Audio System X-ION 100.2

[Bertje]

Working on a Audio System X-ION 100.2
Found two defect transistors from the mute circuit (Q8/Q9:BC850/BC860). They prevent to switch on the bias circuit, shutting down the Vpre power supply lines. When Q8/Q9 replaced still no normal working amp. Green led power on, is on.

When one transistor from this muting circuit removed, the amp is working normal.

Try to find out why it is still muting, but can't trace the muting circuitry easily... There is no LM339 in here.
Any information is welcome, thanks!

 

[Perry Babin]

Are you sure that you do't have those transistors reversed?

 

[Bertje]

Both are in the correct position, checked twice ;-)
Here a part of a scheme from a different amp from the same manufacturer. I think (...) it is the same circuit. So Q43 and Q44 for the muting of Vpre. Without Q44 mounted the amp is working.
The muting circuit itself is different...

 

[Perry Babin]

Just to make sure, Q8 is a PNP and Q9 is an NPN?

 

[Bertje]

Yes, correct. Q8= 4G BC860 PNP and Q9 = 2G BC850 NPN.
And from the scheme: Q43 like Q9 and Q44 like Q8.

This morning I removed the pcb from the housing, I see the mosfets from PS and amp were replaced before. So...this is a amp that has been repaired in the past.

 

[Bertje]

And built it up again without the mute transistor, but it tested on a 4 ohm load just 25w rms before clipping.... To be continued, but first working on other amps.

 

[Bertje]

Right....today removed Q9 again, and amp worked with 4 load normal, up to 17v unclipped output.
The new transistor I put in for Q9 was coded 2Gh and the original 2Gw.
After measuring it appeared to be PNP...!! So this new transistor was a FMMTA56 or KST56, not the BC850 (NPN 2Gw).

Now put in a 2X (NPN KST4401) and amp is working ok. Thanks to Perry!