[TMC] GPR 90

[w3jn]

Oscillators need (1) a high Q resonant circuit at the desired frequency and 
(2) adequate feedback.

That is a crystal oscillator.  The crystal element inside the holder may be 
bad.  Easier to check is the feedback network.  THis is a rather bizarre 
oscillator in that it uses a tapped coil AND a crystal for feedback.  The 
crystal is part of the feedback network.  I would attack this in the 
following manner, ASSUMING all other parts are good in the V4 area AND a 
good V4 (did you try and sub a new V4 yet?):

1)  Disconnect the cathode resistor from V4 (a 180 or 150, can't tell which 
from the fuzzy skizmatic) and try various resistors from say 68 ohms to 400 
ohms to see if you can make it a bit more reliable.
2)  Try shorting out the 47 ohm resistor going to pin 1 of V4 - this is also 
in the feedback network and could have gone inductive or something.
3)  Last resort is to remove the cover from L19 and move the tap up one turn 
on the coil, thereby increasing the feedback voltage.

73 and good luck, John

----- Original Message ----- 
From: "sebdesn" <sebdesn at ecentral.com>
To: "TMC" <tmc at mailman.qth.net>
Sent: Sunday, March 27, 2005 11:39 AM
Subject: [TMC] GPR 90


> Ok guys, got a report on the GPR 90 repair. Found one 6ba6, so only have 
> one
> in the if. chain. Replaced the 150K r25 resistor with a 10K and the
> oscillator started right up, for a while.
>  As was mentioned the 2nd if osc. is really squirley...It would run for 
> ten
> minutes or so then quit. When it quit, while it was on, I would pull the 
> Osc
> tube and replace it with the same tube and it would restart. I let it run
> for an hour or so and then reset L19 to the center of the very small area
> where the thing worked.Its getting better but still craps out after a
> while...I put another tube in the osc. that tests  good, and there is no
> change. Must be the capacitor in the L19 can that is changing value and
> causing the osc. tank circuit to go out of operational range????
> I am sneaking up on the proper adjustment...
>   Wonder if anyone has done an upgrade on the osc. circuit so its not as
> delicate?
> Thanks for the guidance!
>
> 73,de Bud WØHG
>
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