Need help diagnosing H-Bridge

[eKretz]

OK, I have been waiting on desoldering braid to get here and finally got it yesterday. I also managed to get my hands on a known good replacement circuit board. Finished the cratered board/blown trace repair and replaced the blown transistor. Here are photos showing BCE voltages on all the transistors in the area. The photos are in the following order: CW direction-good board; CW direction-bad board; CCW direction-good board; CCW direction-bad board.

CW - Good Board:

CW - Bad Board:

CCW - Good Board:

CCW - Bad Board:

From what I can see, the problem area is T1006? I have 15V on both legs where I should have 6V and 0V. I checked out IC1000 and IC1001 and they are both reading identical voltages everywhere on all boards. T1010 is also 15V when it should be 6V but that is because it's getting the voltage from T1006. T1012 is also getting 15V which is causing the legs to go high, right? So now I have to figure out why T1006 is getting 15V. I also found it odd that T1000 is getting 24V on the CCW Good Board and 0V on the CCW Bad Board, even though the CCW direction seems to function fine.

 

[(*steve*)]

OK, first thing. You show A connection of T1000 on the bad board as 0V. Can you re-check that it is 24V either CW or CCW. I think this was a bad measurement.

Then re-annotate your images, but this time omit any annotations for the same voltage. But leave these photos unchanged, they will be a good reference.

I suspect you're right in suspecting that it's something around T1006, although I'm a little concerned with T1005 as well.

With the power off, use a multimeter to compare the resistance of all of the resistors around those areas, and also do a diode check on the diodes.

 

[(*steve*)]

It would also be very useful if you can annotate another copy of the board with the types of all the transistors.

 

[eKretz]

OK, here are some updated photos, I'll start on the checks right after this.

All the transistors are either 2TY (s8550) or J3Y (s8050) as we discussed earlier, here they are marked:

 

[eKretz]

OK, I've checked all the diodes and resistors in-circuit with no power, and all check the same on both boards. I did find a dead (open) 1 ohm resistor on the bad board (the tan one visible in the full board photo). The funny thing about the recent checks though, is that CR1005 and CR1000, well, pretty much all the CR's except the 330's showed open when I tried to check their resistance. Even on the good board. Is that just an issue with checking them in-circuit?

 

[(*steve*)]

OK, those images look great. That's going to make life a lot easier

did find a dead (open) 1 ohm resistor on the bad board

Fix that, then check the voltages again (then check the resistor again )

The funny thing about the recent checks though, is that CR1005 and CR1000, well, pretty much all the CR's except the 330's showed open when I tried to check their resistance. Even on the good board. Is that just an issue with checking them in-circuit?

What range was your meter on? Do you get a reading other than 0L if you hold the probes in your fingers?

In general, I'm not really concerned with the actual readings, it's any significant difference which will highlight where things are likely to be going pear shaped.

 

[eKretz]

Just ran across this new N-channel MOSFET from ST, I wonder if it would make a good upgrade?

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00071594.pdf

 

[(*steve*)]

Looking at those voltages now, I think there's a source of 18V on the good board that is shorted (or something) and giving close to the full 24V to parts of the circuit.

 

[eKretz]

I tried the meter on both auto-ranging and on 600 Ohm manual when I got no result. I do get a reading when testing my fingers in the MOhm range. (It's a Fluke 87 V).

 

[(*steve*)]

Just ran across this new N-channel MOSFET from ST, I wonder if it would make a good upgrade?

Possibly, but let's get it working again before we add more variables into the mix...

 

[eKretz]

OK, this is very weird. I just re-checked the resistance, and now it is getting a reading. Could checking the diodes have somehow blocked the meter from getting a reading if it charged up a FET? Now I am showing one of the 1k resistors (CR1001) reading only 42 Ohms instead of 1k. That could be the whole issue, eh?

 

[(*steve*)]

I did find a dead (open) 1 ohm resistor on the bad board (the tan one visible in the full board photo)

Just read that more closely. Tan coloured components are generally capacitors.

The funny thing about the recent checks though, is that CR1005 and CR1000, well, pretty much all the CR's except the 330's showed open when I tried to check their resistance. Even on the good board. Is that just an issue with checking them in-circuit?

330 is 33 ohms
102 is 1000 ohms

Both should be easy to read when the meter is set to auto-range. Make sure the probes are pushed into the solder at the end of the component.

They could read lower than the marked value due to other things in the circuit.

 

[(*steve*)]

OK, this is very weird. I just re-checked the resistance, and now it is getting a reading. Could checking the diodes have somehow blocked the meter from getting a reading if it charged up a FET? Now I am showing one of the 1k resistors (CR1001) reading only 42 Ohms instead of 1k. That could be the whole issue, eh?

It's possible that taking a reading on a component could turn on (or off) a mosfet. Hopefully though, their gates are not floating.

You can always short the gate to the source of the mosfets between measurements if you see something odd happening. Use a mA range on the meter and probe across gate and source. You won't get a reading,but if there's any charge stored on the gate, you'll bleed it away and the mosfet will turn off. (This really shouldn't be required)

edit: the 1k resistor reading 42 ohms is only an issue if it reads substantially higher on the the good board.

 

[eKretz]

It does, it reads 1k on the good board. And the tan component is definitely a resistor, it's a through-hole component (R1004) with brown, black, gold, gold stripes. So unless I'm even newbier than I thought, that one's definitely a resistor.

 

[eKretz]

Grr, nevermind, can't find a spare 102.

 

[(*steve*)]

It does, it reads 1k on the good board. And the tan component is definitely a resistor, it's a through-hole component (R1004) with brown, black, gold, gold stripes. So unless I'm even newbier than I thought, that one's definitely a resistor.

hahahah, you're right. I couldn't spot it.

If the 1k resistor reads different to the good board, replace that too. If it doesn't fix the problem, then you'll have to go looking for other components.

I would not be surprised of one or more of the transistors in the lower part of the board is damaged (did you have a few spares?).

However the lack of a correct 18V is concerning me somewhat... And I'm not certain where that's generated.

 

[eKretz]

Yeah, I have a spare damaged board with some spare parts on it, I was hoping to find some components in my spare parts bin, but it looks like I'm going to have to rob Peter to pay Paul. I think the 18V is coming through one of the resistors and getting cut down from the 24V.

 

[(*steve*)]

there is a 6V as well as an 18V rail there somewhere. Possibly they get created near the same place. I think it's unlikely that it's done with resistors.

It could be a small three terminal regulator, and that could be in a package as small as SOT-23

 

[eKretz]

Ahh, OK. Well, I just got through removing the 102 and when I checked it out of circuit it is reading 1k. There's nothing wrong with the resistor, the problem is elsewhere. After I put it back in it's reading 96 ohms in-circuit. Let me see if I can chase down the rails and reg. That couldn't possibly be the FQP19N20C, could it? Isn't that just a MOSFET?

 

[eKretz]

Found it. It's an ON LM2576D2T-5. I'll be checking the outputs shortly. It's supposed to have 3.3, 5, 12, 15, and an adjustable, from 1.2 to 37V. Looks like this version is the 5V model after looking at the datasheet. Not sure where the heck the 18V is coming from.