• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

SX-750 loses sound

M

Misha S

New Member
Hi, folks! Before I plunk down repair money, hoped the experts would have some input on this. I've had an SX-750 that seemed to work pretty well, and sounded great. (Second owner after my mom.) But now the sound will spontaneously cut out. Typically it goes out for a few seconds, then I hear the protection circuit click and a moment later sound returns. Then it will happen again, faster than previous with a longer delay until it returns. And again. And again. And eventually it never returns. Turning it off for long enough lets the cycle start over, but as time goes on, the first round in the cycle is further along that cascade to failure each time.

This happens regardless of input, and I believe there's no pattern to where it drops out in relation to the musis playing. (In fact, I left it running on FM with the speakers in the off position and I still heard the circuit occasionally click.) The problem first showed up when I switched to big honking speakers of unknown impedance (custom jobs that I found) but persists on tiny 8 ohm Sonys. Timing could have been a coincidence. When this happens, all lights stay on, no flicker, and while the unit is a little warm in the front center if I try long enough, I don't think it's overheating, seems like normal operational heat. It's unobstructed, and in the most recent tests the sound cut out while the unit was still cold.

Thoughts? Is it just the protection circuit slowly failing? How much should this run me in the LA area? It's been ages since I soldered anything, and I have none of the equipment to do this myself.

Thanks in advance, everyone!
 
Register to hide this ad
50 / 50 one of the AMPS is failing, and the protection is doing it's job. But the protect could be getting jumpy.

It would be instructive to take voltage readings to try to identify the channel, and the protection system's reason for the disconnect.

But the best was already covered, running it without speakers - and it still did it.

A bad solder joint generates it's own (localized) heat. Arcing...

You could use a can of "air" keyboard cleaner, to cool small areas, to try to find the culprit.(it ain't air, but it ain't freeze spray either).
Don't generate frost at the spot, just cool it.
 
I agree with markthefixer, you can’t go wrong with a good clean. If there’s a lot of dust, it might be causing a short.

I also have an SX-750 and had a similar issue that the group here helped me fix. Let me dig up that thread and share it with you. Mine was rather complicated. But once you get it, and dial that amp in, you’ll be pleased with it. I certainly am, I’m listening to it right now!

your mom had good taste, it’s really a sweet sounding amp.

be extra careful with the off / on / speaker selector knob. They can easily be over twisted and break off in your hand. Finding a new switch is next to impossible.
 
Look for TINY ring cracks in the solder around the leads of the three TO-220 transistors in the power supply circuit (the ones with small heatsinks). They frequently will cause the symptom you are describing. I would just re-solder them anyway.
 
Thanks all for the tips! Looking through the grating it is fairly well CAKED with dust, so hopefully the cleaning alone will do it.
 
yeah, the wooden cover comes off easily. Undo the screws on the sides (2 on each side) and carefully pull upwards and slightly towards the rear of the unit to remove it. You can take the bottom cover off too. There's a lot more screws however, but I think in the neighborhood of 7. The feet can stay on.

Once the cover is off, take it outside onto a work table or a surface. Put an old towel down to protect it. Then blast away with a can or air. Maybe two cans of air. You really won't damage it by blasting away. You can use Q-Tips with a little rubbing alcohol on stubborn spots then air blast it again. If the knobs are scratchy when you turn them, you should use some deoxit (found on amazon) to get inside of those and clean them up. It's not hard, just spray it as best you can and turn the knobs back and forth 30 times or so. You can also use deoxit on any areas that look grungy to you after it's cleaned. Hopefully there's no water damage from someone putting a potted plant on top of it or anything like that.
 
Do NOT get Deoxit or anything else on the fins of the variable capacitor in the tuner section, it will mess it up. If you do get something on them, get a 'no residue' cleaner to wash them down.
There are stronger safe methods to clean it up if it is still really grungy, even after blowing it out. Let us know for further info . . .

EDIT: Deoxit D5 for the switches, Deoxit Faderlube F5 for the pots.
 
Last edited:
1333631-3ae2100f-pioneer-sx750-stereo-receiver-in-excellent-condition.jpg

I've worked on a few 750's and own one. I haven't seen one where capacitors C17 and 18, (330uf 50v), were good. They are on the protection board in an area that gets hot and they have always measured low. I replace with 330uf 63v 105° replacements.

Honestly, that whole board is something that could use a recap and resoldering of the transistors that run hot.
 
Unfortunately, no change to behavior after a good air-can blow out. Looks pretty clean inside now, but exact same behavior.

Now, operating with the case open for the first time, I might be crazy, but I think there's a smell coming off the protection board.

Latest test, running it from the start with speakers off, tone off and centered, volume all the way down, input to aux with no aux device attached, it seems to last a lot longer initially, but eventually the same pattern emerges: recloses and reopens repeatedly, with longer open periods and shorter closed periods until it just stays open. Protection board heat sinks get very hot (and the light bulbs, obviously), immediately surrounding components get warm, relay gets warm, nothing else does.

Is this a possible clue: when speakers are enabled, with small 8Ω speakers hooked up, input set to FM, sound still comes out even with the volume set to 0. Mean anything?

I read through that whole thread, @kyledp, and this is all way beyond my comfort level, nor do I have any of the necessary equipment.

Bob, I assume to check the solder on the TOs I'd have to remove the whole protection assembly and get to the backside of it. Without knowing more what I'm doing I'm hesitant to do that also.

With no glaringly obvious culprits, I think I'm gonna call in a professional. I'm in no hurry, but too busy to sink time in, plus worried I'd damage it further.

Thank you for all the troubleshooting, everyone! (And any recommendations for LA area techs, much appreciated.)
 
If you are not able or willing to work on it yourself, your only real option is to take it to a competent tech, if you want to keep it.
If you are getting a 'smell' and heat from the unit when it is turned on, leave it off and unplug it. Further attempts, and protection circuit cycling, will only cause more damage. The unit needs attention from a tech, troubleshooting, and repair/refurbishment. Aside from the family history . . . it is very repairable by someone who has the proper skill set, and well worth some repair expenditures.
 
The next guess I would have is noisy Q3 or Q4. I have seen this defect several times in the past. You need an oscilloscope to see the issue. It will show up as noise pulses at the anode junction of diodes D6 and D7 (or on the collector of Q3 or Q4). You might be able to see a voltage variation on the anodes of D6 and D7 with a multimeter (but I have never tried). Each time the noise pulse occurs it trips the protector. If the noise continues frequently enough it resets the timeout with every pulse, so the protect relay just stays open. Once the unit stays in protect, you may be able to spray the two transistors with freeze spray (or canned air held upside down), and get them to quiet down so the protect relay will close. If that occurs, you have found the issue. You could also just replace the transistors, and see if that corrects the issue (but I hate shotgunning).
 
Last edited:
I forgot to mention that I tried chilling basically everything on the protection board -- and later other places as well -- when the speakers clicked off, and it never made them click back on. Of course I was just doing general blasts, maybe targeted ones would do the job.

If anybody's confident they can do this and are in the LA area, please hit me up with a price!
 
You must log in or register to reply here.
Back
Top Bottom