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Bose 1800 Amp Spare Parts (Amp PCB)

R58 passes but as I probed it, it easily came detached. It measured 23.8 Ohms roughly. R28 measured 5.2 Ohms with circuit. I repaired the connection, no notable change in DC offset, op-amp output or supply voltage to. Q3 is not in circuit.
 
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JamesDean59 said:
R58 passes but as I probed it, it easily came detached. It measured 23.8 Ohms roughly. R28 measured 5.2 Ohms with circuit. I repaired the connection, no notable change in DC offset, op-amp output or supply voltage to. Q3 is not in circuit.
Click to expand...

Q3 has got to be in there, I believe. Part of the negative feedback path, feeds back into the inverting input of the opamp. Is it still +10V offset? If the minus rail, of the opamp is still minus 8V or so, you might want to take a look at C7. It could be that the IC is bad.
 
I'll swap back Q3 and retest. When I posted that last it was still +10. I dont have a capacitance meter here at the moment, I'll pull C7 and check resistance.. capacitance later.

U1 is a MC1556G, metal case 601. Looks like T0-39 but with 8 legs.
 
Jon_Logan said:
Make sure CR2 is not leaky. You can lift a leg. Those are there for ESD protection and possibly to avoid latch-up.

Yeah, I'd go along with "it's probably okay to go with +/-16V zeners".
Click to expand...

Guys, I've been following this with great interest and its very instructive how you both are troubleshooting this amp back to health, so nice job both of you.

Having said that, I can't believe you actually wrote the above!
 
Alright more progress:

I've restored the +/- 16 volts to the op amp by replacing Q2 a 2N3906. The op amp is now outputting 6.0V

DC offset is now -70.8V. When probing the output transistors I get 0V from the positive side and -70.8V from the negative set.
 
I was reading back in the thread and recalled this:

Jon_Logan said:
The +12 at the output of the op amp, while there is a large negative voltage at the output, tells me that the differential amp and/or the VAS transistor is not behaving correctly. The +12, at the output of the op amp, is "instructing"
Q1 to turn off. Q1 might be leaky. Can you measure the voltage across R5 and R6? The voltage should be higher across R6.

Measure the voltage across R9. If there's a voltage > 0.28V, it sounds like Q3 or Q24 might be leaky.
Click to expand...

With Q2 being replaced/bad perhaps Q1 is bad as well. The op-amp is sending out +6 telling Q1 to turn off.

I retook the measurements across:
R5: 0v
R6: .16v
R9: 2.18v

According to the schematic I should see a 2v drop across R9 as its connected to the -85V rail and the schematic shows -83 on the other side.. That seems to add up.

Voltages at Q1:
Base .7v
Emitter: -1.4v
Collector -86.9v

Voltages at Q2:
Base -2.3v
Emitter: -.16v
Collector -16.0v

I was tracing the source for R5/6 and measured a 90.3 voltage drop across R4. Giving -1.4v on the side R5/6 are connected to..Why is that negative? R4 measured 18.6K in circuit.
 
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Well I swapped Q1 from the other board and it made no difference.

Do you think I should put the old Q2 back in and swap U1s? What makes you think replacing Q2 was a mistake?

If U1 is bad... I'm not sure of a replacement for it..
 
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JamesDean59 said:
Do you think I should put the old Q2 back in and swap U1s? What makes you think replacing Q2 was a mistake?

If U1 is bad... I'm not sure of a replacement for it..
Click to expand...

Yes, I think Q2 should go back. When the original Q2 was in there, the offset was following the input offset. That's what you want. After replacing Q2, your input had a positive offset, but the amp offset was negative. Check your pinout on the 2N3906. It sounds like it's not connected right.
 
Jon_Logan said:
Yes, I think Q2 should go back. When the original Q2 was in there, the offset was following the input offset. That's what you want. After replacing Q2, your input had a positive offset, but the amp offset was negative. Check your pinout on the 2N3906. It sounds like it's not connected right.
Click to expand...

I just double checked the connections and they're correct. I suspected that Q2 was leaky/bad as it seemed to be the only other thing on that -16 bus, so I replaced it.

I just tested the old Q3 and I get conductivity both directions when going across collector and emitter. BE/EB and BC/CB were OK though only conducting one way. This makes me thing it was in fact bad, am I wrong on that?
 
I was following the troubleshooting guide that corresponded with >2 DC Offset and opposite polarity at the op-amp. I realize we're <2V but I was just curious.

It said to suspect:
Q1-8,24
Cr16,17

I checked:
Q1 was swapped out and nothing changed
Q2 was replaced
Q3 was temporarily replaced
Q4 was removed and tested, I was unable to get any conductivity at all on any path. Not sure what that's all about. 2N2219. Still available from Central Semi.
Q5 was removed and tested OK.
Q6 has been previously tested and was deemed OK
Q7 was tested and passed.
Q8 was bad. Conducted in all directions both ways on all legs.
Q25 was bad. Conducted in all directions both ways on all legs.

Cr1,2 tested out OK as well.

Cr16,17 are 1N4148. Both were conducting in both directions in and out of circuit. So they're bad. Not quite sure of the effect of that.

Wild thought, I've no idea if this is anyway valid or not: If Cr16,17 failed, it was probably due to high current. If they had high current going through them they probably sent it down the line to Q3 and Q24. Q4 and Q9 might have been effect/hit by this large current as well. Might explain why Q4 has no conductivity in any paths outside the circuit? So far we did find that Cr16,Cr17,Q3,Q24 were all bad or leaky. Maybe. I could be full of it. My experience with amplifier behavior and circuitry of this level is low.
 
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Edited previous post with new information.

Grabbing ammeter and powering up after some food. We'll see if those replacement parts did any difference.
 
Alright, so at around 80-90v on the variac, I'm getting a 4-5 amp draw. DC offset is around flirting around 0v most of the time. Never got higher than -0.1mV

"If the amplifier does draw a lot of current, that would point to a short or leak on the negative half."

So this appears to be the case now...The output transistors are getting warm as is the heatsink. Between 70-80 volts on the variac the current draw goes from .9A to 2.5A...

I found this bit in the service manual:


Removed the screws on Q9 and the high current draw disappeared. DC offset is at -44VDC. Op-amp output is 4.3VDC
Installed the Q9 screws, removed Q11 screws. The current draw was still present but shift until 2.5amp @ 95-100v on the variac.

Q11 driving the negative side output transistors. Disabling its collector disables all output transistors. So reconnecting it and going disabling individual output transistors one at a time in hopes of finding the offender...

Installed the Q11 screws, removed Q23 screws. No current draw issues. DC offset is at -63.3VDC. Op-amp output is 5.5 VDC.
Installed the Q23 screws, removed Q21 screws. No current draw issues. DC offset is at -63.3 VDC. Op-amp output is at 5.5 VDC.
Installed the Q21 screws, removed Q22 screws. Current draw 2-3 amps at 80V on the variac. DC offset -0.01mV.
Installed the Q22 screws, removed Q19 screws. Current draw 2-3 amps at 80V on the variac. DC offset -0.01mV.
Installed the Q19 screws, removed Q20 screws. No current draw issues. DC offset is at -63.3 VDC. Op-amp output is at 5.5 VDC.
Installed the Q20 screws, removed Q18 screws. Current draw 2-3 amps at 80V on the variac. DC offset -0.01mV.

So the current draw occurs when the negative side output transistors are enabled. When they are disabled there is a high negative DC offset. How is negative DC getting to the output line? Could one of the outputs be leaky?

In short:
With everything connected up as normal, we have large current draws before even hitting 100V mains power. Offset is within normal range. If I disable the negative side output transistors starting with Q9, the current draw vanishes and a negative offset at the output terminals appears.
 
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I was testing for conductivity between the output transistors and the large heat sink. I ended up finding a washer that had fallen into the screw hole on Q12 and was conducting. I verified that nothing else was conducting to the heat sink.

I removed this washer and re tested.

Current draw remained. :(

If I remove Q9s screws, the current draw goes away and I have 85VDC, Op-amp: -7.0 VDC
If I remove Q6s screws, the current draw goes away and I have -22VDC, Op-amp: 2.4VDC

Being that the current draw disappears, that tells me the output transistors are not shorted or shorting to the heat sink (which has been confirmed multiple times)
 
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Removing Q9 screws disconnects collector to outputs and no negative drive.
Removing Q6 screws disconnects B+ from Q6 but it is still acting like a diode connecting the drive to Q8 and the outputs.
Sorry - I don' have any good ideas where to go from here. Maybe q6 7 8 9 and/or problem with bias circuit Q4 (bias problem might explain current draw, but not offset problem) good luck.
 
Allright, let's try this. Take a jumper wire and attach:

The collector of Q5 to

the collector of Q3.

Variac that way and see if the current disappears.
 
Before I read your reply I swapped Q4 out with the other board and got the following results:

Variac at 120V = 1.5 amp draw.
DC Offset = -3.5VDC, Op-amp is at 0.65VDC

I think we're on to something here. I am going to try the Q5-Q3 jumper thus bypassing Q4 and the bias circuitry.

Jumper wire installed.

Variac at 120V = 1.2 amp draw.
DC Offset = -4.0VDC, Op-amp is at 0.68VDC.

Without the jumper wire:
I took some measurements, the positive side output transistors are dumping the the negative DC into the output. Back probing found -6 to -7 VDC on the base of the outputs, their driver and the emitter of Q5. Also found negative voltages all around Q4 as well.

I just swapped op-amps with the good board. Not the easiest task. Stupid 8-pin can. However the non-jumpered numbers still remain. I think we're making progress though! The high current is less high and the DC offset is lower than before.
 
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JamesDean59 said:
Without the jumper wire:
I took some measurements, the positive side output transistors are dumping the the negative DC into the output..
Click to expand...

No. Where would the positive side dump negative DC from?


IMO, you need to make one change and make measurements and try to understand what's really going on. I feel that you are changing too many things at one time and (possibly) missing the clues.
 
Jon_Logan said:
No. Where would the positive side dump negative DC from?


IMO, you need to make one change and make measurements and try to understand what's really going on. I feel that you are changing too many things at one time and (possibly) missing the clues.
Click to expand...

You're right. I wasn't very thorough in probing voltages before I posted that. The negative DC is at all the output transistors because they're all connected to the output bus.

I did a more thorough probing of voltage across the board:


Key:
Purple = Incorrect Voltages
Blue = DC Offset Voltages
Yellow = Correct Voltages

I believe there is a typo on Q10, the collector should be +84 and the emitter should be +56V.

I'm curious about Q3's base. Why is it at -49V?
 
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