I'm wondering if the bias issue I am experiencing could be due to the new driver transistors. The originals have a much lower hfe as measured with a Peak Atlas tester.
I've had no luck finding data sheets for the original outputs but I do have one for the RCA1A10 & RCA1A09 which is attached below.
Original transistors in bold
Q701/Q801....N2484 (NPN)..... hfe=175 & 185
.....replaced with: KSC1845..... hfe=414 & 414
Q702/Q802....RCA1A10 (PNP)..... hfe=40 & 46
.....replaced with: TTA004B..... hfe=179 & 183
Q703/Q803....RCA1A09 (NPN)..... hfe=29 & 60
.....replaced with: TTC004B..... hfe=180 & 184
The new output transistors have an hfe of about 75, originals about 22
ORIGINALS
NPN Replacement
PNP Replacement
I've had no luck finding data sheets for the original outputs but I do have one for the RCA1A10 & RCA1A09 which is attached below.
Original transistors in bold
Q701/Q801....N2484 (NPN)..... hfe=175 & 185
.....replaced with: KSC1845..... hfe=414 & 414
Q702/Q802....RCA1A10 (PNP)..... hfe=40 & 46
.....replaced with: TTA004B..... hfe=179 & 183
Q703/Q803....RCA1A09 (NPN)..... hfe=29 & 60
.....replaced with: TTC004B..... hfe=180 & 184
The new output transistors have an hfe of about 75, originals about 22
ORIGINALS
NPN Replacement
PNP Replacement
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Hoping someone can answer my previous (stupid?) question.
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In the meantime, I discovered a conflict between the schematic and the driver circuit board.
C706 / C806 (470uf) in the schematic shows the POS leg of the capacitor connected to ground, but the graphic on the circuit board shows the NEG leg to ground. The factory installed caps follow the schematic. This is true for both amplifiers I have in my possession. I'm assuming the graphic on the circuit board is incorrect but I thought I would point it out anyway.
The other odd thing I found is that both amplifiers, R721 (left channel) had been replaced and both driver boards are charred where R721 is located.
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In the meantime, I discovered a conflict between the schematic and the driver circuit board.
C706 / C806 (470uf) in the schematic shows the POS leg of the capacitor connected to ground, but the graphic on the circuit board shows the NEG leg to ground. The factory installed caps follow the schematic. This is true for both amplifiers I have in my possession. I'm assuming the graphic on the circuit board is incorrect but I thought I would point it out anyway.
The other odd thing I found is that both amplifiers, R721 (left channel) had been replaced and both driver boards are charred where R721 is located.
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Your question about drivers is not stupid. This design uses Darlington output transistors so, in effect, the "drivers" are the input transistors of Q711 and Q712. Designs more commonly seen have separate, non-Darlington packages implementing these functions. BTW, I don't believe you state explicitly what devices you used for replacements?
Graphics for electrolytic caps are irksome, often ambiguous in interpretation. In the C706 spots, the applied DC is ideally 0V.
The burned R721 resistors are likely collateral damage from a previous failure. Preferably, they would be of the "fusible" variety and have at least 1/4 inch space above the PCB to minimize possible damage. A possible failure scenario is an open collector of Q711; the amp output goes to the negative rail and the current through R721 skyrockets as feedback tries to correct the negative output voltage.
Details of interest: what's the origin of this amp? Is it commercial or DIY? What are supply voltages on the opamp? You might post details of the protection circuit and power supply also.
I believe the Vbe multiplier is under biased, based upon installed resistor values shown. You need about 2.4V spread between the bases of Q711 and Q712, with some adjustment range available. Marcel has a valuable comment, but have you made any changes in this area so far?
Graphics for electrolytic caps are irksome, often ambiguous in interpretation. In the C706 spots, the applied DC is ideally 0V.
The burned R721 resistors are likely collateral damage from a previous failure. Preferably, they would be of the "fusible" variety and have at least 1/4 inch space above the PCB to minimize possible damage. A possible failure scenario is an open collector of Q711; the amp output goes to the negative rail and the current through R721 skyrockets as feedback tries to correct the negative output voltage.
Details of interest: what's the origin of this amp? Is it commercial or DIY? What are supply voltages on the opamp? You might post details of the protection circuit and power supply also.
I believe the Vbe multiplier is under biased, based upon installed resistor values shown. You need about 2.4V spread between the bases of Q711 and Q712, with some adjustment range available. Marcel has a valuable comment, but have you made any changes in this area so far?
Thank you @BSST . It's a Sherwood SEL 400 integrated amplifier from around 1975, I'll attach the remainder of the schematic in the next post.
I purchased two different types of Darlington outputs,
Multicomp 2N6052 (PNP) & 2N6059 (NPN) from Newark or Mouser, can't recall
&
Onsemi MJ11021G (PNP) & MJ11021G (NPN) from Digikey
I used the cheap Multicomp for my testing and am saving the Onsemi for when I work everything out. Two sets of the Multicomp were fried while I was simply listening to the amp and not probing. As you can see, there are holes burned through the 2N6059's
I purchased two different types of Darlington outputs,
Multicomp 2N6052 (PNP) & 2N6059 (NPN) from Newark or Mouser, can't recall
&
Onsemi MJ11021G (PNP) & MJ11021G (NPN) from Digikey
I used the cheap Multicomp for my testing and am saving the Onsemi for when I work everything out. Two sets of the Multicomp were fried while I was simply listening to the amp and not probing. As you can see, there are holes burned through the 2N6059's
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Hi bedrock602 and members,
I've never seen anything like this before! Holes burned through the lid of a TO3 ?! I'm very suspicious about what's inside.
Anyone seen anything like this? Any familiarity with Multicomp?
My guess is these are counterfeit devices, and it's likely they're an interesting distraction from the real problems with the amp--- which aren't nearly so dramatic.
Thanks.
I've never seen anything like this before! Holes burned through the lid of a TO3 ?! I'm very suspicious about what's inside.
Anyone seen anything like this? Any familiarity with Multicomp?
My guess is these are counterfeit devices, and it's likely they're an interesting distraction from the real problems with the amp--- which aren't nearly so dramatic.
Thanks.
I thought the same thing but they were purchased from a reputable seller, Newark or Mouser.
I thought about trying the Onsemi transistors to rule out the possibility of the Multicomp being an inferior product. I'll just take a chance, $20 is nothing compared to the hours and hours of trying to track down the problem.
I thought about trying the Onsemi transistors to rule out the possibility of the Multicomp being an inferior product. I'll just take a chance, $20 is nothing compared to the hours and hours of trying to track down the problem.
Bias is back, baby !
I installed the Onsemi MJ11021/22 pair in the left channel which was blowing holes in the Multicomp transistors, I kept the Multicomps in the right channel.
I was able to set the bias in the left channel to 20 mV per the schematic, the right channel with the cheap Multicomps maxes out at around 9 mV.
I have two driver boards which are socketed so removal is easy. One board has the original driver transistors, the other has the new transistors with much higher hfe, they both work in the circuit and don't seem to make much difference with biasing since the bias pots of both boards are in roughly the same position, just east of the halfway mark.
So far, so good. Will swap out the right channel and report back.
I installed the Onsemi MJ11021/22 pair in the left channel which was blowing holes in the Multicomp transistors, I kept the Multicomps in the right channel.
I was able to set the bias in the left channel to 20 mV per the schematic, the right channel with the cheap Multicomps maxes out at around 9 mV.
I have two driver boards which are socketed so removal is easy. One board has the original driver transistors, the other has the new transistors with much higher hfe, they both work in the circuit and don't seem to make much difference with biasing since the bias pots of both boards are in roughly the same position, just east of the halfway mark.
So far, so good. Will swap out the right channel and report back.
I doubt the MJ11021’s are really any lower hFE. 100@15A vs 100@12A. They are probably pretty comparable at 5A. The old 2N6052’s really aren’t up to the task, and could very well be fakes. Multicomp’s come from a variety of sources. Some of those sources are reasonably good, others not so much. Kind of like, well…. NTE.
Try to twist the can off the dead ones. If it pops off they ARE fake. I’ve gotten Mospec branded “Multicomps” before and they were decent. Those are NOT, and there is no clue as to whose they really are. I also bought EXL’s (the old Mospec) back when Mouser sold them as such. The top hats didn’t come off without cutting, and they survive as well as original Motorola or RCA of the same types.
Speed wise, all those old Motorola-type monolithic darlingtons are all about the same 3 MHz.
Try to twist the can off the dead ones. If it pops off they ARE fake. I’ve gotten Mospec branded “Multicomps” before and they were decent. Those are NOT, and there is no clue as to whose they really are. I also bought EXL’s (the old Mospec) back when Mouser sold them as such. The top hats didn’t come off without cutting, and they survive as well as original Motorola or RCA of the same types.
Speed wise, all those old Motorola-type monolithic darlingtons are all about the same 3 MHz.
Both channels now have the Onsemi MJ11021/22.
Left channel bias is set to 20mV & -20mV as measured across R751 & R752 (one probe on one side of the .3 ohm resistor, other probe on the other side of said resistor)
Right channel bias is not so cooperative. I can get 20mV across one resistor, but only about 12mV across the other.
Left channel bias is set to 20mV & -20mV as measured across R751 & R752 (one probe on one side of the .3 ohm resistor, other probe on the other side of said resistor)
Right channel bias is not so cooperative. I can get 20mV across one resistor, but only about 12mV across the other.