Sx-626 bad distortion on left channel

I have just bought a Pioneer Sx-626. The left channel has severe distortion. I can hear (on the right channel) the potential of this little receiver and would like to repair it.

I read through this thread (http://www.audiokarma.org/forums/showthread.php?t=104150) and measured the voltages. I get readings like
26.3, 59.5 and 20.4 and 19.9. I tried to adjust the bad channel but figures go hand in hand up and down. What could I do next? Visually all the caps seem to be fine.

This is an updated amplifier parts list, there is a multitude of things that can either go wrong, or have gone wrong. In the process of upgrading the board we will check the resistors to spot any damage.

I could try to steer you around to spot the damage (invariably some blown transistors, from the readings), but this is a surer, faster path, with long term reliability. You would be removing the transistors from the circuit anyway, to test them.

don't worry, I've talked many people through this, with as much hand holding as each needed. some little, some a lot.

mouser.com parts numbers:

Q13 2sc938 150v 0.05a 0.6w
512-KSC2383YTA to-92L ecb 160v 1a .9w 50mhz 160-320hfe $0.17

Q1, Q2 2sc732 50v 0.15a 0.4w 200-700hfe 150mhz
512-KSC1845FTA (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea

Q7, Q8 2sa497 100v 0.8a 0.6w 40-240hfe 50mhz
512-KSA1013YTA to-92L ecb 160v 1a .9w 50mhz 160-320hfe $0.17


Q3, Q4, Q5, Q6 2sc497 100v 0.8a 0.6w 40-240hfe 50mhz
512-KSC2383YTA to-92L ecb 160v 1a .9w 50mhz 160-320hfe $0.17

Q9, Q10, Q11, Q12 2sc793 100v 7a 60w 20-300hfe 9mhz
863-MJ21194G npn to-3 250v 16/30A 250W 8-75hfe 4mhz $4.23


before replacing them but after you have them, we will talk about how to get the transistors connected correctly, the replacement's layout can be DIFFERENT than the originals, so the originals are NO guide as to how to install the new ones.

caps:
awh-002 c1 3.3 10 cea 3.3 50 647-UPW1H3R3MDD $0.14
awh-002 c2 3.3 10 cea 3.3 50 647-UPW1H3R3MDD $0.14
awh-002 c3 100 50 cea 100 50 647-UPW1H101MPD $0.28
awh-002 c4 100 50 cea 100 50 647-UPW1H101MPD $0.28
awh-002 c5 3.3 50 cea 3.3 50 647-UPW1H3R3MDD $0.14
awh-002 c6 3.3 50 cea 3.3 50 647-UPW1H3R3MDD $0.14
awh-002 c7 100 50 cea 100 50 647-UPW1H101MPD $0.28
awh-002 c8 100 50 cea 100 50 647-UPW1H101MPD $0.28
awh-002 c11 100 6 cea 100 10 647-UPW1A101MDD6 $0.15
awh-002 c12 100 6 cea 100 10 647-UPW1A101MDD6 $0.15
awh-002 c13 2200 35 ach-004 2200 35 647-UPW1V222MHD $1.34
awh-002 c14 2200 35 ach-004 2200 35 647-UPW1V222MHD $1.34
awh-002 c21 100 50 cea 100 50 647-UPW1H101MPD $0.28

trimpots:
vr1,vr2 30k ohm center voltages adjust
652-3386H-1-503LF bourns 50k single turn trim pot $1.30

vr3,vr4 100 ohm idle current adjust
652-3386H-1-101LF bourns 100 ohm single turn trim pot $1.30


512-KSC2383YTA
512-KSC1845FTA
512-KSC1845FTA
512-KSA1013YTA
512-KSA1013YTA
512-KSC2383YTA
512-KSC2383YTA
512-KSC2383YTA
512-KSC2383YTA
863-MJ21194G
863-MJ21194G
863-MJ21194G
863-MJ21194G
647-UPW1H3R3MDD
647-UPW1H3R3MDD
647-UPW1H101MPD
647-UPW1H101MPD
647-UPW1H3R3MDD
647-UPW1H3R3MDD
647-UPW1H101MPD
647-UPW1H101MPD
647-UPW1A101MDD6
647-UPW1A101MDD6
647-UPW1V222MHD
647-UPW1V222MHD
647-UPW1H101MPD
652-3386H-1-503LF
652-3386H-1-503LF
652-3386H-1-101LF
652-3386H-1-101LF


that's about 32.00, 16 for everything else and 16 for 4 new output transistors

Ok. Thanks a lot. I will order those parts you mentioned. It might take a week or a bit longer...

I have the parts now.

I think I should first loosen the screws and clean surfaces of the PCB. It is rather dusty The incoming and out going wires are nisely wound around the posts so is it feasible to leave them tightened when I remove the parts? Is methanol and Q-tip the right choice for cleaning. The big caps (35V/2200uf) have white wires coming from the PCB and one end is taped on the side of the caps. What is that for?

post a picture of these wires, we'll see where they come from and if it looks "factory".

Here is a picture. It is a white wire on right hand side of the cap. It is taped on the side leading nowhere (the other end goes in the PCB close to the heat sink). The tape is so dusty that it has been there for ages. It is the same thing with the other sama sized cap.

looks like it goes back to the 0.5 ohm emitter resistor, which would then make it an easily clipped to test point to measure Idle current (expressed as a voltage across the 0.5 ohm resistor, to ground) while doing the factory adjustments, THEN the exposed portion of the wire would be cut off to reduce the possibility of short circuits.

djmtascam said:

<snip>
There are 2 test points on my circuit card that had wires soldered to them and hanging off the board.....I know another weird item huh...!??
TP1 is at the junction of R33 (.5ohm 2W) and the emitter of Q11 (2SC793) and TP2 is at the junction of R34 and the emitter of Q12.
I was wondering if these were potential locations for taking meter readings while doing adjustments. I have found nothing on using these anywhere and they are also not shown on the schematic, but are shown on the board layout drawing.

<snip>

Click to expand...

markthefixer said:

voltage to ground at those wires will give you the idle current expressed as a voltage across the emitter resistance. 20 millivolts across 0.5 ohms is 40 milliamps.

this is the idle current of the output drivers, and it is the point where the amp will transition from class a operation to class b (push pull) operation.

the idle current increases from zero, and the crossover distortion drops, until a "knee" is reached where distortion does not decrease with corresponding idle current increases. this only results in extra heat dissipation in the output drivers.

vr3 and vr4 are adjusted for 20 millivolts and this is the recognized procedure. as evidenced by many many other later Pioneer manuals.

<snip>

Click to expand...

I have now done the cleaning. So I guess next thing is to desolder and resolder the new parts. All the old smaller caps have the polarity marked on them but I don't seem to find the markings on the biggest ones. And now I need help with the orientation of the new transistors.

big caps look like they will be marked by the leads .. black negative

Transistor lead identification and installation

Iivan said:

I have now done the cleaning. So I guess next thing is to desolder and resolder the new parts. All the old smaller caps have the polarity marked on them but I don't seem to find the markings on the biggest ones. And now I need help with the orientation of the new transistors.

Click to expand...

markthefixer said:

every Mouser transistor has a data sheet available for download.... download them and look them up. then install as per the symbol printed on the pc board at that location.
never ever use the original transistor to determine how the replacement will go in.

from an earlier post:



First you need to learn about what the three transistor leads are about, and identifying which is which and where it goes according to the symbol. Then you will see that my parts list include lead arrangements - which assume the transistor lettering (and flat) are facing you and the leads are facing down, then go left to right.

Q13 2sc938 150v 0.05a 0.6w
512-KSC2383YTA to-92L ecb 160v 1a .9w 50mhz 160-320hfe $0.17

The big TO-220's with the metal mounting plate are hard to reverse, but the smaller ones are, but my specified replacements are usually E C B as a rule of thumb BUT we ALWAYS CHECK...

But you really should start here and follow ALL the links...

http://www.audiokarma.org/forums/showthread.php?t=331932&page=5

here's a preview, DO YOU SEE these symbols on the boards you are working on?:

e is always the arrow on one angled line (direction can be out (npn) or in (pnp) towards the base)
b is always the flat line the other two lines come out if at an angle
c is always the angled line without the arrow

letters can be smudged or illegible, while the symbols are hard to mistake and pretty language independent
pioneer doesn't use letters, if there is a symbol. sometimes in a linear hole arrangement, there is only an 'e'.... with no room for a symbol.

-----------------------------------------------------

now a quick lesson on installing a small transistor in a traditional triangular hole arrangement with a symbol printed in the center of the triangle

1. find the LEFTmost lead on the package and identify it's designation (emitter? base? collector?)
2. look at the symbol on the board, find the corresponding hole
3. insert that lead into that hole
4. find the RIGHTmost lead on the package and identify it's designation (emitter? base? collector?)
5. look at the symbol on the board, find the corresponding hole
6. insert that lead into that hole
7. the center lead will now need to bent forward or back to go into the leftover hole.

DON'T THINK "WHERE IS THE FLAT FACING"
think
"outside leads" and THEIR holes...

this one's ecb....

and two transistor layouts, ebc and ecb with the same triangular target hole arrangement

Click to expand...

markthefixer said:

well, you do need to get the parts, first....

and when ordering the transistors, mouser has links to the transistor's data sheets, DOWNLOAD the data sheets!!! THEY are your BEST guide as to the correct lead connections for THAT particular transistor.

Do you understand the transistor symbol? enough to identify the emitter, base and collector from it? Because most boards (Pioneer) have the symbol printed on it, with holes around it, and it is orientated so there is an emitter hole, a base hole and a collector hole. And obviously, only the transistor's emitter goes into the emitter hole, the base into the base's hole and the collector into the collector's hole. IF you want it to work...

oh, and by the way, the symbol usually DOESN'T have the E, C and B letters ON on it... :stupid:
there are two flavors,
pnp: arrow pointing in, 2sa,2sb, ksa, ksb part number prefixes
npn: arrow pointing out, 2sc,2sd,ksc,ksd part number prefixes

hint: if "rotating" it in your mind is a puzzler, orient it in the hole this way(transistor hole arrangements are usually triangular):

choose the leftmost lead, use the data sheet to identify it... for example it is the emitter
put it in the emitter hole

choose the rightmost lead, use the data sheet to identify it... for example it is the base
find the base hole, rotate the transistor line of leads around the inserted leftmost lead to point at the base hole
put the rightmost (base) lead in the base hole.

the THIRD hole is now either in front of or behind the transistor, in this example the collector is "left over"
bend the center lead forward or back to go into the hole.

easy...

notice that this example had the lead order of emitter, collector, base which is a common lead arrangement

that means that:

looking at the transistor, with the leads pointing DOWN, and the flat part (or lettering in the case of some zetex transistors) FACING YOU
going from left to right has the lead order of emitter, collector, base

Click to expand...

Thank you so much for this exellent and very informative explanation. I think that I even understood it. So I can trust that the markings on the PCB are orientated correctly?

I have now removed the big 2sc793s. One of them had two mica insulators on top of each other. No grease in between. None of them had thermal grease. There was small amounts something that looked like glue. The screws were also loose. Are the leads in these 2sc793s similarly orientated as in the new replacements?

I had some hard time in trying to remove one of the two big caps. It seems that it is glued as well as soldered in place. How can I remove this glue without braking the PCB?

The whole point is that the old and new transistors PROBABLY WILL NOT have the same orientation.

Part of the "lesson" above, was to be able to identify the leads of the ORIGINAL OLD transistor before removing it from the board.

and to check that the information sources all agree. The big assumption is that whatever is IN the board is WORKING, and is the truth of the matter.
Whether the manual may list a lead arrangement incorrectly, or the board symbol is mismarked, is important information.

The 2sc793 transistors WILL have the same lead layout, with THAT specific type of package (TO-3), the lead layout is standardized.

The glue was something they tried many years ago for faster manufacturing, but when the glue cracks the consequences are much worse than dried thermal compound.

I'm sorry I didn't include the parts numbers of thermal compound or the insulators for the new transistors.

www.mouser.com
534-4662 to-3 mica insulator $0.16
532-249 1 oz thermal compound $4.71
526-NTE303 1 gram thermal compound $1.77

Glued in caps: once the solder joints are open, twisting a bit helps, it will release before the board breaks.

I found thermal crease and mica locally.

I have tried today a long time to remove the big caps without any success. I can at first suck the biggest amount of the solder away but there is always leftovers that keep the pins soldered and these are hard to remove. Should I replace my soldering iron for a stronger one? I am using an old Weller with 25W.

Try wiggling the cap and tilting it away from the joint (and even rotating it along it's axis) while the solder is molten, the glue ISN'T that strong. The solder joint itself isn't plated through either, making access to the complete solder surface easy, so perhaps you should try some (de)solder braid.

Also try moving the lead in the hole (to the center) with the iron itself.

I finally was able to remove those big caps. This was after I realised that they have four legs. This fact is new to me that this kind of caps exist. One of the legs have a red dot. So this must be positive leg. The new ones have two legs. How do they go together?

Have a look at my thread here :

http://www.audiokarma.org/forums/showthread.php?t=459128

You might find the pictures useful. If you have the thing apart, I would strongly urge you to replace the trimmers. When they go bad, they cause the fuses to start blowing, who knows what it will do to the speakers?

Lee.

Thank you for the link.You have done it very nicely.
Pictures certainly are of help and explains also what to do with the 4 legged caps. I also took the PCB out. It is much easier to work with. Should have done it earlier and might have also noticed earlier those legs of the caps. I will also replace those trimmer.

I have now replaced half of the parts. Many of the old caps are leaky

I still need some help with the orientation of the trim pots.
VR 1 and VR2 center voltage adjusts: which of the legs go to gether? 2 and 3? If you have the PCB in front of you the big heat sinks up is leg 1 for left or right hole?
VR 3 and Vr 4: is the hole most to the left for leg 1?

I have now replaced all the parts. What is the procedure for adjusting center voltages and idle current? The trimpots are now at zero. Is the correct procedure to first adjust center voltages and then the idle current? Or is there some idle current needed?

I found a couple of threads explaining the procedure.

The emitter resistor R32 is in a difficult place to make the bias measurements so maybe I can use the white wires that the also were use at the factory for the same purpose. But how does one do the measurement if there is only one wire and the measurement should be done across the resistor? Sorry if this is a stupid question.