UNIDEN GRANT PC409 Slow AGC

Sorry for the obvious question. You are not running something downstream amp/ QRM Eliminator/etc. that has a sniffer circuit or a SSB delay and is switching in/out your RX?

Almost sounds like the AGC is still being over driven. Does turning down the rf gain help?

390 Wagonmaster said:

When I unkey the mic on USB or LSB it takes a second or more for the receiver to come back to full gain. AM works perfect. (Kind of acts like a SSB delay on a amplifier) Does this on all mics tried. Also this started happening when the SSB AGC started sounding overloaded. Changed out some 10v caps, that did help on the overload issue but still has a long return to full RX after unkeying. Never seen this before...........
Any suggestions?

Click to expand...

Since it works Ok on AM but not SSB, I would suspect the issue being in the RX chain after the point where the signal splits off and AM is separate from SSB.

I have seen where front end protection diodes have been blown out and the front end overloads, but that would affect AM too.

This is merely one possible direction to take, others may have different ideas.

First step IMO is to see what is attenuating the RX signal or RX audio. The attenuator diodes at the front end, the squelch, something else?

Edited previous post after additional thought. Been trying to wrap my head around this. The problem clearly occurs around TX.RX switching, so I don't see how the AGC can be a part of this. It takes signal from the RX IF stage to drive the fet gate negative in order to activate the AGC, and until you let off the key, there is (or should be) none.

I am now leaning toward something that is supposed to turn off when PTT is released (like a signal shunting diode that protects the receiver during TX), or something that comes up when PTT is released (like RX IF amp biases that are 0v during TX), or maybe even a switching diode that turned into a resistor keeping an RX stage on during TX.

Gotta see where the response lags.

Ill dig into it more tomorrow. I can tell its getting too late for me to be trying to problem solve. I don't think clearly at 3am.

C23 will affect AGC timing.
might be worth checking.
LC

I would like to make a separate thread about my understanding of this AGC circuit before I continue here to be sure I am understanding it correctly.

Just want to double check with the experts to be sure I understand what all those diodes and capacitors actually do. I think I know, but I want to be sure before further speculating on this problem. Maybe someone already made a thread on the AGC, and if so, please link it to me.

Be back in a bit, got some drawing to do.

check D12 & D13

I'll put it here.

This is the AGC from the Grant, same thing the cobra 148 uses. I want to make sure I am understanding how all of this works. Here it goes.


Starting at the front end, there are two control signals that can shunt incoming RF by forward biasing D12, the AGC control voltage, and the TX supply voltage (so the RF from the finals during TX don't fry the front end). A positive voltage on either of these will forward bias the D12 and cause a reduction in signal making it to the RX preamp.

That AGC voltage comes from TR10. For there to be a positive-going voltage on TR10's collector (increasing attenuation at the front end), TR10 must be turning off allowing more of the RX supply to go to D12. For that to happen, the base of TR10 must be going lower. To make the base of TR10 lower, the gate of the FET must be going down with respect to it's source. A negative-going voltage at the gate of the FET increases front-end attenuation and simultaneously alters the bias point of IF AMP TR16. In other words, making the gate of the fet more negative causes attenuation at the front end AND at the IF amp, a common technique to increase dynamic AGC range according to Lou's book.


With RX RF coming in to the anode of D10 from TR18, the positive peaks of the RF are clipped at 0.3v leaving the negative-going parts unaffected. This shifts the DC average at the anode of D10 negative with respect to ground. This negative voltage is transferred to C23 and C25 via D8 and D9 respectively, and for the moment, D5 is not conducting. This suggests to me that the initial response time of the AGC is governed solely by C23. However, the decay of the AGC action (in AM) is governed by C25. As C25 discharges through the shunt resistor next to it, that negative voltage on C25 moves back up towards 0v. When the voltage on C25 goes higher than the voltage on C23, D5 begins to conduct and starts turning off the AGC.

In sideband, D6 passes SSB voltage to D7 bringing C24 to (nearly) ground and in parallel with C25 changing the time constant for SSB AGC decay, making it take longer to deactivate the AGC function.

So, going back to the issue at hand. It would seem to me that C23 would have to attain a negative voltage in SSB TX for the AGC to be at blame for the momentary absence of RX when unkeying. With the RX IF chain supposedly off when PTT is pressed, I am struggling to see how this can be the case. Where does that negative voltage come from?

Therefore, unless my understanding of the AGC circuit is flawed or there is RF in the IF chain during TX, I do not believe we can blame any of the AGC caps. I think we need to look elsewhere.

Dr_DX said:

check D12 & D13

Click to expand...

If D12 were bad, wouldn't all (or no) RX shunt to ground? RX level changes, so do bad diodes change behavior with signals hitting them? I was looking at that too, but I couldn't rationalize it.

edit: this doesn't mean don't check. Definitely check, I've been wrong more times than I can count

I was thinking last night that an analog voltmeter would work good here to see if there is a DC control changing in step with the RX coming back.