Power Acoustik lfa4-840, no RCAs protect mode - diyAudio
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Old 30th July 2012, 07:48 PM   #1
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Default Power Acoustik lfa4-840, no RCAs protect mode

Hey All-

Got this amp a few days ago. Did a 'proof of concept' install (temporary wiring) and it was working fine, did the actual install last night and now I get protection mode forever.

Its a Power Acoustik lfa4-840. 12.9v between B+ and ground, and 12.8 at remote. Protection light, so I started disconnecting things one at a time, but it never goes off.

Odd but possibly unrelated note - the protection light is bright until I remove the last RCA cable, then it gets dimmer. Also, I can remove the ground and the amp will still have ground through the RCA. Odd, maybe normal.

I have electronic experience, but not with amplifiers and such. Does anyone have a schematic? Can you point me to where the output transistors would be?

Stupid question, but how do I get the board out of the case? It looks like it slides in, but with all the transitors clipped to the heat sink of the case it looks impossible. At the same time, I can barely get to half of the transistors to measure them-

Thanks!
-rj
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Old 30th July 2012, 11:41 PM   #2
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Never remove the ground before all other wires are disconnected. For some amps, that will damage the head unit.

I don't have the schematic diagram.

Read the basic amp repair page (link below in sig line).

Click HERE for clip removal.
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Old 31st July 2012, 04:19 PM   #3
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Gotcha - Removing the ground was an accident - I had already removed B+ and ground, but the remote was still live and the protection light was lit, presumably with ground from the RCAs.

Thank you for the clip advice - have them off, board removed, and have reread the amp repair page. Broke out the Fluke and started measuring. I was hoping for a simple 'output transistors blown' scenario, but I don't think I'm going to be that lucky.

I have 4 groups of C4466 and A1693. All pin combos read very high resistance. Only odd duck is with the A1693 showing a climbing resistance between base and collector starting around 80K Ohms. It is consistant and still rather high, so I am assuming it is fine-

I'm starting to check the power transistors now - that seems like the correct procedure, right?

Thanks for your sagely wisdom, all!
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Old 31st July 2012, 05:35 PM   #4
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Trying to double check my testing - forgive my ignorance, I've looked at the datasheets, but still can't 100% figure out what kind of transistors C4466 and A1693 are. I am finding references to them being MOSFETs as well as BJTs. Not sure which testing procedures to use-

Thanks-
-rj
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Old 31st July 2012, 07:52 PM   #5
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Do I need to remove the components to test them?
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Old 31st July 2012, 08:04 PM   #6
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One last thing - Most of the tutorials are showing the DMM indicating OL. My Fluke 77 is showing .9-3 M ohms unless I turn off auto ranging. That sound right?
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Old 31st July 2012, 10:33 PM   #7
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When doing initial tests with the transistors in the board, you'll look for very low readings. If they're all reading near 1M ohms, they're not shorted. If they were shorted, they'd read approximately the same as when you touch the probes together.

The OL readings are for transistors out of the board. When in the board, other components can skew the readings.
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Old 1st August 2012, 03:20 PM   #8
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Just for kicks, I pulled out the banks of IRFZ48n and tested (FETs, right?), then pulled out the C4466s and tested (as BJT). I assume that the C4466 are my output transistors and IRFZ48s the power ones.

Everything is testing good so far - do I just start pulling out rectifiers and such and testing them??

-rj
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Old 1st August 2012, 05:58 PM   #9
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Removed all output transistors, nothing plugged in but power, still goes into protect mode.

Checked the pins of the 494:

Pin 1: 0
Pin 2: 5.02
Pin 3: 4.89
Pin 4: .189
Pin 5: 1.54
Pin 6: 3.55
Pin 7: 0
Pin 8: 13.68
Pin 9: .001
Pin 10: 0
Pin 11: 13.67
Pin 12: 12.27
Pin 13: 5.03
Pin 14: 5.02
Pin 15: 5.03
Pin 16: .86

Seems in line with others....

Any ideas as to next steps?
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Old 1st August 2012, 07:55 PM   #10
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Put it back together, and lo and behold green light comes on. Put it in the car, and it goes from red to green in a regular cycle, with a nice whump every time it does. Now I'm just confused. Unless I solved some sort of cold solder joint issue, I didn't do anything to fix it....
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