Icom 765 PA section Q6 voltages

Discussion in 'General Technical Questions and Answers' started by W4MEC, Oct 17, 2020.

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  1. W4MEC

    W4MEC Ham Member QRZ Page

    From another site I got onto checking the 765 driver and final idle currents as a possible cause of distorted SSB audio and found a problem. In the Icom service manual for the 765, it gives base voltage for Q6, the final transistor bias control, of 1.3 vdc, and gives the final base volts as 0.7, but does not say what is supposed to be on the emitter of Q6 which feeds the bases. The emitter volts goes through several components before getting to the final bases, guessing it might be in the 1.3 volt range. Does anybody know what the voltage here should be? My resting pa idle current is 1 ma when it should be 500ma, and the rig still puts out full power. The bad idle current may be my distorted audio problem, and this is the only thing I have found wrong in the rig so far. In case someone suggests PLL issues as a distortion issue, it has been fully overhauled with trimmer and other poor quality parts being replaced and an amplified speaker used as as test tool, when hooked to the input of the balanced modulator IC, reveals perfect sounding, distortion free audio getting to that point, FM audio is clear, and CW note is perfect. Any help would be appreciated
  2. W4MEC

    W4MEC Ham Member QRZ Page

    Answering my own question after continuing to work on the rig. I thought I had a rig with a shoddy repair, seeing that Q6 was tack soldered to what I thought was the leads from the original transistors someone cut out. Not so. Icom solders a loop of wire from under the board up over the top and back down through the board to be soldered to the same spot. This loop is where you tack solder the leads of the transistor screwed to the heatsink. After removing Q6 and cleaning up the board, it was evident this was the case. So, since the transistor checked good, reinstalled it, made sure I had good connections, and my 500ma idle current was normal after setting. The red herring I was chasing had been noted before, in one Icom servicing comment, it was stated to check Q6 solder connections. I fooled myself, since using the meter probe to test the voltage, I was pushing on the lead and making the connection to the collector which was 12 volts, but when I removed the lead and went to the emitter, 12 volts disappeared from the collector which made teh emitter volts disappear making me think the problem was elsewhere. The only way I could get secure solder connections here, was to raise the rig and put it on its face, any other way and the solder would not stick to the wire loops on the board. The heating and cooling cycles of other 765's over the years, probably has broken a lot of those tack soldered connections in the PA.

    The good emitter voltage on this rig was 1.45vdc going to the finals. To set PA idle current, Icom tells you to lift R20, and it is already cut and re-soldered, so it is not hard to do, and when you open it up, put your milli-ammeter in the open resistor lead. If your rig is working and you want to check your PA idle current the easy way, I noticed that when in Ic positon on the front panel meter, you can get a fairly accurate reading on the meter Ic/V scale, the first hash mark is 1 amp, so it should be a bit below that. Use USB, power and modulation at minimum. If it is not there, or is above 1 amp, you may want to check it per the service manual.

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