VWaddict

If the transistor really gets hot (and they certainly can!) then it might be worth looking to see if there's a white, powdery, chalky-looking residue where the transistor meets the heatsink. -If so, then its a wonderful opportunity to replace that stuff, which would be dried-up silicone heatsink compound.

I don't know if they use any or not, (since I've never had to even pull my DME) but if so, then you can get the old stuff off with rubbing alcohol on a cotton swab, and replace it with new stuff which is a buck or two at Radio Shack.

If anyone has info, it's be great. However, a hot transistor on a warm-ish heatsing usually means poor thermal conductivity at the junction which either means no thermal compound, or dried-up-and-worse-than-useless thermal compound. Either way, smearing a thin layer on halps heat transfer and cools down the transistor, easing both the solder joint strain and the transistor's service life expectancy.

There may also be an insulating washer on these transistors (known as "TO3" when they're that shape) because there are two pins for connecting to the 'base' and the 'emitter'and the third connection, the 'collector' is made to the case. Sometimes the collector may be at +12V and the heatsink may be at chassi (ground) potential, so there's a washer and some equally vital insulating shoulder-washers which prevent the mounting screws from touching both the transistor case and the heatsink... illustration here:

...-in which case you clean and re-grease BOTH sides of the insulating sliver. -be gentle, it can break, although the Rat Shack should also have replacements if you do.

By the way, a "2N3055" is a particular NPN version of a TO3 transistor, different to this one in all likelihood, but the mounting method and electrical insulation/heat conduction approach is universal to all TO3's.

-Just in case this information is of any use to anyone...

Cheers!
Keith

Ed 944T

I have already's wondered what a "rebuilt" DME was... Maybe all they do is resolder that transistor? No wonder they want a core...

gregeast

My understanding is that they resolder all the components and, one would hope, go through a final test procedure to ensure everything is connected properly. That said, I'm liking free for essentially the same thing as opposed to the $375 for a rebuilt one .

Ed 944T

Being involved with electonics for the past 35 years has taught me that most units of this nature exhibit a common failure mode. I have never had problems with the DME's in any of my 3 944's, but if I ever do, your problem will be the first place (and hopefully the last) that I look at. Thanks, Greg..

Mike B

Ditto...Great pics Greg...Thanks!

alordofchaos

Greg, Keith, good info!

I figured that if I had DME problems, I might tackle the solder job. But thanks to your pics and Keith's info, I might do the DME as a preventative maintenance some winter.

Cheers!

VWaddict

If you were to to a 'shotgun' resoldering job, I'd caution you to be VERY cautious about some other components on the circuit board. -The black rectangles with little rows of silver 'legs' running down both sides are IC's, which will be VERY sensitive to heat from a soldering iron. You're much better off being fairly swift or at least reasonably experienced before you try too many of those... heat will kill the chips if you leave the iron on too long.

-Also, a tip which I usually share with those who aren't absolutely confident in their soldering skills, is to try and do no more than one pin at a time on anything which is heat sensitive (transistors , diodes , IC's... basically any semiconductors). -SO rather than doing all 14 (the number varies) pins on an IC and then moving on to the next IC, do a pin on one IC, then give it plenty of time to cool down while you're doing one pin on a second IC, and then a third and maybe a fourth IC... then go back and do a second pin on the first IC and so on. -It takes only marginally longer, but the benefit is that you're not heating up a second and a third pin on a chip which has also gotten very hot from a slow soldering job on the first pin... the added heat soak of pumping more and more heat into a sincle IC with no cooling period can be the thing that kills it.

The big silver TO3 power transistor is designed to dissipate (and therefore tolerate) a good amount of heat, so there's little risk there, but if you were to extend the resoldering job to other devices which can't cope heat quite so easily, it's much better to be quick. -And DON'T EVEN THINK about using a soldering gun like this: ... Use a soldering iron of about 25 Watts power, or a temperature-controlled iron of about 45-55 Watts... The guns WILL burn components! -They're for 'welding' rather than small component soldering

-If you're in any doubt, you may be able to call around a couple of local TV repair shops to see how much they'd charge for a quick freshen-up of the solder, or even a decent local technical college which teaches soldering... just make sure that it's a reasonably experienced instructor and not a student doing the work!

-Anyone in Orlando is welcome to bring a DME round to my place and I'll do any number of them for free.

Keith

gregeast

Based on my research on the list it appears that those 3 pins off that transistor are the common failure point. It would certainly be worth a look and maybe even preventative resolder on those leads the next time you have your DME out.

Keith: Thanks for all the good insight into the process and technique! I hate intermittent problems and I'm quite pleased that we may have slayed the DME dragon for everyone.

testarossa_td

What he said^^^

joes

I have several bad DME's. Is there anyone who repairs them?

gregeast

Systems Consulting http://www.systemsc.com/

No experience (thankfully) but others have used with good success.

ibkevin

Here's a file photo of the back, looks like the insulated style.

VWaddict

Yes, peering closely at the first picture, it looks like they do use an oversized mica (or more likely synthetic) insulator. -I see no signs of any heatsink compound however, which I would expect to result in the transistor running hotter. -It's then entirely posible that the heat travels down the thick leads to the board, where the solder jooints are warmed up... not actually melted, but warmed up. -This places the solder closer to its eutectic temperature region, and CAN result in this sort of embrittlement, as I mentioned earlier.

-If anyone wants me to take a look at a DME or two and try a rework, I'd be more than happy to. -I can photograph the procedure and include Radio Shack catalogue numbers to help complete the write-up.

The only car that I have to test the DMU's out on afterwards is my 951 which is an '89 with APE chips in it, -if something like a NA 944 would be needed, I'd have to call on teh services of someone else locally, or I could ship them back to whoever resoldered and with heat transfer compound added, but untested. -Of course there may be OTHER things wrong wiht the unit, but -as someone else mentioned earlier, there's some recurring "usual suspects" as soon as you look inside ANY piece of electronic equipment, and 'heat-generating' or 'outside-world-interfacing' components take the brunt of this.

Other than the heat and current flowing through a low-side coil drive transistor (which this presumably is) there's sometimes a shunt-diode which is there to stop the back EMF from the low side of the coil destroying the power transistor. Sometimes these can fail 'open', leaving the transistor vulnerable. SOmetimes they can fail 'hard-short', which means that the whole thing is prevented from working... -Easy tests either way.

-So if anyone wants to see if any dead DME's out there can be resuscitated by simply resoldering, adding some thermal transfer compound, tidying up and teting (and if necessary) replacing the "usual suspect" components, I'll happily give it a shot for free. -If it works, we'll all have a documented write-up for everyone to follow. -If not, nothing lost.

Keith

alordofchaos

More good info - thanks!

Any reason I shouldn't use Arctic Silver compound on the DME? Stuff is normally used between computer processors and heatsink/fans, thought it should be OK for the DME.

VWaddict

I don't know that stuff specifically, but it may possibly be snake-oil in that I doubt it's better than the white-silicone-heatsink compound. -It -on the other hand- may be no worse either, so who knows... but either is probably better than nothing. -It does have to be NON-conductive electrically (seeing that there's an insulating washer), and whereas this may not be an issue for CPU heatsinks and the like, (if for example they're coupling to a plastic or ceramic chip surface) it WOULD be an issue here, and often things which are metallic in colour tend to be electrically conductive, though this is a rule-of-thumb and not a hard fact.

With TO3 and TIP transistor packages, every manufacturer that I've ever seen uses white silicone compound when there's an insulating washer, the exception being the soft, rubbery-feeling beryllium insulators, which are 'self-moulding' and are sometimes used without any sort of coupound.

If it's electrically conductive though, don't use it.

Keith