PPI PC2150 (c.1997/8) Power Supply Noise - diyAudio
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Old 16th September 2006, 10:00 PM   #1
17Peaks is offline 17Peaks  United States
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Default PPI PC2150 (c.1997/8) Power Supply Noise

I have a PC2150 and I seem to be having two problems:

First: The power supply makes an audible noise when powered on, not through the speakers, it's an actual static crackling noise coming from the toroidal transformer. The pitch of the noise changes when I adjust the input up and down using a signal generator.
It's pulling a lot of current so that is most likely why it’s making the noise. It sounds like bacon cooking, and it gets extremely hot even with no input.

The output works, and doesn't sound too bad, although I've only done some brief bench testing.

Anyone have schematics for this amp? Any ideas on where to start?
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Old 16th September 2006, 11:53 PM   #2
17Peaks is offline 17Peaks  United States
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Default Forgot #2

Sorry, I forgot the second problem. One of the output power amps seems to be bad, it has a couple of shorted pins. I removed it from the board to continue testing....
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Old 17th September 2006, 10:41 AM   #3
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Find your shorted power devices first. Pull the center pin on questional devices and recheck them with a ohm meter.
Clip out all shorted devices.
Then fire it back up and check the current draw.
Look for no more than 2 amps at idle, no signal and thats a bit high there. It will be less when you have removed all the shorted devices.

Then tell us what you got

hope this helps
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Old 17th September 2006, 05:04 PM   #4
17Peaks is offline 17Peaks  United States
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Thanks for the advice.

I've narrowed the problem down a bit. The positive rail voltage is at fault for drawing the massive amount of current.

I've disconnected the rectifier diodes and one of, what I think are the regulators, for the positive rail and now the transformer is no longer whinning. The other regulator is getting warmer than the negative rails equiv.

I've checked the output devices and other misc devices but cannot find any short.
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Old 17th September 2006, 09:45 PM   #5
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Quote:
Originally posted by 17Peaks
Thanks for the advice.

I've narrowed the problem down a bit. The positive rail voltage is at fault for drawing the massive amount of current.

I've disconnected the rectifier diodes and one of, what I think are the regulators, for the positive rail and now the transformer is no longer whinning. The other regulator is getting warmer than the negative rails equiv.

I've checked the output devices and other misc devices but cannot find any short.


You have a short somewhere in the amp. You must find this and clear it , and then restore the the regulator right away!

This a balanced design amp and when you yank one of the rails out of the picture the circuitry is now lopsided as far as proper balance is concerned.
You will get all kinds fo offset DC issues, and it will mislead you. So try not to rely on static voltage measurements with one side of the 16 volt rail removed, it will mislead you !

I have seen similar problems and you need to check the power regulator circuit for a shorted Zener diode or a burnt resistor or a bad cap. Tech the regulator fist. If thats the issue then fix it and retest the amp. ( this is usually a symptom not the main issue)
If it draws exces current on you again, then you have a short in the amp somewhere ( this is the hard part) you must check each component.

I start with the blue caps, They like to go shorted, if they all pass muster then move on to any other caps then on to each transistor.

And finally if you have not found the short by them try removing the ceramic driver cards. I recommend this last because if you damage one of those cards YOUR SOL. PPI dose not support amps this old as far as our local rep told us. So there no possible replacements unless you buy another amp with good ones and salvage them.

and if by chance the ceramic card is bad, again your SOL, unless you find a junker for parts.

Good luck !
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Old 17th September 2006, 10:33 PM   #6
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Has someone else been in the amp before you and set the bias too high?

How much current is it drawing?

If you allow the amp to remain on (bottom cover/clamps on transistors), is there one area that gets hot the quickest?

For example, the left and right channels are generally on opposite sides of the sink. Does one side get hot quicker than the other? This may lead you to the defective channel.

Does the power supply end get hot quicker than the audio end of the heatsink?

If you can't leave it on long enough to allow any part of it to warm up, measure the DC voltage across the emitter resistors. The voltage will be in the millivolt range so set your meter to the lowest DC voltage range if it doesn't auto-range. If one channel has higher voltage across the emitter resistors than the other, that will tell you what channel is drawing the higher current. If you do this and your bottom cover is the transistor clamp, it's very important to monitor the temperature of the outputs. Since they will not be clamped down tightly, they will be in danger of failure from overheating.


If none of the above helps and this amp has a large filter inductor, try unsoldering two of the wires (one of each color). If the inductor has shorted windings, that can cause excessive current draw.

Again, this is very important... If you have the amp powered up and the transistors are not clamped down, monitor their temperature very closely. It takes only seconds for components to fail from overheating.

One more thing... If you have to remove the cover screws repeatedly, be sure to find the original threads each time. To do this, set the screw in the hole and turn it counter-clockwise until you feel it drop into the lead-in thread. Then tighten it. This is especially important on these amps because they typically have only a small number of screws so they all have to be intact to insure that the transistors are properly clamped.
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Old 19th September 2006, 02:43 AM   #7
17Peaks is offline 17Peaks  United States
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I'm not making much progress.

Yes, someone has been in the amp before. The inductor was silicone’d to the circuit board in an effort to reattach one of the loose leads.

Some of the windings were shorted from rubbing etc. I've fixed all of that and measured to ensure that they were separated. I wrapped the inductor with polyglass tape to ensure that they don't short again.

Attached is a picture of the board with a few components removed. The symptoms that it has currently are:

TO220 (2N6488) on top right gets very hot at idle and the resistor attached to the collector is getting extremely hot and has charred slightly, but still reads the correct resistance.

The Red/Green LED turns red when first powered up but then goes out (it used to turn green). I caused this when I accidentally shorted a couple of pins on the TO220 (2N6491) on the bottom second from right when probing with my O'Scope. The amp seems to still be powering up but not completely sure.

I've tested nearly everything I believe to be relevant. I'm getting a bit discouraged.

I've looked for shorts everywhere, but have found none so far.
Attached Images
File Type: jpg pc2150a.jpg (83.5 KB, 121 views)
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Old 19th September 2006, 03:32 AM   #8
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If there is a mylar cap on the output of the regulator, check to see if it's shorted.
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