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durwood 25th February 2010 01:11 AM

PPI PC2300.2 power supply issues
Got another power supply problem this time on a PPI 2300.2

Trying to draw excessive current and the transformer is singing to me.

I had replaced 3 blown outputs and the drivers seemed good. The power supply was blown up and I didn't have any IRFz44's so I replaced them with IRFZ44R (supposed to be a drop in replacement according to the datasheet). Gate resistors are good.

No drivers for the power supply fets this time, driven directly by the PWM chip through 2.2K ohm but I suspect it is something on the non-inverting input.


I traced pin 2 back a bit, I think there was a MPSA06 connected there somewhere but it seemed to check ok. I didn't attempt to measure voltage there yet though.

Perry Babin 25th February 2010 01:18 AM

Did you try reducing the bias current on the channel you repaired?

durwood 25th February 2010 03:42 AM

There is no bias adjustment POT on this amp. I'm getting a whopping +/-1.15V rail voltage.

Perry Babin 25th February 2010 04:03 AM

Double-check the outputs to make sure that one hasn't failed.

If you don't find any defective outputs, remove the rectifiers to see if it will power up without drawing excessive current.

durwood 25th February 2010 04:39 PM

Pulled rects power up was ok. Checked the channel I repaired and I had a dead 6491 again. Pulled all the 6491's on that channel, powered on now it pulsates between ok and excessive current. Of course I didn't order any extras of these but 2 of the old ones were still good so I figured it was worth it to try. No go. Something else is boogered up.

Perry Babin 25th February 2010 05:42 PM

Pull all of the outputs in that channel and check each one carefully for leakage. A leaky output can cause this problem and may not be detectable in the board.

Did you check the driver transistors that drive the outputs?

durwood 25th February 2010 06:33 PM

Pulled all outptus on the channel and it powered up ok, for a few seconds I get proper rail voltage but then after the amp is fully on it strains again. Outputs check ok, pulled drivers (MPSA06) since there was a short across collector and base. Drivers are ok, currently trying to trace down what is shorted across those terminals.

Perry Babin 25th February 2010 07:11 PM

With the outputs out of the defective channel, the defective channel isn't likely to cause excessive current draw. The problem would have to be in the power supply or in the other channel.

durwood 25th February 2010 07:34 PM

I am reading +/-32V on the rails when triggered on, then as it soft starts that is when rails drop to +/-14V. It's not a dead short across the terminals where the E an C of the driver connect, it's about 23 ohms across the board terminals with the MPSA06 removed. So you don't think there is some problem feeding that channel even though it is not allowing the driver transistors to work on half the channel? The other side does not measure like that. You don't happen to have a schematic do you?

Perry Babin 25th February 2010 08:16 PM

1 Attachment(s)
I don't have a schematic diagram for this amp. The attached circuit may be close. If it is, which transistor on it corresponds to the one reading 23 ohms?

Are you sure that there are no solder bridges between pads on the board?

You need to measure the DC voltage on the other channel's emitter resistors. None should read more than ~0.001v DC as the amp tries to power up and the rail voltage begins to drop. You'll have to do this quickly if the transistors aren't clamped down.

Clip your meter probes on the emitter resistor. Power it up and read the voltage. Then power it down. Move the probes to the next resistor and go through the same procedure. Do this for each emitter resistor in the good channel.

Check the temperature of all of the heatsink-mounted semiconductors between the times when you power it up.

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