[R-390] Gain drift

[David Wise]

If the gain change is accompanied by AGC activity, which direction does the AGC line go?  If it moves in the positive direction (i.e., less negative) when the gain drops, your problem is in the gain of the signal chain.  (This is also the case if the gain change persists in MGC mode.)  If the AGC line moves in the negative direction (i.e., more negative) when the gain drops, your problem is in the AGC system.

How's your RF GAIN pot?  Keep an eye on one of the cathodes it controls.

Another suspect is the small molded mica caps that tune various inductors, and on early contracts, the mechanical filters.  The foil electrodes in these caps are connected to the lead wires through pressurized mechanical contact only, and after fifty years the contact areas are prone to oxidation, which causes the capacitance to change or "spoil" with high dissipation factor.  This detunes the inductor or kills the circuit "Q". Sometimes voltage transients will temporarily heal this.

My own R-390A had two bad mica caps, one inside one of the RF transformers (so this part of the problem was unique to one band), and another on a mechanical filter (so it was unique to one bandwidth).  But some caps (mostly in the IF deck) are used on all bands and bandwidths, so if your problem occurs everywhere, you should still be suspicious.  

Ceramic trimmer caps aren't immune, since they too rely on mechanical pressure for electrical continuity to the rotor electrode.  My radio may have had dirty trimmers, but I moved them all during alignment, which usually clears them up.

A more subtle way to spoil the gain is via oscillator injection level.  If one of your oscillators goes weak, the conversion gain in the associated mixer will drop.  If this occurs in the 17MHz oscillator, the problem will only manifest on the 8-31MHz bands, but the crystal deck and PTO are used on all bands.  You can sniff the oscillators with a second radio.

I had yet another intermittent gain shift.  My IF deck had a wire supplying screen voltage to one of the 6BA6's which went tightly around the edge of a metal shield which eventually cut through the insulation.  The insulation carbonized and dragged the voltage down without quite grounding it.

HTH,
Dave Wise
SWL in Hillsboro, Oregon
________________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Jacques Fortin <jacques.f at videotron.ca>
Sent: Saturday, October 13, 2018 8:45 AM
To: 'dog'; r-390 at mailman.qth.net
Subject: Re: [R-390] Gain drift

Hello Dave,

Two ideas:
1_ What can be seen if you monitor the AGC line when this happens ??
Say by connecting a meter to TB102, screws 3 or 4 vs GND ?
In which AGC mode this happens ? In all or only Med and Slow ?
Have you changed C551 ??
2_ Possible self-oscillation of the RF stage....
This was a trouble in the early Collins RF decks, especially when the 6DC6
is "hot" - meaning high in transconductance.
From the Moto '56 and up this was solved by the addition of E212, E213 and
C267 around the 6DC6.
I do not know which vintage your RF deck is, so this one is purely a shot in
the dark....

Let's us know about anything found...

73, Jacques, VE2JFE

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