[R-390] Gain Drift
Roy Morgan
k1lky68 at gmail.com
Fri Oct 19 10:58:50 EDT 2018
> 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>
More information about the R-390
mailing list