[TMC] GPR-90 6BE6

[Dave McDonald]

John, Richard and Jim,

While I agree that the crystal could be the culprit, I don't see how it could "come and go".  Like PG!  Now, it has been playing for a few hours.  I pulled the V4 again and re-plugged it.  I tweaked L19 again, tight range to oscillate.  Earlier today, it would quit oscillating and come back on.  Hasn't done that again.

I'll let it play and see what goes on.  Digging out the crystal is major effort!

Thanks for the lead.  I'll get back with any permanent fix.  Still love the receiver.  Quite stable on 20M after it gets warmed up.  I don't even use the GSB-1 since it sounds so good with the BFO.  I have all this Collins gear around , '380. 'A4 and 'SB-3 but I migrate back to the GPR-90. Shows what us TMC folks like! Barcelona IY was coming in earlier.

tnx agn,

Dave




From: John Poulton 
Sent: Sunday, August 28, 2016 4:31 PM
To: Dave McDonald 
Cc: tmc collector's group 
Subject: Re: GPR-90 6BE6


Dave,


We're talking about the 2nd mixer, V4, right?  I have actually encountered a variation on this problem on multiple GPR-90's, but in all cases found that it was the 3.5 MHz xtal, Y2, not the 6BE6, that was causing the problem.  I wonder if it's the xtal cooling down that's causing the behavior you're seeing?


If you do need to replace the crystal, International can make them for you.  This xtal is used in a series-tuned arrangement..   I'm sorry that I don't know the fixture capacitance in this circuit, but perhaps you can find a way to measure it.  Again... always assuming it's the crystal, and not something else!  The xtal frequency isn't terribly critical, in any case--you can "align around it" if it's a few Kc off.


The crystal is "conveniently" located in the subchassis that sits on top of the main chassis on the left side of the radio, viewed from the front.  You can (barely!) get at the crystal by removing the mounting screws for this chassis, and gently prizing it up to expose the underside of the chassis.  Not really the most fun thing you could be doing, but easier than the dreaded RF-compartment repair on an SX-28!


I'd be very interested to hear what you find out once you bottom out on this problem.


73, John K4OZY





On Sun, Aug 28, 2016 at 4:13 PM, Dave McDonald <jdmcd at ij.net> wrote:

  John and TMC Net

  Has anyone experienced the converter tube, 6BE6, quitting after about an hour from being turned on?  Swapping tubes doesn't help.  The pentagrid converter works properly but after about an hour quits oscillating and receiver goes dead.  OK when switch to lower bands where dual conversion turns off doesn't help.  I tried with IERC heat dissipating shield but no help.  I leave the shield off so can swap tubes to see if helps.  Tube cools off and starts oscillating again.  I haven't tried swapping the 220 pf cap, C33, yet and don't really want to.

  Any help?

  Tnx,

  Dave K4EWC


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