samgraysound
Well-known member
JohnRoberts said:
Bias adjust is working now, and Vbe on output transistors is looking normal too.
Tandberg TR2080 troubleshooting
- Thread starter samgraysound
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Bias adjust is working now, and Vbe on output transistors is looking normal too.
sweet.. 8)samgraysound said:
JR
samgraysound
Well-known member
JohnRoberts said:
load testing it...
the "working" channel can put out 45w without distortion, the rebuilt channel can put out 100w. Amp is rated for 75w.
time to rebuild the other channel I guess.
samgraysound
Well-known member
alright I'm confused by these symptoms
The L channel puts out a max of 45w into 4ohms.
It begins flattening the top of the waveform well before the bottom.
It mirrors the Right Channel in output until it reaches that 45w output then it flattens while the Rch has another 50w of headroom.
All transistors test ok.
Tracing back through the circuit with an oscilliscope the problem seems to start at the output of Q901/903, but those transistors test good, and the voltages look fine.
I'm thinking maybe Q901/903 is failing only with a large input but works fine with a smaller one?
The L channel puts out a max of 45w into 4ohms.
It begins flattening the top of the waveform well before the bottom.
It mirrors the Right Channel in output until it reaches that 45w output then it flattens while the Rch has another 50w of headroom.
All transistors test ok.
Tracing back through the circuit with an oscilliscope the problem seems to start at the output of Q901/903, but those transistors test good, and the voltages look fine.
I'm thinking maybe Q901/903 is failing only with a large input but works fine with a smaller one?
The drinking lamp has already been lit here so don't expect good answers...
but... you did have a bad transistor in the current limit circuit, if that circuit is still wonky it could cause the premature clipping in one direction.
JR
but... you did have a bad transistor in the current limit circuit, if that circuit is still wonky it could cause the premature clipping in one direction.
JR
samgraysound
Well-known member
JohnRoberts said:
This is actually the other channel that I thought was working.
The symptom is consistent with bad current limiting in one direction.samgraysound said:
JR
samgraysound
Well-known member
JohnRoberts said:
Ok. Then I will check all the current limiting transistors, replace all those diodes and check the associated resistors. Are Q919 and Q921 current limiting as well?
samgraysound
Well-known member
Alright. Replacing the diodes seemed to solve it. Back to load testing.
Sam
Sam
Diodes are pretty easy to test with modern VOM but cheap... back at Peavey we paid less than 0.01 each for simple signal diodes, but we were using more than 1 million every month.samgraysound said:
Glad it is working .
JR
samgraysound
Well-known member
JohnRoberts said:
A mentor once told me that diodes can test good but still fail under load, so if they are suspect just replace them as they are so cheap anyways. Don't know if that is good advice or not honestly. In this case D911 and D913 both tested dead out of circuit.
I paid $0.019 for my 1n4148s. Nearly double what Peavey was paying!
I can imagine "soft" failures that express differently when reverse biased with significant voltage but these are generally not seeing much reverse voltage (from memory). The most common failure mode for diodes is short circuit in any direction.samgraysound said:
That was what they paid back last century, but what I found remarkable was using over a million per month with no reported failures that I could find (I looked). It is entirely possible that some were bad but did not reveal themselves as used.
I agree our time is worth more than $0.02... so replace them if you got em...
Labor is significant in repair math, even if it's your own labor.
JR
