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Old 24th July 2011, 12:21 PM   #1
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Join Date: Feb 2008
Default Linx Vega power amp...

Hi,

I'm facing a very interesting problem:

The amps run ok, then they made a "plopp" and the relais disconnected.

For me things seemed clear: One output fet may got killed.

Measuring DC Values, J50 seems to be open all the time, so DC offset was around 50 Volts.

I then tried to figure out what got wrong by measuring with my Fluke 27 the DC values of the bases of the output fets:

Connecting the measuring point (I took the mass point from input cinch!) the relais reconnected the output, the output came back to 0 Volts. (So the error seemed to have disappeared)

As soon as I disconnected the measuring point, DC arose and the relais disconnected again.

It was very interesting that even measuring the base voltage of the K135 fet made the same effect!!!

DC on J50 is 0,27V (stationary mode), DC on K135 is 0,27V as well.

This is even more confusing, because the output stage is a complementary design!!!

Ok, lots of words...

My questions:

- How can I explain the measured base voltages?
- How can the tip of my Fuke change the results? It's around 10MOhm or more...
- How can I adjust quiescent current in the end (no emitter/source resistors) Do I have to introduce a measuring resistor in the pwr supply lines???

Next: The amp uses helper voltages, regulated by lm 317/337.
(+50,5, -51,1). No possibility to adjust, fixed resistor values.
Can this imbalance harm - or was it may made by intention???

The power supply delivers +-52V

Any help is very welcome, schematics are also highly appreciated.

Regs, Dirk
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Old 24th July 2011, 06:52 PM   #2
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Join Date: Feb 2008
Hi,

I made some further investigations:

The "ok" channel proves Vgs K135 of +0,9V, J50 of -0,6V

Comparing this with the dead channel, Vgs (+0,33) of J50 does not make it conducting (P-Type)

J50 seems to have some „restconductivity“

Ok,

Measurement of gate voltages on the dead channel changes the behaviour, it amplifies signal with lots of distortion and offset goes to 0.

I need to replace J50 as it seems...

How can I adjust quiscent current?

My plan:

Put 1 ohm in the power rails of the working channel, calculate current after measuring the voltage.

Put 50 ohm in the rebuilt channel power rails (this high value to prevent damage), then find out in which direction to turn the pots...

Turn pot to lowest quiscent current.

Replace 50 ohm with 1 ohm in the power rails, adjust current to the value (measuring the voltage) of the „ok“ channel.

Replace 1 ohm, → PRAY.

Any ideas??? Comments??? Suggestions???

Thanks, Dirk
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