[R-390] Gain Drift

[Barry]

If you happen to have a variable-output DC power supply, that might be handier (and allow you to advance the AGC voltage further as well).

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "Roy Morgan" <k1lky68 at gmail.com>
> To: "dog" <agfa at hughes.net>
> Cc: r-390 at mailman.qth.net
> Sent: Friday, October 19, 2018 9:58:50 AM
> Subject: Re: [R-390] Gain Drift
> 
> 
> > On Oct 19, 2018, at 10:20 AM, dog <agfa at hughes.net
> > <mailto:agfa at hughes.net>> wrote:
> > 
> > Larry,
> > 
> > If I disconnect tb102 3,4, the agc rectifier still works but doesn't send
> > agc to the RF/IF tubes.
> 
> That is the link that sends the AGC voltage from the rectifier off to all the
> controlled stages.  It may reveal something to hook up a couple or three
> flashlight batteries in series, and put negative 3-5 volts on the terminal
> that is the AGC buss (not the rectifier output) with the two terminals not
> connected.  (Positive side of batteries to ground.) The radio should show
> steady fixed gain, steady output level with steady RF input level.
> 
> A variable resistor hooked up to a 9 volt battery gives you the chance to
> vary the voltage applied to the AGC bus at will.  This can tell you a lot
> about how the whole radio is working.  Was there an AGC voltage vs. RF
> signal input voltage for standard output table included in the recently
> posted document:
> R390A CRYPTOLOGIC TECHNICIAN CLASS A CRYPTOLOGIC MAINTENANCE COURSE.pdf
> 
> 
> > What does grounding tb102-4 do? I know it grounds the agc line, but I guess
> > that puts full gain on all the agc tubes?
> 
> Yes, with the GC bus at zero voltage, the radio is at full gain.  Testing
> voltages (VTVM) at the test points may reveal a leaky coupling capacitor
> feeding that point.
> 
> (Note: most Digital meters have high input resistances on DC, but the test
> leads have capacitance to ground, so the test lead applied to an RF test
> point may “suck the signal out”.  Put a 100K or 1 meg resistor at the test
> lead tip, and don’t expect accurate DC voltage measurements.  Most old time
> VTVM’s have a one meg resistor IN THE DC PROBE and that solves this problem.
> The AC-DC switch on some probes switches this resistor out for AC
> measurements, if I remember right.)
> 
> The Y2K manual has diagrams that show just the AGC system.  A bit of time
> studying those diagrams may help any of us understand the AGC system.
> 
> > And measuring pin 3 tells me if the agc voltage is varying with constant
> > gain tubes in the RF/IF? Seems to me if I have varying voltage on tb102-3
> > with no agc action there's something wrong in the agc circuit. I'll try
> > it.
> 
> I don’t remember the nature of the two terminals: one is the AGC rectifier
> (is there an AGC amplifier?  Can’t remember) output, the other is the line
> that feeds all the controlled stages in the radio.  With them disconnected,
> one should vary (AGC ON) with the radio RF signal level, and the other one
> should not change at all.  Varying or occasionally changing voltage on the
> AGC buss with those terminals not connected indicates intermittent or leaky
> capacitors some where (likely signal coupling caps).
> 
> Roy
> 
> Roy Morgan
> k1lky68 at gmail.com <mailto:k1lky68 at gmail.com>
> 
> 
> 
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