ALC problem


Pasquale S
 

Hi guys

I’m repairing an IC-7000, I put 2 new finals RD70HHF1 and I set bias 100% (1 A for each final)

But I have a strange behaviour ; the power goes up slowly, for example if I set 100 watt , power is 20 watt then goes up until 100 watt; same thing if I set 50 or 70 watt for example.

I change the cable, I use now one without fuses. I have done adjustment 3 times, no effect.

In VHF / UHF my 7000 is perfect, no trouble.

Any ideas ?

Thanks in advance

73s Pasquale IW0HEX

 

 

Inviato da Posta per Windows 10

 

Da: J.D. Barron
Inviato: mercoledì 15 luglio 2020 13:30
A: ic7000@groups.io
Oggetto: Re: [ic7000] ALC and audio Tx problem

 

I had this problem with one of my IC-7000s. My symptoms were full ALC on SSB and 100% + output on all other modes.
Turning the power down has no effect.
The ALC circuit gets one input from the SSB logic on the UNSB line to the Q1606 FET that changes the ALC bias.
In my case the FET was bad it I recall correctly. What happened was the voltage on the line that gose to the op-amps was low (?). What happened was Q1606 shorted and the two 2.2 meg (series R1641 & 1636) were always pulled low and did not change when switched to SSB mode by that signal from the NUSB line. if I remember correctly it is high when not in USB. 
Your mileage may vary.
I have attached a copy of the schematic for this part of the circuit.
IC1601 are the op-amps that vary the ALC bias to control power and fold back due to SWR etc.