Yet another Threshold STASIS bias thread

[Redwingnine]

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.

 

[Nelson Pass]

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.

 

[Redwingnine]

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]

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]

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

 

[Redwingnine]

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]

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

 

[Redwingnine]

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]

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]

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

 

[Redwingnine]

Thanks, Nelson

I'll go through and install two with reasonably high gain, along with the two new pots.

 

[Redwingnine]

Much thanks, Nelson

After changing out the bias pots, along with one of the bias transistors, the amp now biases up perfectly, and is making very nice sounding music.

I've put it to use as a rear channel amp for the home theater setup.

By the way, Nelson, you were spot on when you said that the STASIS-3 and S-200 would sound different once they were both biased correctly (they very much do, more than I would have thought). The STASIS-3 is very nice, but the S-200 is truly remarkable sounding by comparison. (The S-200 is one of the most natural sounding amps I've ever heard).

Now, I've got to get to work on finishing the 400A.