[R-390] Gain drift

[Larry H]

Dave, The idea for disconnecting the 3/4 link is to allow the agc circuitry
to work but not affect the IF gain.  This will provide for a constant gain
(if all is working correctly) for easier testing.  Although grounding # 4
creates max gain in the IF, if less gain is desired, the RF gain controls
the gain of the 1st 2 IF tubes.  The 3rd IF gain is controlled by the
internal IF gain adjust.  If C551, C547 and C548 are original, I'd look
very closely at them.  C551 can leak like a normal cap, but it can also
leak to it's case, which is gnd.

While testing, I suggest using the lowest level of input as possible, as it
is possible to overdrive the 4th IF and obscure your results,.  Although
the IF should handle input creating a diode load of 15 vdc, 7vdc is safer.
The issue is driving the 4th IF or the agc amp into clipping.  This can
happen in mgc mode, but in agc mode the agc helps prevent this from
happening.  Since the agc is disabled for testing, you need to be careful
of this.

Regards, Larry

On Fri, Oct 19, 2018 at 1:46 PM dog <agfa at hughes.net> wrote:

> Roy,
>
> At the moment, I'm just feeding 455 RF into the IF module. That appears
> to be where I'm getting the gain drift. It's even doing it with the AGC
> jumper disconnected. Need to fix this before I get into the RF section
> which actually may be OK. Thinking about measuring along the 455 RF line
> through the 4 tubes with the Boonton to see if I can find the area it's
> changing. Slow process since I have to let the RX cool off between
> sessions. I may start at V502 which breaks the IF section in half.
>
> I did replace the recommended caps in the RF section, 3 of them. The IF
> has all the recommended ones replaced, but not C551, it seems OK at
> least measuring, I don't have a ESR tester.
>
> The test equipment I have is a Moto 2005D sig gen/calibrated rx/spectrum
> analyzer/scope. All purpose monitor, it will put out +13dBm into 50Ω and
> a step attenuator with adjust pot down to -120dBm. Also a Boonton
> milivoltmeter and a Fluke 198 along with an old HP427A, and a Tec 545A
> that's still working.
>
>
> On 10/19/2018 11:24, Roy Morgan wrote:
> >
> >> On Oct 19, 2018, at 11:15 AM, dog <agfa at hughes.net
> >> <mailto:agfa at hughes.net>> wrote:
> >>
> >> Interesting. I disconnect TB102, 3-4, ground 4 and 3mV into J513 is
> >> way too much RF for the carrier meter in AGC mode. Reduce the input
> >> to about 250uV and that gives me 50 on the carrier. 0V on TB102-3.
> >> About 5 minutes later the carrier drops to 0. I have to increase the
> >> RF input to 1.3mV to get 50 on the carrier again.
> >
> > I suspect you have an intermittent capacitor or coil in the RF system.
> >
> > Insert the RF signal into the various test points in the RF deck.
> >  Look for the table of voltages (they will be APPROXIMATE) for the
> > various stages (and appropriate frequencies).
> >
> > I assume you have ruled out the IF deck with an IF signal at it’s
> > input - and perhaps set the IF gain while doing that.
> >
> > Roy
> >
> > Roy Morgan
> > k1lky68 at gmail.com <mailto:k1lky68 at gmail.com>
> >
> >
> >
>
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