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Old 3rd January 2008, 02:00 AM   #1
Orbus is offline Orbus  United States
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Default MOSFET questions

Hi all.

I recently got ahold of four Ashly FET-500 power amplifiers that were retired from a movie theater. When I got them, three were in perfect working order, and one had a dead channel (I got a hefty discount on that one). I've had one hooked up in my living room for a few months, until I can get a rack built and set them all up.

Unfortunately, the other day, misfortune struck, and one of the channels on the one I've been using just up and died. No warning, one day it just stopped working. I'm not an electronics whiz by any means, but my dad has some amplifier experience and I'm not afraid of a little experimentation.

Anyway, I just finished doing some swapping - I moved the entire output section for the good channel out of broken amp A into broken amp B, so I now have one completely working amp, and one completely dead amp.

So far so good. Now the question is, how do I fix the completely dead one? My dad has suggested the first place to start is testing all the transistors and replacing any that test out of spec. This amplifier uses a K135/J50 transistor combo, which I gather are a bit hard to come by these days. If anyone's got any they could part with for cheap, I'd be interested in hearing about it.

My main question right now though is... how do I get the darned things off? There are screws holding them on, but even after I removed the screws, the transistor appears to actually be stuck to the PCB with some kind of adhesive. Looking around online, I found some adhesive gaskets for TO3 type transistors - looks like that might be what was used here, as I can see gasket material peeking out around the edge. Should I just pry them off with a screwdriver or what?

Cruddy cellphone pic:

Click the image to open in full size.


The other question is, should I even bother trying to test for bad transistors, or would I be better off simply replacing them all? That might get pricey though...

Oh, and for those who are interested, here's a pic of the whole amp with the hood off:

Click the image to open in full size.

The whole heatsink/pcb assembly on each side is only held in by a couple of screws and some (very tenacious) wires, so I was able to swap the whole module out from the other amp. So far, so good!
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Old 3rd January 2008, 04:21 AM   #2
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Two things:
1) Heat sink compound (aka "grease") has a tendency to dry out due to heat and age and can cause parts to stick. It's not likely that anyone actually glued the devices in.
2) If the transistors are mounted in sockets underneath, then all is well. They should simply pull out unless the heaksink compound is holding them down. On the other hand, some designs solder them to circuit boards or to wiring underneath, in which case you'll have to work at it a bit.

Grey
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Old 3rd January 2008, 05:54 AM   #3
Orbus is offline Orbus  United States
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Thanks for the reply. It's too late tonight, but as soon as I get a chance I'll tug at it a bit and see if I can make any headway. It seemed like they were stuck on there pretty good, but I'll give it another go.

These amps have got to be at least 20 years old. All things considered, they're still in pretty good shape. And they sound great, at least when they're working.
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Old 3rd January 2008, 11:42 AM   #4
djk is offline djk
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You can pretty much see in the photo that they are soldered in.

Right next to the outputs are the driver/Vas, they look like MPSU10/U60. Check those first.

This is not a first-time project.
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Old 3rd January 2008, 12:41 PM   #5
Orbus is offline Orbus  United States
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Well, it's alright - my dad built a heathkit amp back in the day, (nice one, and it still works too), so he knows what he's doing, even if I don't. I just wanted to see what kind of headway I could make on it while learning, before I haul it over there and hassle him with it.

What makes you say they look soldered in? I don't see any evidence of that from the picture. Inspection of the bottom reveals you may be right (although I'm not sure - I have no basis for comparison to other TO3 transistors, so the solder points underneath may simply be where the receptacle is joined to the PCB), but I would like to understand what you see on the top side that makes you say that.

You are correct about the MPSU10/60. There are two pairs of them. I'll check them as soon as I get a chance.

This is all a learning process for me. And the amplifier's already broken, so it can't get too much worse at this point, as long as I'm reasonably careful.
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Old 4th January 2008, 09:30 AM   #6
djk is offline djk
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"Well, it's alright - my dad built a heathkit amp back in the day"

That's what we used to call a 'shake-the-box' kit, no technical knowledge required.

"I don't see any evidence of that from the picture."

I'm guessing, but I don't see enough space between the board and heatsink for the socket to be. The back side photo is too out-of-focus for me to see.

"You are correct about the MPSU10/60. "

I thought so. If you find any for less than $15 a pair, let me know.
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Old 4th January 2008, 01:29 PM   #7
Orbus is offline Orbus  United States
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I would agree with you about the heathkit, but you don't know my dad. He spent three months putting it together simply because he didn't want to finish until he understood everything that was going on inside. That's far from his only electronics experience anyway - just his only amplifier experience that I am aware of.

I'm not trying to say assembling one amplifier makes him as qualified as some of the folks on this board, because I know that's not the case. I just mean to say that he does have some idea of what he's doing.

Anyway, I'll ask back if I have any further questions. The solder points on the bottom suggest you are probably right (I don't have time to take more pics right now). Next step is probably to desolder one and see if it comes off. I'll also test the MPSU10/60 transistor pairs. You can't really see from the pic, but the thermal grease that was on the heatsinks has broken down over the last 20+ years and made a bit of a gooey mess on the board. Nothing too bad, but I should probably clean that up and apply some fresh compound.

With the amplifier being this old, I'd probably be smart to think about replacing the capacitors as well. I know one looked a bit suspicious to my untrained eye.
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Old 4th January 2008, 02:31 PM   #8
Bobken is offline Bobken  United Kingdom
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Hi Orbus,

For what it is worth, I agree entirely with djk's comments.

It doesn't look to me as if there are any sockets between these Mosfets and the PCB, and their 2 legs are most likely simply pushed through this board and soldered (as normal) on the underside. To3 sockets are quite substantial devices, which, even with an average pic quality like this, should be obvious

Before you begin, I hope that you are aware that these Hitachi Mosfets are sensitive to static, so suitable precautions should be taken when de-soldering them. Otherwise, they will blow for sure, whether they are already damaged now or not.

Good luck.

Regards,
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Old 4th January 2008, 03:51 PM   #9
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You can purchase replacement "Exicon" lateral MOSFETs in TO3 from Profusion PLC -- the Renesas units are either TO3P or TO-220, I've ordered from Profusion and the service was quick and not expensive. Here's a link to the data sheet:

http://www.profusionplc.com/pro/gex/...teral%20mosfet
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Old 4th January 2008, 03:53 PM   #10
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Quote:
Originally posted by djk
find any for less than $15 a pair
Mr Kleitsch,

they're still on sale for $13/pr at AMS in New Jersey.
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