ppi 2350 dm not powering on - diyAudio
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Old 5th August 2007, 09:23 PM   #1
twon007 is offline twon007  United States
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Unhappy ppi 2350 dm not powering on

i just got finished replacing all power supply transisters,gate resisters, and rectifier diodes, and the cap bank completely all six.
this was after it failed due to a 2ohm mono. all was working fine in stereo but when i tried to bridge it (the right impedence this time) audio stopped and then sub started thumping in a repeated fashion. i opened her up and tightened down all the edges with a homemade strip and screws on each side so aqs not to fry it anuy further, and i could hear a little click noise every time the thump sounded. tested around to find that there was two transisters mps u01, and two transisters mpsu51cracked open. found some at a nearby store and puut them in. it never fixed the problem and after another minute it shut off completely.
there seams to be a problem in the area of where the remote lead connects to the board. the pwer supply all looks ok and the outputs do too. please, any suggestions would be great. this is a very nice amp . don't wanna throw it away.
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Old 5th August 2007, 09:38 PM   #2
twon007 is offline twon007  United States
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ok i figuered out that the fan + lead grounded out and shorted to-92 switch but still makes that weird noise at the npn- pnp's. help me perry babin. i know you are the guru of ppi!!
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Old 5th August 2007, 10:10 PM   #3
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If I'm not mistaken, TO-3 is the PPI guy.

I've only worked on one of those amplifiers. It only had a shorted rectifier so I didn't have to study the design.

It uses an SG3525 so it should be relatively easy to determine why it's not powering up. You may have to defeat the protection circuit and force it to power up. If you do that, you have to be very careful/observant so you don't do more damage.

If you get it working, make sure that the heatsinks on the driver transistors are not loose. The one I worked on had two of the sinks rattling around in the amp when it arrived.

It's nice to hear that you took the time to make the clamps. I had to do the same thing for a PPI 1200 that was overheating power supply transistors.

P.S. I don't think you're going to throw that amp away. Even broken, it's likely worth $300 on ebay.
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Old 5th August 2007, 10:30 PM   #4
twon007 is offline twon007  United States
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hey thanks for getting back and do take the compliment anyway. i see that you help alot of people. you are brilliant in this area, and i would guess that your time is very valuable. so thanks.btw i got it to power up. it was the fan + lead dangling around. had to clip it to work, know what i mean>? so i fixed a little t092,voila! but i am back to square one with the damn thuumping noise. it is simotanious with that little klick noise i can acuuualy here in the amp. any ideas? it would'nt be the tranny?
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Old 5th August 2007, 10:31 PM   #5
twon007 is offline twon007  United States
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oh and another thing, how much heat compound do you really nead?
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Old 5th August 2007, 10:49 PM   #6
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If you have thumping in both channels, you may have damaged the muting circuit or one of the regulated power supplies. If it's only in one channel, I'd suspect a driver (assuming the outputs are OK). I'm not sure if these amps had DC offset or overcurrent protection. If they do, either could be causing the amp to continually go into protection.

Thumping can be caused by several things. It's generally produced when there is a fault in the audio section. The amp will try to power up. If the DC offset protection sees too much DC on the outputs, it will shut the amp down. The slight DC causes a thump.

If the outputs are shorted or there is a fault causing too much current to flow through the output transistors, the overcurrent protection could be shutting the amp down. Generally, this type of problem is due to shorted output transistors. Generally, the negative feedback keeps the DC on the outputs near 0v. When you have a shorted output transistor, the shorted transistor tries to apply rail voltage to the speaker outputs. To compensate, the feedback circuit turns on the opposite output transistors. Since the other output is shorted to the rail, the current flow is excessive. The overcurrent protection shuts the amp down to prevent further damage. The thump is due to the slight delay in shutting down the amp.

You don't need much heatsink compound but it must be fluid. If it's dry, it's not much good. Good heatsink compound will be displaced almost completely. The only compond that should remain is the compound filling microscopic voids in the metal of the transistor, the insulator and heatsink. Dry compound will not be displaced and will cause the components to run hotter than they should.
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Old 5th August 2007, 10:57 PM   #7
twon007 is offline twon007  United States
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the thump is in both channels. and it will do this with no input gain. rca's removed even.
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Old 5th August 2007, 11:28 PM   #8
twon007 is offline twon007  United States
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Quote:
Originally posted by twon007
the thump is in both channels. and it will do this with no input gain. rca's removed even.

hey i just remembered that it did thisu just before i popped the supply. but it would only do it if i turned the amps gain up alittle past half way.
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Old 6th August 2007, 03:27 AM   #9
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Quote:
transisters mps u01, and two transisters mpsu51cracked open
If I remember correctly those amps used MPSU-56 and MPSU-06. You might get by with MPSU-05 and MPSU-55 but nothing lower voltage wise. Or maybe MPSU-60 and MPSU-10 < max voltage for this series of transistors> The numbers you listed are the lowest voltage version of these, and that does not sound right for PPI's biggest highest voltage amp for that era ???
Both of these drivers can be fun to find nowadays, I have a few of them.

Those devices don't sound original. They used the MPSU devices right in front of the output stage of 2n6487/88 and 2n6490/91 on these amps.

Its been many years since I have seen the inside of these but it was just the same as other PPI amps of that era, just bigger.

Could you post a interior pic just to jog my memory of the lay-out... THX

Oh as for PPI's use of huge amounts of heatsink grease well they are all that way. I am restoring a A1200 right now and its a mess. This one was used in competition and was water cooled but someone buried all of the power devices in this stuff < untrained person I guess as this didn't help anything but to make a mess.


Perry is 100% on target about the possible scenarios of your troubles. I would also check those MPSU devices as they are not the correct part numbers. Too low of ratings for this amp, I know for sure the lesser amps of this series used what I mentioned above.

Good Luck
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Old 6th August 2007, 08:18 AM   #10
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Here's a photo. It's 2700 pixels wide so you'll have to tell your browser (IE) not to resize it. If you're using firefox, simply click the image to toggle between full size and sized-to-fit views.

http://bmpt1.com/tutorialimages/temp/IMG_9836b.jpg

The MPSU01/51 are the power supply drivers in the amp above.
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