[TMC] GPR90

[John Poulton]

Bud,

I'm probably not the "John" you addressed this to, but.. what the heck..

I've had some very interesting (read "bad") experiences with some of
those CM-15 mica capacitors (the little rectangular jobs, usually
red body, 6 color dots) lately.  They seem to be particularly troublesome
when they operate with large DC biases and small AC.  I've "fixed" a
couple of Collins R-388's by replacing a particular one of these things.

The caps fail by becoming leaky, but in a very strange way.  They actually
act like really crappy semiconductors.  The failures are due to the growth
of "dendrites" in the tin metallization inside the caps.  The failure
tends to be time- and temperature-dependent, which is what makes me think
this is a possibility in your case.  In the R-388, for example, the offendi=
ng
cap is in the AVC circuit.  The radio'll play fine when you first turn
it on, then gradually begins to fade out, and finally dies altogether,
when the AGC is driven to -40 volts by the shorted cap.  This same style
of cap is known to be one of the "7 deadly caps" in the Collins 75A-4 as
well, and I've seen a couple/three bad ones of those, too, used as coupling
caps.

ANYhow..  why don't you try replacing that 220 pF cap in the oscillator,
and see what happens?  It's a relatively easy fix, most likely, and can't
hurt.  If you need a cap, and can't find one exactly like the CM-15,
it's proably fine to use an epoxy-dipped (brown) silver mica instead.
If you use one of these, be careful when bending the leads not to break
the epoxy seal around the lead--that'll let water into the cap and induce
failures.

I'm currently in the process of overhauling 2 GPR-90's, so I'm interested=
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to read your progress on this.  Good luck!

Best 73, John K4OZY Chapel Hill, NC


On Wed, 6 Apr 2005, sebdesn wrote:

> John,
>  Well here is the latest,but still not good news on this beaste...
> I did have a new tube in v4.
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> Changed R24 (the 150 or 180 ohm) to 250 ohm.(went lower and higher and di=
dnt
> work)
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> Changed R25 (15K) to 75K.(the original 15k is in the can and someone cut =
it
> loose, and put a 150k outside the can that I changed to 75k).
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> Changed R23 (47 ohm) to 10 ohm.(went to 30 and 23 with no effect)
> Aligned  T2 per the instructions
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> (A)If I get the oscilator playing, and set L19 so it keeps playing for a
> couple hours,and is in the middle of the range, It won't start up after
> sitting overnite.
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> (B)If I get L19 set so it starts in the morning, after sitting overnight,=
 it
> quits after 15 or 20 minutes.
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>   If it wont start after A or B , the only way I can get it to start is t=
o
> pull v4 while its on,and plug it in again.It wont start by adjusting L19,
> which seems strange,like something else is kaput..
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>    I pulled the cover off of L19 and looked at the two caps (120 and 220)=
=2E
> The 220 is a rectangular squaresided  silver mica tub type unit.
> The 120 is one of those tubular jobs that looks like a 1/2 watt resistor =
and
> looks like its some sort of sandy composition stuff.
> Since the circuit seems to be changing with temperature, do you think it
> would be reasonable to change the 120 cap in the can, or would something
> else cause the thing to change that much????
> I can't see anything else except the two capacitors and the tube that wou=
ld
> change with temp.
> I am totaly lost at this point...
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> John,I appreciate your help very much,as I seem to be over my head in thi=
s
> project.
> 73,de Bud W=D8HG
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