Phoenix Gold ZPA 0.5

[Bertje]

I have here a defect PG ZPA 0.5. Power supply is OK, when I removed the 4 diodes for the high voltage supply, amp powers up and the idle current is ca. 2A. Red LED's in the amp in both channels are burning (OK, think so)
Measured all the fusible resistors, one replaced, others are OK. Output devices measures OK, drivers also..
When putting back the diodes I still have a shortcircuit....

Question:
What is the normal idle current, I read 750mA, but that seems very low for this big amplifier. I'm testing with a 6A power supply.
Any ideas or experience with reparing?
Rob.

[Perry Babin]

If the problem isn't having the bias set too high and the rectifiers aren't shorted, insert a current limiter (automotive headlamp, large 2 ohm resistor...) in the B+ line and measure the DC voltage directly across each of the emitter resistors for both channels. Do you see more DC across one resistor or the resistors in one channel? The voltage isn't going to be significant (less than 0.01v likely) so accuracy is critical.

If the voltage 0.000v on all of the emitter resistors, you could have shorted rail caps. If you measure the resistance across the rail caps and the resistance is constantly changing, they're not likely shorted.

Maybe 1moreamp can help. He has more experience with PG amps than I do.

[Bertje]

The bulb trick is a good tip, I can't see which channel causes the problems.

Rectifiers measure OK, and the capactors primairy side and secondairy rail capacitors are new.
I was recapping this amp, the primary caps were all leaking. But there are more problems...
Rob.

[Perry Babin]

Did it power up before replacing the caps?

Did you measure the voltage with the current limiter in series and found that all emitter resistors measured 0.000v DC?

[Bertje]

I didn't power up before recapping, because caps were badly leaking. I removed them, cleaned the pcb ans put on new caps. Dubble checked for polarity...must be OK.
Tomorrow I continue with the bulb-current-limiter.
Rob.

[1moreamp]

Bertje, I could write a small book about bias alignment of this amp. It has a history of being used for bassline and overheating issues when this is done along with being driven into failure at its 1600 watt maximum output rating.

from the PG literature spec sheet for this amp:

All measurements taken at 12 volts DC supply as per IASCA regulations.

this amp has a MAXIMUM current draw at idle of 7.5 amperes @ 12 volts DC. No signals in and no loads connected.

If it is bias'ed to its maximum rating your 5 amp supply will fold over on itself before the amp can achieve stable idle. I highly recommend a 15 to 20 amp supply when dealing with this amp just to get it turn on properly without dumping your Dc supply.

This is for total both channels since this is a Dual Mono supply design. Plus they do spec temp as ambient ~ 20 C. But no idle temp for the amp


PM me if you want more info. this was the most sophisticated all discrete high SQ design amp PG ever released, and not cheap to buy either. It did not sell long before it was replaced with the more affordable ZX product line. It can be a time sink pain to adjust properly due to thermal tracking issues associated with its all discrete design.
Oh and I own two of these also....

[Bertje]

Thank you for this valuable information. I will test it with 50A power supply and current limiter. I't's tricky, I know...
The history of this amp is unknown, the client bought it and didn't test it very well. So I think it was allready broken when buying. But there are no visual damaged parts on the pcb and cannot find any wrong parts when measuring.
Rob.

[1moreamp]

You can always mark the current settings for the bias pots and then turn them counter-clockwise to zero the bias out. The amp should only draw about 2.25 amps then. This would at least allow you to do a no bias test of the amps operation...

[Bertje]

Well after few days doing nothing with the PG, today again some measurements. I compare each component channel by channel, and found one driver transistor defective, 2sd1763a. Search continous...
Rob.

[Bertje]

Today I put the new driver transistor in, amp is working again! Idle current is 2,5A.
Some ampguts, with new caps and green LEDs...