[ICOM] IC-765 Auto Tuner Problem

[David Hammond]

Hi Everyone,

By beloved IC-765 developed a nit over the weekend that has me stymied.  Looking for help.

All of a sudden, while running split on 80 meter CW, the autotuner will not match any load on 80 meters on any mode or at any place on the entire band. It appeared to be a sudden failure.  On all other bands the autotuner continues to work fine and nulls out the SWR as always.  But on 80 meters it searches and searches and then goes into the 'wait' mode (blinking tuner light) with SWR pegged on the meter. This occurs whether I am using my antenna or a 50 ohm dummy load. Same symptoms regardless. 

I am aware of the 765 Autotuner service bulletin issue, so I went in an rebalanced the reference voltages by adjusting R5 and R10 on the TUC board.  Afterwards still only the 80 meter band will not find match via the onboard tuner. (BTW that adjustment is very touchy. Should it be?)

In the 765 service manual I noticed that on the 'Tuner Relay Unit' (which is located in the metal box under the 'Tuner Unit Control', along with the motor drive variable caps) there is a relay (RL-2, one of many) which on 80 meters switches in additional values of fixed capacitance via C2, C3, C4, and C5 which are all 3KV ceramic caps. But the service manual does not give a schematic for this circuit and includes only a parts placement diagram. So I cannot tell which two cap(s) go with 80 meters and which two go with 160 meters.

The RL-2 relay seems to be clicking but I cannot tell for sure until I figure out how to get the whole tuner assembly exposed and out of the chassis.  

So....does anyone have any experience with this symptom?  Does my logic seem correct?

I thought I'd give this a try here before I send the radio to MTS and let Scott give it a go. I'd like to avoid the perils of shipping gear. I called ICOM and they still have these parts available.

Thanks in advance...

73 de N1LQ-Dave