Quick PHD2 question

[spooney]

One half of the outputs are clearly blown. The other side looks to have survived the blowout. I was planning on replacing all of the outputs and ps fets but I think the power supply is ok. Would it be safe to remove all of the outputs and try to power the amp up to see if the power supply is still functioning correctly? I'm thinking I can but these amps are touchy and I'd hate to cause more damage than has already been done.

[nigelwright7557]

I dont see why not.

I have done it a few times.

I usually feedback the VAS output back into the LTP so the loop is made.
If the amp is stable and the bias is OK and there is no dc on the output it should be ok to replace the blown parts.

[spooney]

Originally Posted by nigelwright7557

I dont see why not.

I have done it a few times.

I usually feedback the VAS output back into the LTP so the loop is made.
If the amp is stable and the bias is OK and there is no dc on the output it should be ok to replace the blown parts.

Thanks for the response but I have no idea what the second part of it means. Is the bias controlled by the small pot in the output section of the amp? Is it typically turned almost all the way up because thats where mine is if thats even the bias pot.

[nigelwright7557]

Originally Posted by spooney

Thanks for the response but I have no idea what the second part of it means. Is the bias controlled by the small pot in the output section of the amp? Is it typically turned almost all the way up because thats where mine is if thats even the bias pot.

You really need a service manual to reset the bias.
If you replace the output transistors the bias pot needs turning right down before you can set the correct bias.

Incorrect setting of the bias pot will fry a new set of transistors.

[spooney]

The plan for now is to remove the outputs and crank the bias pot down and see if the power supply is producing rail voltage. I will tackle the bias issue when I get the new fets installed and the HIP4080 replaced. I will probably replace all of the zener diodes connected across the outputs as well.

[Perry Babin]

Don't turn any pots.

You can remove the outputs to power it up.

The Zeners rarely fail.

[spooney]

Unfortunately I am a little late reading your post Perry. The pot is all the way down already. It was previously turned all the way up. Hope this doesn't cause any problems. I removed the 6 outputs that were shorted and left the other 6 in the amp for now. The 1N4148 zeners all appear to be ok but three of the 1N4746A zeners on the side with the 6 shorted outputs were shorted or open. I applied power to the amp through a 10 amp fuse. The amp did the usual quick spike in amperage and then settled back down and idled at a normal level. I did have approximately 80 volts at the rails. The power light on the amp never came on though and probing the led it doesn't appear that it was defective. It just wasn't receiving any power to light it up. There are two resistors in the power supply that have been burnt beyond recognition. I am getting ready to look through some photos in hopes that I may be able to find their value. Maybe replacing these will get the amp to power up normally.

[Perry Babin]

Which resistors are burned?

[spooney]

I will try to get a pic. One of them is connected directly to the remote turn on lead. The other one is directly behind Q9( I think its an mpsa06) near what I think is the sg3525(numbers have been ground off) in the power supply.

[Perry Babin]

The resistor behind Q9 is 10 ohms.

The 16 pin IC is an SG3525.