Pioneer SX-1250 FM muting not working

[Michael Bishop]

My Pioneer SX-1250 has developed a problem where the FM muting between stations no longer works. With my volt/ohm meter, I have verified that the issue is not the muting switch, nor the wiring from the switch board to the FM tuner board. Looking at the FM tuner board, I see no obvious visual signs of bad capacitors, or bad solder joints. There is the Reed relay (part: ASR-005) which supports the FM muting function, and I'm wondering if the trouble lies in there? I'm told the Reed relay is not serviceable, and must be replaced, if that is the source of the issue. One thought is to re-solder around the existing Reed relay - but have not yet attempted any re-soldering. I only have a volt/ohm meter (no other test equipment) and would appreciate any advise on troubleshooting this further.

Many thanks to AK members!

 

[markthefixer]

OK, I'll assume that you watched pin 38 (muting in) actually toggling between 0v and some higher positive voltage.

So, watch the voltage for the reed relay coil, Q10 collector or one lead of C62,
and see if it goes to ABOUT 0v (less than +0.5v) from where it sits when off at about +25v

That would mean the relay coil is getting current - and should pulling in the contacts to mute the sound.

If it doesn't move from +25 at all, measure Q10 base for 0v (relay off) and about +0.6v (relay on, muting engaged and sound off)

If the base gets turned on (+0.6v) either the transistor is dead or the relay coil has lost continuity.

You can SAFELY ground Q10's COLLECTOR to see if the relay operates and mutes the sound - this would exonerate the relay coil.
Don't hit any other leads by accident - they WON'T like it and will reward you with smoke from destroyed components.
If you ground the relay coil's HOT side connected to (and FURTHEST FROM!!) pin 39 you could pop the +25v supply on the power regulator card.

 

[Michael Bishop]

Markthefixer-- Thank you so much for responding. I have not yet watched voltage toggle across pin 38. I have only looked at the continuity of the switch, and the wiring from the switch board to the FM tuner board - with the receiver powered down. With the Muting switch on (front panel button out), I see continuity between pins 1 and 2 on the switch board. With the Muting switch off (front panel button in), there is no continuity between 1 and 2. I have also checked the continuity of the wiring between pin 1 on the switch board to pin 38 on the tuner board (good), and pin 2 on the switch board to pin 14 on the tuner board (also good.) That's as far as I've taken the troubleshooting on this.

So to do the very first step you mentioned, I would connect the positive lead of my voltmeter to pin 38, and the negative lead of my voltmeter to the receiver chassis (e.g., ground) - and then measure VDC across pin 38, with the receiver powered up - and with the muting button toggled on and off? Sorry for such a basic question, but I want to exactly understand each troubleshooting procedure before I attempt it. (Don't want to let that smoke out.)

 

[rcs16]

yes connect your DMM(+ve lead), in DCV mode to terminal 38, it is the mute control signal into the schmitt trigger ckt. Your -ve probe would go to terminal 11(E) or chassis ground. use lead clips(not alligator) to make it safe and easy so you do not short anything out.
FM mode, toggle the FM mute switch, also with your +ve lead measure, Q10(c) to see if it toggles as well (25V & ~0.25V). Check terminal 39 for the presence of the 25V DC supply, without it the relay can not activate.
The voltages shown in the schematic are for a un-muted condition, Q10 is off, reed relay contacts open.
With power off you there is no continuity through the reed relay contacts. When reed relay is activated (FM muting), it shorts the L,R signals (terminals 40,42) to ground, terminal 41.

 

[Michael Bishop]

rcs16: Thank you very much. It will be a few days before I have time to get back to this project. Once I have more data, I will post it here. Cheers.