Help requested: broken amp - 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 10th August 2004, 02:34 PM   #1
nowater is offline nowater  Australia
diyAudio Member
 
Join Date: Sep 2002
Location: south
Default Help requested: broken amp

Hello, I have a basic AB MOSFET amp that I built from an article some years ago. It has one 2SK134 / 2SJ49 output pair per amp, no parallel output FETs. It has worked well but recently the right channel blew. The output of this channel is now jammed to the negative voltage rail, about -37V. There is zero current through either output device (formerly approx 500mA when the amp was working).

I am assuming the problem is one of the output devices. Is that reasonable?

If so, to push the output voltage to the negative rail, is it more likely the 2SK134 (connected to the +ve rail) has failed, or the 2SJ49 (-ve rail)?

Are there any simple diagnostic measurements I can take, to locate the fault?

Many thanks[IMG]Schematic[/IMG]
Attached Images
File Type: jpg aem6500.jpg (84.2 KB, 159 views)
  Reply With Quote
Old 10th August 2004, 02:41 PM   #2
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Quote:
I am assuming the problem is one of the output devices. Is that reasonable?
Yes..it is reasonable...i suspect one or both outputs are bourned.
Inspect also the source resistor , it can be also bourned.

Regards
__________________
Jorge
  Reply With Quote
Old 10th August 2004, 02:55 PM   #3
diyAudio Member
 
Join Date: Jul 2003
Location: Newcastle, Australia
Mosfets are fairly hardy devices, and not easy to damage under normal operating conditions (a short on a speaker line whilst running at high power could). If one or both of the output devices failed, more than likely it would have blown the rail fuses.

Check the voltages either side of the bias pot - if these are also at or near the negative rail, then problem is further forward of this.

Take a number of voltage readings and post the results referencing the schematic, and we may be able to help track it down.

Cheers
  Reply With Quote
Old 10th August 2004, 02:55 PM   #4
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Most probably the neg side device has failed, shorted. But then you should have SOME current through it. Measure the gate voltages of the mosfets, that should give you an initial pointer.

Jan Didden
__________________
I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason - Daniel Dennett
Check out Linear Audio Vol 7!
  Reply With Quote
Old 10th August 2004, 10:47 PM   #5
diyAudio Member
 
Join Date: Jun 2004
Location: Knoxville
Also check the voltages on the VAS transistor and verify that it is saturated and that it is suppose to be, due to circuit function, [or nonfuntion](check the base & emitter voltages.)

If above is true, check the state of the differential transistors to verify that they are set to cause saturated VAS transistor, and then check the negative feedback loop to see if the "neg. input" transistor in diff. circuit reacts to the 'now -60V' output.

Someware in this loop you should be able to track down the mishap.

Unlikely, but one of the current source devices could have shorted.

overall, this is not a really complex circuit...you will figure it out
  Reply With Quote
Old 11th August 2004, 01:32 AM   #6
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
I'd expect a high current flow if a FET had failed. I am looking at the current source(s) as a possible problem.
  Reply With Quote
Old 11th August 2004, 03:39 AM   #7
nowater is offline nowater  Australia
diyAudio Member
 
Join Date: Sep 2002
Location: south
Thanks all, I will check out your suggestions tonight and report back.
  Reply With Quote
Old 12th August 2004, 12:38 AM   #8
nowater is offline nowater  Australia
diyAudio Member
 
Join Date: Sep 2002
Location: south
OK I have some measurements.

Quote:
Originally posted by Centauri
...Check the voltages either side of the bias pot - if these are also at or near the negative rail, then problem is further forward of this.
On the good channel, +1.77V and -1.55V
On the bad channel, -35V and -35V

Quote:
Originally posted by janneman
Most probably the neg side device has failed, shorted. But then you should have SOME current through it. Measure the gate voltages of the mosfets, that should give you an initial pointer.
On the good channel, +1.9V and -1.7V
On the bad channel, -34.7V and -34.7V

Quote:
Originally posted by cunningham
...on the VAS transistor ...(check the base & emitter voltages.)
...check the state of the differential transistors to verify that they are set to cause saturated VAS transistor, and then check the negative feedback loop to see if the "neg. input" transistor in diff. circuit reacts to the 'now -60V' output.
Sorry, but I'm not sure which one is the VAS
Base voltages on the differential inputs:
Good channel, 0V pos input, 0V neg input
Bad channel, -40.2V pos input, -39.5V neg input

Quote:
Originally posted by anatech
I'd expect a high current flow if a FET had failed. I am looking at the current source(s) as a possible problem.
The CCS is set for 200uA. I checked the voltage drop across R12 (should be 0.66V) and R11 (should be 9.4V):-
Good channel, 0.74V and 10.5V
Bad channel, 0.0V and 0.0V

So, it seems the problem is not the output MOSFETs but some kind of short near the input? I checked the input RCA voltages: for the bad channel I measured -40.2V at both inputs (signal and shield).

I get the feeling I'm missing something blindingly obvious!?
  Reply With Quote
Old 12th August 2004, 02:05 AM   #9
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Looks like you are missing your input ground (top side R7). That would shut down your current sources and pull the output negative. This kills your bias as well.
-Chris
  Reply With Quote
Old 12th August 2004, 04:42 AM   #10
diyAudio Member
 
Join Date: Jul 2003
Location: Newcastle, Australia
On the left portion of the schematic (on the previous page of the magazine article - and not shown in the attached scan) is a note saying "shield must be connected to 0V BEFORE applying power".

It would seem that this connection has come adrift.

If your main power supply earth is connected to chassis earth and mains power earth, then I would recommend connecting the input earths to main earth via 10 ohm 5W resistors anyway to avoid earth loops.

Cheers
Graeme
  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
new builder , guidance requested . udderslate Solid State 8 8th February 2009 05:57 AM
Electrocompaniet EC1 help requested BEARD Solid State 26 1st November 2008 10:33 AM
OT - Computer Guru's Help Requested JMB Multi-Way 0 23rd July 2007 06:33 PM
70l Box Suggestions Requested nayr2 Subwoofers 7 25th October 2005 05:52 PM


New To Site? Need Help?

All times are GMT. The time now is 07:33 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