[R-390] T502 caps and Diode Load -VDC

[Larry H]

Hi Dave, Glad you're getting back to this.  Good news on the non-eratic
gain.  Hope it holds.

I assume you've replaced v504 and v506.  Check the dc voltage on v504 pins
6 and 7.  Check bypass caps c528 and c529.  Check resistance of L502.

Regards, Larry

On Wed, Mar 6, 2019 at 12:54 PM dog <agfa at hughes.net> wrote:

> So I finally got around to messing with T502 and the 'faulty' 200pf caps
> in there. Replaced the one that wouldn't tune, the bottom one. That did
> nothing, so I soaked some WD40 into the slugs. The bottom one, which was
> kinda stuck, was all the way up against the top one which does tune.
> With some persuasion, I got the bottom one to tune way down further on
> the bottom core and ran the top slug all the way up so I could get some
> good WD40 in the tunnel. Put it back in the RX and tuned the bottom one
> first from the bottom, feeding 455 into the IF module. Now both top and
> bottom slugs tune properly.
>
> But. According to the book when I have 100,000uV (100mV right?) at pin 1
> of V504, I should have -4VDC on the diode load in MGC. I have -2.2VDC.
> It takes 180mV on pin 1 V504 to produce -4VDC diode load. What
> determines the level of diode load voltage? Just the gain of V504? T503
> seems to tune properly.
>
> I'm feeding 455 from a Moto SM into J513 and measuring the voltage on
> V504 with a Moto RF milivoltmeter (looks just like a Boonton) with the
> low Xc probe.
>
> Or is this within general specs?
>
> I don't think I had a problem with the caps near as I can tell. But I
> was getting jumping gain after warm up that I seemed to track to T502 at
> one point, but don't seem to be experiencing that at the moment and it's
> been running several hours now. I'll let it run all evening.
>
> Dave N3DT
>
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