Need some pointers with finishing touches on mosfet amp repair - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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 17th May 2016, 05:11 PM   #1
Welcome is offline Welcome  France
diyAudio Member
 
Welcome's Avatar
 
Join Date: Mar 2013
Question Need some pointers with finishing touches on mosfet amp repair

Hey dudes.

I just successfully repaired some generic PA MOSFET amp. It had a blown channel and wouldn't come out of protection.

A small fire had developed on the board, ruining a bunch stuff. Replaced all that. Also, one pair of the MOSFETs were blown and I decided to just leave them out entirely.

Anyway. Two things:


1. DC offset is super high (adjustable beteen 600 - 700 mV). Q21 was a component I replaced. Should it be matched with Q20? It's hard to get to so that's why I'm asking instead of doing. How can I improve DC offset?

Idle current seems high. The repaired channel runs way hotter than the original. This is my first foray into MOSFET amps and I don't know where to measure this. The "emitter resistors" seem to give meaningless values even on the stock channel. Where do I measure idle current?

I would love some pointers. Thanks in advance.

Schematic:

Click the image to open in full size.
__________________
  Reply With Quote
Old 17th May 2016, 05:21 PM   #2
diyAudio Member
 
JonSnell Electronic's Avatar
 
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
Send a message via Skype™ to JonSnell Electronic
VR3 is the off set. VR4 the quiescent adjust. Measure the voltage between the source of the top transistor and the drain of the lower one. The do not need to be matched pairs, they are Power MOS Fets.
__________________
Support for Flying Mole Class D, PMC DS-001 (Hypex),Valve Equipment and designs both new and old. www.jonsnell.co.uk
  Reply With Quote
Old 17th May 2016, 05:31 PM   #3
Welcome is offline Welcome  France
diyAudio Member
 
Welcome's Avatar
 
Join Date: Mar 2013
What I meant is, adjusting the DC offset is not good enough. It will only go from around 600 to 700 mV. How do I get the "base" offset down? The stock channel is easily adjustable to 0 mV.

The high idling current is a different issue. It seems way high because the heatsink gets much hotter than the stock channel.

So you're saying I should put the measuring probes on the middle leg (source) of the 2SK1058, and the other probe on the right (drain) leg of the accompanying 2SJ162? I'll try that. Thanks a lot. EDIT: Didn't work. That's just the rail voltage.
__________________

Last edited by Welcome; 17th May 2016 at 05:37 PM.
  Reply With Quote
Old 17th May 2016, 05:44 PM   #4
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
0.1R is a very low value source resistor. Have you measured voltage across both resistors, e.g R71+R72 and then calculated the current for 0.2R?
With possibly 100mA bias per pair, an expected voltage measurement of 20 mV for each pair should be easy enough to read - unless it is unstable for some reason, if you say the measurements are meaningless values. Can you 'scope it?
__________________
Ian
  Reply With Quote
Old 17th May 2016, 05:54 PM   #5
Welcome is offline Welcome  France
diyAudio Member
 
Welcome's Avatar
 
Join Date: Mar 2013
Ah-ha! The only thing I missed to measure, somehow. Measuring across both source resistors, I get around 20 mV on the stock channel. Should be fine then I suppose. I adjusted the fixed channel to the same value. Heat is fairly even now I think.

Now the only problem is getting the base DC offset down. Could a matched pair of Q20/Q21 help?
__________________
  Reply With Quote
Old 17th May 2016, 06:01 PM   #6
Welcome is offline Welcome  France
diyAudio Member
 
Welcome's Avatar
 
Join Date: Mar 2013
No, fixed channel definitely runs hotter still. Burns to touch that part of the heatsink after a couple minutes of operation. Stock channel is hot, but not painfully so.

To clarify: The stock channel has four pairs of outputs, fixed channel only has three. Bias is 20 mV across 0.2R, so 100 mA per pair. I have left the source resistors of the removed pair intact. This may not be the smartest thing?
__________________
  Reply With Quote
Old 17th May 2016, 06:32 PM   #7
diyAudio Member
 
JonSnell Electronic's Avatar
 
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
Send a message via Skype™ to JonSnell Electronic
100mA is rather a lot!
__________________
Support for Flying Mole Class D, PMC DS-001 (Hypex),Valve Equipment and designs both new and old. www.jonsnell.co.uk
  Reply With Quote
Old 17th May 2016, 06:46 PM   #8
anatech is online now anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi JonSnell Electronic,
Quote:
100mA is rather a lot!
Actually, it's the higher end of normal and expected in many mosfet amplifiers.

-Chris
__________________
"Just because you can, doesn't mean you should" my Wife
  Reply With Quote
Old 18th May 2016, 02:04 AM   #9
diyAudio Member
 
mrshow4u's Avatar
 
Join Date: Jul 2006
From the description, it sounds like it could be a leaky device, Q22.
Since your offset adjustment isn't working enough, there could be a leakage current that is dominating the correction current.

From your description, I'm assuming a positive 600-700mV offset. If so, I'd expect a correction current coming from the differential stage. This correction could be measured across resistors, R55&R56. Those 2k2's. With a positive offset, and if the differential pair are doing their job, I'd expect a larger drop measured across R56. This would show that the differential pair is functioning correctly and trying to correct the offset.

If you do see this current difference, then I'd expect a leak in Q22 (or something else wrong with the current source). I'd tend to think that there's not an issue with Q20, since there is bias current (maybe on the high side, if not adjustable).

Good luck.
__________________
".........These go to eleven"
  Reply With Quote
Old 21st May 2016, 09:18 AM   #10
Welcome is offline Welcome  France
diyAudio Member
 
Welcome's Avatar
 
Join Date: Mar 2013
I think I found the problem.

On the both channels there is about 500 mV DC before R54.

On the bad channel, there is about 600 mV DC after it (which is the same value I measure on the speaker output), but on the good one - nothing.

That resistor appears to be open on the bad channel.

It's hard to get to so I haven't replaced it yet.
__________________
  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
Finishing Amp Ripcord Solid State 3 8th September 2015 09:36 PM
Building a Baby Huey EL84 amp - might need some pointers Welcome Tubes / Valves 9 28th April 2014 12:45 AM
Finishing a custom made power amp... Luke123 Solid State 9 18th April 2014 09:57 AM
Amplifier repair, some pointers please? Aragorn Solid State 1 7th September 2011 07:48 PM
Quick question re: finishing up my sub enclosure & mounting driver CUclimber Multi-Way 1 18th April 2004 07:46 AM


New To Site? Need Help?

All times are GMT. The time now is 09:42 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki