I rebuilt this driver with all premium passive components and the original Toshiba transistors. For some reason I can't get it to balance. The bias adjustment has almost no effect (.01-.05v)
Everything seems fine until we hit the darlington. The transistor seems to bench test okay but the hFE is too high for my multi-meters to measure. I swapped it out for a KSD1692-D(Y) and it made no difference at all.
Point 'U' is a constant -2.59v. The pot lets that side of the voltage divider to swing from 1.8-2.53v so it should balance but nothing happens.
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The power rails are all 2v higher due to the Schottky efficiency from the power supply and ultra-low resistance traces. Any chance that would affect the bias due to the diode on the one side of the bridge?
Or is there something downstream that is pulling the bias out?
Thanks in advance.
Everything seems fine until we hit the darlington. The transistor seems to bench test okay but the hFE is too high for my multi-meters to measure. I swapped it out for a KSD1692-D(Y) and it made no difference at all.
Point 'U' is a constant -2.59v. The pot lets that side of the voltage divider to swing from 1.8-2.53v so it should balance but nothing happens.
The power rails are all 2v higher due to the Schottky efficiency from the power supply and ultra-low resistance traces. Any chance that would affect the bias due to the diode on the one side of the bridge?
Or is there something downstream that is pulling the bias out?
Thanks in advance.
Update. I went as high as 4.4k on R749 and it only brought the neg side down to 2.4 until d725 stops conducting altogether, so I'm pretty sure it's not a bias adjustment issue. Something must be pulling more toward neg farther downstream but I haven't figured out what yet.
What are the voltages across R743 and R745? One end of each of those resistors is grounded?
Craig
Craig
Yes the middle of R743 and R745 is grounded. The voltage across them is the same as points (S) and (U) respectively (I double checked to make sure I'm not crazy).
Also I found out that if I lift R749 completely then (U) goes to what it should be (-1.8v). (S) drops to .5v and (T) to -.123. I'm not sure it would hit exactly -1.8v if there was a problem with Q723 so now I'm back to square one.
Is the 2SD1508 transistor Chris Wallace and the KSD1692 Steve Scully? Are both of them biased moderators? Too soon for those jokes?
Also I found out that if I lift R749 completely then (U) goes to what it should be (-1.8v). (S) drops to .5v and (T) to -.123. I'm not sure it would hit exactly -1.8v if there was a problem with Q723 so now I'm back to square one.
Is the 2SD1508 transistor Chris Wallace and the KSD1692 Steve Scully? Are both of them biased moderators? Too soon for those jokes?
Okay I'm finally understanding what's going on and why efficiency is a bad thing. This is not a bias problem but rather a separation problem.
1.8 - (-2.5) is the Zener voltage, therefore D723 is limiting the S-U separation but in reality it's probably more.
The increase in voltage at the rails (42.67-40.7)*2 = 3.94v, so do I need to figure out where to shed that voltage so that the S-U separation is back to 1.8-(-1.8) = 3.6v?
So the question is how? Should I
- Throw some fat diodes on the power rails and just burn the power I recovered?
- Increase R739/R741 or add a couple diodes there? Do I have to increase R731/R733 as well to maintain the interaction between the interlocked transistors?
- Increase the Zener voltage of D723 to 8.24v? If so, how much should I increase R753/R755 by so that Q721/Q723 aren't driven too hard?
This is where I could REALLY use one of you dinosaurs who instinctively now how to adjust a differential circuit. Thanks a MILLION.
1.8 - (-2.5) is the Zener voltage, therefore D723 is limiting the S-U separation but in reality it's probably more.
The increase in voltage at the rails (42.67-40.7)*2 = 3.94v, so do I need to figure out where to shed that voltage so that the S-U separation is back to 1.8-(-1.8) = 3.6v?
So the question is how? Should I
- Throw some fat diodes on the power rails and just burn the power I recovered?
- Increase R739/R741 or add a couple diodes there? Do I have to increase R731/R733 as well to maintain the interaction between the interlocked transistors?
- Increase the Zener voltage of D723 to 8.24v? If so, how much should I increase R753/R755 by so that Q721/Q723 aren't driven too hard?
This is where I could REALLY use one of you dinosaurs who instinctively now how to adjust a differential circuit. Thanks a MILLION.
A Marantz SM6100sa R1B AKA PM8200 without controls.
You could be right but I'm not seeing it. No shorts or open circuits at the 1186. Both output pairs are new because they were still available from Digi-key, and I'm seeing the same problems with both channels. It's entirely possible that I could have blown the 1186 on both sides but I assumed my problem was at the bias stage. I will pull them and test them I guess. I did swap out the 1145 and 1360 already but to no avail. I do want to swap out Q717 tomorrow and see what happens.
Also update: I increased R785/R787 from 10 ohms to 100 ohms (.3v drop before x10 = 3v drop which is what I needed). (B) and (E) are now roughly 40v. The spread is now 3.6v, but that only put point (S) at 1v and (U) at -2.6v.
Q727 Checks out good
From 0v, starting at the top of the totem, D717 cathode:
41.8v, .67v, .62v, 0v, -1.8v, -2.1v, -41.7v
DFV if it matters: .595, .615, .732, .734, .618, .595 respectively
Both left and right are -1.3v. If it matters Q725,Q727,Q729,Q731 are on the primary power rails which are still at +-42.7v. Since R765 is still roughly half the voltage of R761, I would think that the spread is still proportional on the - side to what it should be?
Thank you SO much everyone. I put so much time and care into this project and this is the only thing standing in my way.
Out of desperation I pulled R753/755. Had to be quick because I didn't want to pop the zener but point (S) dropped to -2.2v and (U) dropped to -5.8. At this point I'm focusing my attention on the interlocked transistors unless anyone thinks I'm chasing a red herring.
Q731 must be a problem. If there is -1,3V on the base, and 0 on the emitter, it must conduct about 10-12A. So the problem somewhere there. Maybe the device itself is OK, but some sodering is wrong.
You will not able to identify the problem without R753, and R755. The output stage will floating...
Sorry for the late reply.
My reasoning for pulling R753/755 was to isolate the input rather than the output. It turns out Q715 was really weak which caused the bias problem, but I also had a bad connection on Q731 so thank you for that.
The output is now balanced but it's still running really hot. Output to the power transistors is +-2v. Points (S) and (U) are also +-2v.
Can/should I replace R735/725 1.5k resistors with a 1K resistor plus +-15v regulators (MC7815CTG/MC7915MCG)? Then I can go down the line and increase the resistances for the other stages.
My reasoning for pulling R753/755 was to isolate the input rather than the output. It turns out Q715 was really weak which caused the bias problem, but I also had a bad connection on Q731 so thank you for that.
The output is now balanced but it's still running really hot. Output to the power transistors is +-2v. Points (S) and (U) are also +-2v.
Can/should I replace R735/725 1.5k resistors with a 1K resistor plus +-15v regulators (MC7815CTG/MC7915MCG)? Then I can go down the line and increase the resistances for the other stages.
NM I figured it out. There was a problem with the darlington. My poor board is getting baked from all the tinkering.
However are the regulators still a good idea? Am I trading ripple for noise on the op amp?
However are the regulators still a good idea? Am I trading ripple for noise on the op amp?