I've been working on my TR-2055 for a while and I'm a bit stumped. The problem is a large channel imbalance in the power amp section, with the left channel being weaker than the right. There are no DC issues -- the power supply rails are fine and all DC voltages I could think of checking are correct.
The problem appears to be isolated to the negative portion of the left channel diff amp. I played a test tone through the amp section and compared AC voltages between the good and bad channels. The AC voltage at the collector of Q807 (which connects to the base of Q821, the first NPN emitter follower) is correct and matches the right channel. However, the AC voltage at the emitter of Q809 (which connects to the base of the first PNP emitter follower) is much too low.
I've checked the value of every resistor that seems involved in this circuitry, including R823, R825, R827, R843, R847, R851, R837, R841, R835, and R819, and all test good. Same for D809, D803 and D805, and D817. However, I'm definitely open to re-testing stuff if anyone can spot a component that's especially likely to be the culprit.
I also transistor tested Q809, Q805, and Q813, and all had hFE within datasheet specs and diode tested good. I tested them at realistic currents using a PC-based transistor tester, not one of those $30 ebay ones.
I'm running out of ideas short of shotgunning things or blasting the board with cold/hot air while it's running to try and spot a thermally-sensitive issue, but I'm curious if anyone smarter than me has ideas. I haven't tested C811 or C821 but I might desolder and test those next.
Schematic for reference:
The problem appears to be isolated to the negative portion of the left channel diff amp. I played a test tone through the amp section and compared AC voltages between the good and bad channels. The AC voltage at the collector of Q807 (which connects to the base of Q821, the first NPN emitter follower) is correct and matches the right channel. However, the AC voltage at the emitter of Q809 (which connects to the base of the first PNP emitter follower) is much too low.
I've checked the value of every resistor that seems involved in this circuitry, including R823, R825, R827, R843, R847, R851, R837, R841, R835, and R819, and all test good. Same for D809, D803 and D805, and D817. However, I'm definitely open to re-testing stuff if anyone can spot a component that's especially likely to be the culprit.
I also transistor tested Q809, Q805, and Q813, and all had hFE within datasheet specs and diode tested good. I tested them at realistic currents using a PC-based transistor tester, not one of those $30 ebay ones.
I'm running out of ideas short of shotgunning things or blasting the board with cold/hot air while it's running to try and spot a thermally-sensitive issue, but I'm curious if anyone smarter than me has ideas. I haven't tested C811 or C821 but I might desolder and test those next.
Schematic for reference:
