PACE PRC2000 / TIP107 failures

I've got a PACE PRC2000 Rework station...nice piece of equipment...
A few weeks ago, the microchine (fancy name for a dremel tool type thing) goes nuts, runs at full speed, status LED on the front of the PRC2000 goes out, microchine keeps running, and smoke starts rolling out of the unit. So, I open 'er up and start troubleshooting...
First thing I find is a completely smoked TIP31A (Q23 if anybody actually knows which one that is!) and a handful of charred resistors and capacitors. So, I replace all of the obvious smokes parts, and also the not-so-obvious/slightly charred parts, 'cause that's what a guy does with an expensive piece of equipment. I used exact replacement parts, tolerances, composition, etc. It's all good...
So, I plug the microchine back into the unit and give it power...
The microchine starts running again, full speed, status light is red for a bit, then goes out, microchine keeps running, and...yep...smoke starts rolling out of the unit...AGAIN!!!
Same TIP31A is burned up, but I shut off the unit before it wrecked anything else...
This time, I replaced only the TIP31A (#3), left the microchine unplugged, hit the power switch just for a second or two, and the TIP31A immediately got hot. I hit the power switch again for another couple of seconds and shut it off at the first hint of smoke...
More troubleshooting...
I find a TIP107 upstream from the TIP31A that doesn't seem right.
The TIP107 is a PNP Epitaxial Power Darlington Transistor with built-in biasing resistors. Ok, fine... And according to the datasheet, it's got a body diode (like a power MOSFET) across the collector-emitter.

Now...if I'm reading all the information, and remembering my troubleshooting, I should be able to run the meter (in diode-drop/continuity-check mode) across the collector-emitter pins and get about .6v in one direction, and wide-open in the other direction...Yes?

Well, when I put a meter across it, I got 0v drop & almost 0 ohms) in both directions across the collector-emitter. That tells me that the body diode shorted out somehow, or at least the base-collector junction of the 2nd pair of the darlington pair broke down, either from over-voltage across the B-C on the 2nd pair, or it overheated and ran into secondary-breakdown across the whole thing, which smoked the 2nd B-C junction and probably melted it internally.

What I'm really wondering is if there is a common failure mode of darlington transistors that I haven't read about yet and/or if that body diode is a special thing and makes this TIP107 a special case in some way...

Any thoughts from the analog guru's from 'back in the day' ?

(Notes: That TIP31A is actually a 'braking transistor' that shorts out the microchine's motor when the power switch is turned off or when the 'probe brake' kicks in and stops the motor dead in its tracks. It is 'ON' when the microchine is off, keeping the motor from spinning. But...at the same time, that TIP107 is also supposed to be off, until the power switch is pressed to start the motor, at which time, some logic circuits turn the TIP31A off. It's a bit of a convoluted circuit, but it's obviously worked well for years. But apparently that's what smoked everything. All the juice is running thru the TIP107 all the time due to the shorted body diode, even when it's supposed to be off, and the TIP31A is ON at that same time. And the TIP107 has a much bigger heatsink on it, hence the TIP107 doesn't smoke, and the TIP31A does...anybody follow this at all?)

What you've done is a classic Tekkie line of thinking "Let's replace what's gone brown and crispy and it should all work fine thereafter".

You probably have a simple component failure, but what's smoked isn't the CAUSE - just the end result.

Before doing anything, check the Zener across the motor. It's there to stop any back-emf and over-voltage spikes going back and upsetting the drivers. We don't want to put fresh parts in to have them die immediately.

The Brake Q23 should never be on at the same time as the Driver Q22 - that much you know. Their respective circuits (Q16/17 for Q23 and Q13/14 for Q22) ensure this never happens because the Drive enable and the Brake are both driven from a common source. Obviously this isn't happening. Ignore the centre bit of the circuit Q1/Q4 and everything to the right… that 'convoluted logic' is absolutely NOTHING to do with motor control and brakes – believe it or not, that's all just pretty lights.

Remove Q23 that will stop any brake/short. Put in a fresh good Q22. Run the Motor and check you have speed control. If it's running full speed with no control, check Q13/14. When I say check Q13/14, it doesn't mean it's those parts and they have to be immediately replaced. I expect you to look at their respective voltages and determine if what you're looking at is sensible or not. You could have a dry joint, or a defective piece of track, or a Resistor that's died.

If you CAN vary the speed (remember you have no brake so it'll be relatively unresponsive), then your fault probably lies with either Q16/Q17 keeping the Brake on when it should't be. Remember, when finally returning the removed Q23, that you put back a good one.

The fun part is following the clues and finding the cause. You've got a circuit diagram - it should be a relatively simple task.

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Originally Posted by Melanie

What you've done is a classic Tekkie line of thinking "Let's replace what's gone brown and crispy and it should all work fine thereafter".

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Well, I figured I've got a few handfuls of parts laying around...WHY NOT :) And besides, I've got another PRC2000 to do the rework when I REALLY let a few traces waft across my nose :D

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Before doing anything, check the Zener across the motor.

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Tried the same microchine(s) on the other PRC. Works fine. (Have 3 total microchines, 2 PRC's, and other assorted handpieces.)

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When I say check Q13/14, it doesn't mean it's those parts and they have to be immediately replaced.

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Actually, I already checked those out of circuit, as well as Q16/Q17, on the Huntron (signature analyzer) and a meter. Sig's on the Q's look like any other 2904/2906 I've got laying around and diode drop's practically match as well. Resistors check good enough, cap's are close enough to marked values to call good (+80/-20%, +/-20%, whatever they happen to be. I've done enough desoldering in the past couple of days to replace the filter in my desolder tool and scrape the baffle more than a few times).

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The fun part is following the clues and finding the cause. You've got a circuit diagram - it should be a relatively simple task.

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You know how that ends up sometimes.
I guess the main issue is that this is the first time I've dealt with this type of transistor in practice...the NPN Epitaxial Power Darlington Transistor with built-in biasing resistors and a body diode. I realize it's just a fancy way of saying they crammed some extra stuff into the same TO-220 case, but it's just a bit different trying to shoot those extra bits that you can't get at.
And I can't find any extra TIP107's laying around...Trying to find a substitute to troubleshoot with. I've got a few TIP32's, not a darlington, doesn't have the same hfe, less current capacity..same basic function, different spec's. I see no real reason not to put that TIP32 in Q22 and try the circuit out without the microchine plugged in. With a TIP32 in the Q22/TIP107's place, I shouldn't see any Vce across the Q23, hence the circuit should perform correctly. Right now, with the old Q22 in place, I get a full 14v across Vce in the empty Q23 after power on without the microchine plugged in.

>>I guess the main issue is that this is the first time I've dealt with this type of transistor

Just treat it as a high-gain NPN transistor. You can use any NPN power Transistor, but it's not going to work as well, and you'll need a reverse Diode across it for protection. For the purposes of test and playing with you could try it.

>>And I can't find any extra TIP107's laying around...

Yes you have... "I've got another PRC2000 to do the rework".

>>Actually, I already checked those out of circuit

Noooo... this is still Tekkie line of thinking... remove each part, one by one and eventually I'll find the one that's duff (probably adding faults that weren't there in the first place). Whilst I'm at it, I'll use a milliohm meter to check each piece of track... trouble is, it was probably a bad joint which you've eliminated, the problem goes away but you never found the cause... Yup... I'm confident enough in my proceedures to take that job in the Pacemaker repair shop...

My line of reasoning is this... the Motor Drive and Brake both being ON simultaneously cause a short. So remove the Brake - Do I have Motor Control? No - look in the Motor Driver Circuit. Yes - Look in the Brake Driver Circuit. Let's eliminate whole chunks of the circuit and concentrate on looking at where the problem could lie.

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Originally Posted by Melanie

Just treat it as a high-gain NPN transistor. You can use any NPN power Transistor, but it's not going to work as well, and you'll need a reverse Diode across it for protection. For the purposes of test and playing with you could try it.

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That's what I'm talking about. Just get control of the workings back. Worry about the complete fix after I get a TIP107...

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Yes you have... "I've got another PRC2000 to do the rework".

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And smoke another one! :) Maybe not...

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remove each part, one by one and eventually I'll find the one that's duff (probably adding faults that weren't there in the first place).

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Thing is, I had to pull a load of parts out anyways since they had been charred up by the initial burnout. Bad solder joint? Sure...maybe. While I was at the 2M class, they showed us some great X-ray video of cold solder joints. Shiny on top, cracked in the middle. I've got a decent handle on those...probably moreso than 99.9% of the rest of the hobbyist world.

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My line of reasoning is this...

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Oh, I agree completely...if it weren't for those charred bits/pieces that I had to pull in the first place. It could've easily been a bad joint on one of the biasing resistors that locked out the drive motor that that started the whole chain of events in the first place. But, I'll never know since that particular part was charred up...

The PRC is actually on hold for a couple of days. I blew up my tractor a couple of weeks ago and just happened to come across a new engine late last night. Putting it in tonight.
The final fix will come next week....at least it had better!!!

I think I found the ROOT cause of the problem.
LM324, U1, pin 7 seems to be stuck high. Can't get it to go low no matter what I do to the + and - inputs, pull to ground, pull to V+, whatever. I can get the other 'sections' of U1 to do what I want using the same methods.
Swapped it out with a new one. The output at pin 7 acts like I want it to, as do the rest of them. Can't check it out completely yet, still don't have a high-gain replacement for Q22 on hand.

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Originally Posted by skimask

I think I found the ROOT cause of the problem.....
Can't check it out completely yet, still don't have a high-gain replacement for Q22 on hand.

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Update to the update...
Finally got my hands on actual TIP107's.
Swapped 'em out, ran the microchine for about 15 minutes, works like a champ...
The original problem (besides the bad LM324) might have also had a little something to do with the fact that Q22 originally didn't have any heatsink compound under the TO-220 case at all...don't know...don't care...it's all working...