Phonic PWA-2400, protection circuit malfunction.

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I got this amp from a friend. He had shorted the outputs on one channel with the result of blowing some mosfets.
I switched the original SJ115 and SK405 against 2SJ201 and 2SK1530.
There was also space for some moore so it now has 12 pairs per channel.

The problem is that the protection Circuit reacts when I connect doubble speakers , around 4 ohms, but not with 8 ohms, and only the ex. blown channel.
It sounds really good thou. :)

If someone has a schematic, or good knowledge of this amp it could save me a lot of hard thinking...


Best regards.

Figge.
 
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Not familiar with the amp at all... but a couple of thoughts.

Protection circuits often monitor current in output transistors via a low value resistor (FET's so probably in the source). If the resistor used for this has gone high in value, maybe due to previous overload/failure, then the circuit will trip to early for a given output current.

Bias current. Is that adjustable and is it correct ? If the bias is way to high then its well on the way to being an overload condition.

Just ideas... as I say, I'm not familiar with the amp at all.
 
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Thanks for the replys.

When I switched the mosfets I also swapped all caps, sourceresistors and output relays.
Bias was adjusted to around 150mA per device and DC offset was low, ~ 10 mV.

The protection Circuit are all descrete Components, there are only 2 IC's in the preamp.
If I remember correct there's a lot of SA970, 2235 and SC2240, 2235 all around.
In the IPS there is a SK240, and drivers are SD1148 and SB863.

I tried to reverse engineer the driverstage and it's almost done, but I guess I have to put it back on the Workbench and do the rest.
I Think that the only thing I didn't check was the resistors at the output.:(
Anyway - It worked ok when tested with 2 ohms and dummyload so I guess this can be intresting ;)

Meanwhile I can put in my Altec 9440 to drive my four RCF-boxes in my workshop... :cool:


//Figge.
 
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