[TMC] GPR90

[John Poulton]

Bud,

I'm delighted to hear that you've made some progress, and that my suggestio=
n
actually helped!  One thing, though:  you replaced BOTH of the caps in the
can, right?  That may not have been a great idea, depending on what kind of
cap the other one was.  It's possible that TMC put in a negative-temperatur=
e
coefficient capacitor in parallel with the 220pF silvered mica to help=20
with temperature stabilization.  If so, the "other" cap would have been som=
e
sort of ceramic capacitor, and if you examine it carefully, you may discove=
r
that it has a negative tempco.  The TMC parts list'll tell you what the cap
is, most likely.

ANYhow, if the 'other' cap was of a different type, with negative tempco,
then you'd be better off putting it back in, rather than having two dipped=
=20
silver micas with the SAME tempco.

So, there are three things to check, and I'll do it this evening, if I get =
time: =20

1. the tempco of the original CM-15 220pF mica cap=20
2. the tempco of the "other" capacitor in the can, and=20
3. the tempco of the dipped silver mica that you used to replace the CM-15.=
 =20

Hopefully, the tempco of the dipped mica is the same as for a CM-15.

One thing that's definitely worth taking away from this experience, and pre=
vious
encounters with these little micas:  THEY'RE TROUBLE!  Probably not in the =
same
league as the dreaded Black Beauty's, but still:  if you're experiencing a
time- and/or temperature-dependent problem in a radio, check those micas!

Best 73, John K4OZY



On Fri, 8 Apr 2005, sebdesn wrote:

> Yeeeessssss!
> Report on the oscillator problem...
>   The replacement of the two caps in the can for L19, with silver micas,
> (the brown epoxy ones), did the trick. It seems to be working  good, both=
 at
> start up, and in constant operation.
>  As John k4oxy mentioned the old ones get strange. The failure seems to b=
e
> time and temperature sensitive. I would adjust the thing to work and put =
it
> outside for a couple hours in the 40's and bring it in and it was ok. But=
 if
> it was left overnight inside (12-14 hrs)the thing would not work...
>   As John w3jn suggested will go back and tidy up the resistor values tha=
t I
> changed to try to get it to play...
>   So thanks to all who weighed in on the fix, I really appreciate it...Th=
ing
> was making me crazy (er?)...
>=20
> 73,de Bud W=D8HG
>=20
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>=20