Pass X250.5 right channel hum problem

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Nelson Pass

Nelson Pass

The one and only
Joined 2001
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Depends on the version. Stereo units have 4 output stages (2 balanced for
each channel) and thus 4 adjustments.

Since we don't know the version, the generic procedure is:

Locate the Source pins for N channel and P channel for each of the 4 stages
and measure the voltage from the N Sources to the P Sources. You should
see numbers in the region of 100 to 200 mV.

Check all 4 output sets and find out which one is different. Then you adjust
the bias potentiometer for that one.

Clearly all this implies you know what you're doing. If not, there could be
problems.

I recommend that you simply arrange for the factory to look at it.

:cool:
 
rubli

rubli

Member
Joined 2008
afaudiolab.com
thank you Mr. Pass for the indications.

sending such a heavy and expensive amp from Mexico, is simply a nightmare.

there are 5 adjustments on the left board, 4 of them I presume are dc and gain for each phase, and there is a 5th which is the meter adjustment.
on the left board, there are only 4 adjustments.

there was a dc imbalance which I corrected, but no adjustment varies the source to source DC ( I have a lecture of 45 mv on each channel)
the right heatsink gets warmer than the left side.

here are the photos of each channel of my unit

thank you for your help

best regards

Alexander
 

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wayne

wayne

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Joined 2002
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You can also measure just across the .47 Ohm resistors and compare the values. Measure one in each corner of the amplifier and then adjust using the small multi turn pots. A couple of meters are helpful and you must be careful if you turn and see no change stop you are looking at the wrong spot. The two pots on the small boards are used for overall offset and differential offset. One controls the offset from ground and the other sets the difference between red and black terminals also note the boards may face different directions due to the layout.
 
rubli

rubli

Member
Joined 2008
afaudiolab.com
Dear fellows, for an unkown reason one of the phases of the right channel went up to smoke, I replaced all six OP mosfets with Vishay/siliconix, and the gate resistors with 1% metal film resistors (as well as the original .47 panasonic resistors).
I adjusted the bias to 50mv on each phase, the dc on the small board, and everything was going OK. the heatsinks were getting comfortably warm, after 15 minutes

I conected speakers and listened to it for 5 minutes, and that phase went smoke again, the mosfets weren´t hot, but all were gone as well as 3 of the gate resistors ( the ones at teh IRFP240´s), and of course all .47's

what bothers me most is that I have no clue why did it happen. I have another bunch of repalcement parts , but I am afraid that same thing would happen.

I suspect a high frequency oscillation, maybe i should use carbon film gate resistors ?

any hint will be appreciated

thanx

A.
 
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