amplifier troubleshooting question - 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 16th August 2003, 06:04 PM   #1
diyAudio Member
 
Join Date: Mar 2003
Location: Willmar, Minnesota
Unhappy amplifier troubleshooting question

Hi,

I am currently building a clone of the M250 pictured here: http://www.astro.uu.se/~marcus/private/m250.html

but have stumbled into some difficulties. At first I build the front-end (everything but the 8 transistors in parallel and everything worked perfectly. I applied a 1khz signal to the input and got a beautiful replica of the 1khz signal on the output. Once that was working I decided to connect the front-end to the current amplifier but when I applied power the current ballast resistors started smoking. I am using 5W .33 OHM resistors like the schematic shows but no luck. While Iím doing this test I have no signal feeding the input nor do I have a speaker or dummy load connected across the output. Any ideas would be greatly appreciated!

Thanks,
Randy
  Reply With Quote
Old 16th August 2003, 09:00 PM   #2
diyAudio Member
 
john curl's Avatar
 
Join Date: Jul 2003
Location: berkeley ca
When first testing, USE A LIGHTBULB in series with the AC line. No kidding! This is a life saver. You will have to make up your own lightbulb socket, but make sure that it is in SERIES of the amp. This is a very old technicians' trick. Think about it: A 100-250W 120 volt bulb will have a very low COLD resistance. If the needed idle current is reasonably low, the lightbulb will stay relatively off and still have a low resistance. However, if the amp requires lots of idle current ( a dangerous condition) then the bulb will heat up and become high resistance, limiting the absolute current through the whole amp. It makes the amp last much longer, before breaking and usually you can even determine what the problem is, while still running the amp (sort of). I hope that this helps.
  Reply With Quote
Old 17th August 2003, 04:07 AM   #3
Diode is offline Diode  United States
diyAudio Member
 
Join Date: Feb 2003
Location: Oklahoma USA
Hey Randy,

I wish I could see the damned schematic. It's too big for my poor video card I guess. 32MB video card should be enough shouldn't it? Oh well..... Check for DC on the output. What polarity is it. This sometimes gives clues where to look. The lightbulb trick is a good one or a Variac will do also. If no DC on the output, check output for oscillation. If oscillation, put a 100pF cap from base to collector of your drivers.... This will get you there quicker. If you set the bias without the output transistors in circuit, this will cause big biasing problems. Turn the bias all the way DOWN, connect the output stage, and slowly power the unit. Sorry I can't be more helpful. I can't see the diagram.....

Chris
__________________
The best audio is clean audio!
  Reply With Quote
Old 17th August 2003, 04:24 AM   #4
diyAudio Member
 
Join Date: Jul 2003
Location: Newcastle, Australia
Hi,

Suggest you check your wiring VERY carefully. As there is no connection between R22, T8 or T9 and the output line when the output transistors are not connected, you should not have seen a signal on the output line during your initial test.

Suggest you remove the output transistors and connect two 100ohm 1W resitors, one from T8 emitter to ouput line and one from T9 emitter to output line. You should see a maximum of 0.6V DC across each of these resistors, and 0V DC on the output line. The pot P1 should adjust the voltage across the extra resistors. If all ok here, amplifier should amplify input signal with no load.

let us know results of this check

Cheers
  Reply With Quote
Old 17th August 2003, 05:11 AM   #5
diyAudio Member
 
Join Date: Sep 2002
Location: California
I would check the setting of P1, which generates the bias voltage for the output stage and thus controls the its idle current. P1 should be set to its maximum value. If it is set to its minimum value, the idle current will be excesive and could blow up the output stage. To adjust P! you will need to put an ameter between one of the power supply rails and the amp. Decrease the resistance of the P1 until the desired idle current is achieved. Let it run for a while to stabalize thermally and then reset it if the current has drifted. It may take several itterations to get it stabalized at the value you want.
  Reply With Quote
Old 18th August 2003, 10:41 AM   #6
diyAudio Member
 
Join Date: Jun 2001
Location: Flanders, Belgium
Quote:
Originally posted by Greg B.
I would check the setting of P1, which generates the bias voltage for the output stage and thus controls the its idle current. P1 should be set to its maximum value. If it is set to its minimum value, the idle current will be excesive and could blow up the output stage. To adjust P! you will need to put an ameter between one of the power supply rails and the amp. Decrease the resistance of the P1 until the desired idle current is achieved. Let it run for a while to stabalize thermally and then reset it if the current has drifted. It may take several itterations to get it stabalized at the value you want.

indeed I guess this is the problem!

there's also a thread about this amp: link

best luck!

HB.

EDIT: I remember this amp has a short-circuit protection, why isn't it working in this case??
  Reply With Quote
Old 18th August 2003, 04:38 PM   #7
diyAudio Member
 
mrfeedback's Avatar
 
Join Date: Feb 2002
Location: Perth, Australia.
Default Hey, Who Said I Am An OLD Technician...

Quote:
The lightbulb trick is a good one or a Variac will do also.
Use both with a 60W lamp and power one channel at a time for initial no load testing.
Up the lamp wattage when connecting a load - if you have cross-conduction or oscillation problems the lamp will limit the fault currents and save your expensive output transistors.

Eric.
__________________
I believe not to believe in any fixed belief system.
  Reply With Quote
Old 19th August 2003, 12:47 AM   #8
diyAudio Member
 
john curl's Avatar
 
Join Date: Jul 2003
Location: berkeley ca
Dear old technician, I am one as well, from so many decades ago. I learned this trick from a REAL technician, and sometimes I forgot about it, over the years. Well, once, in 1978, I had a new amp design that I had to make it work for a CES. It ALWAYS blew up. In frustration, I hired the tech who had trained me and he brought his own light bulb assembly. Guess what? My personal tech, FORGOT to put the transistor insulators between the driver transistors and the heatsink. We found the problem and it went to the show.
  Reply With Quote
Old 19th August 2003, 04:37 AM   #9
Diode is offline Diode  United States
diyAudio Member
 
Join Date: Feb 2003
Location: Oklahoma USA
Damned techs....

Yup, I know the feeling. At least he got the polarity right. Sometimes I get the polarity wrong especially when there is no silkscreen and the mechanical drawing is ****-poor. (Galian-Kruger) is terrible about that as well as OLD Peavey stuff! Oh well, Sometimes it is my job to let the smoke out of parts that have too much smoke pressure. My boss once told me "Well, there ain't any ****** left in it anymore...." I scratched my head and he said "Yeah! you blew the ****** out of it!" Funny-guy, funny-guy........ He always says that when people blow up their speakers and are in for re-cone.

Chris
__________________
The best audio is clean audio!
  Reply With Quote
Old 19th August 2003, 04:42 AM   #10
diyAudio Member
 
mrfeedback's Avatar
 
Join Date: Feb 2002
Location: Perth, Australia.
Default Oldie But A Goodie....

Hi John,
Yeah I fix a lot of big Pro PA amplifiers, and I do not dare to run them up after a rebuild without the Lamp/Variac current limiting - if all those output transistors go splatt I get to buy a new set out of my own pocket and that blows any profits on the job.
I also have a very large Variac (3kVA) and this I use to run up things like Krell and other high current Class A amps.
I have never left the insulators out, but a fine solder bridge left can wreak expensive havoc, and I am not interested in the anxiety factor when running up a new or repaired amplifier.
Also these big amplifiers are needed as soon as they are repaired, and a few days delay getting more parts can be a problem too.
Glad to know that I am not the only 'old tech' who makes the rare mistake.

Eric.
__________________
I believe not to believe in any fixed belief system.
  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
Cello Amplifier troubleshooting morrish Solid State 76 8th January 2014 02:11 PM
Amplifier Troubleshooting. 5th element Solid State 181 15th April 2009 05:20 PM
Help me troubleshooting amplifier module fosforo Solid State 15 7th November 2006 11:59 AM
help troubleshooting amplifier/receiver please JMB Solid State 6 25th October 2004 05:14 PM
amplifier troubleshooting sriegel Solid State 0 6th April 2002 06:28 PM


New To Site? Need Help?

All times are GMT. The time now is 11:21 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