[TMC] GPR 90

[w3jn]

Bud - first, repair the faulty components.  A 6AU6 is a poor sub for a 6BA6 
unless it is in oscillator service - then, it probably doesn't matter.  A 
6BA6 is a remote-cutoff pentode that is generally used in AGC-controlled IF 
stages and the 6AU6 is a sharp cutoff so your AGC will not work worth a 
hoot.

Check the plate and screen voltage on the oscillator toob.  You should see a 
voltage drop across the plate load resistor which indicates that the toob is 
drawing current.  If there is very little drop, the oscillator ain't 
working.  A scope comes in handy here also.  Could be that the crystal is 
dead, one of the capacitors in the oscillator circuit is bad, etc.

I don't have the skizmatic handy, but if that resistor you're talking about 
is associated with the conversion oscillator plate or grid, change it to 
what the schematic sez it oughta be.  THe busted bypass cap might mess 
things up too.

If it still doesn't work, try subbing the oscillator toob.  Once while 
fixing a National FRR-59 (60 toobs in that beast) I replaced some 6C4s with 
brand new Raytheons.  These were all oscillator circuits, and not a single 
one would oscillate.

Good luck and 73 - John


----- Original Message ----- 
From: "sebdesn" <sebdesn at ecentral.com>
To: "TMC" <tmc at mailman.qth.net>
Sent: Tuesday, March 22, 2005 7:13 PM
Subject: [TMC] GPR 90


> Hi Folks,  I am working on a GPR90 that will not work on the 2nd if
> bands...I have had it come on and work a couple times, and then once it 
> just
> "faded out" and another time it would not come on after I switched the 
> send
> receive switch. I adjusted L19 in an attempt to get the 2nd if  oscillator
> working but to no avail. Have checked all the tubes and found two bad 
> 6ba6's
> and replaced them with 6au6.(all I had on hand), the buffer is still a 
> 6ba6.
> Did the resistance measurements on v4 and v5. all ok except for pin 1 on
> v4, it is 152K and supposed to be 15K. According to the schematic I have,
> r25 is supposed to be 15K and it is 150K. Would that cause the osc. not to
> play???
>   There is also a cracked disc cap (the red disk ones that say 10K) on 
> tube
> v8..
>
> Any way I am stumped..Any light anyone could shed on this would be greatly
> appreciated....
> 73,de Bud WØHG
>
> ______________________________________________________________
> 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