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Old 31st December 2003, 04:27 AM   #1
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Default Jaycar Playmaster Subwoofer Amp Kit

Hey guys.

This would be my first post/thread in this forum and hopefully not the last.

I purchased, a few years ago, one of these amps and have had it connected to a single 4 ohm Rockford Fosgate 18in sub. It has been working perfectly since I built way back when.

That was until the other day when I moved it and touched the RCA input leads whilst it was on. It made a VERY loud buzz and then nothing. All it does now is blow fuses.

I have basic knowledge on electronics but that would be the limit. I have ever-so-slowly pulled out/replaced basic parts (e.g. resistors, diodes and caps) with no luck.

My question to you all knowing Gurus is how to test the power mosfets? The amp uses three 2SK1058 and three 2SJ162.

Please help - DVDs are just not any good now without it.



Thanks, Liam.
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Old 31st December 2003, 05:13 AM   #2
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Sorry guys I did a further search and found a link to a really good site - bcae1.com and found out how to test the FETs.

Whilst I still have your attention; I won't be able to test these for a couple of days so I'd like to hear your opinion on what might be wrong with the amp other than the FETs.
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Old 31st December 2003, 05:32 AM   #3
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I don't know how to check the mosfets, but I'd be looking for a short on the output.

If it's a classic playmaster design it will have an RLC ciruit on the speaker output (a cap and a coil). Check the cap and make sure that it hasn't gone short circuit. also look for any loose wires that may be shorting (at the speaker).

I've blown the fuses in my playmaster 200 a number of times when I've had a short on the speaker terminals (even a single strand of speaker wire will do it).

Tony.
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Old 31st December 2003, 06:48 AM   #4
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I have found some time and pulled a couple of the suspect Trannys out and as per the "Output Transistor Failure" guide on http://www.bcae1.com It seems all of them are faulty. They are all reading shorts, or near enough, between pins two and three (with metal back facing away)
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Old 31st December 2003, 10:25 PM   #5
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You might want to read these two links as well.

http://www.tpub.com/content/neets/14...s/14193_65.htm
http://www.uoguelph.ca/~antoon/gadgets/mostest.htm

The second one suggests that the drive transistors may also be blown, since they will likely be much cheaper than the mosfets would probably be a good idea to replace them as well. At $18 a pop for the mosfets, it could get very expensive trouble shooting this amp!

Tony.
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Old 1st January 2004, 03:41 AM   #6
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all of the output devices blown? you sure? lol maybe you should find another website...
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Old 1st January 2004, 04:47 AM   #7
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From the tone of your typing I'm guessing you think this website that has shown me how to test MOSFET may not be all its cracked up to be.
Maybe you could let me know how you would test these parts?

Thanks for the help Y'all
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Old 1st January 2004, 07:12 AM   #8
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Quote:
Originally posted by SkinnyBoy
all of the output devices blown? you sure? lol maybe you should find another website...
Hey skinny boy, the second site I linked to pretty much says that if one goes it usually takes the rest with it, and often the stage before the outputs too! Nasty stuff. I too at first thought maybe the test methodology was dodgy, but both the sites I found had pretty much the same test as Liam found. And the first sites test I think has been sourced from the Military!

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Old 2nd January 2004, 09:20 AM   #9
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I removed all the 2Sks and 2SJ and turned the amp on and guess what?

The fuse didn't blow.

BUGGER!!!!

Looks like I'm up for a hundred plus dollars to replace these dang trannys.


Thanks for your help people.

Liam
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Old 2nd January 2004, 10:08 AM   #10
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You probably still need to determine what caused them to blow in the first place! You don't want to replace them all and have them blow again!!!

touching the inputs wouldn't IMO be enough to blow the output mosfets, sure it will cause hum, and maybe you will act as an arial for RF but....... I assume that the fuses that are blowing are in the + and - rails of the actual amp, and not in the AC before the power supply. Generally speaking I think that the fuses should blow before any damage to the output devices occurs (assuming it's designed properly).

I'm no expert on this BTW, maybe you should seek another opinion! And maybe even see if you can find someone who's willing to test your mosfets for you.

Tony.

edit: Maybe you could ask the mods to change the title of the thread to Help finding fault with Playmaster Sub AMP. Might get more (of the right) people reading it.
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