[TMC] GPR-90 6BE6

[John Poulton]

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
>