Electronics > Repair

Percul gokart laptimer repair and teardown

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adinsen:
My son recently bought an old Birel MX32 kart. It came with a Percul laptimer which I'm trying to resurrect. The timer is based on an H8/538 processor with an external EEPROM and RAM. It has a four cell NiMH battery managed by an onboard MAX713 battery charging IC. A 7W74 single flip flop is controlled by a power switch. This flip flop works, but there's no power on the rest of the circuit when it toggles. I'm trying to trace the circuit from there. There's a separate MAX708 reset generator on the board, which is not recieving any power.

The board has some corrosion from water which has ingressed through the charging port. The battery was more or less dead so I've cut the leads and is powering the thing with 4.8V from my lab supply via them. I have attempted ot remove as much grit as possible.

I haven't been able to obtain a schematic or other service information from Percul, which today, is a contract manufactorer near Matsuyama, Japan, but they're not responding to my e-mail. Their contact form on their website does not work. They do show a picture and a brief description of their laptimers as discontinued products on their website https://percul.co.jp/service/discontinued/

I've resurrected lots of circuits before even without schematics, but I've always tried to avoid compact SMD boards like this as tracing and identifying connections is difficult. I feel stuck now. I'm poking around looking for dead capacitors and short circuits, but as I don't know what I'm looking for, it's hard. Should I give up?

/Anders

Gyro:
Looking at the power supply end of the board, I'm not surprised that you're not getting power to the MAX708. There is corrosion on both sides of the board so I assume it will have attacked the vias too. If there is any suggestion of leakage from the NiMH battery, you should wash the affected end of the board with a dilute acid - citric acid, white vinegar, lemon juice etc. to neutralize the alkaline residue. Then scrub the board in water until you have removed as much of the corrosion as possible (from the photos, you have a long way to go) followed by an IPA wash. You need to get the board clean (back to green solder resist) before you can start following tracks (including the ones under components), checking continuity of vias, re-making solder joints etc. Finish off with another IPA wash and dry. Edit: the buttons may also need replacement if they have become contaminated.

Sorry, there is no short cut. At least the damage seems to be restricted to the lower tech end of the board, so you have a reasonable chance without schematics.

jchw4:
Quoting myself:


--- Quote from: jchw4 on May 01, 2024, 09:11:01 am ---That's some serious corrosion here and it looks like an exact reason for the fault.

So it could be either a leakage (i.e. a new current path) or a corroded trace or via (i.e. a missing path).

I would first clean up all the corrosion making sure there is none left under the ICs.

Personally I would submerge the PCB in vinegar (regular 5% or so) and leave it there for a 2-3 minutes. It's risky, though, but the corrosion is also pretty bad.

Then blow the liquid away with compressed air. Wash in tap water, blow with compressed air again. An then leave fully submerged in pure 99% alcogol for a few days. I personally found kitchen ziplock-style bags to be very useful here.

If it does not help, I would start hunting for some coroded trace. I.e. taking photos, tracing as usual...

--- End quote ---

In this particular case you have this huge PLCC socket that may have damage underneath. The chip was glued that does not help either.

I would suggest $20 ultrasonic cleaner with vinegar, then soapy water, then clean water. Take it out of the cleaner, blow out all water with compressed air. Dry with a hairdryer  and submerge in pure IPA as described above. You may want to refresh IPA after a day. Blow with compressed air, use hairdryer again....
(See https://www.eevblog.com/forum/repair/advice-on-aperture-repair-on-camera-lens/msg5478658/#msg5478658 too.)

Then you can start debugging.

Hopefully it's a two layer board. You can take PCB photos with your phone, align both sides on a computer and start tracing.
You probably don't need to trace too much, but having both sides on a PC screen helps a lot.

adinsen:
I made a dilution of citric acid and submerged the pcb. The visibly corroded areas and the switches, reacted very clearly as can be seen in the first picture. I then cleaned it with tap water and IPA from a spray can, and dried everything with compressed air. I found that the glue on the H8 PLCC does not seal in the corners, so I blew extra air in the corners. It now looks better although there is still residue on the RAM-side near the battery connector. And I have seen the first signs of life as there is light in the LED's, the display is showing something, and the CPU clock oscillator is running. Unfortunately the unit is unresponsive, but at least the power supply problem was solved just by this cleaning.

Thanks a lot both! 

Gyro:
That sounds more hopeful  :-+

Be careful to make sure that you have dried the board completely. Electrolytic corrosion can still occur in tiny gaps like the underside of SMD passives. You can't beat a decent spell in the sunshine and breeze.

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