Bose 1800 Amp Spare Parts (Amp PCB)
- Thread starter JamesDean59
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JamesDean59
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I replaced R2 with an appropriate resistor and voltage is now at 15v.
However the negative voltage supply to the op-amp is still at -9.6v.
The output pin of the op-amp is at 12.2VDC.
I dont have all of the output transistors installed, I'm wondering if this is having any effect. I noticed that without Q17 there was no -85 supplied to R2,C2,Cr9...I'm curious as if that could be the same for other parts as well...
Installed remaining transistors, no change. still -44VDC at output. Still -9.6 at pin 4 and 12.2 at pin 6.
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Jon_Logan
Addicted Member
Watch out for Bose doing weird things like making B+ connections through the cases of the output transistors.
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.
JamesDean59
Well-Known Member
im not all that familiar with the topology of solid state amp, trying to learn more. Which transistor is the differential amp and which is the vas? I'll take these measurements and get back to you later tonight. I have an order of 2n3904/6s on the way..ill have to find suitable replacements for some of those transistors
JamesDean59
Well-Known Member
Alrighty,
Voltage across:
R5 = .65 V
R6 = .84 V
R9 = .2 mV
Those R5, R6 voltage values appear to be on par with the schematic's numbers. Does that then indicate Q1/Q2 might be leaky?
I did replace Q24 as the part that was installed appeared to be a weaker transistor than the part that was supposed to be in there...
Jon_Logan
Addicted Member
You have a negative output offset and the output of the op-amp (pin 6) has +12. These R5/6 voltages make sense. You see that there is more current through R6 than R5. Q2 is turned on harder than Q1. That means that the differential amp (Q1, Q2) is trying to turn off the flow from the negative side. Without making more measurements, it looks like the differential amp is doing it's job.
This value (0.2/20 = 10mA), indicates that the differential amp has asked this path to conduct less. The complementary path of R15 (0.75/51 = ~15mA) is trying harder to put some positive current into the bias string. To me, this is indicating that there is a short, or leak on the negative side or an open on the positive side. if the amplifier is not drawing a lot of current, it would point to an open in the positive driver side. If the amplifier does draw a lot of current, that would point to a short or leak on the negative half.
What is the voltage across R23? Check Q6 for open.
Ah, Q24 is just another output transistor. For troubleshooting, you could run it without that one, if you're concerned about it causing a problem. You can measure R44 and see if it's similar to R's 41, 42, 43, 45, 46.
JamesDean59
Well-Known Member
I had to return the meters for the day, I'll get back tonight and check these. The amp has been drawing under 1 amp each time I power it up. So This points towards an open on the positive driver side
I removed Q6 from the board and tested it. It showed B-E junction drop of 0.54v. B-C drop of 0.53v. Open E-B. Open C-B. Open C-E/E-C. So this NPN seems to be good.
There is a disagreement between the the PCB layout in the pdf and the amp schematic in the pdf.
The silk screen says Q11 drives Q18-23, the PCB layout in the pdf agrees. So there's a type on the schematic. Q8 and Q24 in the output transistor section should be labeled Q18,Q21.
The Q24 I replaced it attached to Q3 and R8/R9 near the -85V rail.
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JamesDean59
Well-Known Member
I measured voltage across R23, I got 0V. Measured either side of the resistor with respect to ground, was -44.5VDC
there are 2 Q 24s on the schematic - it looks like the output Q24 should be Q21 (based on the numbering of the other outputs and that there is no Q21)Make sure you have good grounds, especially around the input areas.
Quick question about the voltages on Q7 - you have -43 on the base and -44 on the emitter - This transistor is part of the current limiting circuit and should be shut off (less than .1 volts difference between base and emitter - if you measured correct, R34 (one of the .8 ohms) is open and may cause some of your problem (on the positive half the current limiter is based on the current through q15 emier resistor - on the negative side it is done through the parallel R37, 38 and 39. Check voltages base to emitter - sometimes difference of large voltages makes analysis difficult.
Q7, Q8 and Q25 are all part of the current limiting protection and normally should be shut off (little or no voltage drop across the b-e junctions). Good luck.
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Jon_Logan
Addicted Member
0V across R23, eh? I would lift one leg of CR7. This will open the positive voltage clamp of Q7. Tell you what, before you lift one leg of CR7, measure the voltage across R34. If you get 0V, go ahead and lift one leg of CR7. If you get something near 1 V (or not 0), record the number and write back.
If Q7 is shorted, you lose all positive drive and the output section cannot fight the offset that exists.
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JamesDean59
Well-Known Member
Measuring R34, I got 0v. I then lifted a leg of CR7 and was still seeing -44VDC at the speaker terminals. Still 0V across R34 and -44VDC on either side of it. Today or tomorrow I should be getting a bunch of 3904/3906s in I was planning on replacing all the ones on this board.
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Jon_Logan
Addicted Member
Okay, good. No voltage across R34. It's okay to leave CR7 unconnected during the troubleshooting.
I would just push through and continue with the troubleshooting before replacing a lot of stuff.
I know it's aggravating that you're trying a lot of stuff and the amp isn't working, but you're making great progress. ....even if it isn't obvious at the moment.
I would just push through and continue with the troubleshooting before replacing a lot of stuff.
I know it's aggravating that you're trying a lot of stuff and the amp isn't working, but you're making great progress. ....even if it isn't obvious at the moment.
JamesDean59
Well-Known Member
I only bought them because I've used them elsewhere and was out. I plan on continuing the troubleshoot on this amp. I'm a very patient person, especially with this stuff.
So removing CR7 disables part of the current limiting circuitry? Do you still suspect an open on the +85 side? All the output transistors have been replaced. Even when I had only one output transistor installed it still had -44VDC.
This would seem to eliminate Q12-17. What would we look at next?
So removing CR7 disables part of the current limiting circuitry? Do you still suspect an open on the +85 side? All the output transistors have been replaced. Even when I had only one output transistor installed it still had -44VDC.
This would seem to eliminate Q12-17. What would we look at next?
Jon_Logan
Addicted Member
It's a little hard to know, for sure if there's an open causing the offset, or a leaky/shorted path.
Usually, if there's something like a shorted negative output in an amplifier, the differential amp will try to force the positive side to correct the offset. This results in a large current depending on the design.
I think you can go ahead and lift one leg of CR5. That only comes into play (ought to) when the over current sensing is triggered.
I would measure the voltage across R27 (22 ohm). It may indicate that there's a leakage path through Q9
Usually, if there's something like a shorted negative output in an amplifier, the differential amp will try to force the positive side to correct the offset. This results in a large current depending on the design.
I think you can go ahead and lift one leg of CR5. That only comes into play (ought to) when the over current sensing is triggered.
I would measure the voltage across R27 (22 ohm). It may indicate that there's a leakage path through Q9
JamesDean59
Well-Known Member
Measuring R27 before removing CR5 yielded 0V. Sheet calls for -83V.
Removed CR5, no change in offset, R27 is still 0V.
Removing CR5 and CR7 disables the current limiting circuitry right?
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Jon_Logan
Addicted Member
Does lifting Cr5 and Cr7 disable the current limit circuitry? kinda sorta. It doesn't function (at least the positive half), but there's still some resistive paths into the output point.
0V across R27. Okay, that's helpful.
So, here's what we know:
There's a ~ -43V offset at the output
There's a 15mA current (positive) flowing into the bias string.
There's a 10mA current (negative) flowing into the bias string.
The differential amplifier is trying to force a positive shift into the output.
With this, I think you have a leakage path from the -84 rail to the output line.
Measure the voltage across R's 41~46. I think you'll find a voltage drop across 1 (or more) of those.
JamesDean59
Well-Known Member
I measured R41-R46, 0V across each. I also measured R31-36, 0V across them as well. Hrmmm.
Jon_Logan
Addicted Member
Okay. It gets interesting.
So, we know we have a -10mA "feed" from R9
We know that there is 15mA "feed" from R15
There is a -43V offset at the output
There is no currents detected at either set of emmiter resistors.
CR5 is currently open.
Cr7 is currently open.
It's true that this is kind of a "hrmmm" moment, but there's always a reason. It could be a more esoteric kind of a failure.
Is C9 leaky?
Is CR10 leaky?
Is Q3 leaky B-C?
There must be an additional negative current path. You've confirmed that the rail voltages are equal and opposite. These are "fighting" currents. There's more current coming down from R15, but the offset is negative.
So, we know we have a -10mA "feed" from R9
We know that there is 15mA "feed" from R15
There is a -43V offset at the output
There is no currents detected at either set of emmiter resistors.
CR5 is currently open.
Cr7 is currently open.
It's true that this is kind of a "hrmmm" moment, but there's always a reason. It could be a more esoteric kind of a failure.
Is C9 leaky?
Is CR10 leaky?
Is Q3 leaky B-C?
There must be an additional negative current path. You've confirmed that the rail voltages are equal and opposite. These are "fighting" currents. There's more current coming down from R15, but the offset is negative.
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