[TMC] GPR90

[John Poulton]

Um.. never mind!  The second capacitor inside the L19
can is C32, a 120pF ceramic that is used for grid
coupling.  It's tempco probably doesn't matter much,
so replacing it with a dipped S/M should be fine. =20

Furthermore, since this is a crystal oscillator, the
temp stability of the tank isn't critical anyway.

Sorry for the wasted bandwidth.  Continued good luck
with the GPR-90, Bud!

73, John


On Fri, 8 Apr 2005, John Poulton wrote:

> Bud,
>=20
> I'm delighted to hear that you've made some progress, and that my suggest=
ion
> actually helped!  One thing, though:  you replaced BOTH of the caps in th=
e
> 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-temperat=
ure
> coefficient capacitor in parallel with the 220pF silvered mica to help=20
> with temperature stabilization.  If so, the "other" cap would have been s=
ome
> sort of ceramic capacitor, and if you examine it carefully, you may disco=
ver
> that it has a negative tempco.  The TMC parts list'll tell you what the c=
ap
> is, most likely.
>=20
> 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 dippe=
d=20
> silver micas with the SAME tempco.
>=20
> So, there are three things to check, and I'll do it this evening, if I ge=
t time: =20
>=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-1=
5. =20
>=20
> Hopefully, the tempco of the dipped mica is the same as for a CM-15.
>=20
> One thing that's definitely worth taking away from this experience, and p=
revious
> encounters with these little micas:  THEY'RE TROUBLE!  Probably not in th=
e 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!
>=20
> Best 73, John K4OZY
>=20
>=20
>=20
> On Fri, 8 Apr 2005, sebdesn wrote:
>=20
> > 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, bo=
th at
> > start up, and in constant operation.
> >  As John k4oxy mentioned the old ones get strange. The failure seems to=
 be
> > time and temperature sensitive. I would adjust the thing to work and pu=
t it
> > outside for a couple hours in the 40's and bring it in and it was ok. B=
ut 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 t=
hat I
> > changed to try to get it to play...
> >   So thanks to all who weighed in on the fix, I really appreciate it...=
Thing
> > was making me crazy (er?)...
> >=20
> > 73,de Bud W=D8HG
> >=20
> > ______________________________________________________________
> > TMC mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/tmc
> > Help: http://mailman.qth.net/faq.htm
> > Post: mailto:TMC at mailman.qth.net
> >=20
> ______________________________________________________________
> TMC mailing list
> Home: http://mailman.qth.net/mailman/listinfo/tmc
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:TMC at mailman.qth.net
>=20