I have purchased B&K 200.2 that has one channel with ~30Vdc on the output. After removing the channel, checking all the resistors and diodes (and fuses), the Lateral Mosfets measured off in diode and resistance compared to the other channel. I removed all 5 and tested out of circuit and 4 Lateral Mosfets and their drain resistors were bad. I replaced all 5 Lateral Mosfets with what I'm 90% confident are originals I bought from a reliable source 4-5 years ago and have used in 2 other amp builds.
That said - here's the bias issue... possibly, because it doesn't match the other channel. The replacement K1058s are only getting to 6-8mV of bias, when the J162s are getting to the specified 22-25mV. All 5 J162s are within +/-2mV and all the k1058s are within +/-2mV, just low by 15mV compared to the J162s.
I have gone so far as replacing the 5 J162s with the corresponding replacement Mosfets (all K1058s and J162s have all been replaced from same source) assuming there was a mis-match with the 20+ year old J162s. Exact same Bias after replacement.
I have since pulled all TO-92s and TO-5 drivers and measured out of circuit and replaced the J162 (Q13) driver that was low on Hfe and now both drivers match Hfe (~70). Again no change to the bias and amplifier channel performs exactly the same.
I can adjust bias, it plays music, doesn't overheat - but my OCD can't get around that there is something I'm missing, because the other channel is +/-5mV across all 10 outputs (22-27mV)
Am I going crazy, does this difference really matter or possibly a simple explanation and I can fix the problem? Can't get specific schematic to extract, so I'm including the entire Service Manual for 200.5-200.7, amplifier board schematic matches my amp boards 99% (only difference is there is 3k gate resistors on K1058s, not the 1.5k on the schematic).
Thanks for reading the long post and looking forward to gleaning some of this group's vast knowledge to figure this out (or confirm it doesn't really matter).
That said - here's the bias issue... possibly, because it doesn't match the other channel. The replacement K1058s are only getting to 6-8mV of bias, when the J162s are getting to the specified 22-25mV. All 5 J162s are within +/-2mV and all the k1058s are within +/-2mV, just low by 15mV compared to the J162s.
I have gone so far as replacing the 5 J162s with the corresponding replacement Mosfets (all K1058s and J162s have all been replaced from same source) assuming there was a mis-match with the 20+ year old J162s. Exact same Bias after replacement.
I have since pulled all TO-92s and TO-5 drivers and measured out of circuit and replaced the J162 (Q13) driver that was low on Hfe and now both drivers match Hfe (~70). Again no change to the bias and amplifier channel performs exactly the same.
I can adjust bias, it plays music, doesn't overheat - but my OCD can't get around that there is something I'm missing, because the other channel is +/-5mV across all 10 outputs (22-27mV)
Am I going crazy, does this difference really matter or possibly a simple explanation and I can fix the problem? Can't get specific schematic to extract, so I'm including the entire Service Manual for 200.5-200.7, amplifier board schematic matches my amp boards 99% (only difference is there is 3k gate resistors on K1058s, not the 1.5k on the schematic).
Thanks for reading the long post and looking forward to gleaning some of this group's vast knowledge to figure this out (or confirm it doesn't really matter).
Yes - I have ~1-2mV DC offset. The circuit appears to be working, the only issue I can't figure out is the K1058 outputs bias is lower then both the original working channel K1058s and ~15mV lower than the same channels J162 outputs (which bias correctly up to 25mV, +/-2mV)
Figured it out...
I tested the drain resistors when I put the replacements in, but either my DMM was inaccurate or they immediately went out of spec, not sure. Supposed to be 0.47r 5w resistors, and I replaced with the same spec., but they are now measuring 0.22-0.26r, so I think I just happen to catch it before they completely went open. They were NOS units, but not old enough I was worried about the spec. or being out of tolerance.
What I learned...
Original amp after first replacement of one side of outputs was giving 8-10mV across 0.22r resistors, which is ~45mA, with other half of outputs having 25mV across 0.47r resistors, or ~50mA - so there wasn't actually anything wrong with the bias in the amp channel, which is why it was playing fine and no overheating.
So my OCD could rest tonight, I torn it back apart and replaced all those resistors with new 0.47r (double checked with two DMMs this time) - put it all back together and I get 20-25mV across ALL drain resistors on all 10 outputs. Back to factory condition, and the only things left to is a thorough burn-in and scope check again to make sure both channels output is within tolerance.
I tested the drain resistors when I put the replacements in, but either my DMM was inaccurate or they immediately went out of spec, not sure. Supposed to be 0.47r 5w resistors, and I replaced with the same spec., but they are now measuring 0.22-0.26r, so I think I just happen to catch it before they completely went open. They were NOS units, but not old enough I was worried about the spec. or being out of tolerance.
What I learned...
Original amp after first replacement of one side of outputs was giving 8-10mV across 0.22r resistors, which is ~45mA, with other half of outputs having 25mV across 0.47r resistors, or ~50mA - so there wasn't actually anything wrong with the bias in the amp channel, which is why it was playing fine and no overheating.
So my OCD could rest tonight, I torn it back apart and replaced all those resistors with new 0.47r (double checked with two DMMs this time) - put it all back together and I get 20-25mV across ALL drain resistors on all 10 outputs. Back to factory condition, and the only things left to is a thorough burn-in and scope check again to make sure both channels output is within tolerance.
