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Pioneer SX-626 Distortion In Right Channel

basicblues

basicblues

Well-Known Member
I've been working on a little SX-626 that I am getting ready to sell and noticed some distortion in the right channel. Liberal shots of Detoxit in the pots and switches didn't correct the issue. Any suggestions before deciding what to do with this unit would be appreciated. The right channels plays, just not as loud as the left and with with distortion, otherwise this is a nice unit but I just sold the other one I had and this one is next.
 
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Hi,

First I would try to isolate the fault to either the pre-amp or the main-amp.
Fortunately that's easy on the SX-626. Just remove the two connectors on the back between pre-out & main-in. Get hold of a normal RCA stereo phonocable and use it to reverse the connections, i.e. Pre-out Left goes to Main-in Right and vice-versa.
If the distorsion moves to the other speaker, you should look for the fault in the pre-amp section. If not, the main-amp is defective.
In both cases a good bet would be on electrolytic capacitors being off value and a re-cap is necessary.

The manual for SX-626 is available from the AK library.

Kind regards,

/tri-comp
 
tri-comp said:
Hi,

First I would try to isolate the fault to either the pre-amp or the main-amp.
Fortunately that's easy on the SX-626. Just remove the two connectors on the back between pre-out & main-in. Get hold of a normal RCA stereo phonocable and use it to reverse the connections, i.e. Pre-out Left goes to Main-in Right and vice-versa.
If the distorsion moves to the other speaker, you should look for the fault in the pre-amp section. If not, the main-amp is defective.
In both cases a good bet would be on electrolytic capacitors being off value and a re-cap is necessary.

The manual for SX-626 is available from the AK library.

Kind regards,

/tri-comp
Click to expand...

Well, I crossed the pre-out and main-ins and the distortion stayed in the same channel which is actually the left channel instead of the right channel as previously mentioned. So the problem appears to be in the main amp section. I am not sure this will be worth the time to fix in my situation unless there's an easy target to shoot at. Otherwise I'll trade or sell it as-is but this is probably a simple fix.
 
If you download the service manual, I can point out a few places to test. But until you have the manual to make sense out of my instructions, they will be cryptic.

This series of amps used a circuit called quasi-complementary, which means that there is a capacitor between the amplifier outputs and the speaker connections. The voltage on one side of this capacitor should be half (29?) of the voltage found at the collector of the output transistor (58?).

The receivers back then were made before there were decent PNP output transistors, thus the fooling around with half voltages.

If this half voltage is wrong, there could be clipping on just half of the signal, after it hits a particular volume.

The voltage can be adjusted.
 
shrinkboy said:
uh, oh, basic. is it the one i sold ya last summer? it was all well then....
Click to expand...

No, thanks for asking but this is another one that I picked up at a local auction house.
 
markthefixer said:
If you download the service manual, I can point out a few places to test. But until you have the manual to make sense out of my instructions, they will be cryptic.

This series of amps used a circuit called quasi-complementary, which means that there is a capacitor between the amplifier outputs and the speaker connections. The voltage on one side of this capacitor should be half (29?) of the voltage found at the collector of the output transistor (58?).

The receivers back then were made before there were decent PNP output transistors, thus the fooling around with half voltages.

If this half voltage is wrong, there could be clipping on just half of the signal, after it hits a particular volume.

The voltage can be adjusted.
Click to expand...

Alright, I have the service manual to view. So when you get time point out those places you mentioned to test. Thanks
 
basicblues said:
Alright, I have the service manual to view. So when you get time point out those places you mentioned to test. Thanks
Click to expand...

This actually turns out to be easy at this point, the power output transistors are TO-3 types, the big metal ovals with a screw on each end.

The collector (of each channel) of one will be at half of the voltage as the collector of the other. It's a nice big recognizable component, with a large area to probe. The entire metal body of the transistor is the collector!!

The same will apply for the other channel as well. They are on opposite halves of the board.

These are 2SC793 transistors, NPN's rated at 100 volt, 7 Amp, 60Watt and 5 MHz .

Check the manual to see what I mean, or post a picture, the two potentiometers closest to the center of the board adjust the half voltage point, one to a channel. GO S L O W L Y ! ! ! !

The OUTER two potentiometers adjust the bias - don't touch them
 
markthefixer said:
This actually turns out to be easy at this point, the power output transistors are TO-3 types, the big metal ovals with a screw on each end.

The collector (of each channel) of one will be at half of the voltage as the collector of the other. It's a nice big recognizable component, with a large area to probe. The entire metal body of the transistor is the collector!!

The same will apply for the other channel as well. They are on opposite halves of the board.

These are 2SC793 transistors, NPN's rated at 100 volt, 7 Amp, 60Watt and 5 MHz .

Check the manual to see what I mean, or post a picture, the two potentiometers closest to the center of the board adjust the half voltage point, one to a channel. GO S L O W L Y ! ! ! !

The OUTER two potentiometers adjust the bias - don't touch them
Click to expand...

Found exactly what you're talking about, but I am not sure what adjustment you want me to make at this point?
 
First post the four voltage readings, and if one of the two lower voltages is not exactly half of the other, we need to see if it is on the distorting channel.

Then an adjustment should be tried to see if the distortion can be eliminated by getting the voltage to the correct value.

It would have to be a very large deviation in voltage to cause distortion, and it would be very obvious.

The lower voltage should be exactly half as high as the other voltage for each channel, and the amplifier is DEPENDING on that ratio to operate correctly. It could have drifted over the years as components aged.

We're looking for that easy target to shoot at you were talking about, as well as the easy fix.................

There will be one other measurement and adjustment that can be checked that would contribute to the distortion if this one either is not needed or doesn't cure the distortion.

That other measurement and adjustment involves the OUTER two potentiometers that adjust the bias.

This bias adjustment DIRECTLY impacts distortion, but it CANNOT be adjusted correctly until the first adjustments I talked about are CORRECT.
 
With the front of the receiver facing forward measuring directly across all 2SC793 transistors with DVM set at DC volts. The voltage readings are from left to right.

.543 .544 .595 .591
 
Take a picture of where you are making those readings and post it, because I was expecting things like , say , 57 volts and 28.5 volts... and the black meter lead should be on a metal chassis ground point.

There is a cover held down with 4 screws over the heatsink area, protecting the transistors from being shorted out inadvertently, sorry I neglected to mention it, but it is in the manual.

You will need to remove the cover to take the readings off of the cases, rather than finding corresponding points on the printed circuit board.

The power amp assembly mounts vertically THROUGH a hole in the chassis, for the chimney effect....

Otherwise, it's my turn to be confused.
 
The previous readings were taken with the metal cover removed, directly on the 2SC793 transistors on the soldered side with red on one pin and black on the other pin.

Now I've basically done the same thing this time except the black lead is grounded to chassis and I used the red lead on each of the 2 transistor pins where they are soldered on the vertical board you mentioned.

Reading left to right as the receiver front is facing you.

.545V on the higher pin
0.21mV on the lower pin

8.83V on the higher pin
8.29V on the lower pin

26.6V on the higher pin
26.0V on the lower pin

.617V on the higher pin
31.4mV on the lower pin

Let me know if I am not measuring these voltages in the proper place and I can add photo's if necessary later this evening. Thanks
 
While those aren't the exact voltages we need, they are close enough to show that the adjustment is badly needed.

Your measured values that were good are in green, and the 8v instead of 26v is in red. the green circle is the adjustment pot for the good side (if you want to tweak it a bit) and the red circle is the adjustment pot for the side that REALLY needs adjustment..

The places to measure are the yellow circles with the red question marks. These are the places that have a V and 1/2v relationship.

I had been talking about the metal bodies of the four transistors on the other side of where you took the measurements, I wanted to avoid poking where you did (at least at first...:D ). THe yellow dots, and the entire case of the transistor on the other side are connected electrically.

The purple rounded squares are the bias resistors and the purple circles are the bias adjustments for that side. IF WE ADJUST the bias, the dots are where you will measure, and and the circles are where you will adjust the measurement. Knowing what the voltage(s) across the bias resistor(s) ARE, (at the purple dots) will help determine if bias adjustments are needed. The measurement for the bias should be taken after about 15 to 30 minutes of warm up time. The readings will be in the millivolt region. I think the good side will be 31.4 millivolts (that one reading IS across the resistor, one side of which is grounded), but the bad side will have to be read AFTER adjustment on BOTH resistors as it is showing 0.21 millivolts, which right now is contributing greatly to distortion.

edit 30 milliamps through 1/2 ohm is 15 milli VOLTS...

the 626 shows 70ma on the 1973 manual. 10ma for the rest of the circuits, 30ma apiece for idle current. 15mv across the resistors.

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Finally got chance to do some more testing...

I printed off the diagram provided so I would be exactly sure I measured in the correct places. I really had a heck of time trying to figure out what was going on until I realized that the good and bad sides are flipped flopped. By the time I figured this out I had made all kinds of measurements and adjustments.

Got back on track and finally put things somewhat back like they were. I grounded my black meter lead then used the red lead at the metal bodies of the transistors. The closest settings I could obtain on the good side were 28v and 57v. Regardless of the adjustments, I could only obtain 6v to 9v on the bad side. I guess these are the yellow dot readings.

Purple dot readings:

Good side: 11.2 mA (outer) 47.1 mA (inner)
Bad side: 7.2 mA (outer) 1.2 mA (inner)

Any attempt to get the bad side values higher were unsuccessful regardless of any adjustment made.
 
Well, something has drifted beyond the ability to adjust. Since the entire circuit is a see-saw type balance, it won't be quick and easy to figure out what has changed to upset the balance. Dead parts are easier to find by remote...

If the distortion appears as you increase the volume, you are gettng asymetrical clipping, no surprise at 9v vs 28v. Plus with the balance that far off, one of the power output transistors is probably contributing crossover distortion.

You COULD watch the voltage (9v) while you hit the components on that side with freeze spray, you might aggravate the bad component to into shiftiing even worse (or better...) than you could affect the voltage with any other frozen component. Watch out for too much ice or moisture shorting things out though....
 
markthefixer said:
Well, something has drifted beyond the ability to adjust. Since the entire circuit is a see-saw type balance, it won't be quick and easy to figure out what has changed to upset the balance. Dead parts are easier to find by remote...
Click to expand...
Don't forget that this is supposed to be adjusted under load, at the clipping point. It may act quite different under those conditions.
 
Hi,
I have this same problem with my Sx-626 (earlier model). Bad distortion in the left channel. I have measured the voltages across the 2sc793 as per the attachement earlier in this thread. The values are starting from left (as in that attachement) 26.3, 59.5 and 20.4, 19.9. When I carefully try to adjust the bad channel they move hand in hand up and down.

Visually all caps are fine.Does anybody have an idea what do next?
 
Welcome to AK :yes:
and
Yes,

PLEASE start you very own, brand new thread :D

Later, post a thread in the introductions forum and be properly welcomed to AK.... :thmbsp:
 
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