kiev
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Re: Troubleshooting and repair for On-board Charger (OBC)

Mon May 31, 2021 8:00 pm

That's excellent testing and much better than drilling vias. What's interesting is that all 4 of those ICs are the same logic chip, a quad 2-input NAND gate from Toshiba, TC74VHC00. Pin 14 is the Vcc and pin 7 is Ground.

Maybe you could just add soldering iron heat to pin 14 and gently pry that pin up with the tip of a sewing needle. Wipe your hot iron with a damp paper towel to remove excess solder from the tip, put some flux on pin 14 and apply the heat; have your needle in position to pry it up. Use a wooden toothpick as a pry block under the needle and use the needle as a lever under the pin to pry it up as you press down on the needle. With the Vcc pin disconnected you can check if the short is still present.

IC504: The output of IC504 on pin 11 is routed up to the via into R721 and input to pin 6 of IC509 and the area hit by the metal splatter, so it could have been damaged.
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iOnico
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Re: Troubleshooting and repair for On-board Charger (OBC)

Fri Jun 04, 2021 6:43 am

I was a bit short on time to proceed with testing these last days, but now I’ve been able to realise the next steps.
I’ve used the needle-toothpick leverage method you had suggested. Thanks for the detailed description. It worked well. I had started with the IC that warmed up the most (IC502) and lifted its VCC leg. I then measured resistance VCC-Ground and it was a bit disappointing: it just had gone up by some Ohms. So, I went on also lifting the VCC legs from the other 3 ICs (always measuring in between if there had been a major change in resistance after having desoldered a contact). After having disconnected the 4 ICs (IC502, IC503, IC506 and IC504) from the VCC circuit, the resistance between VCC and Ground had gone up from 4 Ω to 19 Ω. According to Ohm's law (I = U/R), this would mean that there still would be (5V/19Ω =) 260 mA of current flowing through the VCC circuit, what seems too high. So, I again connected the voltage source and applied a voltage of 3.5 V to the VCC circuit to measure which component was heating up the most. It was IC518, of which I then desoldered the VCC leg also. That made the VCC-Ground resistance climb to 30 Ω. That would correspond to a 60 mA current through the VCC circuit at 5V. Again, I applied the 3.5 V to the VCC circuit and the components now warming up the most (but way less then the other components before) are the IC704 and IC717.

These were the facts, now starts the interpretation:
The observed temperatures of IC704 and IC717 aren’t that much above the temperature of the rest of the board and for IC704 (a TJA1050 from NXP (was Philips Semiconductors before)) it seems normal that it dissipates some heat according to its datasheet (bottom of page 5). The datasheet also says that in dominant state it sucks 50 mA of supply current (top of page 6), what could explain the biggest part of the 60 mA flowing through the VCC circuit (at 5V).

Concerning IC717 (marked “VHC, 14-S, B1 14”, if I correctly identified it, it’s a 74VHC14FT from Toshiba, out of the “74” family like IC502, IC503, IC506, IC504 and IC518 but with a Schmitt Inverter logic) it’s probably more unusual that it heats up, as the datasheet says that these chips have low power dissipation and a very small current between VCC and Ground (quiescent supply current of 2 µA).

As all the mentioned Toshiba ICs were exposed to the same abnormality, it doesn’t seem unlikely that all of them were damaged in a similar way. Maybe some a little bit more, some a little bit less.

Due to these last findings, I’d say that a short between 5V layer and ground layer seems unlikely.

The difficulty I’m now facing is to know if this obtained resistance of 30 Ω between 5V-VCC and Ground is the resistance we should observe in this situation, or if it should be higher. That would determine if all damaged parts have now been disconnected from the circuit, so the repair could be limited to these parts, or if the search for other potential shorts needs to be continued.

kiev
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Re: Troubleshooting and repair for On-board Charger (OBC)

Fri Jun 04, 2021 9:23 am

i checked on 2 boards, but there are some large capacitors on the upper layer that filter and hold-up the switched 5V supply above ground which might influence the readings.

When i measured from 5v to ground with the meter leads + to - the reading settled out to 667 to 677 Ohms. With the leads reversed - to + then the reading was about 227 Ohms for both boards. Using the diode function + to - settled out to 0.357 to 0.408V, and reversed leads it was 0.112 to 0.117V.

i think you may be right that the layers are not shorted, otherwise removing the Vcc leads would not have changed the readings.

Measure resistance between the lifted Vcc pin and its ground pin on the chips to see if the ics were internally shorted.
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iOnico
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Re: Troubleshooting and repair for On-board Charger (OBC)

Fri Jun 04, 2021 10:13 am

These are the measured resistance values through the ICs (disconnected Vcc pin to Ground):
IC503: 20 Ω
IC502: 23 Ω
IC506: 33 Ω
IC518: 52 Ω
IC504: 56 Ω

I also measured the voltage drop with the diode check function (5V-Ground): 0.02 V

On my board it makes no difference when I’m switching the leads, neither for the diode check nor for the resistance (5V-Ground).
Hmm... 30 Ω is not exactly 227 Ω or even 667 Ω... looks as if other parts are having too low resistances. Next on the list to be disconnected would be IC717.

kiev
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Re: Troubleshooting and repair for On-board Charger (OBC)

Fri Jun 04, 2021 11:48 am

Wow all those CMOS chips are blown right thru.

Maybe IC707 would be a good one to check, although i don't exactly know how to do it on the board with something shorted . That creates the 5V from the 12V supply.

With several devices taken off the Vcc load you might have to bump up your test voltage slightly to get enough current thru whatever is causing the short to heat up.

Actually just work thru all the logic-type ics removing the Vcc pin regardless of heating--i count about 15 possible. Of course the worst case would be if the big microcontroller has also blown.
kiev = kenny's innovative electric vehicle

iOnico
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Re: Troubleshooting and repair for On-board Charger (OBC)

Sat Jun 05, 2021 7:09 am

If I disconnect the 5V Out of IC707 I should (when measuring resistance of the 5V circuit to Ground) at least be able to observe if this has an influence on the short of the 5V circuit, no? Even if this doesn't tell me the resistance through the device itself. You were mentioning a switched and a HATT 5V-supply in an earlier post. Am I still just checking for issues on the switched circuit? I'm asking because so many devices (15 logic ICs, maybe the microcontroller) could be concerned, that I'm wondering if there are devices left that are powered by the HATT supply...

Concerning the logic ICs to be checked, I'm able to identify easily some others from the same Toshiba parts family ("74") as the last ones I have desoldered. Would be IC717, IC716 and IC705. But that's only 3 out of around 15 logic-type ICs you've mentioned. I don't know if you have the information handy, but it would be a big help if you could tell me the IC code on the board and the part number. So I could just lookup in the data sheets which are the Vcc pins of the devices.

And I also wanted to thank you for having taken these measurements for reference on your board!!!

kiev
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Re: Troubleshooting and repair for On-board Charger (OBC)

Sat Jun 05, 2021 8:50 am

So you already know some:
3793, Monitor with W/D, IC#: 509
74VHC00, NAND gate, IC#: 502,503,504,506,518
74VHC14, Hex Inverter, IC#: 705,716,717

Others:
74VHC123A, Multivibrator, IC#: 505,706 Vcc pin16
ST95320W, eeprom, IC#: 508 Vcc pin8
TJA1050, CAN Transceiver, IC#: 704 Vcc pin3
JRC2746, Dual OpAmp, IC#: 515 Vcc pin8

Power Supply ICs would use a different technique to troubleshoot
JRC2374, PWM DC/DC Regulator, IC#: 707,708
MO33, 3.3 supply chip, IC 718
TACQ, Voltage regulator chip, IC513

The PWM regulator chips don't output a voltage, they switch a transistor at high frequency to chop the input voltage Up or Down to the desired output voltage which gets filtered and stored in the big metal can capacitors. So check the easy chips with Vcc pins first, and if the short hasn't been found then move to the supply chips.

i'm sure your soldering skills will be improving as you get the hang of lifting the pins; it would be great to check the rest on the list if you can, just to get a feel for the range of damage across the board.
Last edited by kiev on Sun Jun 06, 2021 9:21 am, edited 1 time in total.
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coulomb
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Re: Troubleshooting and repair for On-board Charger (OBC)

Sat Jun 05, 2021 3:44 pm

Personally, if I discover more than a few chips loading a power supply, I'm ready to discard that board. Obviously, something bad has happened to that supply rail, meaning that all chips on that rail could be affected. It's even possible that some will be damaged in subtle ways that are nearly impossible to detect.

In this case, since a bunch of relatively inexpensive, presumably easy to replace "garden variety" chips are involved, I might stretch my pain threshold, but from your description, I think that board is toast. I'd be looking for a second hand replacement, sorry to say.

iOnico
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Re: Troubleshooting and repair for On-board Charger (OBC)

Mon Jun 07, 2021 3:48 pm

Wow, that was a quick and very complete answer, Kenny. Thank you. I’d wish I could test and trace as fast as you are providing information to help me! ;) I haven’t come much further, just desoldered 3 other ICs from the 5V supply. The resistance between 5V and Ground changed from 30 Ω to 34 Ω after that.
The resistance readings through the 3 chips are:
IC717: 61.4 kΩ
IC716: 356 Ω
IC705: 63.7 kΩ
Looking at these, I’d say that IC705 and IC717 are likely ok, while IC716 is probably not ok. With all the “likelys” and “probablys” on so many chips, this makes a lot of assumptions and I get the point you mention there, coulomb: how to be sure to not have overseen or not detected a damage. Another risk that you haven’t pointed to, probably for reasons of politeness, but that I see as a real threat, is that it’s not that unlikely that I damage the board additionally while working on it... :roll:

With my little experience in repairing damaged PCBs, it’s difficult for me to estimate if it’s worth carrying on with the repair initiative or if it’s just a waste of time. I can’t look on experiences made to predict out of the current situation what is likely to come next... Of course, it’s a bit disappointing to read, that the board can’t be repaired, but from the beginning this possibility was part of the scope of possible outcomes. And it’s less disappointing to have the insight now, then after 3 other weeks of repair efforts for the same outcome. I’ll finish unsoldering the VCC pins of the list Kenny has provided to me and if I don’t find surprisingly clear and easy to interpret results after having done that, I’ll put an end to the repair. At least I’ll have trained my SMD-soldering skills...

I’ll continue to keep my eyes open to find a replacement board. Until now, all I came across were some complete chargers, but no firm lead to just the single boards. I’m slowly getting used to the idea to have to buy a whole charger.

kiev
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Re: Troubleshooting and repair for On-board Charger (OBC)

Mon Jun 07, 2021 9:46 pm

Thank you for chasing down this problem. As far as i can remember this is the first time to see such a failure on the control board due to a snubber failure, so in my opinion it is never a waste of time to discover and share new findings and technical knowledge.

Checking the ICs may help for repairing your board, but even if not, it may help the next guy that comes along.

Send me a PM and we can discuss getting a replacement board.
kiev = kenny's innovative electric vehicle

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