US Amps 2000x

[Achim1409]

Hello,
i received this amp, with blown ps, and one bank output.. Repaired ps, removed the whole damaged bank, and resolderer the other bank, because of bad solder joints..

The problem is now, that it pulls me up the -15V for the op amps.
On the board are only two op amps, they measure ok..
The LM7915 gets hot realy quick. Op amps stay cold. Between +and- on the op amps i measure 9Ohm.. Removed the lm 7815 and 7915.. Anybody has a schematic, where the voltage is used?

 

[Achim1409]

Ok, it seems that both op amps are burned.... Removed them, now voltage is ok... Very strange amp...never had this, that everything on one amplifier is burned.

 

[Achim1409]

can i replace a tl072 with a njm4556?

 

[Perry Babin]

For testing, yes. Op-amps, when subbed, can do some strange things. They typically work but not always.

 

[Achim1409]

Continue on this amp.
Repaired PS, installed new output transistors, pre stage, and op amps.
I have Pos. DC on the output of that channel, and the other only disorted low signal.


For the first i want to fix the dc.


Iam now at the stage before the bias.

MPSA 92, i have on all three pins rail voltage.


On the side, with the disorted signal, i have rail on base and emitter. Collector is nothing.
Ground is sec. center.

Marked is the collector. How does it should work?

 

[Achim1409]

Base from the A92 is connected over a 1,2k resitor to rail....

 

[Achim1409]

Fixed this problem. Removed now the mute circuit. Got clean sine wave on both channels.

 

[Achim1409]

So, maybe somebody can assist with the mute circuit.
If I remove uf301 and 401 I have good signal. If I put back, the mute circuit is not releasing..
How this circuit works?

 

[Perry Babin]

Is this the type of muting circuit it has?

Do you have to remove both of those optocouplers to get audio on the defective channel?

Is one optocoupler for one channel and the other optocoupler for the other channel?

 

[Achim1409]

Each optocupler is for one channel. There is one optocoupler in the center, maybe this one drives both. If I remove the n Chanel jfets, I get audio.

 

[Perry Babin]

How are they using the jfets?

For the center opto, what is the DC voltage across pins 1 and 2 before and after the mute delay?

 

[Achim1409]

Na, iam wrong. The center optocoupler is the Mute. Pin 5 fromt the Coupler goes to the gate of the J Fet.


Pin 1 and 2 has only 1.2VDC after a second. When power of, its slowly goes down.

 

[Perry Babin]

How are they using the jfets?

Do they correspond to any components on the diagram posted?

 

[Achim1409]

See attached Picture. The J Fet is in the off state bridging this resistor, and pull the signal down to ground.
I think, when gate is active, the bridge is open, and the resitor is working.
S is Source, G should be drain... Sorry

 

[Achim1409]

Its a 4n25....
Pin 4 has 15 V, Pin 6 only 0,5V

 

[Perry Babin]

What's the jfet part number?

Is the voltage on pin 4 positive or negative?

 

[Achim1409]

J-Fet PN4391
Pin4 is +15V ground is sec. center.
If i Bridge pin 4 and 5, the mute is off, because the J-Fet gets 15V on Gate.
So i expect a damaged Optocoupler, if the 1,2V at Pin 1 and 2 is enough.

 

[Achim1409]

Changed with a used coupler... Still the same
How many Volt i must have at Pin 1 and 2?
Is 1,2VDC enough?
Otherwise, pin 6 pulled down from something

 

[Achim1409]

My mistake . New Optocoupler works.. Now i have to check bias ...

 

[Achim1409]

Ok.... Next Problem... The old output bank has a good sine wave, up to 20khz.
The repaired one shows above 2khz sawtooth wave.. could this be caused from bias?
I changed the output transistor from original MJE4343 and MJE4353 to FJA4310OTU and FJA4213OTU