Sansui 4000 With A Couple FM Issues - Need Help

bhuskins

Super Member
I found a really nice 4000 that I've started to restore.

It'll eventually get a full recap and such but before I do that I always try to get the unit working with 100% functionality first.

All the inputs work great and output is strong and clear
All the controls have been cleaned...including the ones on the back
AM tunes perfectly with nice signal deflection and clarity
FM tunes great as well with nice strong signal and tuning deflection and a good quality sound and performance.
Spot on pointer dial alignment for both Tuners

The issues are:
-FM does not tune stereo even though a strong signal is received and sounds very good, no matter what VR401 (mpx adjustment on mpx board) is set to
-FM muting will not engage to mute static, no matter what adjustment is set to
--As a result, no difference in sound when MONO is selected and no difference in sound when the MPX filter is engaged. Also, no stereo light illuminates either (the bulb has been replaced)

I have spent most of my effort on the MPX board (Sansui Board F-1185.) I went ahead and replaced all of the electrolytic capacitors because I had them on hand and I also replaced the 6 transistors that are on the board. I used KSC1845/KSA992 for these. I have not replaced TR407 which is underneath the chassis but I don't think it is the issue because of the muting problem and lack of change to the sound when VR401 is adjusted...it causes no change in the sound no matter where it is adjusted to. I removed and cleaned VR401 and tested it. It tested fine and adjusts from about 5 ohms to about 9.5K ohms. I have not replaced CR401 and CR402 which look similar to orange dip caps but have three legs.

So, I'm at a bit of a loss as to where to look next. It's like the MPX board isn't doing anything no matter what its adjusted to. Pretty frustrating when the FM tuning is spot on and strong and the board doesn't have any IC chips.

Thoughts from the experts?
 
The issues are:
-FM does not tune stereo even though a strong signal is received and sounds very good, no matter what VR401 (mpx adjustment on mpx board) is set to

This is problem #1
-FM muting will not engage to mute static, no matter what adjustment is set to

This is problem #2 check voltages all over this circuit, something is not right. It's on the FM board, not the MPX board.


--As a result, no difference in sound when MONO is selected and no difference in sound when the MPX filter is engaged. Also, no stereo light illuminates either (the bulb has been replaced)

Correct, all this is as designed. Without the MPX working, there will be no difference between left and right, and hence no change from either switch, and no stereo light.

I have not replaced CR401 and CR402 which look similar to orange dip caps but have three legs.

Don't. You can't find a replacement anyway. They are definitely working at this time, anyway. ALL sound goes through them in the Source selector FM Stereo position, so they must be working. Pioneers are much more likely to develop bad Low Pass Filters than Sansuis, in my experience.

The service manual has lots of troubleshooting/alignment info. Use it.
A partial alignment will often fix problems. If you don't have the proper test equipment, don't adjust that step, but read it, understand the objective, and it's effect.

A suitable signal generator is your local FM station, or the inter-station hiss.
A sensitive voltmeter/VTVM or oscope is needed for measuring/peaking signals.

The schematic has LOTS of voltages, check/verify them all.

With that said, I believe that your MPX problem is a detuned discriminator tank circuit. I've regained stereo on many tuners by getting this adjustment right. Try CAREFULLY peaking the coil. Return it to where it was if Stereo doesn't pop on.

Here's why: the coil will never change value. But those little caps across the coil inside the can that make it a tank circuit, WILL. When that happens the high frequency intelligence is lost or attenuated. MPX needs strong 19 khz pilot to operate.

Is your center meter centered? Did you (or someone else) center it with the pot vr201? If it's off to one side, that is a dead giveaway that the discriminator is off. Tune in between FM stations and see if it is centered.
Edit: the meter, not the pot.

Of course the drifting cap theory applies to the tanks on the MPX board so they may be to blame.

If you have a scope you can get a decently close alignment just using off the air stations.

If you don't have a scope, check all the voltages on the schematics. If they are all nominally correct, then it needs an alignment.
 
"Here's why: the coil will never change value. But those little caps across the coil inside the can that make it a tank circuit, WILL. When that happens the high frequency intelligence is lost or attenuated. MPX needs strong 19 khz pilot to operate."

And they will usually be stable after "the change". Kinda sounds like menopause...

But changing them is really too much if they are anywhere near usable values for the circuit. If the coil core CAN be adjusted to make it work right, as far as center and all that, the Q is not affected enough to worry about. It may be just slightly higher distortion, but most people will never hear it. In fact, i the caps decrease in value it should be a slightly lower Q which will give less distortion but possibly more interference from adjacent stations. if they go resistive too much, the whole damn thing won't work, and I suspect this is what happened to alot of nice Marantz out there. I call it FMitis. The detector simply WILL NOT align correctly. And those quadrature detectors are no better.

Another thing is that sometimes, especially after vibration, the core can slide down. so if you go in there to tweak, go counterclockwise. Also, use a BRAND NEW alignment tool. You don't want to use worn out ones because the forces they presen can cause the core to crack in half. When that happens you got a real problem that is bad enough you just might not have a tuner anymore. I have fixed a few, but would not anymore these days at gunpoint. Just too many wayds to screw it up and too little of a chance for success given the state of me, can't see. At least worth a shit. But I have in the past destroyed the core with vaious implemets that honestly were not meant for destruction, and then replaced the core from another similar coil.

Found them then, SOMETIMES. Find one now.

bottom line I owuld rather add a variable capacitor to the circuit than to adjust the core of a coil, especially a discrimintor or ratio detector.
 
Thanks for the replies guys...I've tweaked it a bit and have solved issue number 2...the FM muting works great now. I still have no stereo indication (or sound separation) and for what it's worth the MPX noise canceler does not impact the sound at all either.

Adjusting the 5 coils on the main FM board with both tuned and untuned stations corrected the FM muting and the tuning is excellent.

However once tuned to a strong FM signal if adjustments are made to any of the 5 coils on the MPX board or to VR401 on that board...no matter how extreme...no affect to the sound is heard either positive or negative. It's like the board is dead but passing sound. Is that possible?

So, I have the schematic and am looking it over. What voltages should I verify and focus on for the MPX stereo issue?

Oh and I should say that the discrimator adjustments did not affect the MPX situation at all and when a station is locked in and peaked the tuning meter is centered. When there is no station tuned it is also centered. It does however properly deflect while working on the tuning and centers when the station is peaked.

Thoughts?
 
Another quick observation...jumping the Collector to The Emitter of TR407(underneath the chassis) engages the stereo bulb...so my new bulb is good.

And when I take a voltage reading off of the Base of TR407 it reads 7.93v whether a station is tuned or not. The emitter reads 8V which is straight from the transformer. Could TR407 be bad? It's the Ge transistor 2SB324...it's the only transistor I didn't change in the MPX circuit. Suggested replacements?
 
I've just been lurking in this thread because I think my approach is incompatible, but I can't help myself:

There's approximately zero chance that TR407 is the problem (at least, not the whole problem).

Try grounding the MPX board terminal 4C, which is the driving signal for TR407. My guess is you'll find that the stereo lamp comes on, which will mean that TR407 is fine.

If you're lucky, you might also find that the sound now has glorious stereo separation, which will mean that the problem is in the stereo sensing/switching logic, TR404-TR406 and friends.

Cheers,

chazix
 
I know this is an old thread, but has this been solved? My 4000 has the exact same issue, and I would like to know possible solution before I tear into it.

Thanks

Ron
 
Honestly not sure...I think I replaced TR407 but with what I can’t say at this point...once I did so it fixed it. BUT, I had already replaced most everything else as well on that board. Definitely a shot gun approach though.
 
Honestly not sure...I think I replaced TR407 but with what I can’t say at this point...once I did so it fixed it. BUT, I had already replaced most everything else as well on that board. Definitely a shot gun approach though.

Thank You for your reply, I appreciate it. Looking at the schematic appears to me that all 407 does is turn the Stereo light on or off determining whether or not a stereo signal is being received. My 4000 has no separation, there is no difference, even on strong stereo signals, no difference whether Mono switch on or off. I was just trying to avoid your shotgun approach, guess I'll have to just dig in.
Thanks again
Ron
 
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