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Sx-626 FM Stereo problem - Does this sound familiar? Need some help.

tusntuk2

tusntuk2

Active Member
Just finished recapping a lovely little SX-626, and the FM board doesn't want to go stereo. The Muting switch sometimes works, sometimes cancels all strong signals and goes silent. The Stereo light flickers while tuning, sometimes indicates stereo on weaker stations, but often does not glow on strong stations when meter centered. As you tune off center you get more static/hiss noise but then the Stereo light glows weakly. I'm using a little dipole ant. The FM sounds great - in Mono - and when in Auto position that's what I get, Mono.

So - lamp is OK, FM tuner works, problem suggests another circuit. I'm not an engineer, I have had success recapping things by doing one piece at a time. On the FM board I only changed 4 blue Sanyo caps, I left the rest alone and the FM sounds good. Just not Stereo.

The PS is fully rebuilt, but the schematic has very few voltage measurements displayed. For instance, it says B+ or B-, but no value.

This is the 1971 version, with an AWE-011 pcb and a Motorola chip, no HA 1142 or other HA ic's. The 1971 manual seems to take a lot for granted (about the serviceman's skills!)

Is there any easy fix or adjustment? The amp sounds so good, but it would be great to have stereo FM too.
 
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Sounds like the FM MPX is out of alignment or FM signal isn't enough from the detector / discriminator circuit (Tuner) to the input of the MPX. You can replace the rest of the electrolytics (CEA Types) on the board with Nichicon UKL series. 16V bump to 25V. 25V and 50V leave voltage same. All values as noted on board. C38 & C53 you can use a Panasonic ECQ type film or a Cornell Dublier DME type. Both are physically small enough to usually fit depending on the location on the board. As they are both on the outer edge should be no problem.

Don't make any adjustments to the coils or trimmers. Recheck after recapping the rest. If still not working right, contact your local HAM group, and ask if they have any older operators who can do alignments.
 
Thanks, Larry. I was kinda afraid I'd hear that advice (realign!) and hoped for a simple adjustment. Maybe attention to a Mpx circuit? I don't have lots of gear, just scope and DMM, and even less knowledge! I have replaced four caps on the FM board (the Sanyos), all in the signal path, I think, and the sound is fine. I can replace the rest easily enough.

What about upgrading transistors? I would expect more influence on the tuning with Tx changes than with caps. Er, no?!

Do you have any voltage information? Figure I should try to confirm that first.

Thanks so much for the help!
 
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It might also help to verify that the tuning and MPX sections/boards are clean, along with the tuner VC. Dirt accumulation, excess flux, and other contamination can have a remarkable effect on RF circuits.
 
Thanks for the cleaning tip - cleaning more as we speak. There is dust and liquid residue (flux?) on the boards. Seems to be clear, dried, puddled around each through hole, so likely flux instead of Coke.
 
Could very well be flux as these boards were generally wave tank soldered and some flux wicked up the holes.
 
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