Wondering if I can fix this board myself.

[MRdolt2u]

Hi! I have a very limited understanding of electronics, almost entirely automotive in nature. I am trying to repair a PCB from an electronic racquet stringing machine. I blew something by inadvertently shorting the motor as the unit was powering up. I am inserting (I hope) a photo of the board below. It is not a large board but it's mostly greek to me.

The leads to the 12v DC motor are at the bottom of the board (2 arrows). I figured this would be the region of the board likely to be affected. None of the components on the board are discolored or burnt, but the resistor with the 1 arrow is marked 0.51 ohms. So, I did a quick google and figured this is a fusible resistor and sure enough, it was blown. Bought replacement and installed. While the board is powering up the control panel, the motor is still not getting power. Normally, you turn the machine on and the unit does a self diagnostic cycle and the motor runs forward till it trips a limit switch and then back and comes to rest. The motor works though.

I know the motor works because I have 2 of these machines. I am thinking that even with my limited knowledge I can compare the good board to the bad board component by component. But all I have is an old Radio Shack analog Multitester, so basically an ohmmeter. Is this feasible? Do I need a digital tester with diode and capacitor testing capability?

I would prefer to do any testing on the unit without powering it. I have already blown one board. If I blow both I am up a creek.

[james_s]

The picture is too dark and blurry to see what's going on. There's gonna be a transistor of some sort to control power to the motor, they usually fail shorted however it may have blown the bond wires inside it if there was enough current. You'll have to trace enough of the circuit to figure out how the motor is controlled.

[MRdolt2u]

Well, thank you James for taking a look. I wasn't expecting anyone to analyze the board. My limited experience with electricity and electronics basically boils down to testing for continuity, resistance, and live wires. So my question I think really boiled down to: Can you safely and effectively test semi conductors with an old analog ohmmeter or is it just going to blow more of them?

Now, for instance, I am posting another picture of the board (below) from a different angle since you mentioned transistors. I take it you think it's likely I have blown one or more of these? The 4 on the left are K2116's and the one all the way to the right is a TA7805S. I googled them and looked at the spec sheets but I don't understand any of it other than I've got 4 MOSFETS and 1 Voltage Regulator. What I am wondering (and this goes back to my initial question) is whether I can test these semiconductors with my old ohmmeter or will it fry them? I am especially concerned because I don't know the correct way to hook an ohmmeter up to anything but a resistor.

I have looked at some videos on youtube on how to test MOSFETS since I started this post and everyone is talking about setting up in diode mode so I am thinking at this point I need to purchase a DMM.

Anyway here is the pic.

[greenpossum]

7805 is a voltage regulator that in all probability supplies the logic circuits. Since the logic is apparently working, it's probably ok, but you can check for 5V on the output.

You'll need to trace out the circuit around the power transistors. You should be able to measure the voltages on their outputs with a voltmeter in various phases of the initialisation after that and go from there.

[Manul]

Yes, you can test them (not fully, but still) with any ohmmeter. Very often mosfets fail by shorting drain and source. Sometimes also gate and source. If there is quite low resistance (0-100 ohms) between drain and source - it is very likely failed. You can also try gate - source. No worries in damaging by measuring. You can in most cases test them while on the board, no need to remove.

But turn of power before doing that.

[james_s]

While it's possible to do with the tools, it sounds like you're likely in over your head. Unless you want this as a learning opportunity and are willing to invest more time than it would cost to pay someone more experienced to repair it, your best bet is to send it out.

[MRdolt2u]

While it's possible to do with the tools, it sounds like you're likely in over your head. Unless you want this as a learning opportunity and are willing to invest more time than it would cost to pay someone more experienced to repair it, you're best bet is to send it out.

Normally, this time of the year I would be working 70-80 hrs/wk and would not even have the time to post on a message board. Thanks to the pandemic I have time. It used to be possible to get repairs from the manufacturer in Japan but they stopped responding to my emails years ago. The unit is 20 yrs old now, btw. Point is, going forward, it will be useful to understand the electronics of this device because the manufacturer is leaving me on my own and it is aging. It is however better than anything being made today.

If I were to send this out, is there someone in particular you would suggest?

[james_s]

I really have no idea, I've never tried to find someone to repair things but you could post in the work wanted section of this forum.

If you do want to try to fix it yourself, we're happy to offer tips and hints, although it's very difficult to diagnose something remotely while also providing an electronics technician education.

[Renate]

I'll have to admit my ignorance, I have no experience with "electronic racquet stringing machines". 

So we've got a 6 pin connector that goes to the motor.
I'm thinking a center-tapped stepper motor.
But, as I noted above, I don't know what I'm talking about.

First thing. Easy to do.
Use your ohmmeter on the motor end of that connector.
See what is connected to what.
I'm guessing that the 6 pins will measure like this:

Code: [Select]

1        2        3        4        5        6
  3 ohm    3 ohm   infinite  3 ohm    3 ohmIf that's correct, try measuring the board.
Measure pin 1 and see if it's directly connected to the center pin of one of the big 4 transistors (not the 7805).
Do the same for pin 3, 4, 6.
Can you see a transistor for each pin?
Check between pin 2 and pin 5, it should be a direct connection (you might also be able to see this directly on the board.)
If this is all correct, then we know what is going on and fixing should be easy.

Measure each of the six pins to the aluminum heatsink (is it grounded?)
Check with the probes in both polarities.

[james_s]

I assumed it was a DC motor, perhaps with a shaft encoder. It might not hurt to get a picture of the motor though.

[NivagSwerdna]

I blew something by inadvertently shorting the motor as the unit was powering up.

How exactly?

I think there is good advice already ^ but to add... likelyhood is an output transistor... measure the big ones for continuity between the pins and report back.

Are you happy to resolder one or many of the transistors?  Because that's probably where you are heading.

Hi res image of top and bottom would be fun to look at.

[MRdolt2u]

I'll have to admit my ignorance, I have no experience with "electronic racquet stringing machines". 

So we've got a 6 pin connector that goes to the motor.
I'm thinking a center-tapped stepper motor.
But, as I noted above, I don't know what I'm talking about.

First thing. Easy to do.
Use your ohmmeter on the motor end of that connector.
See what is connected to what.
I'm guessing that the 6 pins will measure like this:

Code: [Select]

1        2        3        4        5        6
  3 ohm    3 ohm   infinite  3 ohm    3 ohmIf that's correct, try measuring the board.
Measure pin 1 and see if it's directly connected to the center pin of one of the big 4 transistors (not the 7805).
Do the same for pin 3, 4, 6.
Can you see a transistor for each pin?
Check between pin 2 and pin 5, it should be a direct connection (you might also be able to see this directly on the board.)
If this is all correct, then we know what is going on and fixing should be easy.

Measure each of the six pins to the aluminum heatsink (is it grounded?)
Check with the probes in both polarities.

Thanks for the detailed instructions. I checked the resistance between pins 5 & 6 and got 15 ohms. I know these pins go to the motor which is not a stepper motor btw. The motor is regulated by a pair of optical encoders (I guess for forward and reverse).

Before you invest any more of your time (much appreciated btw) I want you to know I have a digital multimeter on the way and my first move when it arrives is to test the mosfets. As I said in my initial post I have 2 machines and therefore 2 boards, 1 good & 1 bad. If necessary I will test every component on them both to figure out what is damaged.

Thanks again, and I will definitely post my progress.

[MRdolt2u]

I blew something by inadvertently shorting the motor as the unit was powering up.

How exactly?

I think there is good advice already ^ but to add... likelyhood is an output transistor... measure the big ones for continuity between the pins and report back.

Are you happy to resolder one or many of the transistors?  Because that's probably where you are heading.

Hi res image of top and bottom would be fun to look at.

How exactly did I short out the motor? You want details? It's embarassing you know.

The long story is that the brushes had worn down to nothing. I found someone in China selling something with the right dimensions. It took months to arrive. The springs on the new brushes were too wide and the caps were too large to fit so I took my old brushes apart and soldered the old parts to the new brushes. Then I thought I should just check to make sure the motor was working. If I had been smart I would have mounted it to the housing but I didn't. And if I was smart I would have put electrical tape on the two solder joints I used to rejoin the leads where I was forced to cut them. I thought I could hold the motor in my hand, flick the on switch and then flick it off. Instead it jumped out of my hand like a king cobra and I guess when it went as far as the leads would allow, the two leads shorted where I soldered them.

As far as soldering the transistors, I don't feel that is beyond my abilities. I am more concerned about how long it will take to get them if they are indeed the problem.

Hi res photos I think would beyond my equipment. This is a two sided board though. There have been at least 2 posts where people suggested tracing the circuit. With the paths being on both sides I just find this is not something I can follow. But, as I said in the post above, I have 2 boards and I will figure this out.

[james_s]

As far as soldering the transistors, I don't feel that is beyond my abilities. I am more concerned about how long it will take to get them if they are indeed the problem.

Hi res photos I think would beyond my equipment. This is a two sided board though. There have been at least 2 posts where people suggested tracing the circuit. With the paths being on both sides I just find this is not something I can follow. But, as I said in the post above, I have 2 boards and I will figure this out.

Digikey or Mouser should have suitable replacements for the transistors, you can get them in a couple of days or less once you know what you need.

Do you not have a mobile phone made in the last 10 years or so? Most of them have quite good cameras, getting a high quality photo is mostly down to adequate lighting.

[Renate]

The long story is that the brushes...

Oh! It's a brush motor then.
I presume that it goes backwards and forwards?
So we're looking at a single H drive from the four transistors.

So get out your ohmmeter and see if we can identify stuff.
Measure from your new resistor to the middle lead (drain) of all four transistors.
(You'll have to try both ends of the resistor to see which gives the lowest reading.)
You should find two transistors that are directly connected, those are the "top" transistors.

Is the heatsink connected to the circuit? Or another ground?
Try measuring between ground and the right lead (source) of the four transistors.
You should find two transistors that are directly connected, those are the "bottom" transistors.

Then you've got the two pins that feed the motor (I think they are #5 & #6 pins?)
Each of those should connect directly to the middle lead (drain) of a "bottom" transistor and the right lead (source) of a "top" transistor.

If this all works out, you should now have a complete layout.
The four transistors are #5 top, #5 bottom, #6 top, #6 bottom.

If you find something without this symmetry, that's probably our problem.
You should check to see now to see if either motor connector pin is shorted to the (more active end of the) resistor or ground.

Next, you can do some diode tests.

[james_s]

A multimeter with a diode test function is very useful for testing transistors. It would also be good to know whether they're BJTs or MOSFETs which can be determined by the part number.

[NivagSwerdna]

If it was me.... and I knew this to be an expensive bit of kit... I would just change the four driver MOSFETs and see what happened.  Given the history of the short it is probable that one has blown.

I'm not the best person to advise... but maybe... SiHA180N60E which Mouser stock?

[SL4P]

I think we’ve almost reached that point where we ask...

Are you confident desoldering and removing the existing transistors.  Cooking the pads or board might lead to worse long term problems.

Any competent tech should be able to read the above, and save you a lot of pain within a couple of hours total.

[themadhippy]

Are you confident desoldering and removing the existing transistors.  Cooking the pads or board might lead to worse long term problems.

theres enough exposed leg on the existing transistors to snip em off well above the board and  attach the new ones .

[KL27x]

IME, you're probably looking at:

FETs are gone. Driver of the FETs also likely dead, since the control circuitry is on a 5V bus, and the motors probably 12+?

The key thing if this is me, is gate voltage. If the gates traces are doing what they're supposed to, then your uncle is Bob and you replace FETs. If not, you have to trace back until you find a signal, and then replace (or re-engineer) all the stuff in between.

It might go a little something like: snip out the FETs, and probe voltage of the gate traces while turning the controls of the control board. When the motor is supposed to start/stop moving or changing directions, the gate traces should change voltage. If this ain't happening, then don't put in new FETs. No point.

It looks like the gate drivers might be standard lego-brick buffer/logic IC's (or actual dedicated transistor driver IC, maybe even H-bridge-specific IC), so you might be in luck and be able to replace damaged parts. You can trace a double sided board using a continuity tester. You might find it handy to wrap a wire around your ground probe so you can solder it in place while you flip the board to find where the other side pops out. First time you run into a microcontroller, you're pretty well done with an easy fix.

AFAIC, if you can't find this drive signal, then there's nothing you can replace to get it back. Any damaged parts that may be found between this signal and the FET gate are most likely stuff to strengthen/sharpen the drive signal and/or for back EMF or whatnot. Voltage buffering, amplification, a diode here or there. This stuff quite often involves logic inversions, but you can cross that bridge if you get there. So that stuff can be re-made from jelly bean parts, with some homework. As long as you can find the signals what fires when the motor is supposed to do something. You can also work forward from the controls. If you can't find it there, doh. Start pulling suspected damaged parts that might be shorting the signal.

[SL4P]

theres enough exposed leg on the existing transistors to snip em off well above the board and  attach the new ones .

That’s a classy, reliable fix if I ever saw one. 

[Renate]

theres enough exposed leg on the existing transistors to snip em off well above the board and  attach the new ones .

That’s a classy, reliable fix if I ever saw one. 

If you cut off the old transistor's leads, it's easy enough to do this properly and solder suck the old bits out.
Yes, if you were experienced enough cleanly soldering it might make sense to just "shotgun" the four transistors.
But for a beginner if we can narrow it down to one transistor that might be helpful.

[SL4P]

ohhh ok.., !
I thought you were tacking the new leads onto the old stubs !   

[MRdolt2u]

Digikey or Mouser should have suitable replacements for the transistors, you can get them in a couple of days or less once you know what you need.

Do you not have a mobile phone made in the last 10 years or so? Most of them have quite good cameras, getting a high quality photo is mostly down to adequate lighting.

I found a company in Hong Kong that has stock of the original part. As a neophyte I feel ill equipped to select replacement parts on the basis of data sheets. If there was an online tool that spit out replacement part numbers when you input the part number of a discontinued item, that would be my kind of tool.

I can get up close to the board with a camera but then it's only possible to see a small section. Would this actually be useful?

[SL4P]

BTW when you do install replacement transistors, secure them to the heatsink BEFORE you solder the legs.
This eliminates stress on the parts for tiny misalignment etc.