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Pioneer sx-1000tw Distorted Left Channel

V

vmedia

Active Member
Hello.

I have been working on repairing vintage gear as a hobby for about 2 years with reasonable success - mostly because of the great information and guidance here on AK.

I have read and re-read the existing threads here on sx-1000 troubleshooting and have made great progress on a unit that came to me DOA (from *bay) - I've been able to bring it mostly back, essentially, left channel now sounds good, right channel has lower level and is badly distorted. Hoping you good folks can help.

I should mention that this unit was clearly worked on by someone else previously - plenty of bad solder joints and lifted solder pads (main amp board and power supply board) that I have re-floated/fixed.

Other work done to date:
Replaced all .7 power resistors with .68
Rebuilt power supply board (all but rectifier diodes) voltages:

1 - 0
2 - 35.4vdc
3 - 35vdc
4 - 0
5 - 76vdc
6 - 37.2vdc
7 - 44.9vdc
8 - 11.4vdc
9 - .2vdc
10 - 0
11 - 20.5vdc
12 - 0

Main amp board: left side has all new transistors (followed advice here - check and rechecked that they are correctly installed) replaced resistors that had fried (R846/R848) or were out of spec. All new lytic caps as well on left side only.

Replaced Q1/Q3 (have also swapped with Q2/Q4 to see if distortion followed - it did not) I have another set of matching Transistors for Q2/Q4 which I will install once I'm certain all is well.

I am seeing drastically different voltages between channels on the main amp board pins- 7,8,9,10 are much lower on right side - I believe this is a clue to what's amiss (?)

Here are the power readings:
Left (good) side:
3- 45vdc
4- 0
5- 0
6- 76vdc
7- 42.3vdc
8- 42.3 vdc
9 - 41.8vdc
10- 41vdc
11- .4vdc
12- 0

Right (Bad) side:
3- 45vdc
4- 0
5- .1vdc
6- 76vdc
7- 10.7vdc
8 - 10.8vdc
9 - 9.8vdc
10 - 8.7vdc
11- .8vdc
12- 0
Note: I have not tested/replaced D1/D2 diodes as I'm not clear on how to proceed.

Look forward to your collective wisdom - Thanks in advance
 
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i have an sx-3000 with the same symptoms you describe, i havnt gotten around to fixing it yet but i did determine that the problem with it was in the preamp (first by using a diff pre amp to drive the power amp section, which worked as it should), and then comparing resistor values to their counterparts for the other channel, things are very weird on that board for mine. on mine, the voltages on the power board for the rails going to the pre are also out of whack.

yours may or may not be in the same situation but if you're having a problem that you cant track down on the power amp, its a place to start.
 
Thanks - since I have one channel working, would the simple test be to swap the inputs going to pin 2 on the main amp board? If the problem follows, this would point to the control amp? I've traced the signal and it seems clean coming in, but I have read enough about this unit to know that I should keep a very open mind!
I'll try this morning and report back.
 
OK - swapped the signal coming from the control amp and the problem stays on the right side. So, I can confidently say that the signal is clean coming from the Control amp and becomes distorted on the main amp board.
Next steps?
 
You didn't mention setting the idle bias on the amps. The voltage missing, is from the SV-3A compensating diode circuit.
 
yes .check bias and dc balance both channels ..p.s one can and does affect the other .
2 matching meters helps if you dont have a scope . i personally get on better with 2 meters .
pin 7 seems to be the one to follow back from .
 
Left side I was able to adjust VR801 to give me ~36 on pin 9 , half of the ~73 on pin 6
Right side will not adjust higher than ~10, will go lower, but not any higher.

As for adjusting VR5/VR6 - which resistor do I measure across and what mv reading am I shooting for?
Thanks
 
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firstly you are aiming for 40v at pin 9 . without this it wont operate very well at all . sorry i cant read this schematic too well to see the numbers . 2sc485 seems to need more voltage at its base or something is pulling that voltage down .. all voltages for that transistor in the bad channel may help here .
 
I trust you're looking for voltages on Q806? Swapped out to a ZTX694b per advice from another thread.
Base:.05
Emitter:.05
Collector: 24.1
 
Replaced all .7 resistors with new .68 - I believe this was key to getting it this far - was frying resistors on main amp board (R846 and R844)
In case it matters - I checked caps C7/C8 in circuit - 1389uf .36esr and 1212uf .08esr (These are 1000f 50v caps)
 
Update:
I swapped D1/D2 thinking the SV-3A may be causing the voltage drop on the right side - the problem did not follow - stayed on the right side.
Any thoughts on whether this has to do with the larger caps c7/c8 or c12/c13?

In the meantime, I broke one of the Transformers on the FM MPX board (hand slipped...) so am working on getting a used working board off #bay.
Am still able to work on the unit using aux input - hope to have the original problem solved before I break something else!!

Appreciate everyone's help.
 
Thanks for the suggestion. Quick question: Are you thinking that the loose connection might be on the main amp board? If so, what path does the power take to get to the collector (sorry - I'm new to this level of troubleshooting)
 
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Thank you.
In the meantime, here are the main amp voltage readings:

Left (good) side:
3- 45vdc
4- 0
5- 0
6- 76vdc
7- 42.3vdc
8- 42.3 vdc
9 - 41.8vdc
10- 41vdc
11- .4vdc
12- 0

Right (Bad) side:
3- 45vdc
4- 0
5- .1vdc
6- 76vdc
7- 10.7vdc
8 - 10.8vdc
9 - 9.8vdc
10 - 8.7vdc
11- .8vdc
12- 0
 
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