PPI 2150M - amateur repair time

[damage]

Hi!

PPI-2150M draws has bad outputs in one channel. I removed those, but the other channel makes a farting noise when powered up and was drawing about 8amps, maybe more. I can't quite remember how much now.

I lifted the rectifier legs, and I get ~45VAC there @ ~10VDC input. However, the opamps don't seem to be getting any voltage. The FETs and gate resistors measure fine. Not sure where the -/+ 15V comes from. Still draws quite a bit of current when remote is connected.

I've read the other threads, but my problem doesn't seem to be the rail FETs like others had.

Thanks!

[Perry Babin]

With the rectifiers in the circuit but no RCAs plugged in and no load (on the speaker terminals), does the amp draw excessive current?

With speakers connected but no RCAs plugged in (again, with the rectifiers in the circuit), does it make the noise?

If so, does the noise change when you adjust the gain control?

With the rectifiers out of the circuit, the regulators probably won't have any input so they can't have any output.

Be very careful when the amp is powered up and the bottom cover removed. Unless you've made some sort of clamps for the power transistors, they can overheat and fail within seconds.

[damage]

hi!

Yes, still draws excessive current. Still makes noise on the channel that still has outputs installed.

However, in the middle of testing, R29 self destructed. Current draw went away after that.

[Perry Babin]

Is R29 near the channel that was producing the noise?

If R29 failed, it probably means that the outputs have failed. Recheck all of the outputs near R29.

Can you post or send me a few clear photos of the bottom of the board?

[damage]

Hi again!

As usual, I have made another amateur mistake. The channel that had noise was not the working channel, but the bad channel that had no outputs. R29 is a 47ohm resistor across the base and emitter, feedback I assume.

So I shall replace it. And then, like I should have done in the first place, check all drivers and pre-drivers. I imagine one of them is bad causing a dead short.

I'm getting better, but still making too many mistakes.

[Perry Babin]

In this amp, one of the to-220 case transistors is the driver. R29 was connected between the emitter of the driver (same as base of output) and the speaker output. R29 turns one half of the outputs off when the other half are on.

[damage]

So here it is, almost a year later and I've made some progress on this amp that I bought broken....

• Replaced all outputs on one half of amp
• Fixed crappy soldiering job on replaced PS fets (not mine)
• changed out Gate resistors for said crap job from 470ohms to 68ohms to acxommodate the previously changed FET to IRFZ44Ns
• found solder bridge
• recapped everything

So amp works, as long as voltage stays below say ~12V. Go over that voltage, and the amp shuts off but the rails stay powered. I get increased voltage on the outputs' emitter and base, ~5V. Also lots of noise at that point.

+/-15V 7815 outputs goes to 1V and -10V.

Measured voltages on the SG3525A ~12.8V

1 5.6mV
2 5.07
3 30.7mV
4 155mV
5 2.02
6 3.75
7 2.02
8 4.8
9 5.79
10 67.9mV
11 5.19
12 6.9m
13 11.4
14 5.01
15 12.82
16 5.07

Looks like 11 and 14 should be ~1.9V and 13 should be 4.67V as measured on the working amp.

voltages on Q58 are messed up. I replaced this before as well as all resistors. Voltages on those are messed up as well. The circuit appears to bias the outputs(?) at a quick glance or switching you said? It's late.

Lastly, CR17 which is labeled as a Zener Diode on the board is missing but is there on another 2150M I have. Not sure of it's value or purpose. There is actually another CR68(?) that is missing also. Both go from each side of the primary to GND. Input voltage regulation?

[Perry Babin]

If you have an adjustable supply, what is the rail voltage from the working amp and the defective amp at ~14v.

Post the DC voltage on all 3 legs of both regulators (7815/7915) before and after the amp shuts down.

Does the voltage on pins 11, 13 and 14 change significantly after the amp shuts down?

Where is Q58? On the photos of a Rev. D amp, I can't find it.

CR16 and CR17 aren't critical. They aren't used in the Rev. D amp.

[damage]

Ok, now I checked these before I changed the gate resistors on the PS Fets and they were fine, but now it's not. I suppose mainly because I was having cross conductance problems before.

Q58 is by the speaker outs.

7815
On compared to chassis GND:

1. 55V
2. .685V
3. 15.89V

when not working right

1. 61V
2. .797V
3. 1.299V

7915
On compared to chassis GND:

1. .685
2. -54
3. .538

when not working right

1. .799
2. -60
3. -11.07

Rails:

• defective one 61/-59.4
• working one 39.6/-40.9

[Perry Babin]

The ±15v regulators are probably shutting down due to high voltage. Normal rail voltage is approximately ±40v.

Did you have the voltages (working/not working) reversed for the 7915?

Find the transistor that drives pin 13 of the 3525. It may drive it through a resistor. Is it shorted or leaking (electrically)?

If it's OK, find the transistor that drives that one and check it.

If you send me high resolution photos of the bottom of the board, I may be able to help you trace the circuits.

babin_perry@yahoo.com