Hello. A friend of mine recently purchased a Pioneer SX-1000TD with some problems, and asked me to take a look at it. I don't normally work on other people's units; I have more electronics knowledge than the average person, but no real expertise. He asked that I fix the immediate problems, and also "do whatever you would do if it was yours and you were going to keep/use it".
The right channel was dead, or almost completely dead - it may have had some very low output, but I think what was coming through was bleed-over from the other channel. Additionally, on DBT, the unit would fire up, bulb would go bright them immediately dim, then go bright for just a moment, then dim back down again and remain dim.
In any case, I isolated the problem to the tone/control board and found two bad capacitors (one swollen, one open). I replaced those, and the right channel seemed to be working. I did notice that it seemed a bit lower in output, but didn't give it much thought at the time, since I was going to recap the unit, adjust the center voltage, adjust the bias, clean the pots and switches, etc.
I proceeded to do that work, testing with DBT and brief sound checks after each half of each board was completed. No problems. BTW, once I addressed the caps on the tone board, the DBT test worked normally (power on, bright for a moment, then immediately dim...no second cycle of bright then dim).
Last night, after getting some needed parts, I completed the last board. Fine on DBT, so moved on to firing it up for an extended period to make sure all was good, and to give it a serious listen.
Initially, I though it sounded pretty good. Then, in comparing R and L channels, I realized that the right channel does indeed have a reduced, thinner-sounding output. After some investigation, I realized that neither the loudness switch nor the bass tone control have any effect on the right channel. It seems that the output is not so much reduced as it is missing the lower frequencies. Tone control and loudness seem to work just fine on the left channel.
I made an assumption that the problem is on the tone board, and did the following:
I don't know where to go from here. I am not experienced/knowledgeable enough to come up with any theories on what would keep lower frequencies out of the right channel signal. Unless the AK brain trust has other suggestions for isolating the problem, my next steps will be:
EDIT: The manual and schematic I am using are from hifiengine.com, here:
https://www.hifiengine.com/manual_library/pioneer/sx-1000td.shtml
Paging @merlynski.
The right channel was dead, or almost completely dead - it may have had some very low output, but I think what was coming through was bleed-over from the other channel. Additionally, on DBT, the unit would fire up, bulb would go bright them immediately dim, then go bright for just a moment, then dim back down again and remain dim.
In any case, I isolated the problem to the tone/control board and found two bad capacitors (one swollen, one open). I replaced those, and the right channel seemed to be working. I did notice that it seemed a bit lower in output, but didn't give it much thought at the time, since I was going to recap the unit, adjust the center voltage, adjust the bias, clean the pots and switches, etc.
I proceeded to do that work, testing with DBT and brief sound checks after each half of each board was completed. No problems. BTW, once I addressed the caps on the tone board, the DBT test worked normally (power on, bright for a moment, then immediately dim...no second cycle of bright then dim).
Last night, after getting some needed parts, I completed the last board. Fine on DBT, so moved on to firing it up for an extended period to make sure all was good, and to give it a serious listen.
Initially, I though it sounded pretty good. Then, in comparing R and L channels, I realized that the right channel does indeed have a reduced, thinner-sounding output. After some investigation, I realized that neither the loudness switch nor the bass tone control have any effect on the right channel. It seems that the output is not so much reduced as it is missing the lower frequencies. Tone control and loudness seem to work just fine on the left channel.
I made an assumption that the problem is on the tone board, and did the following:
- Triple checked orientation of all the new caps.
- Examined all joints (new and old) under magnification and re-soldered anything even remotely suspicious (only a couple).
- Compared voltages right to left at all the of the external connections of the board...both sides are same.
- Measured voltage at just about all points in all circuits on the board, comparing right to left. No appreciable differences.
- Measured resistors, in circuit. Got same results on both sides (right and left).
- Checked resistance on bass pot tabs...seem good, and are same on both right and left halves of the pot.
- Used diode test function to check the four transistors on this board (in circuit). All tested fine.
I don't know where to go from here. I am not experienced/knowledgeable enough to come up with any theories on what would keep lower frequencies out of the right channel signal. Unless the AK brain trust has other suggestions for isolating the problem, my next steps will be:
- Verify it is not a preamp problem by reversing inputs from preamp to tone board, under the assumption that if problem remains in the right channel, the preamp section is not the problem.
- Reverse the inputs from the tone board to the power amp board to see if the problem follows.
- After getting the boards recapped, but before centering the voltage and setting bias, I changed the voltage selector from 117V (where it apparently has been for a long time) to the 130V position. I did this under the assumption that with the normal 122-123V service from the local provider, it is better for the transformer to get less than expected than more.
- I have not yet replaced the PS filter caps nor the output coupling caps (waiting for parts). I did measure the coupling caps in place...spec is 1000uf, and they both measured around 1500uf. I don't have a means of testing ESR. I would not think this is a big problem...in fact, I would think that higher capacitance here would be a good thing?
- As part of "tuning up" this unit, I cleaned and tested the "center voltage" and bias pots, and set per service manual. Bias seems to be holding fine.
- I also pulled the outputs, cleaned them, installed new insulators and thermal compound, and re-installed. While out, used diode test function of meter...all good.
- I do not have a scope. I thought I was getting a used one later this month, but that deal fell through.
EDIT: The manual and schematic I am using are from hifiengine.com, here:
https://www.hifiengine.com/manual_library/pioneer/sx-1000td.shtml
Paging @merlynski.
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