amplifier fix help needed

[martbot]

hi everyone. so, i'm trying to fix a mixer that a friend gave me. its nothing fancy but something mildly disposable to learn on, traynor 6400 series 2. right now i'm focusing on getting the main amp in it working again as it seems the preceeding sections are still working well. i've been going at it for a while and am stumped as to where to look next. here's what i know.

it broke because my friend accidentaly plugged a powered signal into the line in, there was smoke and it wouldn't power up at all. i replaced all the obviously once on fire components and it powers up and you get sound through but it sounds like its overdriven/distorted. i unplugged the mixer stages and put a signal direct to the main amp and it sounds the same.

on the main amp board i replaced a couple resistors that tested way low R37, R38.

it appears to have good power 44.1v on the rails

LD1 turns on when signal is applied

the signal on the emitter of Q1 (sinewave) is missing the top half

signal on the base of Q2 and just about everywhere else is more like a squarewave

the test voltage indicated on the schematic is way way high, the lowest it can adjust is 330mv when it should be 7mv

Q6,7,8 seem to have little voltage across them the largest reading is 2.12v compared to 45v everywhere else.

it does occasionally amplify well for brief moments, like a half second

all parts appear to test well except ZD1 which only measures a voltage drop of 15.5v where i would expect 30v and D1 which is dropping 36v. both of these i have replaced but nothing changed.

the output transistors are not the part number indicated on the schematic, they are actually mjh11021 and mjh11022, the signal at that point looks like a squarewave on one, the other is square on the top and kind of an elogated square with a slightly rounded end on the bottom

thats all i can think of right now. any thoughts would be gretly appreciated.
thanks!

 

[unclejed613]

R37,38 are 0.15 ohms, they should read low. if they read lower than that, then the output transistors are shorted B-E.

don't worry much about the waveforms on Q1E or Q1C, same goes for Q2, and the Q3B and Q3E. the "signalss" in these parts of the amp are currents, not voltages, and voltage waveforms are going to look distorted.

the idle current of the output transistors is what you are measuring when you measure the voltage at R37 and R38. yes, your current is too high and is apparently about 10A. Q5 and the resistors, etc around it are your bias transistor. either the transistor or one of the resistors around it are open (or Q5 is shorted and R14 or R26 is open

Q6,7,8 are protection transistors, and since they are in full-on operation seem to be working.

ZD1 looks to be a method of protecting the input stage from large negative viltages

Q1,2 form a differential pair Q1 is the non-inverting input and Q2 is the inverting input. the current from Q1C sinks through Q3 and operates in it's linear range. as the current through Q3 changes, the current into Q3C determines the voltage at the bases of the output transistors. Q5 acts as a variable zener to keep the bases of Q9, Q10 biased between 0.7 to almost 3 volts different, while passing the AC signal to the bases of Q9,10. solve the reason for the high current in the outputs, and you'll probably solve all of the other problems

Q5 or the resistors around it are probably the key to the whole thing.