Busted tankless electric water heater control board

[nop]

Hi. I have an old Bosch/Powerstar AE125 water heater in my cabin whose control board died for the nth time. This time around, I figured I'd try to diagnose and repair it. At the end of the board's life its power indicator LED will quickly cycle on and off as water passes through the heater, rather than remaining steadily on as it normally would, and water at the tap remains cold. Slightly increasing pressure in the system by closing the outlet valve seems to delay the inevitable for a little while.

Here's all the info I found from Bosch regarding troubleshooting the board:


My meter (a cheap one) shows over-range between the leads marked in the troubleshooting diagram for each triac on the broken board. However, it does the same on the new board I replaced it with (which works perfectly). The resistance between the outside leads on each thyristor on both boards is about 29 ohms. So it seems the thyristors are not the problem?

Here are some photos. FWIW, I can't see any cracked joints or other obvious problems.





Any advice? Thanks!

[sam_sam_sam]

Re: Busted tankless electric water heater control board

Do you have any voltage on the triac side of optic sensor ic chip
Do you have any voltage on the controller side of the optic sensor ic chip

[R_J]

Re: Busted tankless electric water heater control board

I would suspect the power supply circuit, based on the way the led is flashing.

[petehall347]

Re: Busted tankless electric water heater control board

i would be working out what switches the light on and working backwards ..
my guess its some sort of driver issue .

[CapLeaker]

Re: Busted tankless electric water heater control board

When the board is dead, the light is off?
Maybe upload that troubleshooting guide so we can have a look at it?

[eccerr0r]

Re: Busted tankless electric water heater control board

Capacitive Dropper PSU? C32 still good?

Anyone find datasheets for the thyristors? Teccor/Littlefuse branded it seems, but is it a real part? fake?

[kaboom]

Re: Busted tankless electric water heater control board

Replace the electrolytics with the same voltage/value, but Nichicon PW/HE, or Panasonic FC/FR. I see at least 3 on that board.
Some look like 'noname' junk.
No wonder these "regularly" fail, with bosch "ready" to sell replacements; the scam then is to upsell to the more expen$ive model at that point, which is unacceptable.

Check that the line dropper cap (blue box type) really is 2.2u.
They can loose value over time, yielding a "weak" power supply that sags under load.

Don't fixate on the triacs. The problem (caps) is right out in the open, especially those two black electros. Possibly the 2.2u line dropper cap...

And don't worry about Bosch's so-called troubleshooting, which is procedure-based vs result-based.
You'll most likely get a good result by recapping.

[nop]

Re: Busted tankless electric water heater control board

Wow! Thanks, everyone, for all the replies!

@sam_sam_sam, is there a way to test the optocouplers without connecting the AC lines? I'm a total novice and squeamish working around all that juice. (The water heater is on three 40 amp breakers.)

@CapLeaker, when the board is dead, it's not completely dead. The heater fails to raise the water temperature, but the LED on the unit blinks on and off when water passes through the flow sensor (somewhat randomly, it seems). The full troubleshooting guide from Bosch is here: https://www.bosch-thermotechnology.u...guide_us_1.pdf

@kaboom, if I test the capacitors without removing them from the board with my DMM, can I expect valid results? Like I said, I'm a total novice with this stuff. Apologies.

[CapLeaker]

Re: Busted tankless electric water heater control board

Hmm… thought there would be more info given. I looked at a replacement board and saw it comes with a new thermistor and other sensors? For shits and giggles measure the resistance on the sensors on the old and new board. I also see an xtal around U7. If the crystal isn’t working so would not the control IC either. To measure the crystal you’d need an oscilloscope. Check the little chip caps for short. I don’t think the problem lies on the HV side. A power supply issue on the low voltage side like RJ said, surely would sour your day too.

Capacitors should be removed to test. It depends on the circuit.

Yes, you can also check the optocouplers with your DMM in diode mode. Download the data sheet and it will show you the pin out. However I don’t think that’s the issue, but doesn’t hurt to get some experience in.

[sam_sam_sam]

Re: Busted tankless electric water heater control board

You might want to watch this video on how to troubleshoot a device when you have the exact same device board ( a known good board and a non-function board ) it is very interesting and the concept can be used for troubleshoot and figure out where is to narrow it down to a certain portion of the circuit board

https://youtu.be/pRpyHF2dfq0

On note that this video is very long but she does finally get to where the issue is on the board

I have personally use this technique before but using an ohm meter but she explain why this is not such a good way to do but using the the diode test function is a lot better

[kaboom]

Re: Busted tankless electric water heater control board

@kaboom, if I test the capacitors without removing them from the board with my DMM, can I expect valid results? Like I said, I'm a total novice with this stuff. Apologies.

The heat from desoldering them can temporarily make the electrolytics test "better," leading to false positives.
If you're at that point, just change them.
Don't buy into the ESR testing- given they're cheap caps, they're likely marginal by now.
Why waste time "holding out for a false hope" and pay/wait for an ESR meter that'll only confirm what I already suspect?

The 2.2u film cap should read the same out of ckt, not being affected by temporary (de)soldering heat.
This probably has a bleeder resistor across it, so must be tested out of ckt.

Again, let's not "disaster plan" and put our bets way out there, like on the optos or MCU.
Just because the MCU is a "complicated" part does not mean the odds of it failing suddenly increase.
Keeping the big picture in sight, let's focus on the power supplies that everything on that board needs to run!