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Bose 1801 Monster Power Amp Restoration!

When you have an amp with a dc offset, there are generally 3 causes:
1. Something is shorted on the half of the drive that drives to the polarity that the offset is; a positive offset could be a short in the positive half of the drive.
2. Something is open or a transistor isn't working like a transistor anymore in the opposite half of the drive; a positive offset can be caused by a lack of drive in the negative half of the amp.
3. Failure of the feedback circuit or differential pair.
You have a negative offset. The first thing to check would have been the voltages on Q5 to see if it was being turned on. If replacing Q5 doesn't fix the problem, measure the voltages on E, C, B.
 
dr*audio said:
When you have an amp with a dc offset, there are generally 3 causes:
1. Something is shorted on the half of the drive that drives to the polarity that the offset is; a positive offset could be a short in the positive half of the drive.
2. Something is open or a transistor isn't working like a transistor anymore in the opposite half of the drive; a positive offset can be caused by a lack of drive in the negative half of the amp.
3. Failure of the feedback circuit or differential pair.
You have a negative offset. The first thing to check would have been the voltages on Q5 to see if it was being turned on. If replacing Q5 doesn't fix the problem, measure the voltages on E, C, B.
Click to expand...
Thanks!

The collector voltage on Q5 was wrong. Two volts off the -ive rail (-80V), instead of +2V. This transistor is supposed to be supplying that voltage, right? (Open loop voltage gain?) And it isnt. But it tested good on diode check. Just placed a mouser order, which includes a replacement Q5.

I'm getting the impression that:
-these issues can be hard to track down, and they can throw lots of voltages out of wack, making it hard to track down. Spent time trying to figure out why the + supply to the opamp was too low. Didn't seem to get me anywhere.
-a part could be causing the issue yet test good with diode check.

Remember that marantz quad you helped me with? That was fixed by replacing parts that tested good on diode check.
 
bktheking said:
Sometimes a diode check isn't enough if the transistor is leaking between junctions.
Click to expand...

I think that's what was happening in the marantz.

Can a semiconductor test good on diode check, but then break down (or go open) when it is under greater load in the circuit?

How common is it for semiconductors to fail open?

It would be helpful to know more about the various ways components can fail. There is a lot of background knowledge that goes into diagnostics.
 
Raccoon1400 said:
Thanks!

The collector voltage on Q5 was wrong. Two volts off the -ive rail (-80V), instead of +2V. This transistor is supposed to be supplying that voltage, right? (Open loop voltage gain?) And it isnt. But it tested good on diode check. Just placed a mouser order, which includes a replacement Q5.

I'm getting the impression that:
-these issues can be hard to track down, and they can throw lots of voltages out of wack, making it hard to track down. Spent time trying to figure out why the + supply to the opamp was too low. Didn't seem to get me anywhere.
-a part could be causing the issue yet test good with diode check.

Remember that marantz quad you helped me with? That was fixed by replacing parts that tested good on diode check.
Click to expand...
Need to know the base and emitter voltage in order to know if the transistor was being turned on by the circuit. It is common for transistors to measure good on the diode check, but they won't pass current from collector to emitter, or vice versa.
 
I recently repaired some Carver amplifiers, all of the commutator transistors checked OK with a DMM but when tested on a transistor analyzer they were were dead. Definitely screws with your head.

Craig
 
Mannegizen said:
Q1 and Q2 are a differential pair, my bad.
Click to expand...

A bit confused because although Q1 and Q2 appear to be a differential pair, they are not even the same part #.
I want to replace Q1/Q2. Trying to figure out what part to use and how closely they need to be matched. Q1 is MM-4001, Q2 is 2N3906. The 2N2609 is rated for 200mA. I'd use a pair of ksa992 if it would meet that spec. Would it even see that much current in an input stage? KSA1013 would meet that spec, but gain is not high.
 
Raccoon1400 said:
A bit confused because although Q1 and Q2 appear to be a differential pair, they are not even the same part #.
I want to replace Q1/Q2. Trying to figure out what part to use and how closely they need to be matched. Q1 is MM-4001, Q2 is 2N3906. The 2N2609 is rated for 200mA. I'd use a pair of ksa992 if it would meet that spec. Would it even see that much current in an input stage? KSA1013 would meet that spec, but gain is not high.
Click to expand...

Let's take this a step at a time. Replace Q5 and see if that fixes it. If not I need the BCE voltages on Q5.
Q1 and Q2 are a diff pair. I don't know why they are different parts but they should be the same and matched. I can give you a part number to use tonight.
 
dr*audio said:
Let's take this a step at a time. Replace Q5 and see if that fixes it. If not I need the BCE voltages on Q5.
Q1 and Q2 are a diff pair. I don't know why they are different parts but they should be the same and matched. I can give you a part number to use tonight.
Click to expand...

Q5 is in a box that is currently at the local fedex center, according to tracking. Will be in the unit for tonight. I will be resolving the DC issue before replacing any other parts, I do want to know what was responsible.
 
Damn! Because apparently it is next to impossible to order $300 worth of parts and get it all right, I have two types of NPN and no PNP for Q5. But I borrowed from the other channel and no dice.

Q5: Emitter: 79V approx. Base is about 0.6V (diode drop) below E. C is -76V. (So B and E are correct, C is not) (With 200W DBT)
 
Raccoon1400 said:
Damn! Because apparently it is next to impossible to order $300 worth of parts and get it all right, I have two types of NPN and no PNP for Q5. But I borrowed from the other channel and no dice.

Q5: Emitter: 79V approx. Base is about 0.6V (diode drop) below E. C is -76V. (So B and E are correct, C is not) (With 200W DBT)
Click to expand...

Look for a shorted transistor in the lower half, then.
 
I'm thinking now Q3? EBC: -79.5, -78.9, -79.6
Meter doesn't read short though. Replace and test? But I don't seem to have a good replacement handy.

2N6175: 250V, 2A. TO-225

I have a blowup of the schematic pinned to the wall over my bed.
 
Last edited:
Should be -1.4 volt on collector according to chematics, but doesn't mean Q3 is faulty. it's just probably feed with -79v.

Check voltages on Q25 and Q8.
 
Hang in there Racoon1400! You will get it! You are on the right path thinking about how components can fail, and what they do when they fail. I'm very glad it's you and not me troubleshooting it. I don't think I have the patience for this kind of work anymore. Too many years away from component troubleshooting.
 
Raccoon1400 said:
I'm thinking now Q3? EBC: -79.5, -78.9, -79.6
Meter doesn't read short though. Replace and test? But I don't seem to have a good replacement handy.

2N6175: 250V, 2A. TO-225

I have a blowup of the schematic pinned to the wall over my bed.
Click to expand...

This means Q3 is being turned on hard. There is 0.6V B - E. That means it's on.
This is driven by the front end, which means the problem is there. What voltage do you have on U1 pin 6, U1 pin 3, TP7, next to R1, and Q1 and Q2, EBC?
 
Mannegizen said:
Couldn't a short on Q9 give the same voltages on Q3?
Click to expand...

The fact is that the .6V B-E indicates that Q3 is being turned on. If there was a short on Q9, and it caused the negative offset on the output, then the feedback would force Q1 and Q2 to try to correct for the offset, and turn Q3 off. Since this isn't happening, the likely cause of the problem is Q!, Q2 or U1.
 
dr*audio said:
The fact is that the .6V B-E indicates that Q3 is being turned on. If there was a short on Q9, and it caused the negative offset on the output, then the feedback would force Q1 and Q2 to try to correct for the offset, and turn Q3 off. Since this isn't happening, the likely cause of the problem is Q!, Q2 or U1.
Click to expand...

U1 is good. I swapped them left to right with no change to either channel.

So maybe Q1 or Q2. What is the best way to proceed? Measure voltages on Q1/2? Replace? If so, what part to use?
 
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