agc problems

[John Poulton <jp@xxxxxxxxxx>]

Folks,

I think this may be another example of the infamous C-204 failure.
C-204 is the AGC coupling capacitor.  It's a CM-15 mica that is
mounted on the tube socket for V110 (AM detector, AGC rectifier).

A bit of lore previously reported on this list suggests that
Collins bought a large lot of capacitors from ARCO that had
manufacturing problems, and these problems are causing the capacitors
to fail after a mere 50 years of service life!  Failure mode is
the growth of tin dendrites within the capacitor, driven by
voltage across the capacitor and fed by moisture invading the
innards.  It's the same failure mechanism that kills some of the 
7 deadlies in the 75A-4, the 470pF coupling caps in the RF section. 
Basically, these little micas tend to die when they have a DC bias imposed 
on them.  If you remove the capacitor and measure its leakage, you'll
probably find that there is considerable conductance through the
capacitor.  It may exhibit all kinds of entertaining behaviour,
if you put a voltage across it, and heat/cool it.

Typical symptoms:  radio works fine when first powered up, but
gradually looses gain as it warms up, eventually becoming deaf.
Symptoms vary considerably, however.  Ultimately, the problem
is that the AGC is pinned at a very negative voltage.

Replace the capacitor, and there's a good chance your radio will
be heal-duh, as the tent preachers would say.

73, John K4OZY


On Sat, 27 Oct 2007, Chuck wrote:

> Greetings,
>     I recently acquired a very nice condition R388.
> It was working fine up to a day or so ago. Works as it
> should with the agc off. When agc is turned on voltage
> rises to about -20 (there abouts). That is measured at
> the agc tp on the rear of chassis.
>     I checked the voltages for the avc amp (V111
> 12au7). With the agc on, the voltages on pins 2 (grid)
> and 3 (cathode) are just about the same at -45vdc. The
> chart says -52 for pin 2 and -44 for pin 3.
>     Resistance checks yields very close to what the
> charts call for.  Tube was substituted. C208 was
> subbed also, no change. I checked some of the values
> of the resistors in the voltage bias divider, all
> within tolerance.
>      Anyone have a similiar problem, that mabey give
> me some idea of what to try next?
>      The rx is a later manufactured unit. Most of the
> tubes are original and are dated '63, also one switch
> had a '63 date on it. The schematic I am using is from
> a manual dated '52. I guess only minor changes were
> made over the years, all the values in the avc
> schematic jive with what are in my unit.
>     Any help would be appreciated. Thanks.
>
> Chuck.....wa2onk
>
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SHARE THE FUN: JOIN THE COLLINS COLLECTORS ASSOCIATION TODAY !
     http://www.collinsradio.org/membership/    -paypal accepted-
.
NETS: Tues & Thur: 3.805 MHz-2000 Central + Fri: 3.895 MHz-2000 Pacific
 SUNDAYS: 14.263 MHz-2000 UTC  +  AM Net: 29.050 MHz-1200 Central
      1st Wednesday AM Net 3.880 MHz-2000 local (ET, CT, MT, PT)