Where to start diagnosing a 19.2v NiCd battery charger?

[kynar-jones]

Hi, I am a n00b. I have several old Porter Cable 19.2 volt power tools that still work great. I rebuilt four batteries a couple years ago with 3800mAh packs, which was somewhat expensive, but since then both of my Porter Cable chargers (model 8624) have stopped working. One of them had a couple questionable solder joints, so I re-did all joints on both units but it didn't help either of them.

Upon being plugged into mains power (USA), a properly functioning unit should make a brief audible relay click along with the green LED lighting up, and then both the relay and the LED should deactivate. After that, the unit should wait for a battery to be inserted and when one is sensed it should make another relay click sound and begin testing & charging the battery.

Unit A doesn't show any signs of life when plugged in, no click, no LED. When I attach a battery, nothing happens.
I put the multi-meter on this unit and found 120vac at various points, and 10-20vdc at other points, but I get lost diagnosing it after that.

Unit B sometimes shows signs of life when plugged in... maybe 1 out of 15 times I plug it into mains power, it'll click the relay for a split second and maybe the LED might flash too. In these cases, nothing further happens when connecting a battery.
Maybe 1 out of 30 times it WILL actually start charging a battery, but if it gets that far it usually won't actually complete a charge cycle...I come back and the unit is dead to the world again and the battery might or might not have gotten a partial charge.
Maybe 1 out of 60 times, it will fully charge a battery without going into a coma. The last time I got it to charge one battery, it also charged the other three consecutively without any problems.

I could not find schematics for this charger, and both units (same model 8624) are different inside anyway... Unit B has a green PCB, while Unit A is beige and has older looking components laid out differently.

I sure would like to repair both of these units, because they are scarce and way overpriced on ebay ($50-70 US, even if UN-tested). I doubt there is anything seriously wrong with either of mine - probably just bad capacitors, but the caps are not leaking/bulging and I don't know how to diagnose anything further. If I had schematics, I would at least know what does what and have a rough idea of where to start looking, but I don't know enough about electronics to just look at a PCB and know how it works.

Would anyone be willing to help me save these units, so I can charge my perfectly good batteries so they don't go to waste? I would want to fix Unit B first since it sometimes actually works.
Thanks in advance!

[tunk]

It may help if you post photos of the PCBs (both sides).

[kynar-jones]

This is Unit B, the one that sometimes still works. I took these pics before doing more than one or two solder joints. This one didn't really need any touch-up but I redid some of them anyway.

The rated power output is 19.2 volts at 2.3 amps.

Some pics might look like there is goop/crust around a capacitor or two, but it is just adhesive.  In the last picture, that green component gets quite hot when a battery is charging. The rectifier to the left of it gets fairly warm. The green component gets what I would call the bottom end of hot, where it becomes uncomfortable to pinch it for more than a few seconds... but it hasn't fried yet. It gets much hotter than the two power transistor heat sinks do. The big L2 coil gets somewhat warm. R25B doesn't have a damaged trace (pics 4 and 5), that is just flux from a re-flow.







Seems basic enough that it should be easy to fix, eh? Thanks!

[Lifeborne]

Hello Kynar,

After viewing the pics, the relay and LEDs look to be controlled by the micro-controller (MC) MDT2010EP. If the initial startup consists of the relay clicking and LED flashing, and that is not working, I would start by checking the signals from the micro-controller to the LEDs and the signal to the MOSFET that activates the relay. If those two signals are not present for the duration that you remember them to be, I would then check to see if the MC is getting the proper power and clock signal. If they are good, then check the MOSFET Q5, Also check D10 diode to make sure it is not shorting.