High voltage wrecked my old school amps - diyAudio
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Old 17th February 2006, 01:45 AM   #1
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Join Date: Feb 2006
Location: portland OR
Default High voltage wrecked my old school amps

Hello all,
I have been reading the various threads regarding amplifier repairs. PPI etc.. and it sounds like there are some very knowledgable folks here on this board.. I have a problem I would like to share with you all.

Untill recently, I have had a SQ installation in my "baby" for like 10 years now. Phoenix gold MS250, 2 orion 280GX's, a baby Orion 220sx, Audiocontrol EQT's, Epicenter, and ESP-2.

It is all toast now, because of a bad alternator that was pumping out 17V at idle and who knows how high at cruise. I was on the freeway when it finally took out my ECU and the car ground to a halt. The system was never on during any of this time, so I had assumed that it would all be OK. Boy was I wrong.

When I cracked open the amps, all 3 of the big amps had one burnt capicator right next to where the power leads come into the board. I cant see any other damages besides some traces that seem to have lifted a bit.

They probably all saw 20+v, think there is any chance of saving them? I have not even looked inside the signal preocessors yet, all the processors had 1A fuses and the all popped. The amps had 20A fuses that did not pop.

Thanks


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Old 17th February 2006, 09:16 AM   #2
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If possible, post some close-up photos that show the damaged areas in detail.

For the Orions, the damage should be minimal. The FETs in the supply have the gates protected by Zeners. The FETs themselves can take at least 60 volts. The control IC can take more than 30 volts. If the cap failed and it's rated at 35 volts (most common), it was likely destroyed by ripple current not voltage. At high voltage, the battery wasn't likely as good a filter as it is at normal working voltages.

If you need to buy replacement caps, try digikey.com. The 'FC' series should work. You'll need to measure the old caps (pin-pin distance, diameter and length -- usually in mm) to get the correct size.
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Old 17th February 2006, 08:13 PM   #3
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Thanks for the reply,

I will post some pics of the damged amps tonight. I looked at the caps and they are rated at like 16 -18V, cant quite remember.

The amps draw power as soon as the leads are connected, no remote wire needed any more

Thanks again
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Old 18th February 2006, 03:24 AM   #4
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One pic of the most damaged area. I snipped the lead from the burnt cap. Traces look lifted and there is a burnt something to the right also.
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Old 18th February 2006, 10:44 PM   #5
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lets try that again


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Old 18th February 2006, 11:32 PM   #6
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Wow!

That looks like the pre-MOSFET version.

If that's the case, it uses 2N6488s in the supply. 5 per primary winding plus one to control the power supply to the drive windings. There's no control IC so there may be very little damage beyond the 6488s. The one that's charred is the one used to switch power to the drive windings. There's a good chance that the other 6488s in the supply are also damaged.

I can't see the board as well as you but I can't see any lifted traces. I see a few areas where the solder has flowed under the solder mask but that was normal on those old boards. When traces lift due to excessive heating, the solder mask on the trace usually turns black.

One problem you may have is getting the transistors off of the insulator. If they are too difficult to break free, you may want to leave the board in place and only remove the damaged components.
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Old 19th February 2006, 06:01 AM   #7
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Thanks for the reply. I got the bar off the top, and found the the other
2N6488s, they are all right next to each other. The other 5 do not look burnt, but I should replace all 6, correct?

Here are a couple of pics of the other one. The only damage I can see here is the cap is swollen and leaking some sort of fluid. Also I have a feeling that someone has been in this amp before. Some of the resistors look different.

Click the image to open in full size.

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Old 19th February 2006, 06:11 AM   #8
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When you remove the clamps, try doing it as shown in the attached image. Start close to the end of the clamp and work your way to the other end.

The fluid is electrolyte and it's corrosive. All of it needs to be removed with a cotton swab and a good solvent like acetone. Read the warnings on the acetone container if you've never used it.
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File Type: gif orionampclamps01.gif (16.0 KB, 106 views)
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Old 19th February 2006, 06:25 AM   #9
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Wow! I was editing my post to let you know I got the clamp off, and meanwhile you posted the instructions on how to do so.

Is there any reason I should take the clamp off the other side?

Also for amp#2 do you think that replacing that cap may be all that it needs?

Thanks again!
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Old 19th February 2006, 06:41 AM   #10
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If the transistors are not stuck to the insulator, it will be easier to work on out of the sink. If the transistors are too difficult to remove from the insulator try to repair it in the sink.

I'd suggest replacing all of the power supply transistors if even one has failed (other than the power control transistor). It's imperative that all that are running in parallel be matched. Both sets of transistors were subjected to the same conditions. If they didn't fail, they were likely pushed to the edge and may be weakened. The parts are less than $1 each.

On the second amp you can see if replacing the cap will get it going but it's likely to have more damage.

Be careful with the insulators. If they are torn they will need to be replaced.

When taking the screws out you may see some tiny metal shavings near the screw hole. If 1 tiny fragment gets under any transistor, it will cause the amp to fail again.

All of the double-sided tape needs to be replaced. All of the old tape must be removed from the transistors AND the clamps. A bench grinder's wire wheel works well (don't use the abrasive wheel). When replacing the tape, leave the backing on the tape so it doesn't stick to the transistors.
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