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 and way smaller than i would expect
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 greatly appreciated.
thanks!
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 and way smaller than i would expect
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 greatly appreciated.
thanks!
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You have some bad parts still. Shut the thing off and test every part, especially the semiconductors. There may be an open resistor too or perhaps a shorted capacitor. Only serious patient testing with an Ohmmeter will unearth the defective components. The best Ohmmeter to use is an analog type, which seems to be a rarity these days.
I would whip out all the semiconductors and test them on a multimeter.
Check the hfe too, I got caught out with a transistor that had an hfe of 1 once !
While they are out check all electrolytics for shorts and resistors for value.
Might be worth testing all diodes and zeners while it is still powered up.
thanks for the input,
here's how i spent my afternoon....
tested all resistors for a second time, taking a lead out for any that seemed off. all tested well.
i replaced the electrolytic caps just cuz i had them on hand and to get that out of the way
i tested all the other caps, pulling one lead out, none are short.
i tested all the transistors with a multimeter and they all read the same as unused parts when out of circuit. since i had them out anyway, i put new ones in just in case. (this project is getting to the point where i'm okay with brute force)
and after all that, i'm still getting the same results. one thing i didn't mention before is that the signal i'm putting in i made about the same level as a guitar signal but if i bring it down lower i can get it to a point where it sorta pops back and forth between clean amplification and the crazy sqarewavey output.
i've never tested regular diodes with the power on before. would i just be looking for an appropriate voltage drop? i checked that and they are all over the map, between 0v and 3v for the 1n4148's, the bav21 reads 43v, the zeners all have the right voltage except the 33v one which measures half that.
thats again for any help you have to offer.
here's how i spent my afternoon....
tested all resistors for a second time, taking a lead out for any that seemed off. all tested well.
i replaced the electrolytic caps just cuz i had them on hand and to get that out of the way
i tested all the other caps, pulling one lead out, none are short.
i tested all the transistors with a multimeter and they all read the same as unused parts when out of circuit. since i had them out anyway, i put new ones in just in case. (this project is getting to the point where i'm okay with brute force)
and after all that, i'm still getting the same results. one thing i didn't mention before is that the signal i'm putting in i made about the same level as a guitar signal but if i bring it down lower i can get it to a point where it sorta pops back and forth between clean amplification and the crazy sqarewavey output.
i've never tested regular diodes with the power on before. would i just be looking for an appropriate voltage drop? i checked that and they are all over the map, between 0v and 3v for the 1n4148's, the bav21 reads 43v, the zeners all have the right voltage except the 33v one which measures half that.
thats again for any help you have to offer.
What did you replace R37,38 with? They are 0.18 ohm resistors and should check less than an ohm. That could be your problem if you replaced them with a higher value you would not be able to set the bias current ( 7mv test voltage ) and your output would be very distorted. You said that you could not lower the test voltage below 330mv if R37,38 are the correct 0.18 ohm your idle current would be 1.833 amps and your output transistors would be dissipating 80W each (very hot). At least you now have RT1 (bias adjustment ( 7mv)) set to the minimum so that when you replace R37,38 with the correct value you just have to turn it up to 7mv.
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You're not the only one. Last year, i struggled for over a week with an amp that just wouldn't get maximum output on one channel, and it was one of those pesky little transistors that ran out of hfe.
that's why we write 0r18 or 0.18r to differentiate that from .18r and 18r
Never drop the leading zero from values less than 1
yep, that was it. feeling like an idiot right now but happy its working again. thanks for the help. it would have taken me a while to go back and have a second look at that. i had switched those out right neart the beginning of this process when it was still starting fires when you turn it on. never would have thought twice i read the number wrong....
thanks everyone.
thanks everyone.
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