Getting to the point on this one. Below is a summary of the parts I’ve changed out (quantities are per-channel). The replacements I’ve made since the previous post are the ones labeled “Replacement 2” only:
- 2SA970 (PNP, 5x)
- Replacement: ZTX795A
- 2SC2240 (NPN, 2x)
- Replacement: ZTX694B
- 2SA1360 (PNP, 3x)
- Replacement: KSA940
- 2SA1015 (PNP, 1x)
- Replacement 1: KSA1281
- Replacement 2: KSA1015(Y)
- 2SC1815 (PNP, 2x)
- Replacement 1: ZTX968
- Replacement 2: KSC1815(Y)
- 2SC3423 (NPN, 3x)
- Replacement: KSC2073
- 2SC2238 (NPN, 1x)
- Replacement 1: KSC2298
- Replacement 2: KSC2690A(Y)
- 2SA968 (PNP, 1x)
- Replacement: KSA1220A
This time, I made sure to use complementary transistors whenever the originals were complementary as well.
After making this round of replacements, I fired the amp on again. Same issue – no runaway current (good), but a still a 1.7 - 1.8 VDC offset. After probing it, however, I noticed that the input sine was being rectified!
Given that this issue was present on both sides, it’s unlikely that it’s caused by a component failure. I’ve also confirmed that (1) the supply rail voltages are checking in as nominal, and (2) this issue was occurring when bypassing the input board.
Now I’m a little stumped. I’ve already checked that the complementary NPN/PNP’s are matched whenever possible and that their gains are binned in the same range as the originals. Maybe this is a mismatch between transistor stages? Is that even an issue? There aren’t many leads on this specific line of amplifier on audiokarma, and the amount of time I’m wanting to further spend on this is dwindling.
At this point I may just wait until I can find a working P2150/2250 for cheap and make a complete swap. Which is effectively, just buying one that works. We’ll see if a Part 3 will follow…
