Bose 1801 Monster Power Amp Restoration!

Raccoon1400

Raccoon1400

Super Member
Calling all Bose 1801 experts!
Here is a monster on the bench for restoration, for AK member BilboBaggins.

I will be replacing all the carbon comp resistors, all electrolytic caps, zener diodes, all outputs, and bias pots. I will also be adding DC protection circuitry. New main filter caps will be installed too.
The right driver board just about done. I tested it today, successfully. Checked bias, offset, and did a sound check.

A couple questions:

Anything else on the driver board I should change? I have heard I could change the input op-amps. Any benefit to this?
Offset in the right channel is 70mV. How can I get this down?

Thanks in advance for the help.
 

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Hmm, -75V DC on the left channel output.

EDIT:
Found Q24 B and E shorted. Changed Q24. No Change to offset, but fixed a bias issue. New Q24 still tests good.
As discussed in the troubleshooting section of the SM, the voltage at point A is opposite of the DC offset. All the parts then mentioned test good in circuit, so no shorts I found. Haven't tested out of circuit.

Also, previously to powerup, I repaired a trace burned in half. It was the -ive supply, burned off after J1.

EDIT 2:
Swapped op-amps left to right, no change.

EDIT 3: + supply of op-amp is 7V instead of 16V. But why?

EDIT 4: The point between CR1 and CR2 is connected to the point between R55 and R57. This is not shown in the schematic. Also, in the schematic, R57 is shown as a second R55.
 
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I also replaced CR1, no change.
Got the schematic printed today.
I can't find any shorts. I guess something could test fine but still fail under load?
The voltage on the output is the -ive rail voltage. Less than 85V because I'm using the DBT with a 200W bulb.

Any ideas?

EDIT: there is a couple volts between the output and -ive rail.

EDIT 2: The collector of Q5 is at -72V. Should be +2. Could Q5 be going open?
 
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Raccoon1400 said:
R2 is correct, and new.

Tested Q5 out of circuit with diode check, and it is good.
Click to expand...

Could be one of the neighbour components that are fried and thus moving the voltage.

I would start checking Q3 and Q4. If Q24 was fried it's not unlikely that Q3 passed on along with it.
 
after studying the chematics it could be any of Q8,19,20,24,22,23 that causes the DC-offset of -75 volts by being shorted.

If it was CR10 that was shorted offset would be higher I think, but might be worth to check also.
 
Mannegizen said:
after studying the chematics it could be any of Q8,19,20,24,22,23 that causes the DC-offset of -75 volts by being shorted.

If it was CR10 that was shorted offset would be higher I think, but might be worth to check also.
Click to expand...

I've checked everything I can think of. No obvious shorts. Q3 is good.
Thinking I should replace a bunch of transistors. Better parts are available even if not the issue. Would Q1 and Q2 need to be matched? I do notice they are not the same part, and they aren't high gain parts.
 
There must be a typo on the chematics, the -56 voltage on Q10,15,16,17 should be +56 also the -84 on Q10 should be +84 volts.

But I guess you already seen that.
 
Q8,25,9, and 11 are also OK?

CR6 tested, if it's leaking to the ground it could cause the wrong feeding to the OP-amp.?

If the OP-amp is unevenly feed by the +7 and -16volts, could this cause a small negative bias that then is amplified tru Q1 > Q25 > Q3 > Q9 > Q11?

Q1-Q2 doesnt look like a differential pair to me, I think the OP-amp plays that role, but I could be wrong.
 
R2 is 3k. The zeners are new.
I now need to replace Q5. I broke one of the leads off. Replacement of greater spec and same package on my pending mouser order.
 
Raccoon1400 said:
EDIT 3: + supply of op-amp is 7V instead of 16V. But why?

EDIT 4: The point between CR1 and CR2 is connected to the point between R55 and R57. This is not shown in the schematic. Also, in the schematic, R57 is shown as a second R55.
Click to expand...

According the PCB layout it looks like CR1-2 is connected between R1 and R57. (wrongly labeled 55 on chematics.)

....CR1
......|
R1-+-R57--OP-amp
......|
....CR2

Any voltage at that conjunktion, shouldn't bee any IMO.

CR1 and CR2 are normal diodes and not accidentily exchanged for zeners?

I'm running out of ideas... :worried:
 
Mannegizen said:
According the PCB layout it looks like CR1-2 is connected between R1 and R57. (wrongly labeled 55 on chematics.)

....CR1
......|
R1-+-R57--OP-amp
......|
....CR2

Any voltage at that conjunktion, shouldn't bee any IMO.

CR1 and CR2 are normal diodes and not accidentily exchanged for zeners?
Click to expand...
Between CR1 and CR2? There was about 30mV, as opposed to 10mV in the other channel. Zener diodes are still diodes, but they are 4148's.

I'm running out of ideas... :worried:
Click to expand...
me too...
 
Raccoon1400 said:
I've checked everything I can think of. No obvious shorts.
Click to expand...

Have you checked all the transistor junctions with a 'diode-test' function in your DMM?

Have you checked all the emitter resistors to be sure none are open?

Just curious, as this amp was one I have some interest in - does it have a speaker protection relay?
 
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