Strategy to track down a short - 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 15th April 2015, 01:53 AM   #1
Bcjm is offline Bcjm  United States
diyAudio Member
 
Join Date: Mar 2015
Default Strategy to track down a short

My integrated amp has a short that causes the fuse to blow. I have replaced the output transistors on one channel because they were shorted. The driver transistor seems fine so I did not replace. The light bulb in DBT were bright when I turned the amp on so the root cause is there there somewhere. When I disconnected the driver from the bad channel, the light bulb in DBT was dim. That makes me believe the other channel is working.

I am thinking I can measure and compare the voltage or ohm of two channels at different points to find out the defective component. But since there is a short the readings may not be reliable. Any other suggestions?

Thanks. Bob
  Reply With Quote
Old 15th April 2015, 05:54 AM   #2
diyAudio Member
 
Join Date: Jul 2003
Location: Dona paula, Goa
You have to go towards the frontend, as in some circuits, the input transistors can also blow as a result.

Gajanan Phadte
  Reply With Quote
Old 15th April 2015, 05:59 AM   #3
Cortez is offline Cortez  Hungary
diyAudio Member
 
Cortez's Avatar
 
Join Date: Dec 2002
Location: Hungary
As the first step measure all of your resistors and semis (in diode mode with your DMM) to check for bigger fails.
Most of the cases this is enough to locate the error.
  Reply With Quote
Old 15th April 2015, 02:21 PM   #4
Bcjm is offline Bcjm  United States
diyAudio Member
 
Join Date: Mar 2015
Can a shorted input transistor cause the fuse to blow without any input signal?
  Reply With Quote
Old 15th April 2015, 03:54 PM   #5
Cortez is offline Cortez  Hungary
diyAudio Member
 
Cortez's Avatar
 
Join Date: Dec 2002
Location: Hungary
I would say yes.
  Reply With Quote
Old 15th April 2015, 08:30 PM   #6
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
I would say no but problems like the bias circuit and a shorted Vbe multiplier could drive the output to full bias so yet again the bulb will be bright but the cause will be laying elsewhere ....

So the title of your post might be wrong , your method seems to be also wrong measuring a few things like bias, proper rail voltage, state brand and model and help is on the way ...

remember that a common failure around bipolar transistors is that may measure OK outside the circuit but fail under voltage

Kind regards
Sakis
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 15th April 2015, 09:02 PM   #7
Bcjm is offline Bcjm  United States
diyAudio Member
 
Join Date: Mar 2015
My bias voltage does show significant difference between two channels. What do you think I should check?
  Reply With Quote
Old 15th April 2015, 09:04 PM   #8
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
A common cause of blown fuses is a shorted supply cap...

If you can operate the amp with the short circuit error without blowing fuses (so run it at low voltage or through a light bulb tester) you may be able to find the short by measuring between various points on the supply and ground lines. Basically, you're looking for a large voltage drop across any part of the ground or supply line impedance. That may give you some clues.

A shorted Vbe multiplier would drive the output bias to 0 A. That would work rather well for protecting the amp. An open Vbe multiplier would crowbar the output stage.

Tom
__________________
Modulus-86: Composite amp achieving 0.000067 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com - Engineering : : Done : : Right
  Reply With Quote
Old 15th April 2015, 09:42 PM   #9
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
shorted vbe with a blown resistor next to it or a shorted driver at base is more likely but my suggestion was a pointer not the exact description ...

shorted capacitor ...???/ never found one and today we are just a bit over 1000 audio devices per year at east electronics ..shorted bridge i would say far more common ....
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 15th April 2015, 09:49 PM   #10
Bcjm is offline Bcjm  United States
diyAudio Member
 
Join Date: Mar 2015
I have adjusted the good side bias to 15 mv based on the service manual.
The bad side stayed in 44 mv and the adjustment pot does not change the reading.
  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
Could someone tell me more about the Mesa Boogie Strategy 400? JZatopa Tubes / Valves 14 9th February 2014 08:32 AM
MESA STRATEGY BIAS MOD blackarab Instruments and Amps 7 19th November 2012 05:37 PM
MESA STRATEGY 400 blackarab Tubes / Valves 7 15th November 2012 02:14 PM
Best strategy to convert Aleph 3 to monoblock Sony Pass Labs 6 23rd April 2012 06:51 PM


New To Site? Need Help?

All times are GMT. The time now is 04:15 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2