Anyone with experience with the ALC circuit?


I have an IC7000  that has problems with the ALC circuit.
The ALC voltage is about 3 volts unless USB or LSB is selected and then it drops to 0 and no output.
The ALC is pegged and no power out.
I found a bad 4.7K (R1634) resistor in the UNSB circuit (Q1606), but this made no difference.
D 1606,7,8, all have about 7 volts on the OP amp side and 0 on the ALC side when the mode is SSB.
There is no indication of a short circuit for DC with the radio off.
It seems that something is pulling the line down.
Any guesses or suggestions?


As I understand the circuit a voltage of about 2.8 volts is derived from R1641 and R1636 two 2.2 megohm resistors in series.
The several modifiers of the voltage to reduce the voltage to the IF pull that voltage down when the op amps voltage drops from 7 volts and bias diodes D1606 - 1608.
I think that the FET Q1606 is controlled by the UNSB line from the logic CPU and this bypasses the network of R1626 and R1634  through D1610, if that makes sense. Q1606, when turned on connects R1627 in parallel with R 1641 and 1636.
When not turned on the diode D1610 provides the voltage at the junction of R1626 and R1634 at about 1/3 of the 8 volt buss or 3.1 volts.
Either way Q1606 controls the ALC base voltage and the amount of drop when current is drawn. The voltages are close either way on my working radio.
In my radio the ALC line is pulled hard down when UNSB goes low. Next I will try changing Q1606 as I think that it is letting the line which goes to UNSB pull the ALC voltage down because perhaps the FET is "Leaking" the ALC voltage to ground through this line.
And thoughts on this?
I have included a pdf of the main board ALC circuit for those who are masochists and want to follow along 


Ok I have fixed the ALC circuit!
The low coming from the CPU actually turns off the FET  1c1606 and not on as I thought.
Late at night I thought (while waiting for the replacement FET) that since it needs to be off and it must be shorted and the ALC voltage being pulled to ground by that low from the CPU just take the darned thing out!
So I removed the FET and the ALC came to the proper voltage for SSB.
The replacement of the FET put the ALC voltages correct for the other modes, selecting AM, FM , etc sets the proper power levels and the SWR foldback works.
So if the ALC is going to 0.0 volts on SSB look to the FET!

Steve W3AHL

Good info!  Thanks for posting the resolution.

Steve, W3AHL