STK4050V and it's use in Definitive Tech PF15TL subwoofer - Page 2 - diyAudio
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Old 13th March 2007, 04:08 AM   #11
rkc7 is offline rkc7  United States
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Well, you were right about the resistor. After spending a few hours going over the board with a multimeter and not finding anything strange followed by lots of thinking, I took a few resistors, strung them together to get around 18-20K, put them in and put it all back together and plugged it in. It worked!

I would have hooked up my ammeter to it and used my variac to see if it was going to draw heavy current, but considering it turned on previously with no bad effect and finding nothing but a burnt resistor and everything else OK, I decided to take a risk. Fortunately, it worked out.

I'm still waiting on Definitive to send me any information, hopefully technical, on the subwoofer, which I'll be happy to share. It'd be nice to have for the future, just in case.

I still have to replace my make-shift "resistor array" with a single multi-watt piece, but I'm glad to know this was one of the easiest amp fixes I've ever had.
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Old 13th March 2007, 05:35 AM   #12
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Default well that was easy...

congratulations, it's so nice to get an easy fix every once in a while.

Do you notice the resistor getting hot in normal use or under muting conditions?

My most recent easy 'fix' was to replace fuses in a 'dead' Sony receiver, the previous owner had replaced it before he asked if I wanted to take a look at it...oh well, now I have a nice home theater receiver.

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Old 13th March 2007, 03:56 PM   #13
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I unplugged it over night, but this morning I ran it for about 15 minutes at moderate volume and the resistors were about 125-130 F, or about 52 C.
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Old 13th March 2007, 07:55 PM   #14
rkc7 is offline rkc7  United States
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The Definitive guy got back to me. He sent a schematic. The resistor that blew was R89, which is on page 5.

He also told me they don't fix these amps, they just replace the entire board... which is $225! That's about 3x what I paid the guy for whole thing, lol
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Old 13th March 2007, 08:57 PM   #15
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Default Very interesting...

A note in the schematics mentions a reduction in the idle current at mute...not sure if it matters, since it looks like the resistor in question is most stressed under normal operation...

A couple of things are clear from these diagrams...

Assuming your model has the same voltage rails listed on the schematic, the 19.1k resistor sees 24+66v under normal operating conditions, which gives 0.42w dissipation, a little close to the 0.5w rating for my taste. Inserting a 5w resistor would probably be overkill...1 or 2w should have a long and useful life...

The schematic 'claims' a rating of 250w into 4 ohms, presumably with 66v rails, but then claimes the limiter is set to give a peak of 360w into 4ohms (180w RMS). There is a little comment that suggests there are other versions with different power ratings, perhaps there is some mixing of settings here...

Have fun with your new toy, I'm assuming it is loud enough...;^)

Stuart
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Old 31st October 2008, 07:26 PM   #16
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hi everyone- I know this thread is really old, I found it via a google search. I recently bought a Definitive Tech PF15TL+ and it is in mint condition. It worked great for about a week or so (never had the level above 40% or so, and the guy who had it before me was careful with it as well) but then today it suddenly stopped working. The amp power LED is still on, and it makes a tiny pop when the auto power-on kicks in, but zero output.

Sounds like the exact same problem the original poster stated...

Now..Im not an EE by any means, if I took the amp apart Id understand that Im looking at a bunch of resistors, FET's, and what have you...but I have no idea how they actually work together to make an amplifier. Anyway, I have no idea where to start with diagnosis.

Anyone have any ideas that might be able to help me out? I have a multimeter and can solder anything, but I just dont know where to start checking... ie any advice like "ohm resister so-and-so and if its below X ohms its dead, solder in a new one", etc...

thanks for the help!

ben
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Old 31st October 2008, 07:41 PM   #17
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Just open it up and have a look. If you see something that looks burned or broken, try to identify it or post photos.
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Old 31st October 2008, 08:39 PM   #18
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Here is the only thing that might be wrong?? I dont know if those two resistors are supposed to be that color, they look a little brown???

should I pull the actual amp and transformer out of the sub box? As long as its unplugged its "safe" right? I dont know if any of those capacitors hold a nasty charge even when unplugged like CRT tv's do...

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Old 1st November 2008, 01:18 AM   #19
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the culprit is a resistor, as posted, the R89 resistor that is 19.5 kohms or is it 19.2 kohms? it is quite difficult to get but you can find it at mouser electronics for only $0.10 a piece. Do not touch the STK or the capacitors - they should be ok. happy fixing.
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Old 1st November 2008, 01:30 AM   #20
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in the Definitive Tech PF15TL, this resistor is close to the STK4050v. When tested, this should be open...meaning it will not allow current to flow.
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