SX-737 - definitive answer on replacing tuner board components?

[vulturon]

I am fixing up a nice SX-737. It has the AWE-043 tuner board. I've successfully replaced "problem" transistors (725/726/1451) on the other boards, but have done nothing to the tuner board.

Can you guys help me with a definitive answer about replacing components on the tuner board? Is there a definitive answer? Here's what I've read:

* Don't touch anything on the tuner board (not even the 2SA725 transistors) unless you're prepared to do an alignment.

* It's okay to replace capacitors on the tuner board, but don't touch anything else (especially transistors) unless you're prepared to do an alignment.

* It's okay to replace the C49/C50 capacitors, and Q8/Q9 transistors (markthefixer wrote "replacing those four components will have NO effect upon alignment") but nothing else should be replaced.

And I think I've read other permutations, including guys who've gone ahead and replaced everything with no re-alignment required. If markthefixer's comment above is correct, is it true that the Q10/Q11 2SA725 transistors should NOT be replaced...just the Q8/Q9 2SA725's?

 

[john stumpf]

it's a dice roll i guess. ive replaced everything and got improved sound quality and noticed no difference in alignment, but at the same time remember,its probably drifted over time anyway,it might be in your interest to find someone who can align it,and THEN go ahead and recap and get rid of the junk transistor's prior to having it done.ive since acquired a signal generator so im not intimidated any more.but i can only do front ends and no multiplex adjustment as that can require an o scope a multiplex generator and a distortion meter (big bucks). i guess any responsible ak'er would have to tell you to leave it alone. me well, i got pushed into it as there's NO ONE here who can or can be trusted to pull it off. i thoroughly botched a 1080 tuner about 4 yrs ago as i felt i couldnt possibly make it worse. this turned out to be the WRONG ANSWER. i know this isnt what you wanted to hear, but i would get rid of the 725's or 726's but thats just me.

 

[larryderouin]

I did my 737 about 5 years ago. I replaced ALL of the 725/726 on the whole unit including those on the Tuner board and a majority of the smaller caps on the tuner board (All of the <4uf Sanyo robin egg blue jobs). No alignment needed or done. Meter sensitivity needed to be touched up but that was it and that's covered in the manual.

Mark is worried about the guys who haven't done any recap/work on electronics before and indiscriminite replacement of parts without knowing what the hell they are doing. So his parts list for the tuners is limited to the 4 parts that are totally safe to replace without changing the capabilities of the tuner. And I can't blame him. I usually recommend if the guy is low time/low experience to follow Marks Recomendation. Someone with the equipment and knowledge (ie: a Tech, an EE, Military Electronics Tech, etc.) get a bye and replace all of the caps/transistors and they can align if needed.

 

[dr*audio]

You can replace parts in the audio circuitry and you will not need to align it. If you don't know what part of the circuit is audio only, then follow MTF's advice.

 

[vulturon]

Okay, thanks for the input guys. I understand Mark's concern and advice on limiting most replacement jobs to those 4 parts.

 

[markthefixer]

Q10 and Q11 are 2sC945 the OPPOSITE polarity of the 2sA725.
Q10 and Q11 are used as switching (NOT sound) transistors to suppress and mute the stereo decoder when there isn't a stereo signal to decode.
Thus unless bad, keepen der mitts OFF!!!

The FM audio cannot be hurt by E-recapping - this is a MOTL stereo decoder with NO separation adjustment - or others found on TOTL tuners.

You MIGHT knock off the VCO center frequency adjust - (C38, C39, C40) but that can be set with a strong over the air FM signal.
That is set by finding the OFF to ON stereo lamp transition POINT on CW rotation of the adjustment pot VR2
and then finding the OFF to ON stereo lamp transition POINT on CCW rotation of the adjustment pot VR2
Then the set point on the VR2 POT is the midpoint between the two points you just located.
MUST ONLY BE THE OFF TO ON stereo lamp transitions!!!!!!
NEVER NEVER NEVER the ON to OFF transition!!!!!!