amplifier fix help needed

[mbot]

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!

 

[davenn]

Hi mbot,

audio amps are not my forte, but a few Q's and maybe your answers will help.....

firstly what sort of signal are you feeding into the signal in input ?
a say... kHz tone from a sig generator ? prob only want ~ 1mV or so max before the
following stages are overloaded and possibly produce the distortion you are hearing.

if not that is really what you need to do .... feed a known freq tone at a known level.

I presume you were looking at the signals in those different test places with an oscilloscope ?

cheers
Dave

 

[mbot]

thanks for having a look but i guess i wasted your time. turns out i'm just a poor reader. i had replaced those 0.18 ohm resistors with 18 ohm resistors. caused all my problems. i feel like a bit of an idiot chasing after shadows but i guess i'm a little wiser for it... hopefully.

thanks

 

[davenn]

Ahhhh R37 and 38

yes the resistors in that place in most audio amps are usually very low resistance
less that 1 Ohm and usually hi wattage, depending on the ouotput power of the amp
I have seen some up to 10W rating ... those large white, amd rectangular resistors

good to hear you got it sorted ... hey dont feel like an idiot, we all make mistakes

I'm trying to sort out a 3.5W 10GHz transmit amplifier at the moment that I have damaged by accidentally transmitting with no load on the output of the amp.
Now the amp is going into self oscillation ... ie. its putting out power even without
a drive signal being applied
The amplifier chip a FMM5061 is US$ 110 ouch!!!

take care keep up the experimenting

Dave