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Old 19th March 2005, 09:53 AM   #11
djk is offline djk
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Default W

No variac please.

As someone else said, use a lightbulb, try 75W~100W.

The little arrows indicate connector plug contacts.

Heatsink at speaker out potential.

All small signal transistors with gold leads must have the solder wicked dry, and then replaced. 4% silver if possible.

Measure voltage at Q3 collector, should be -0.7V or so.

Check collector voltage on Q6 and Q4, Q6 should be about 3V higher.
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Old 19th March 2005, 11:07 AM   #12
jaycee is offline jaycee  United Kingdom
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Hmm OK... perhaps MOSFET gates are sensitive enough to pick up nearby charge and switch on when the drivers are removed... pity, that wouldve been an easy test

You could try unsoldering the drivers and soldering 2 wire links from the emitter hole to ground, this would make sure the MOSFETS are definately off. I'm not too sure about this though - anyone else care to comment ?

It might be worth checking neither of the 1N4005 catch diodes have blown short and thats whats putting rail voltage on the output.

Also, you could check (with the power off and caps discharged) if theres any continuity between the heatsink and the -70V rail when powered off - if there is thats a sure sign theres a shorted device.
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Old 19th March 2005, 07:02 PM   #13
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Thanks guys,

I'll check the things you've suggested. I put it away for a couple of days to let my head settle. To many scenarios wheeling around in there right now. I'm forgetting what I've tried and what I haven't. I'll pull it back out on Monday and give it another go.

The traces on the board are a little fouled up. Three of them so far just went away at the pad when I tried to resolder them. I've had to put in wire links. I hope this isn't going to be the case throughout. I may just have a box of parts here. Hope not.

I'll let my head clear a bit over the weekend and dig into this again on Monday.

Thanks so much for all the help. I'm learning a lot if nothing else.

Blessings, Terry
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Old 20th March 2005, 08:00 PM   #14
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Each MOSFET should be insulated from the heatsink with a mica washer and white grease. Can you see the mica washers underneath each MOSFET?

The case of the MOSFET is the source, the two pins are gate and drain.

You seem to be observing drain voltage on the heat sink and at the output.

Check the wiring to the MOSFETs on the back side of the circuit card and the back side of the MOSFET mounting socket. Voltage to the gates should be around .5-.7 volts, polarity depending on channel. The wiring connection to the drain tab is the rail voltge. The source voltage should only be that of the output AC signal.

The --> symbols indicate how the wiring is run over or under something. It relates to the wiring layout and is not an electrical symbol.

You could pull the 2SJ50 mosfets and see if that stops the -65VDC from appearing on the heat sink itself and at the output. The 2SJ50 is a P-channel device and is located on the negative side of the circuit. Generally a MOSFET will fail "open" but if one were shorted to allow drain voltage to get to the source pin then you would see rail voltage at the output.

You would only see rail voltage on the sink if it somehow wiring to the drain connection got shorted to the sink. The sink should be insulated from any circuit connection.
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Old 20th March 2005, 08:03 PM   #15
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Show us a picture of the insides. The circuit cards from a Hafler would work with the voltages and MOSFETs if they can be mounted properly. Just a thought. Can you see how the circuit cards in your P-225 could be wired up inside to replace your present circuit board?
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Old 21st March 2005, 06:14 AM   #16
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Click the image to open in full size.

This is actually a picture of my good PCR800 but they look the same.

As you can see, the heatsink mounts to the PCB. The MOSFET's pins pass through the Heatsink and solder directly to their respective traces on the board. There are no sockets. I checked both PCR800s that I own and in all cases the MOSFET cases are connected to the heatsink without any washers, only grease. I did a continuity test on each MOSFET and they all are a direct connect to the heastink. The mounting bolts for the heatsink are what is soldered to the output traces. There is no other way that the MOSFET case is connected, only though the heatsink. I will pull the MOSFETs one at a time and see if the voltage goes away. I can't really check the voltage at the other two pins right now because I don't want to run it at full voltage again until I get this figured. I just glad I didn't touch that heatsink when I had it a full voltage. Also, I had said there was rail voltage at the output but it is actually less than full rail I don't remember exactly how much less but it is some. I will check that as well and report back.

It would take some special work to make a Hafler board work in one of these.

Truth is, I think these Soundcraftsmen amps sound better than Haflers. At least to my ears they do.

Thanks for all your help. I will keep you abreast of what I find when I pull the MOSFETs.

Blessings, Terry
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Old 21st March 2005, 07:10 AM   #17
karma is offline karma  Canada
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i just picked this one up on ebay. good to here thay sound good


this one works but the case is dented. and the fan needs some work.

i was told the fan has 2 speeds is this true?
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Old 21st March 2005, 07:16 AM   #18
djk is offline djk
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"Each MOSFET should be insulated from the heatsink with a mica washer and white grease."

Why?

The extra thermal resistance would require a larger heatsink.

" I had said there was rail voltage at the output but it is actually less than full rail I don't remember exactly how much less but it is some"

Fine and dandy. How much? I inferred it was stuck to one rail. Is it like 4~5V or so? Replace C9.
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Old 21st March 2005, 02:00 PM   #19
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djk,

Once again, you are correct. Less thermal resistance without the mica washers. To my experience not having the heat sinks insulated is unusual and certainly requires special precautions for the unwary amateur such as myself.
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Old 21st March 2005, 03:39 PM   #20
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Hey thanks guys.

I will replace C9 and see what I get, before I start pulling the MOSFETs. I had thought about replacing all the 1N4005's but I measured them with the diode test setting on my multi-meter and they were all very close to each other. I'm trying to not do any more unsoldering/resoldering than I have to since the traces seem to be so weak. I've already had to put in three jumper wires because of the traces receding.

I agree that having the heatsinks "hot" is a little more dangerous, but these are enclosed inside the case and in a healthy amp they only carry the output current which would be as safe as touching the speaker wires.

I'll rig up a light bulb to do the rest of the testing. I have been using my Variac set at about 12V so far, but if you guys think that is risky I'll stop doing that. Question; with the light bulb setup, do you go ahead and run it at 120v? Can I still use the Variac in conjunction with it so that I'm not running that high of voltage? I'd hate to get shocked because I happened to brush the heatsink.

Thanks again, Terry
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