Pioneer SX-750 - Muting defective

Bert Beukema

Well-Known Member
Hi guys,

This 750 has been in my possession for a long time, restored. This week I installed it in my wife's office and I realised that I had missed a feature. For all this time I had the muting feature off (mute button is pressed). When I disengage the button, complete silence greets me. The signal is coming in very strong, there is a signal tower near by. The signal meter is at it's highest point; the tuning meter doesn't work (not moving).

I checked all voltages in suspect parts of the circuit. The muting board (this is a separate board, it doesn't use the IC muting feature but bypasses it) is receiving the correct voltages. The HA1196 is not on point for pins 4/5/6/7 (roughly 2 volts too low). Schema doesn't say which voltages would be acceptable for the transistors on the muting board though. Diodes all check out fine, normal transistors also fine.

The fact that there is a complete audio dropout when the muting circuit is engaged is worrisome. The tuning meter could possible be an alignment issue and solved by working on the T6 coil. But the complete dropout makes me suspect either HA1201 (discriminator), HA1196 or something else in the circuit. I hope it isn't the filter on the muting board as that one is both unobtanium and difficult to check as the board is soldered in sideways.

Has anyone encountered this issue before working on the SX750 or it's higher level brethren?

Thanks,

Bert

Ps: I've added part of the schema.
 

Attachments

  • SX750 tuner.png
    SX750 tuner.png
    273.1 KB · Views: 6
There are 2 adjustments since there are 2 discriminators. there is the AFC 0 for the HA1137 and then there is a zero for the ratio detector.
Look at page 15 for a description of the muting circuit. the big schematic shows the muting circuit better. Got to go, will get back to you later, good luck
 
Ah, okay please let me check this first thing in the morning. Any advise is welcome, although I have done tuner 'tuning' in the past I can't say that it is a favourite pastime ;-). I thought about simply bypassing the muting circuit with the one that is in the IC already as an option to test the origin of the issue. But as you said, it is most likely that some coil is out of whack. The tuner section of the 750 is slightly odd, mostly over engineering.

Page 15 of the service manual shows the muting circuit. But it doesn't show where to make adjustments; it also doesn't mention which voltages are enough to trigger the circuit.

Bert
 
Looking at the simplified diagram in fig 5 ,the "pnp" muting bjts are held off by 2 diode clamps,-1.3V (D1,2) from the -19V supply. The bases (+0.6v) are lifted to the B+, to conduct/turn on for mute op.
Mute bjts have been known to fail. If you remove them start with Q13 (2sa733) replace with ksa733c
 
Those muting transistors don't even need to be completely failed. If one has some leakage it. May test good with a standard multimeter, but will not provide a complete off switching function.
 
When I first restored the 750 I took extra care in removing the muting board and then rebuilt it entirely as a precaution. The board is soldered onto the main pcb and repeat desoldering will damage it easily. So that muting board was brand new and using much better transistors. Of course stuff happens so I rechecked all components but they were all functioning within spec.

Seeing that the stereo light was out and the tuning needle wasn't moving I believed that to be an indicator of trouble. The stereo light will go into the on position only if the tuning needle is in the center. So I cleaned T6, which is the tuning coil that controls the tuning indicator. Afterwards I injected a signal and tuned to it (signal meter at maximum), then proceeded to work T6. It had seized quite a bit but the cleaning made it possible to slightly start rotating. It took roughly a 3/4 turn to align the tuning dial. The stereo light activated instantly and the muting network also activated.

So, it seems that the tuning meter is rather significant within the ecosystem. If you have a problem with muting and stereo it seems prudent to check on the tuning meter. If that meter doesn't move it is misaligned; aligning it will solve the other issues. If it doesn't then there is another problem of course ;-).

Thanks for the help!

Bert
 
Good work Bert, glad you figured it out. The muting function is controlled by the tuning of the HA1137 quadrature ckt (deviation offset muting) and the IF sig level.
 
Thanks, but your words helped me reassess the situation. Now just need to do some additional fine tuning on the tuner, seems it has gone out of alignment a bit. Some stations are off, a bit off noise and some cleaning are necessary. But overall, I would say that the tuner in the 750 is rather nice. Picks up remote stations fine and the sound quality is much better than this 'little' Pioneer should have. Recommended if you can find it cheaply.

Bert
 
Back
Top Bottom