Yet another Threshold STASIS bias thread - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 30th January 2011, 07:16 PM   #1
diyAudio Member
 
Redwingnine's Avatar
 
Join Date: Apr 2010
Default Yet another Threshold STASIS bias thread

I recently finished a refurbishment on a Threshold STASIS-3. Changed out all the electrolytic caps, new rectifier, new power supply caps, and internal wiring.

Before starting the job, checked the current draw on the unit, and it was drawing six amps at 120V.

The amps sounds fine, except for the fact that the bias is a bit low on the left channel, and the right channel even lower. The problem is, if I just slightly bump up the bias on the right, it quickly gets very high, and the right channel seems to get cooler. I know Jon S. at Vintage Amp Repair has a mod for the bias circuit.

Before I put this puppy back on the bench, was asking for advice as to what to check out on the boards. I know the circuit is a bit finicky, but I seem to be struggling just a bit with this.

Last edited by Redwingnine; 30th January 2011 at 07:18 PM.
  Reply With Quote
Old 30th January 2011, 10:11 PM   #2
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
That 6 amps you measure should be creating some real
heat somewhere. If accurate, that is too much.

The channels should operate at about 50 deg C after a
couple hours. Touchy bias might call for a new bias pot.

  Reply With Quote
Old 30th January 2011, 10:17 PM   #3
diyAudio Member
 
Redwingnine's Avatar
 
Join Date: Apr 2010
Thank you, Mr. Pass

The 6 amp reading was before installing replacement parts (It's nowhere near that now).

Based on your advice, I should probably change out the bias pots.

I'll order a pair, change them out, and try to adjust again.
  Reply With Quote
Old 6th February 2011, 06:51 PM   #4
diyAudio Member
 
Redwingnine's Avatar
 
Join Date: Apr 2010
After checking out the circuit, will also change out the bias transistors as well as the pots. That should resole the issue, (I hope).
  Reply With Quote
Old 7th February 2011, 02:17 AM   #5
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Faulty bias pots will result in low bias, not high. Also,
it's highly unlikely that the bias transistor needs replacement.

  Reply With Quote
Old 7th February 2011, 08:12 PM   #6
diyAudio Member
 
Redwingnine's Avatar
 
Join Date: Apr 2010
Much thanks, Nelson.

I've ordered the parts, so I'll just replace the bias pots first, and verify operation.

On a side note, just got done re-doing a S200 with optical bias, and what a great sounding amp! Put in new electrolytic caps, new rectifier, cardas jacks and posts, and adjusted bias back in spec. One side feels a bit warmer than the other, but they measure the same. Incredible sounding amp.

Now, once the bias on the STASIS-3 is resolved, would reckon it will sound similar to the S200.
  Reply With Quote
Old 7th February 2011, 09:42 PM   #7
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
It's different. You'll see...

  Reply With Quote
Old 8th February 2011, 10:31 PM   #8
diyAudio Member
 
Redwingnine's Avatar
 
Join Date: Apr 2010
I am very much looking forward to comparing the differences once the STASIS-3 bias is fixed.

I'm still amazed just how good the S200 sounds with the refurbishment.
  Reply With Quote
Old 17th February 2011, 09:28 PM   #9
diyAudio Member
 
Redwingnine's Avatar
 
Join Date: Apr 2010
OK, ran into a snag.

Decided to check the bias transisitor. The schemitic says it should be a MPSA42, so I ordered a pair. Well, when I take it out, it's a MPS6571. The hfe on this guy is over 600.

So, does anyone know what the story here is? What transistor is supposed to be in the bias circuit. What should the normal hfe for the bias transistor be? Was there a mod done at the factory, as the transisitor looks orignal. Could this be a manufacturing flaw?

Last edited by Redwingnine; 17th February 2011 at 09:43 PM.
  Reply With Quote
Old 18th February 2011, 12:20 AM   #10
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
No reason to worry about it. A generic NPN with reasonably high
gain will be fine.

  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
threshold stasis 2 docjoe Solid State 3 10th November 2012 12:05 PM
Threshold Stasis 2 Class A Power Amp - bias setting? speakerfritz Solid State 19 12th February 2011 03:28 AM
Threshold Stasis 2. steenoe Pass Labs 52 29th August 2007 06:59 AM
Another Threshold Stasis 2 rebuild AMV8 Pass Labs 2 22nd December 2006 05:13 AM
Threshold Stasis 3, too Roscor61 Pass Labs 3 20th May 2004 09:12 AM


New To Site? Need Help?

All times are GMT. The time now is 11:35 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