77highlands
New Member
Seeking advice from forum members on my Pioneer SX-780 that I’ve owned for 5-6 years. I “lost” my left channel (all sources, both A and B) after recently completing a full restoration. I combined the guidance from Mike at ATO Audio and Oleg at Oleg’s Vintage Audio. They provided easy-to-follow instructions on components to replace. The reasons for the restoration were age of the unit (original components), a persistent “fuzziness” on a 1K sine wave of my oscilloscope, and a “muddy” left channel when volume was turned up past one-quarter.
This will be a long read, but first, let me assure you that I read over a dozen posts on this forum regarding channel loss and checked out those suggestions on my receiver. Both channels were working before the restoration. I have double-checked the polarity of new capacitors and pinouts of new transistors; no errors. I visually checked all of the solder joints (new and old); no problems. I tested for continuity between the speaker jacks and the associated jack tabs (where wires are soldered on) and various pins indicated by the schematics; again no problems. I also switched the speaker wires between left and right channels to confirm my problem is not a blown speaker; not the case. I also cleaned the switches and pots with the appropriate forms of Deoxit. I did not have any problems with switches or pots before the restoration, but decided a thorough cleaning was a good idea just in case. I also cleaned the RCA connectors with Deoxit and a pipe cleaner.
By the way, the protection circuit does click (otherwise the right channel wouldn’t work either). My dim bulb shows no shorts. FM muting is off and bass/treble are mid-way. My Fnirsi 1014D oscilloscope show NO signal on the left channel; the right channel sine wave is “relatively” smooth (compared to before the restoration). I do not know how to safely trace an audio signal path, despite reading several forum posts. DC balance is set a 0mV for both channels.
I replaced the original STK0050s about 2 years ago with discrete replacements from Chris at Pyramid Audio in Austin, TX. I started troubleshooting this left channel loss by checking the voltages at the pins 1,2,3,8,9,10 on both channels; they all compare favorably to specs.
Spec Left Right
pin 1 (1.4) (1.1) (1.04)
pin 2 (40.0) (40.9) (40.9)
pin 3 (0.005) (0.009) (0.001)
pin 8 0.005 0.001 0.004
pin 9 40.0 40.9 40.9
pin 10 1.4 1.1 1.147
I measured the voltages at all the pins on the AF Assembly board I could safely reach. Where the schematics show a voltage, my measurements are very close.
I also measured voltages of all transistors on the AF Assembly board, and they compared favorably to schematics voltages with two exceptions.
Pin 1 on Q3 and Q4 (TA7136P) and Collector on Q15 and Q16 (2SA912). The SX-780 schematic for pin 1 of Q3,Q4 shows 10.4V, but I measured 21.41V on both transistors (three times in a row and after unit had been on over 20 minutes).
The schematic for Collector of Q15,Q16 shows 1.4V, but I measured on average 4.23V and 4.20V, respectively for Q15 and Q16. I do not know if these variances are a problem or not (my audio repair skills are medium at best and I’ve got a lot learn about using my oscilloscope). I then checked voltages up- and down-stream of these pins and did not find any significant variances.
After finding no other transistor voltage discrepancies, I visually examined all of the resistors and diodes for burn marks or other deformities as well as their solder joints once again; none found. I have not taken voltage readings on resistors or diodes yet because I believe I should unsolder one leg of each component from the board to get accurate readings. I’m not adverse to this task, but I wanted to seek the advice of forum members for my “lost” left channel before tackling this chore.
Despite all of my research and testing, I think I’m at the point of not knowing what I don’t know. Voltage measurement results are attached. Can anyone suggest what else to check? Thanks in advance, Greg.
This will be a long read, but first, let me assure you that I read over a dozen posts on this forum regarding channel loss and checked out those suggestions on my receiver. Both channels were working before the restoration. I have double-checked the polarity of new capacitors and pinouts of new transistors; no errors. I visually checked all of the solder joints (new and old); no problems. I tested for continuity between the speaker jacks and the associated jack tabs (where wires are soldered on) and various pins indicated by the schematics; again no problems. I also switched the speaker wires between left and right channels to confirm my problem is not a blown speaker; not the case. I also cleaned the switches and pots with the appropriate forms of Deoxit. I did not have any problems with switches or pots before the restoration, but decided a thorough cleaning was a good idea just in case. I also cleaned the RCA connectors with Deoxit and a pipe cleaner.
By the way, the protection circuit does click (otherwise the right channel wouldn’t work either). My dim bulb shows no shorts. FM muting is off and bass/treble are mid-way. My Fnirsi 1014D oscilloscope show NO signal on the left channel; the right channel sine wave is “relatively” smooth (compared to before the restoration). I do not know how to safely trace an audio signal path, despite reading several forum posts. DC balance is set a 0mV for both channels.
I replaced the original STK0050s about 2 years ago with discrete replacements from Chris at Pyramid Audio in Austin, TX. I started troubleshooting this left channel loss by checking the voltages at the pins 1,2,3,8,9,10 on both channels; they all compare favorably to specs.
Spec Left Right
pin 1 (1.4) (1.1) (1.04)
pin 2 (40.0) (40.9) (40.9)
pin 3 (0.005) (0.009) (0.001)
pin 8 0.005 0.001 0.004
pin 9 40.0 40.9 40.9
pin 10 1.4 1.1 1.147
I measured the voltages at all the pins on the AF Assembly board I could safely reach. Where the schematics show a voltage, my measurements are very close.
I also measured voltages of all transistors on the AF Assembly board, and they compared favorably to schematics voltages with two exceptions.
Pin 1 on Q3 and Q4 (TA7136P) and Collector on Q15 and Q16 (2SA912). The SX-780 schematic for pin 1 of Q3,Q4 shows 10.4V, but I measured 21.41V on both transistors (three times in a row and after unit had been on over 20 minutes).
The schematic for Collector of Q15,Q16 shows 1.4V, but I measured on average 4.23V and 4.20V, respectively for Q15 and Q16. I do not know if these variances are a problem or not (my audio repair skills are medium at best and I’ve got a lot learn about using my oscilloscope). I then checked voltages up- and down-stream of these pins and did not find any significant variances.
After finding no other transistor voltage discrepancies, I visually examined all of the resistors and diodes for burn marks or other deformities as well as their solder joints once again; none found. I have not taken voltage readings on resistors or diodes yet because I believe I should unsolder one leg of each component from the board to get accurate readings. I’m not adverse to this task, but I wanted to seek the advice of forum members for my “lost” left channel before tackling this chore.
Despite all of my research and testing, I think I’m at the point of not knowing what I don’t know. Voltage measurement results are attached. Can anyone suggest what else to check? Thanks in advance, Greg.