[ICOM] IC-765 tuner and total loss of RF from tuner circuit

[Mark J Bailey (mjbtnus)]

My 765 lost all RF output from the RF Out jack some time back. Since I
wasn't transmitting all that much at the time, I only mucked around with the
tuner module a little bit back then to troubleshoot. The receiver seems in
good shape as when I pull the RCA cable off Rx In and Rx Out and attach a
random wire to the Rx In, I get good RF for Rx on all bands. Suspecting a
relay in the tuner module, at the time I swapped out RL201 on the
antenna/accessory board. No go. RL13 appeared OK, so I didn't do anything
with it. 

 

I eventually gave up with other things pressing me for time. Recently, I
started look at this issue again. This time I swapped out RL13. But no go
still. It is almost as if the tuner is stuck on the transmitter circuit. The
tuner tracks with the VFO, but hitting the tuner buttom does not alter RF
for Rx through tuner. Since the Rx is A-OK on the Rx In port, I still
believe the issue is in the tuner module. But, not being a hardcore
electronics guy with all the bench gear to really dig into the block
diagrams of the tuner module and related testing and what not, 

 

I was hoping someone here might chime in to guide me deeper into the
mystery. I love my 765 and really would like to get it back on the air. I
suppose I could send it off, but who these days still services these? Since
I did reasonably well on swapping out RL201 and RL13, I think could tackle
most anything that was not IC or surface mounted, so if I could pin this
down, I'd much rather succeed in fixing it myself. One thought I also had
was could the tuner controller board have fubared? 

 

I know there are several other relays in the tuner module. What is the best
practice for testing relays such as these while they are still inboard?

 

I should add that in receive on the Rx In port, i get strong RF and meter
indication. In fact, it is totally nominal - sensitivity on all bands is
great. you wouldn't know there was a problem. But when you patch Rx In back
over to Rx Out and attach the coax to the SO-239, it is like there is no
antenna attached at all. I've tried hitting the power on and off rapidly in
succession, Transmit / Receive toggled in rapid succession, and Tuner In and
Out in rapid succession - Rx is dead. Preamp and AGC still seem to work fine
too on Rx, FWIW.

 

One other tidbit that might be pertinent.  with the antenna attached to the
SO-239 and if I try to transmit, I get considerable frontend
feedback/overload - not at all nominal! Needless to say, I only tried this a
very limited number of times when the problem first appeared, and have been
reluctant to do so any further for fear of burning something out in the Tx
finals. So, in transmit, it does not appear to be able to get RF out through
the tuner module to the antenna either. Perhaps knowing this might help
narrow this down some?

 

I have posted this also on the IC-765 Yahoo Group, but had the brain fart to
also seek you guys out too! Sorry it was a bit winded. :)

 

Thanks for listening! 73s, Mark, W4MJB.