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Old 5th July 2006, 08:54 AM   #11
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Thumbs up Solution?

Okay, I think I've worked out what is happening. Of all the excellent suggestions above, the one that turned out to be most immediately useful is also, I suppose, the simplest--checking continuity.

When I put an ohmmeter on the heat sink or ground of the unaffected side of the amp (contacting the heat sink itself is not easy, I had to find a place where the paint is scraped off to get a reading, and even scrape a little more off--so what was ultimately easier was finding any good ground point in the amp),... when I check the good side by getting a solid contact on the MOSFET case and the other lead to a clear ground contact or solidly on the heatsink, on each of the 4 MOSFETS I get a reading of around 800-1000 ohms, after a few seconds. This actually seems like a gradually increasing reading, which may be an artifact of my meter, or more likely an effect of measuring resistance which builds up on the transistor from initially around 700 ohms to 1K ohm or more. I found the same readings on all the MOSFETS on the right side of my POOGED amp, as well as roughly the same on all the MOSFETS on both sides of my original unmodded DH-200.

When I put the meter on the problematic left side, I got almost no resistance. Maybe .1 or .2 ohm, essentially a short. I got the same reading on all 4 MOSFETS, not just the "hot" one--which threw me at first. Slowly, I backed off each of the screws on the upper MOSFET that had gotten hot so quickly. The top screw had no effect, and I snugged it up again. I backed off the lower screw, and everything changed immediately. This took only about 1/4 turn of the screw. Once the pressure was off, all 4 MOSFETS gave a "normal" reading. TIghtening or loosening the screw slightly gave a very clear result on all 4 of the cases, which obviously have continuity between them.

Crossing many fingers, I plugged the amp in and fired it up. No smoke, no heat. I wired the speakers and preamp, and played some music. Eureka! She seems none the worse for wear. Well, possibly minimally worse on the right channel, possibly the softest of humming on the right side now, which in any case is unnoticable and totally swamped by even the softest music. Actually, that might only be my apprehension. The faint hum is probably no different than before. I've been told these MOSFETS are sturdy, but with all this shorting and the high temps, I would have thought they would have been toast.

By the way, the modded DH-200 sounds significantly better than the non-modded one, and is much quieter too, even though the mod was done many years ago.

Something I haven't mentioned yet that added to the evidence is that I realized upon closer observation that this MOSFET has had a problem before with the previous owner, and a fix was applied. The insulator (inside the heat sink) is clearly not original, and some kind of makeshift one has been fabricated. It is oversized and obviously does not do the job exactly as it should. Tightening the lower screw down to just after it starts to bite solidly results in a short. So the question arises, do I need a new insulator, or even a new MOSFET?

As I see it, I have three choices: 1) I could do a MOSFET replacement. 2) I could see if it is possible to replace only the insulating materials on this MOSFET. I haven't tried removing it yet, which will require unsoldering leads, so I don't entirely understand yet where the short is made. Clearly, these are designed to have thermal contact to the case, but electrical isolation--I don't know how that is accomplished because the screw goes through part of the transistor case, then through the heatsink and the insulator.

Lastly, I could 3) just leave things as they are and remind myself NEVER to tighten that screw again. With luck, I might get decent service from this amp for years more without fooling with it any more. It is actually pretty musical.

Dick, the PCB is one of the older types, not the newer green one I note you mentioned in one of your posts. These PCBs are pretty beat up already and won't take much more working on them. The transformer has beginning signs of heat breakdown too, so possibly I should use this a little longer until it fails or I'm feeling a need--and the capability--to do something totally new with the case. The serial # is 3111035.

The above is more than enough for now, though I still have some other thoughts rumbling around. For one thing, I keep wondering how this amp's sound compares to the mod with the Musical Concepts boards, and other options I've heard mentioned on these pages. And I need to tackle my other DH-200 next. Does someone have an extra insulator lying around?

Again, many thanks!
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Old 5th July 2006, 09:43 AM   #12
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With the mileage and obvious bad previous maintenance of this Hafler i'd remove all of the TO3 devices and reinstall them with fresh insulators and thermal paste. If you take the mosfets out you also have the chance to check if they are all OK, there are plenty posts here on diyA on how to inspect a mosfet with an MM.

Time and heat age mica insulators and dries out thermal paste, thermal transfer between all of the Hitachi mosfets and the heatsink is likely to be below the optimum. Solder joints also degrade over time, reflowing solder on all joints is a treatment for +20 year olds that would prevent trouble in that department for the next 20. A DH-200 is fairly simple and modular, so it is not a huge undertaking.

TO3 micas insulators are dirt cheap but i gathered mica quality in the US is poor compared to the grade that is common overhere.
In contrast, aluminium oxide insulators are much cheaper and of higher quality in America. The Avid Thermalloy Al-Ox. insulators i got thanks to the highly appreciated help of Mark Gulbrandsen do a great job on my Krell KSA output stages.
I've been using that kind of insulators for Hitachi mosfets output devices also in the 1980s. The Hitachi devices are an extinct breed, i'd give them the best possible care iiwy.

Mosfets do not suffer from secondary thermal breakdown, the resistance between drain and source becomes high when the device is overloaded. That limits dissipation and protects the mosfet from killing itself if the current short situation doesn't last too long. Once cooled down the drain to source resistance goes back to normal and they are ready for showtime again. The case of a TO3 is the source connection of the mosfet. My experience with Hitachi power mosfets is that they are indeed incredibly stirdy devices, no idea what a short does to the remaining life expectancy of those transistors. Everybody was fooling around with Hitachi based DIY stuff overhere in the 80s, i've salvaged many Hitachi TO3 mosfets from horrible diy projects. With exception of a few that had really gotten the trash can treatment they worked like new in a bunch of my fumblings.
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Old 5th July 2006, 11:38 AM   #13
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Hi Michael,
I agree with what Jacco has said. I will add that you need to disconnect this unit now. Do not run it at all until you have remounted the transistors. That one you loosened will overheat if you don't do this.

You must clean all the old grease and debris from the heatsink and transistors before attempting to remount them. The transistors can be static sensitive, so handle them as litle as possible, never by the two leads.

Keystone is one manufacturer that sells TO-3 mica insulators (100 for about $4 USD). I think Radio Shack still sells them in single kits, MCM, Mouser, Newark and DigiKey should as well. New grease should be available from the same sources.

-Chris
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Old 5th July 2006, 01:32 PM   #14
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Michael,

As I read your last message I think you are talking about the socket that mounts on the back side of the heat sink. You can loosen the screws that hold the heat sink to the chassis and also remove the 3 small screws that hold the PCB to the heat sink. Then, you easily can get at the back side to unsolder and install a new, proper, socket for that MOSFET. The other advice about cleaning and regreasing everything and install new mica insulators is spot on!

I did many, many mods to my previous 2 DH-200s (back in the 80s) and still have the original TAA article describing the POOGE process. Later, when I graduated to the Musical Concepts recent PCBs, I realized how much time had been wasted doing DH-200 mods. The MC boards really make a wonderful difference in sound all up and down the musical scale.

Your PCBs (phenolic) do not have through plated holes and their traces are very fragile to work with for modding.

BTW, your amp was sent from the factory the 11th week of 1981 and it was a kit, not factory wired.

If you listen to a lot of rock and heavy metal, mod the DH-200. If you like string quartets and piano concerti and Diana Krall, get the MC PCBs and install them. And, be sure to get the pair of power supply filter caps from MC. You won't be sorry.

Once again, my $0.02
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Old 6th July 2006, 06:32 AM   #15
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I will take Jacco's advice to heart. Actually, I was about to order some stuff from Parts Express. They have an item listed as:
TO-3 SILICON INSULATORS 10/BAG for about $2.

Does that sound correct? Two bags is enough that I could repack all the MOSFETS on both my DH-200's. I also need to check if this is the thermal grease I want:
NTE 303 THERMAL COMPOUND $2.33.
(These are some of those items for which PE has almost no info listed. I also have on hand some thermal grease for CPU processors--I suppose I could use that.)

Do I just coat the hole and transistor bottom with this after cleaning off the old grease, and then replace the transistor? Just so I don't get things mixed up (I have not yet pulled a transistor on these amps), are the MOSFETS on one side of the heat sink the same, and the two on the other side the same--but the two sides different from each other? (NPN vs. PNP??) I will try to label them and put them back where they were--but just in case! I also will try to find the forum posts on testing the MOSFETS with a MM.

Then there's that socket. I am really not sure if I am finding the right thing. I will check tomorrow with PE if they think this is the correct item. --Does it seem to you like this could be what I am looking for?:
NTE 209 3 TO SOCKET $2.22 EA.

Dick, I am the latter type of listener, definitely not rock but classical and yes even Diana Krall. So I see now I need to consider carefully what I will do with my second DH-200. The TAA mod (I have the article too), or buy the MC PCB's. Mmmmmmm.
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Old 6th July 2006, 09:28 AM   #16
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Silicon insulators have to be torqued very evenly and not overtorqued, they're very fragile imo.
Thermal and isolation properties of silicon pads have improved much in 15 years. Thermal resistance went down from 0.5 C/W to 0.15 C/W or less( <1.45 C/mK), electrical isolation up from ~2.7kV to above 6kV. Thickness of silpads went down from 0.25mm to less than 0.20mm, downside is that they damage even easier so they'll have to be handled gently.(bare in mind that i'm a klutz and a brute, i'm not allowed near any wine glass)

For the TO-3 socket you'd have to check what boss height you want.
I think you can get TO-3 sockets cheaper than PE lists,
here's a file with types and pictures=>

www.keyelco.com/pdfs/prod22-trans-sockets.pdf
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Old 6th July 2006, 01:44 PM   #17
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Hi Michael,
Jacco is dead right. He has pointed you to the Keystone website. You can find a dealer list there. I still use mica and grease.

You need to clean the mounting surfaces with a solvent, like lacquer thinner (cheap at a pain store). Get rid of all residue. The grease would be applied evenly to the transistor, stick the insulator on and push out the air bubbles, apply grease evenly to the insulator. Then you mount the transistor. I find an artist's #2 paintbrush works very well for applying grease.

Jacco's tightening advise is valid no matter what you use. Do not overtighten as most people seem to do.

For the socket, don't sweat the boss height. Get long ones. If they are too long, use a razor blade to trim to height. Mount the socket and run the blade along the surface so it trims to the surface. That's the safest way. Even then, all you really need is the screw to be centered. So a short one would locate things okay as well. Minor detail not worth worrying about.

-Chris
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Old 6th July 2006, 10:47 PM   #18
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Okay, it looks like my best bet for small quantities is to order this from Mouser. They have the Keystone TO3 mica insulators #534-4636. I'll get 20 for $2.40, and the Keystone socket #534-4601 @ $1.56. The boss height on these is .09, which is the largest on this series. Well, the heatsink is almost 1/4" thick, so no way is that going through the heatsink all the way--I gather it is just there to position the socket and the screw, right? I've got about 3-4 small tubes of cpu heatsink grease around, which should be plenty to do 8 or even 16 transistors (both Haflers), on both sides of the mica. I've got both laquer thinner and acetone for cleaning the heatsink, as necessary. And what the heck, I'm finally going to get a static control wrist strap, after all these years (even though I've never fried anything yet by handling it).

Jacco, is mica a better insulator thermally and electrically than silicon? BTW, I'll watch out for wine glasses. No drinking and driving or remounting transistors.

BTW, I will be doing some rewiring on the other DH-200. In the old POOGE articles, they used fancy multiple-small wires, a little like Litz wiring (I have this on my modded one), but I've noticed most people just use primary wire inside an amp nowadays. Do you guys have any opinions on the wiring controversey? I was thinking of getting fancy and using a few pairs of CAT5 wire pairs in a braid on the leads to the input and output terminals from the PC board, but just use regular 16 gauge primary wire to the power supply. (The original was 18 gauge wire all around.)
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Old 6th July 2006, 11:02 PM   #19
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While I am ordering from Mouser, is there anything else I should consider getting for my Haflers? I seem to recall somewhere a comment about the protection circuit or adding a thermistor?
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Old 7th July 2006, 12:11 AM   #20
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Hi Michael,
For now, just fix it. Don't use CAT-5 wire for your high current leads. There is no reason why it would be better. If you are truly worried, get some fine strand wire in a slightly thicker size and use that. Much better and you still may not hear a difference.

Well, you might if you listen with a bunch of friends. Wine or beer would help bring out the finer detail.

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