Pioneer SX 1000TD low distorted output in ONE channel

Welp, I have tried a lot to get my SX-1000TD fixed, but it hasn't made it there yet.

I have made progress, and here it goes...

When I received it, the left channel was real bad. It would only fart out a few distorted blurbs every now and again. The right channel sounds great.

I opened it up, found a blown transistor on the main (power) amp board on the bad (left) channel. It was the only PNP transistor (2SC538A). It fried the r848 220ohm and r840 2.2k ohm resistors. I replaced both resistors, and checked other resistors in the area. All seems well with the resistors.

I replaced all of the electrolytic capacitors. I replaced all components on the power supply. I replaced all transistors in the main, control, and head(pre) amps. I replaced the output transistors.

I cleaned all of the potentiometers and variable resistors with deoxit, and followed with fader lube.

The left channel works, and it sounds better than it did. However, it still doesn't sound as good as the right channel. It has a mild distortion, and puts out a slightly lower volume than the right channel. Both channels now have the same components, other than the fried resistors I replaced, but those should be fine.

This prompted me to check the DC offset. I did this by setting the the collector voltage on the output transistors to be 1/2 of each other. This is done by adjusting the 300k variable resistor on the main amp board. The collectors on the output transistors now read 93.7 VDC and 44.3 VDC for one channel and 93.7 VDC and 44.7 VDC on the other. This seems perfect.

I also tried to set the idle current. The schematic says the amp is feeding 20mA into the collector, and the emitter is grounded by a 0.7 ohm resistor.
Therefore, the voltage across the 0.7 ohm resistor should read 0.014V or 14mV. (This is AC voltage?).
V=IR = (.020A)*(0.7ohm) = 0.014 V = 14mVAC
To tried to adjust this idle current, I tried tweaking the 300 ohm variable resistor so that my ohmmeter reads 14mVAC across the 0.7 ohm resistor.
However when I check the voltage across the 0.7 ohm resistor, I get a reading that I don't understand. The channel that sounds good has 3mVAC across the 0.7 ohm resistor, while the channel that sounds bad has 10 mVAC.

So I don't know what I'm doing really when trying to adjust the idle current. It seems that the good channel is running at a lower voltage than the schematic calls for, but both channel's voltage are lower than what the schematic calls for... I have not successfully set the idle current on either channel to 14mVAC like the schematic says.

I also checked the thermisters and thermal diodes for both channel. They seem good. The thermisters read ~70 ohms at room temp, and ~50 ohms when I warm them in my hand. I have a semiconductor tester, and the diodes seemed fine.

One final possibly critical observation.

For the good (right) channel:
The output transistors run warm after running for a few minutes.
In the bad (left) channel:
The transistors don't seem to be warming up. They stay at room temperature, and are cool to the touch.
Also, the bad (left) channel PNP transistor on the main amp board is running MUCH hotter than the PNP transistor on the good (right) channel. (This is the same resistor that was fried when I bought the amp!)
Luckily, I replaced the stock PNP 2SC538A with a beefy ZTX795A, and it hasn't fried yet.

So for the bad (left) channel, the output transformers are running cold, and the PNP transistor on the main amp board is running hot.

Does this indicate something obvious?

I feel like the answer might lie in that PNP resistor.

Any suggestions on things to check/try would be much appreciated.

Here is a picture of the schematic of the main amp unit. I have put a small red circle around the parts of the schematic where there was a failed transistor, and two failed resistors.



Here is an image of how the main amp unit interacts with the output transistors to drive the speakers.

The PNP transistor that was a 2SA537A is the one that originally failed. It has been replaced with a ZTX795A. The failed resistors have been replaced. I also replaced a few other resistors on the board just to be cautious.

Well actually I circled the transistor on the right channel instead of the left channel, but it shouldn't make a difference.

By my interpretation, thee are a few original smaller transistors left in the power amps, on both channels.

If you feel you can do it safely, try swapping the 2sc627's (can't read their numbers reliably) that are connected to the 2sa537 and see if the problem moves to the other side. Also check the resistors in that area: 150, 220 and 33 ohms by lifting one leg before measuring them.


That 2sc627 could have taken damage when the 2sa537 died.

Thanks for taking the time to read my post, Mark. I have replaced all the transistors on the main amp board, but I will definitely try what you have suggested.

However, in messing with the circuit late last night, I blew through the last of my output power transistors. (I have gone through 5 pairs of mj21194 power transistors while working on this receiver! It is never good to work on these things at 1AM) So, I need to order some more backups before I can get back to working on this unit.

Additionally, I have to move at the end of the month. I was hoping to finish this receiver before then, but I need to get it and everything else packed. I will put this post on hold, and come back to it sometime in August.

chuckulaa said:

...
However, in messing with the circuit late last night, I blew through the last of my output power transistors. (I have gone through 5 pairs of mj21194 power transistors while working on this receiver! It is never good to work on these things at 1AM) ...

Click to expand...

5 PAIRS! I thought I had some ad luck now and then. Are you using a DBT?

DBT? No, I suppose I should make one...

The funny thing is that I started blowing transistors on the good channel. I tweaked the idle current on the good channel to try and record a baseline of what potential I see across the 0.7 ohm resistor. I turned the idle current up too high and fried the transistors. I have been trying to restore the good channel, but now I have to wait until I get some new transistors. I have to pack everything up this week, so its going to be a bit before I get back on this.

I suppose a DBT is in order when I do get back to it.

Yes, a DBT has saved me a lot of times.
All it takes is one little mistake.
If you're doing a lot of work on something, its tough to gets everything right the first time.

Repairing without one is flying without a net.

I just got my SX-1000TW straightened out and read your post again.

I'm pretty sure bias adjust is DC.

My thermal diodes tested OK with a meter but were not working well and one was probably the reason I had a blown channel to start with.

If your bias adjustments on DC and way off or won't adjust, you may want to pull those SV-3A and drop in 2 1N4148 in series to replace each one.

Thanks blhagstrom,

I should have some 1N4148s around. Could I clear a couple things up from you?

Did you replace each SV-3A diodes with two 1N4148s? (4 total?)

Did you replace those two diodes in series and mount them against the heatsink in the same way the original SV-3A was mounted?

Did you keep the original thermisters installed?

Because my main amp board seems to be working proper (I have pulled and tested each transistor with a DCA55 semiconductor tester, and all the the resistors seems pristine) I assume that my problem lies in the point-to-point wired components such as the thermistors, thermal diodes, or even the 0.7 ohm resistors.

chuckulaa said:

Thanks blhagstrom,

I should have some 1N4148s around. Could I clear a couple things up from you?

Did you replace each SV-3A diodes with two 1N4148s? (4 total?)

Did you replace those two diodes in series and mount them against the heatsink in the same way the original SV-3A was mounted?

Did you keep the original thermisters installed?

Because my main amp board seems to be working proper (I have pulled and tested each transistor with a DCA55 semiconductor tester, and all the the resistors seems pristine) I assume that my problem lies in the point-to-point wired components such as the thermistors, thermal diodes, or even the 0.7 ohm resistors.

Click to expand...

Yes, you use 2 diodes in series to replace 1 SV-3A, I cut the joining leads short, solder them together, put some shrink wrap over all of it and bend it to be about the same shape as the SV-3A. My SV-3A just "tuck" into a recess in the heat sink and where taped over. I just pressed them into the recess as best as they would and taped over them. I won't be cranking the receiver so I don't expect it to even heat up. (I have BIG amps for getting concert volumes).

I left the original thermistors.

FWIW, I went after the SV-3A because everything else was checked OK or replaced and it wouldn't adjust properly. Usually thermal diodes are pretty stable and working but I've run across a few that were bad and drove me nuts trying to figure out what was wrong. I am just tossing it out here for you since mine were bad and yours are the same age and design.

Yes, you use 2 diodes in series to replace 1 SV-3A, I cut the joining leads short, solder them together, put some shrink wrap over all of it and bend it to be about the same shape as the SV-3A. My SV-3A just "tuck" into a recess in the heat sink and where taped over. I just pressed them into the recess as best as they would and taped over them. I won't be cranking the receiver so I don't expect it to even heat up. (I have BIG amps for getting concert volumes).

I left the original thermistors.

FWIW, I went after the SV-3A because everything else was checked OK or replaced and it wouldn't adjust properly. Usually thermal diodes are pretty stable and working but I've run across a few that were bad and drove me nuts trying to figure out what was wrong. I am just tossing it out here for you since mine were bad and yours are the same age and design.

Click to expand...

Thanks for clearing that up. I will be working on this guy again sometime next week hopefully. :thmbsp:

OH, check all the 0.7ohm resistors on the big caps too.
One of mine, R29 or R30 on the big main filter caps was high.
By replacing them with bigger resistors I had on hand (and accidently using the improper values) I found that they would heat up R847 and R848. (way too much voltage, I don't know what I was looking at when I swapped them).

I see you had a fried R848. Perhaps a bad R29 caused an overload situation.

Thanks again. How high were they reading? I did check one resistor and it was reading about 1.2 ohms. I feel like it will be easy to get to the bottom of this after I move.

I am going to replace the 0.7 ohm resistors, just have to put in another order from Mouser.

I thought my bad resistor tested in the many hundreds when I spot checked it in circuit.

I just found it and checked and it measures 2.1M ohm. Basically open

So many parts on this were AFU.

sneak current paths, giving false resistance readings.

That's why to be absolutely sure, one desolders and lifts one end of the resistor.

I don't remember what model or position, but some resistor in circuit years ago read close enough that I thought it was ok, until I lifted the leg later, then I saw it was open. Reconnected it and the reading went back to the "close enough" one. :gigglemad

Yup, but lifting a leg on every resister can be a daunting task on some boards.
I do a sanity check in circuit and if it's a one side problem, I have a reference on the "good" side, but I realize I am not 100 percent until I am out of circuit.

Alright, I got it working. I replaced all of the 0.7ohm 5W resistors with 0.68ohm 5W resistors.

I did find that the left channel resistor that you measure across, either R28 or R27, was way high at ~3.4M ohms. That explains why I wasn't able to measure a proper 14 mVDC across it. Interestingly, this resistor showed no signs of damage. :scratch2:
That is to say, the resistor had no char, black marks, or swelling.

Perhaps when you work on these receivers, you should just replace the 0.7 ohm resistors. In the future, I will just go ahead and replace all output power transistors of the units I work on. The replacements are cheep and worth doing while your in there.

I also replaced the SV-3A diodes as suggested above for peace of mind. If they failed for hagstrom, then they could fail for me eventually.

It sounds good, balanced. It was a tough one, but I'm glad I stuck through it.

Thanks for your help everyone.

It's okay to use the balance knob

Well just a follow up.

The SX-1000TD has a beautiful sound but, the left channel is now louder than the right channel...

This is an interesting development, because when I started the post, the left channel the quiet distorted channel, and now its louder than the right channel. Both channels sound good, but the left channel is louder. It has more kick with both bass and treble.

I spent the last week obsessing about this problem. I thought for sure I could fix it, and balance the channels.

I cleaned all of the switches about a dozen times. I also thoroughly cleaned the pots and the rotary switches. I cleaned the speaker jacks.

I used a large amount of DeOxit D5 on my receiver. Still didn't help.

I swapped the inputs of the power amp (This was suggested to me by Hagstrom in a private message) by reversing pins 2 on the power amp board. After doing this, the right channel was still weaker.

This tells me the problem is in the power amp section, or it lies in the point to point wiring of the final output transistors, resistors, and capacitors.

However, I pulled a leg on every resistor. I replaced all of the 0.7 ohm resistors, and I checked them before placing them in the circuit. I checked the 4 100 ohm resistors, and the 2 330 ohm resistors.

I checked every resistor on the power amp board that I didn't recently replace by pulling a leg and measuring with my Fluke 177 multimeter. I checked all of the transistors (which are all new) using my Atlas DCA 55 semiconductor tester.

I even checked the gain of my output transistors using my Atlas DCA 55 semiconductor tester. I made sure and put matched output transistors back into the stereo.

The idle current is balanced, the DC offset is balanced, everything seems perfect.

But this amp is still misbalanced... The left channel just kicks harder.

After sinking an additional 10-12 hours into this receiver over the weekend, I finally realized something... The receiver has a balance knob.

The balance knob is there to fix the problem that I am having. Well, it would be more appealing to keep the balance knob in the center, but I'm not that worried about it. Once I became comfortable with adjusting my balance knob to compensate, I released a deep sigh of relief. Now I can start enjoying my receiver. :thmbsp: