ok, i just read back through the thread, as i don't want us to be chasing our tails.
first thing i noticed was that the radio does not do this on SSB.
that is a clue!
second thing i noticed was that you have another 959 that also has this same dual final mod done to it, but is a china built radio, and this radio works just fine in your car on AM and SSB.
please correct me if i am wrong on any of that.
next thing is that the dual final mod you used is for the ranger 6900 series boards, and you have the 696xxZ series board. they are very similar, but im not sure they are exactly identical. it does appear that the mod worked for you, but i am curious to know if you had to change anything in the instructions to make it work.
Wavrider had a really good suggestion that i somehow missed.
he asked you to take your dummy load out to the vehicle, take the antenna off the mount, and connect your dummy load in place of the antenna.
then try the radio and see if it does the squeal thing using the dummy load as your mobile antenna.
if your antenna doesn't have a convenient SO-239 connector on it, you can cut the end off of a piece of coax, put some lugs on the center conductor and shield. (those leads should only be about three inches long or so)
now you can connect that coax to your antenna mount instead of the antenna, and connect the other end of the coax to your dummy load.
let us know what happens.
that being said, i do think you have a problem inside your radio, and right now im thinking that it has to do with poor soldering somewhere on the board. (factory soldering, not necessarily yours)
I have already repaired two of these vietnam 959's and both problems were due to poor soldering.
get out the magnifying glass and spend some time going over the board. especially the pll and vco sections, along with the TX strip.
if you see anything funky, don't just re-heat the connection; flow some fresh solder in it.
you should be able to get a VCO reading at TP3 without transmitting, and that's the reading im interested in at the moment.
HOWEVER!
you really helped things along when you posted what the freq counter was doing during TX!
first, another question of course.
is the clarifier unlocked?
next, i want to make sure that when you are peaking out the TX adjustments, that you are using a modulated tone and not tuning for max deadkey.
re-assure me on that one.
make sure Q39 (amc) is in place and doesnt have a leg clipped. you will have to look closely for that one.
to give you something to mess with till i get back to this thread tomorrow, go through the VCO alignment section on cbtricks, and make sure you have peaked L15 as per the instructions.
now set the radio for channel 20, AM mode, receive mode, clarifier centered at 12 o'clock.
connect freq counter to TP3, and adjust L20 for 16.5100 mhz.
remove the mirror board, and key the radio. the freq counter reading should not move.
it should still read 16.5100mhz.
if the clarifier is not unlocked, and the freq counter does shift to a different freq, but stays steady at the new freq. then adjust VR7 in TX mode for 16.5100mhz.
if the clarifier is unlocked and the frequency changes when you key the mic, or if it jumps around when you key the mic (it seems like it does) then this is a real problem.
bad soldering could cause this easily.
it seems to me right now that your PLL will lock, because otherwise you wouldnt have any transmit, but the VCO frequency isnt staying steady at TP3 for some reason.
if you look at the schematic, and find TP3, you can see that it branches off in two directions, one goes to the receiver mixer, and the other goes to the transmit mixer.
if your freq reading in receive is solid and correct, then that branch is ok.
follow that branch to the TX mixer and check the soldering of that path.
sorry if it seems like we aren't getting anywhere, but this is a wacky problem, and its tough to troubleshoot this type of thing without being there.
more tomorrow after i see the results of your testing along with your answers to my questions.
no hammers!!!!
LC
