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-   -   Yet another Threshold STASIS bias thread (http://www.diyaudio.com/forums/pass-labs/182177-yet-another-threshold-stasis-bias-thread.html)

Redwingnine 30th January 2011 08:16 PM

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. :confused:

Nelson Pass 30th January 2011 11:11 PM

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.

:cool:

Redwingnine 30th January 2011 11:17 PM

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.

Redwingnine 6th February 2011 07:51 PM

After checking out the circuit, will also change out the bias transistors as well as the pots. That should resole the issue, (I hope).

Nelson Pass 7th February 2011 03:17 AM

Faulty bias pots will result in low bias, not high. Also,
it's highly unlikely that the bias transistor needs replacement.

:cool:

Redwingnine 7th February 2011 09:12 PM

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.

Nelson Pass 7th February 2011 10:42 PM

It's different. You'll see...

:cool:

Redwingnine 8th February 2011 11:31 PM

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.

Redwingnine 17th February 2011 10:28 PM

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?

Nelson Pass 18th February 2011 01:20 AM

No reason to worry about it. A generic NPN with reasonably high
gain will be fine.

:cool:


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