amplifier fix help needed - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th June 2010, 03:30 PM   #1
mbot is offline mbot  Canada
diyAudio Member
 
Join Date: Mar 2009
Default amplifier fix help needed

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!
Attached Images
File Type: jpg mIXER_AMP.jpg (423.2 KB, 119 views)
  Reply With Quote
Old 27th June 2010, 08:12 PM   #2
diyAudio Member
 
Join Date: Mar 2010
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.
  Reply With Quote
Old 27th June 2010, 09:25 PM   #3
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by bob91343 View Post
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.
  Reply With Quote
Old 28th June 2010, 12:13 AM   #4
mbot is offline mbot  Canada
diyAudio Member
 
Join Date: Mar 2009
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.
  Reply With Quote
Old 28th June 2010, 01:00 AM   #5
Banned
 
Join Date: Jan 2008
Blog Entries: 2
Are all the tracks intact? No change if you flex the board? All the solder joints good?

w
  Reply With Quote
Old 28th June 2010, 06:52 AM   #6
RJM1 is offline RJM1  United States
diyAudio Member
 
Join Date: Jan 2010
Location: Titusville, Fl.
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.

Last edited by RJM1; 28th June 2010 at 07:20 AM.
  Reply With Quote
Old 28th June 2010, 07:38 PM   #7
mbot is offline mbot  Canada
diyAudio Member
 
Join Date: Mar 2009
i think you may have just figured it out. i replaced those with 18 ohm resistors as i guess i was reading the schematic wrong (its very small in real life) and assumed the low reading to be a fault there. i'll put the old ones in as soon as i can and update.

thanks again.
  Reply With Quote
Old 28th June 2010, 07:56 PM   #8
diyAudio Member
 
Join Date: Feb 2010
Location: Bucharest
Send a message via Yahoo to Th3 uN1Qu3
Quote:
Originally Posted by nigelwright7557 View Post
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 !
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.
  Reply With Quote
Old 28th June 2010, 11:30 PM   #9
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by mbot View Post
i think you may have just figured it out. i replaced those with 18 ohm resistors as i guess i was reading the schematic wrong (its very small in real life) and assumed the low reading to be a fault there. i'll put the old ones in as soon as i can and update.

thanks again.
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
__________________
regards Andrew T.
  Reply With Quote
Old 29th June 2010, 01:43 AM   #10
mbot is offline mbot  Canada
diyAudio Member
 
Join Date: Mar 2009
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help needed to fix channel Conrad Johnson Sonographe amp A Sanchez Solid State 2 14th April 2008 07:43 AM
please help to fix a integrated amplifier Stefanoo Solid State 2 12th November 2007 09:03 AM
Attempting to fix amplifier, guidance needed please! LONG 240z4u Solid State 36 11th January 2006 03:27 AM
help needed to fix amp! marko Car Audio 28 19th October 2005 05:53 PM
trying to fix an amplifier rkc7 Everything Else 1 13th May 2004 07:43 AM


New To Site? Need Help?

All times are GMT. The time now is 02:44 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2