PA3000db in protect after repair - diyAudio
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Old 1st September 2009, 06:26 PM   #1
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Default PA3000db in protect after repair

I had a friend who had burnt this amp up and gave it to me to try and repair it. I had seen that all the power supply FETS were burnt up and then I checked all the outputs and found them all to be bad. I also had to replace two rectifiers that the legs had broke off. I replaced the input and output FETS and also replaced 2 driver board transistors just to be safe as perry had mentioned in an email.

The amp powers up in protect and then the green light comes on which is normal startup for this amp. After it turns green it then goes into protect shortly after. If I'm quick enough I can get it to play sound through the subs for a split second. I haven't opened it back up again, but when I do, does anyone have any suggestions as to what to look for? I put 2 MPSA42's on the driver board in Q125 and Q126. If these are slightly different from the other transistors on the driver board, could this cause it to go in protect?
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Old 3rd September 2009, 01:08 AM   #2
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It's Q124 and Q125 that typically fail.

There should be at least 9 or 10 transistors on that board that look like the MPSA42. Does it look like someone else has worked on that board?

The protect LED should light for 3-9 seconds (typically) then go off. Is that what you're seeing?

If so, is there any DC across the speaker terminals before the protect LED goes off?
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Old 3rd September 2009, 04:58 AM   #3
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Hey thanks for the reply. I did actually type those wrong and meant to put the Q124 and Q125. It doesn't look like anyone else has messed with the board but the owner literally smoked the amp. The power FETS burned badly and cracked.

The protect light comes on at the initial start up but then shortly turns green. After it's green, I can then hook the inputs up and get sound out of the subs for a split second and it turns red. After it goes into protect it will stay that way even if i power off the amp and power it back on. If I wait at least 10 minutes and then power it up, then the light will turn green again and will eventually go to protect. The amp doesn't shut down while in protect either. If left on long enough while in protect....it will eventually turn green again.

I'm gonna open up the amp and test the speaker outputs for DC voltage, but since they played for a split second I'm assuming they seen voltage.
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Old 3rd September 2009, 03:03 PM   #4
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Before the red LED goes out (when you initially power it up), do you have DC voltage on the speaker terminals (connect meter probes across the speaker terminals)?

You don't need to connect a load to the speaker terminals.
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Old 10th September 2009, 01:36 AM   #5
Dimon74 is offline Dimon74  United States
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I'm repairing Planet audio VX2200D right now,same board inside as your Power Acoustik.On driver board in this amp installed 4 pieces of each MPSA42and MPSA56, 3 pieces of each MPSA92 and KTC1027 and one KTA1023.IC is defaced,but I think it is TL072
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Old 10th September 2009, 01:54 AM   #6
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njm4580d

Q124 and Q125 are the only components that fail, generally. The second most common problem is failure of the board itself. It becomes conductive due to excessive heat and has to be milled or drilled out to remove the conductive material.
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Old 10th September 2009, 02:34 AM   #7
Dimon74 is offline Dimon74  United States
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Thanks Perry.In my amp all power suplly fets were burnt and three IRFB31n20D in output.Replaced suplly fets with IRFZ48V which is better then original IRFZ46N.Installed only two fets in output stage.Amp is playing but sound with distortions and outputs fets are heating up very fast.Took out driver board.Remove all transistors from the board.They all measured perfect as new.Replaced all transistors except KTA and KTC.Installed board back.Now amp is playing fine with one pair of IRFB31.
My question to you Perry.May be you can help with your huge expirience.Now I'm trying to replace IRFB31N20D with FQP32N20C (I have 20 of them with matching date code) looks like they have almost same specification as IRFB31.And also Chris db-r install them in same amp on repair example page.I tied one pair of them.Amp playing fine,but fets are heating up much faster then IRFB31.Should I may be change gates resistors from 47 Ohm to 33 or less.Did you try some expirement like this?Thanks
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Old 10th September 2009, 02:50 AM   #8
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I tried this with IRFB42N20Ds and got mixed results. These amps are reliable with the 31N20s so I won't likely be doing any more experiments. If Chris used the FETs you mentioned, they should be reliable but it may be the same type of situation that I had with the 45N20s.

You can try changing the gate resistors but I doubt that it would help. If you do change them, you need to go up in value. The gate resistors are used to turn the FETs on. They are driven off through the diodes (parallel with the gate resistors).

As of now, you're likely having a problem with shoot-through (both banks of FETs on at the same time) because there isn't enough deadtime. Increasing the value of the gate resistor would provide slightly more deadtime.

It took me a long time to get over the tendency to try to redesign every amp to try to make it better. It turns out that the engineers that design amps are pretty good and it's very difficult to make improvements. With only a few exceptions, I repair the amp with the original parts.
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Old 10th September 2009, 02:58 AM   #9
Dimon74 is offline Dimon74  United States
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Thanks Perry.Probably I need to order set of IRFB31.They are expensive
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