Pioneer SX-626 no stereo on FM

[leesonic]

I have a Pioneer SX-626 that does not currently produce stereo sound on FM. I was planning on recapping the whole thing, which I've done, and thought it might cure the problem, which it didn't.

I found this thread with a similar problem...

http://www.audiokarma.org/forums/showthread.php?t=119021

Mine is an early one without the HA1142 as well. The stereo light doesn't come on, but it also doesn't sound like stereo, and switching from mono to stereo doesn't make any difference.

Looking at the schematic for AWE-011, it appears the stereo indicator is turned on by Q19, which is turned on by Q20. It looks like these also feed the M5109P. I wonder if I should try replacing Q18 (2SC711), Q19 (2SC968) and Q20 (2SC711)?

Of course, I could just replace the whole tuner board, I have one from a parts 626 that lights the stereo light. I can't verify it actually produces stereo because the power amp board is bad, but it lit up the stereo indicator when I attached an antenna and powered it up.

Lee.

 

[larryderouin]

Lee. You could pull the Q's but work back from Q20. Test and reinstall or replace.

If the other board is also a AWE 011 and you know that board is good, change it out and see what happens. If it lit the stereo lamp with an antenna connected, and flip flopped (on-off) when you went up and down the band it's proabably ok.

Larry

 

[leesonic]

Getting a strange feeling of deja-vu here. I have another SX-626 that I'm working on, with exactly the same problem. Perhaps the only difference is, the signal meter on this one doesn't work either. It's quite possible the signal isn't strong enough from the simple dipole antenna that I'm using, but it's never been a problem with other tuners and receivers, I can usually get a stereo light and a pretty strong deflection on the meter.

The last one I "fixed" by swapping out the board. This time, I'd like to learn to fix it properly, but tuners always mystify me.

I've attached a schematic of the board. It's a 1971 board, without all the HA chips.

Lee.

 

[leesonic]

OK, so things are really getting confusing. I have two SX-626s, and three AWE-011 tuner boards : one that was in the 626 I'm working on, another that I recapped before to fix a similar problem (which didn't), and another board bought from eBay with all original caps.

The first SX-626, I previously recapped and re-transistored all the other boards according to lists found here on AK. All the other functions work fine, and it sounds really sweet. I thought I would see what the tuner board did before attempting anything on it. It would not pull in stations very strongly at all (hardly any movement on the tuning meter), and would not switch over to stereo.

I recapped the tuner board, but it didn't make much difference. I then replaced all the transistors (except the one FET) with the following list :

2SC382 - 610-2N3904
2SC711 - 512-KSC945GTA
2SC870 - KSC1845EBU
2SC968 - 512-KSD1616LTA

The 2SC738, the only substitution I could find was an NTE108, but I looked on eBay, and found a seller here in the US with good feedback that was selling a bunch of 50 original Mitsubishi 2SC738. With these fitted, the tuning meter deflects a little bit more, and certain stations down the lower end of the band will flick over into stereo.

So, for an experiment, I tried the eBay board, this still had all the original caps and transistors. It was just as bad, the same slight deflection on the tuning meter, but no stereo at all.

By now, I was getting desperate for ideas. The second 626 was bought as a parts unit*, someone had hard wired the two power amp channels together to make it appear that both channels were working. They did such a bad job, dry solder joints everywhere, and wires twisted together and "insulated" with duck tape. I was aware of that when I bought it, so I didn't pay a lot for it. I put the eBay board into this, and after pulling the fuses for the power amp board, I gingerly fired it up. This time, the signal meter was getting almost full scale deflection. I couldn't hear it switch over to stereo, and I stole a bunch of lights from this one to use in the other one, including the stereo light. But what I heard sounded a lot stronger than the other unit, I used a separate power amp in order to listen.

So, my question is, could the FM front end in the first 626 be bad? It certainly would appear so, based on my findings above. Looking at the service manual, it has three transistors in it, a 3SK30, a 2SC535, and a SE3001. I'm not about to claim I know anything about working inside one of these. Perhaps I should try and find one from a parts unit?

The one nagging thought in my mind is the tuning capacitor. It looks very "wet" on the tuning vanes, I wonder if somebody tried to clean it before, and literally gave it a bath in DeOxit, or maybe even something less suitable? I thought about using a small brush with some rubbing alcohol on it, but how would I know I cleaned all the "wetness" off?

Mark the Fixer, I could really use your advice.

Lee.

* BTW, I referred to the second 626 as a parts unit, but I don't consider it as such. I'm not about to rob Peter to pay Paul. Nor am I about to rip apart a complete unit to get another one going. If a unit has all the pieces, as this 626 does (granted, they might be transplants), it's not so much of a parts unit, more a unit waiting to be brought back to life.

 

[patfont]

Pull the 7060P and try in the other unit, or check voltages at the legs of the good one vs the bad one. Looks like there are 3 in there.

 

[markthefixer]

tuner vanes look wet, somebody gave it an oil bath and knocked ALL the adjustments cattywumpus... use zero residue cleaner and hose the thing down, then let it dry for 24 hours UNMESHED.

Later it will need to have the bearings relubed by DRIPPING in the oil and KEEPING the oil OFF the plates!!!

The major adjustments for sensitivity in an alignment are all in that front end.

DO NOT NOT NOT NOT NOT (get the idea???) mess with the transistors or anything else in there - don't %^&^$^% with any coils or fins.

Once the cap fins are clean you should see an increase in signal strength - as long as some other %^& hasn't twisted the variable capacitor screws on the tuning cap assembly.

If not, THEN we check the IF stages. patfont's 7060's...

Thing is, you are getting SOME signal...

Don't be twisting adjustments until you have educated yourself on the theory of FM stereo encoding / decoding and what these specific circuits are intended to do. Even I go VERY slow and carefully when doing them, backtracking a lot and re-checking continuously.

old school tuner and stereo decoder realignments are a PITA to do. Which is why the chip based circuits became so popular so fast.

 

[leesonic]

Since I was going to take the tuning cap out anyway to clean it, I found one on eBay for less than a score shipped. I simply swapped them over, and hey presto, a healthy deflection on the meter, a stereo light that works on almost every station, and that great FM sound these old receivers have.

The old tuning cap, it really does look like someone gave it an oil bath, it smells like oil too. With the new eBay cap, I squirted some deoxit into an old syringe (don't worry, not for me, I keep it just for tasks like these), and then dripped a single drop onto the tuning cap shaft.

I plan on keeping the old one in my "spares box", what would be a good way to clean it now it's out of the receiver? Could I pour rubbing alcohol on the vanes, and leave it to dry outside in the warm sun?

Lee.

 

[larryderouin]

Get a can of CRC "QD ELECTRONICS (CONTACT) CLEANER @ Home Depot. It's NON-RESIDUE, and works great on all my tuners here. Flush the FINS UNMESHED LIBERALLY with the unit on it's side (tuner to the bottom). Let the dregs drain off, and let it air dry for 24 hours. Plug in and fire it up. Don't get it in the coil towers.