Pioneer SX-636.....rushing sound on left channel

are you sure they are 726's? they list 2sc945's for stereo on/off control and 2sc763's for audio amp after stereo decoder chip. look at the numbers carefully.

 

I was wrong, there are two 725 transistors on the Tuner board, not 726s. Here's a picture showing one of them.

 

a992's...

 

Got it....that is what I ordered to replace the A725s.

Power amp adjustment question. Page 18 of the service manual talks about measuring the DC voltage across the power transistor pairs. It specifies to use an 8 ohm dummy load across the speaker terminals while doing this measurement/adjustment. I don't have an 8 ohm dummy load. The speakers on my bench measure 3.5 ohms.

I did the measurement and read 28.5 mV left and 32.4 mV right. The manual says it should be 20.0 mV. Is using my speakers as the dummy load affecting these measurements?

Also measured DC offset now at the speaker terminals. Don't think this is adjustable on the SX-636, it is not mentioned in the manual anyway. DC offset is 17.3 mV left and 22.3 mV right.

 

I've never connected dummy loads to any X3X series receiver when doing the adjustments, per past practice on various units postings per Mark. Having said that tho, I've never seen a posting on the 636. It's similar in practice to the 434 and 535, that it's probably ok to not have dummy loads connected.

No DC OFFSET adjustment. Just the BIAS. Set for AUX, Zero Volume, No input connected. Take off the speakers and leave the terminals free. Set for 20mv.

Offset is fine. Anything below 50mv is very good. It may even come down once the bias is set.

Larry

 

look for an output coupling cap, if there is one (it will be almost as honking big as the power filter cap, and there will be one per channel), then a 10 meg DMM will give you strange and frightening DC offset readings that will "disappear" with a speaker load. It's the electronic leakage current that's unavoidable through a capacitor. The DMM is sensitive enough to detect it. But there isn't enough of it to disturb a speaker, no less start a fire. (unless of course the capacitor shorts out - then ouch, THAT's what fuses are for)

If there IS NO output cap (I don't have a handy dandy model list for that right now) a load resistor is not needed to set DC OFFSET, the dc offset will be the same with it or not. The Idle current may have a slight difference with a load resistor in place as now there is some place for the current to sneak off to rather then go through BOTH push pull output transistors.

 

Thanks again Mark and Larry. Your help and guidance is much appreciated.

Refurb done.....she is singing like brand new!!!

 

Last two caps redone. Fuse lamps replaced. Wood case Feed-n-Waxed. I'm done.

 

Great job! :tresbon:

 

OOO-RAH!!!! Great Job! She looks brand new.

I just read the last of it from my last post. checking the schematic, No output coupling cap. No Protection board or relay. So those two honkin 4A fuses are your ONLY speaker protection. But if it's anything like my 434, it's a great little receiver.

Larry

 

I blew it on the list, the awh-034 diff amp input stage transistors are now correct.

anal cranial insertion err..... pnp - npn inversion ... now fixed...

 

SX-636 power supply board AWR-060 B cap value change

Just a heads up to anyone recapping an SX-636 - You may find that C12 on your power supply board is actually 10uf 16v instead of 220uf 16v. It appears that this was done at the factory and should be kept as a 10uf if that's what was in there originally. My power supply board number was AWR-060 B so I don't know if the non-B version uses a 220uf or not. The schematic for the non-B version appears to match up with the AWR-060 B board except for the value of C12. Here's a thread about the issue.

 

647-QYX1H334KTP

Data Sheet

 

I'm adding one more post to this, don't have a part number yet, but the SX-636 main caps do not have bleeder resistors, they depend upon idle current to deplete them, BUT idle current goes away during shutdown when the regulated DC disappears and these caps stay charged for a long long time - I just remembered that.

So add bleeders, say 3k ohms at 1 watt (35v into 3000 ohms is ~ 0.4 W) across the two main caps 6800 uf 35v.

This is important because the output is DC coupled (no output cap) and there is no protection relay. I remember times when I shut off my SX-636 and a while later there were scratching and rumbling sounds coming from my speakers (the 2sc1451's were probably clearing their throats) so I used to turn OFF the speaker switches before turning off the power.

 

Hey
Could you add the recommendation to the post on page one?
I often wonder about that very issue.
Some use caps (like the 1250) and others use resistors like the 1010's.
This is an example of the posts (of yours) I miss most.
Is the hiatus over?

 

Hiatus not over yet

I do overwatch for for bad things as much as I can, but there's a huge time difference between reading and posting.
Thus the quick post above as the recollection hit. No post backtracking, parts lookup etc.
sorry.

eventually I will be doing MY SX-636, so the thread will eventually be revisited. But that's a long long time off.

 

I have a SX-636 on the bench. Working on the AF Amp AWK-035-0 (Tone Controls) and using markthefixers list in post #6 as a rough guide. Just wanted to point out that my unit has the orange jacket CEANL caps at 13/14 and 21-24 for the 4.7uf 25v stock caps. Therefore I'd recommend using ukl caps over the upw as in the list.

Edit: The stock Caps 9/10 100uf 16v on this same board are the yellow jacketed CE LL caps by Nippon Chemicon and are also low leak types so use ukl here as well.

 

Going through the BOM list, I've found that:
Mouser #: 667-ECQ-E1474KF
Mfr. #: ECQ-E1474KF
Mfr.: Panasonic
Desc.: Film Capacitors 100DC 0.47uF 10% L/S =7.5mm
(x4)
....and
Mouser #: 598-DSF050J334
Mfr. #: DSF050J334
Mfr.: Cornell Dubilier
Desc.: Film Capacitors .33UF 50V 5%
(x2)

......are no longer available.
Any suggestions for Mouser part number equivalents?
Thanks in advanced!

 

Sorry if this is the wrong place to ask the above question but this thread is where I discovered the BOM list for this model.
If so, please have my posts deleted.