Pioneer SX-780 no sound from one channel

strader61

I'm the German craftsman
I've done a little bit of testing and could use some help please.
I tested the Darlington STK-0050's to start with and attached a photo with my readings. also tested the main filter leads.
The main filter capacitor leads read +40 on one side and -40 on the other side as they should. I checked the three larger transistors that have the "U" shaped heat sinks and have been known to get hot and Q25 was 27.14 volts, Q19 was +50.25 volts, and Q20 was negative or - 51.00 volts.
when I remove the right STK-0050 when looking at the bottom I get sound in one channel and protection circuit works as it should, but if I add the STK-0050 back in unit stays in protection mode and have no sound from either channel. The STK-0050's that I am using are new. Thanks for any help with this.
 

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Try the duff module in the working channel. (the working one goes on the side for now)

Then on the duff channel side, check the 0.22 ohm resistors right next to the module connections.
 
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am adding this picture to make it easier to work out the problem ...
STK-0050.png
 
when you say module connections?

when you say module connections do you mean that the .22 ohm resistors are near the Darlington units?
I don't understand Duff module in working side?
 
when you say module connections do you mean that the .22 ohm resistors are near the Darlington units?

Yes



I don't understand Duff module in working side?

we need to know if the module with the strange voltage readings is damaged, or if the circuitry connected to the module is just driving the module crazy and has not destroyed it.

Two modules, one works, one is suspect

Two amplifier channels, one works(good) , one does not.

Remove the working module from the working channel, setting it safely on the side,
Remove the suspect module from the non-working channel
and then install the suspect module in the working channel.

FROM the non-working channel
INTO the working channel,
to see if that suspect module (from the non-working channel) is defective or damaged

if the good channel works with the suspect module,
then the module is good and the circuitry in the non-working channel is the problem.

clearer?
 
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Verifying both modules

Yes





we need to know if the module with the strange voltage readings is damaged, or if the circuitry connected to the module is just driving the module crazy and has not destroyed it.

Two modules, one works, one is suspect

Two amplifier channels, one works(good) , one does not.

Remove the working module from the working channel, setting it safely on the side,
Remove the suspect module from the non-working channel
and then install the suspect module in the working channel.

FROM the non-working channel
INTO the working channel,
to see if that suspect module (from the non-working channel) is defective or damaged

if the good channel works with the suspect module,
then the module is good and the circuitry in the non-working channel is the problem.

clearer?
I removed and reversed the modules and both modules were able to get the one channel to work and sound was present at speaker.
 
looks like its possible its missing negative voltage .or positive is shorting .
do this first and report back .then we can start looking below the dotted line .
if you could get voltages at the empty places with bad side module removed it might give us a clue ...............................................................................................................................................................................................................................................
there are 3 transistors to get voltages for ..if its the right channel q12 q14 q16 ..if its the left channel then its q11 q13 q15 ..
odd numbers on the left even on the right ..
its possible there might be an open resistor or bad solder joint .
 
edit - pete GRRRRRR lol... when I started this post, your reply wasn't there...

Ok, that means the modules are OK, and there is a fault in the amplifier circuitry.

Have you checked all 4 0.22 ohm resistors for continuity? R263, R265, R264, R266 on the main schematic, connected to module pins 3 and 8 on each of the two modules.

Next step is testing specific small transistors by removing them and performing the 6 way diode test (specific example and sample results in post #11) looking for gross faults.

I checked, you have NOT named the channel that gets sound, and thus I must complicate things by naming components in BOTH channels.

Q11, Q13, Q15 for the left channel, and q12, q14, q16 for the right channel - depending upon which channel is not working.

If they test ok, then test these resistors
R259 (200 red black brown gold ) and R261 (100 brown black brown gold) for the left channel
R258 (200 red black brown gold ) and R262 (100 brown black brown gold ) for the right channel
by lifting one leg out of circuit to isolate it and then check it's resistance.

The BOLD ones are the most likely culprits due to the voltage readings in your first post.
 
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edit - pete GRRRRRR lol... when I started this post, your reply wasn't there...


.
sorry mark ...
you explained it better than i could ..at least we are on the same track . :D
was going to get around to the resistors and out of circuit transistor testing later .. i know its better first .
was just being academic again getting voltages first..
looking at the earlier module readings for the other channel i am laying bets on one of the 200 ohm resistors .

just trying to figure out why working side input is about half a volt low on positive side:scratch2:
 
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That's why I lol'd, you are on the right track, and had I waited, I wouldn't have had to type so much...

Starting a post and have someone else post what I had been researching to post makes me feel that I look like one of those "meeeee too, look at meeeee" posters... lol....

happenstance, keep right on doing what you are doing... :D
 
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Side that has sound

edit - pete GRRRRRR lol... when I started this post, your reply wasn't there...

Ok, that means the modules are OK, and there is a fault in the amplifier circuitry.

Have you checked all 4 0.22 ohm resistors for continuity? R263, R265, R264, R266 on the main schematic, connected to module pins 3 and 8 on each of the two modules.

Next step is testing specific small transistors by removing them and performing the 6 way diode test (specific example and sample results in post #11) looking for gross faults.

I checked, you have NOT named the channel that gets sound, and thus I must complicate things by naming components in BOTH channels.

Q11, Q13, Q15 for the left channel, and q12, q14, q16 for the right channel - depending upon which channel is not working.

If they test ok, then test these resistors
R259 (200 red black brown gold ) and R261 (100 brown black brown gold) for the left channel
R258 (200 red black brown gold ) and R262 (100 brown black brown gold ) for the right channel
by lifting one leg out of circuit to isolate it and then check it's resistance.

The BOLD ones are the most likely culprits due to the voltage readings in your first post.

when the unit is upside down and face/front is towards me the module on the left is the one producing sound.
 
That should the the RIGHT channel. The ±40VDC power supplies appear to be intact. The general suspicion is that the cement 0.22Ω resistors (the white ceramic rectangular ones) have a problem.

Please check the resistances of those resistors and report in with results. I expect the readings to be infinite instead of less than 1 Ω.
 
I'll test those .22 resistors and get back soon

That should the the RIGHT channel. The ±40VDC power supplies appear to be intact. The general suspicion is that the cement 0.22Ω resistors (the white ceramic rectangular ones) have a problem.

Please check the resistances of those resistors and report in with results. I expect the readings to be infinite instead of less than 1 Ω.
I'll test those .22 resistors and get back Monday afternoon. Thank you
anywhere I can purchase these resistors if I need to replace them? and do you know the watt of these?
 
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yes we know of places to try to get them, and exactly what they are and their ratings. That's a bridge we'll cross if we come to it...
 
when testing the resistors I'm getting around .271 on each of them. that is with receiver off and Darlington units still soldered in unit. Do I need to unsolder these to check? used my BK precision 875b to verify these resistors.
 
they look ok so ....
If they test ok, then test these resistors
R259 (200 red black brown gold ) and R261 (100 brown black brown gold) for the left channel
R258 (200 red black brown gold ) and R262 (100 brown black brown gold ) for the right channel
by lifting one leg out of circuit to isolate it and then check it's resistance.

The BOLD ones are the most likely culprits due to the voltage readings in your first post.
 
Not that any of these more experiences advisors need reassurance, but I'd agree that the emitter resistors seem to test O.K. and that moving along is a good idea.

As long as you are in there with the power off, it might also be good to check R255/256 to see if they are close to 2KΩ. Any of those with the flame suppression sleeves on them could have been hurt. For a quick/dirty check you may not even need to lift a leg for testing. If any test open or with a markedly higher resistance than their markings would indicate, lifting a leg for verification would be required at that point.
 
resistors tested

Tested the rectangle .22 resistors and came up with each reading around .271
I did this with the power off and Darlington units still in place. Is this ok or do I need to remove the resistors from the board?
I will also test the
R259 (200 red black brown gold ) and R261 (100 brown black brown gold) for the left channel
R258 (200 red black brown gold ) and R262 (100 brown black brown gold ) for the right channel.
looks like I need to lift one leg out of board to check each of these other two?
 
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