Precision Power ProMos450

[Bills amps]

Looking for some schematics for this amp. Has issue with the rear channels. draws excessive current. The power supply fets (IRFZ34) get hot and one bank of the BTJs (2N6487)gets hot. But i cant find the reason. Nothing seems to be out of order. I have removed each transistor and checked for defects and reinstalled. Real head scratcher here. Thank you in advance.

 

[Bills amps]

Sorry needed to add these....

 

[Perry Babin]

The biasing may have drifted on the channel that's getting hot if all of the outputs in that channel are heating (roughly) evenly.

Measure the voltage across all of the emitter resistors in that channel and compare the readings to the voltage across the emitter resistors in the other channels.

 

[Bills amps]

Using the center tap of the transformer for negative the emitter voltage was 2.1v . And on the good side it was 0.5v. AM I correct in this measurement?
If so where do I make the adjustment?

 

[Perry Babin]

You measure the DC voltage directly across each emitter resistor.

Since there is no bias pot, biasing is likely done via transistors and thermistors. One or more may be defective. There may be a way to replace fixed resistors with the bias pots (see locations for what may be bias pots).

How much does heating/cooling the thermistors between make the idle current change?

 

[Bills amps]

I measured the thermisters at room temp. all were the same. in circuit were 2.7k ohm. I applied power and heat to the closest one to the bank and o change to the over current. The thermister did go down in value though. It draws over 10 amps at idle with no speakers attached. Attached is the area of issue.

 

[Perry Babin]

Did you heat the thermistor on the channel that is drawing too much current?

 

[Bills amps]

yes that is correct. heated the one that was drawing the high current

 

[Perry Babin]

Does shorting across the thermistor make a difference?

 

[Bills amps]

i put a 10 ohm resistor across the thermister and the voltage came down to 1.8v. But the amp still draws 10+ amps. i have a current regulated power supply for testing. max current is 10 amps. I also pulled C6, C26, Q64 and replaced the 4 output transistors with new ones. The caps all checked good and Q64 checked good as well. Nothing has changed.

 

[Perry Babin]

Pull the two TO-220 drivers for that channel. Does that stop the excess current draw?

 

[Bills amps]

negative

 

[Perry Babin]

What is driving the outputs? That should have disconnected all drive from them.

Did you try powering it up when the outputs were out of this channel to see if that eliminated the high current draw?

 

[Bills amps]

I did pull the outputs and power up the amp, it burned R54 47ohm resistor. And did so quickly. i have removed both sides of the drivers. just in case. but to no avail. replaced the 47ohm resistor now i will try again and see the bias voltage.

 

[Perry Babin]

That would seem to indicate that you either have a bad output transistor or something is shorted/leaking. With the drivers removed, that resistor should not have had any voltage on it.

Confirm that nothing (like remote voltage) is shorted to the speaker wire.

 

[Bills amps]

all removed. no over current. also removed the dreaded molex connector. just to make sure nothing was crossing.

 

[Bills amps]

now i am going to put the outputs back on and try again. No drivers yet....

 

[Bills amps]

with Q27 and Q34 there is no high current. As soon as i put Q27 in the current is back.

 

[Perry Babin]

That's better than it was, isn't it?

If so, reinstall the drivers and find the pre-drivers and remove them. Do you still have excessive current draw?

 

[Bills amps]

no. Its the same. there is 23v on the collector and 21v on the emitter and base of all the outputs with the driver removed. But no high current. Same as on the driver but no base voltage present. Is that normal? And still no pre-driver installed.