Bosch AL2215CV charger fix

[Erdist]

Re: Bosch AL2215CV charger fix

Thanks momaka. It looks like there is a miniature crack on the conformal coating between the first and second diode, so good idea to check!

[momaka]

Re: Bosch AL2215CV charger fix

Oh, and regarding R5 value... I think 22 KOhms is actually right, because that should provide about 6.76V on the Gate of the MOSFET (just do a voltage divider between R3, R4, and R5 and you will see that in order to get a high enough voltage to turn on the MOSFET, R5 must be at least a few 10's of KOhms.)

Also, I just noticed my post from 2019... and yeah, this 2-transistor self-oscillating circuit does not look very well designed. Heck, the whole PCB looks very cheaply made an separation between adjacent HV traces not good at all.

*EDIT*
When I was mentioning R3, R4, and R5, I was referring to this schematic posted by DannyX:
https://www.badcaps.net/forum/attach...8&d=1547107967

Looks like that R5 designator is not referring to the same resistor as the one on your board. The one on yours (behind the MOSFET that connects the MOSFET's Source to primary-side ground) should be a low-value resistor: 3 Ohms sounds about right for that. Make sure you use fusible flame-proof (Metal Oxide / MOX ) resistor. If you use Carbon film, it may catch fire if it fails again. And for the resistor in front of the MOSFET, 20-30 KOhms will probably work fine. Yours looks like 39 KOhms (orange, white, black, red, brown - 5-band code for 39K 1%.) And the one above it that connects MOSFET's Gate pin to NPN BJT's Collector pin - 30 Ohms seems like an accurate value. The schematic posted by DannyX refers to that resistor as R7, and it is indeed showing as 30 Ohms.

[Erdist]

Re: Bosch AL2215CV charger fix

Momaka, you are a champion! Thanks very much for your help. It starts to make a little sense now, here's what I think looks to be the situation and the reason for my confusion:

- on the schematics only R6 is in the 3 ohm range (3.6), the burnt resistor could possibly be 3 ohm with 1% tolerance, but confused about the location of that black band. Also the slot is marked as R5 on the board, which is not matching with the schematics. So perhaps go for 3.6?

- Right after that, comes F3NK80Z (V6 on the schematics, V5 on the board)

- then comes R5 on the schematics with 22k ohm, and it looks like it is 39k in the board. Seems like a significant difference.

-followed by R7 in the schematics with 30 ohms, and on the board (also damaged) looks like it is 30.1k with 1%.

Do I understand it right?

[momaka]

Re: Bosch AL2215CV charger fix

- on the schematics only R6 is in the 3 ohm range (3.6), the burnt resistor could possibly be 3 ohm with 1% tolerance, but confused about the location of that black band. Also the slot is marked as R5 on the board, which is not matching with the schematics. So perhaps go for 3.6?

Yeah, probably anything in the range of 3 to 4.7 Ohms will work... or at least make the PSU try to turn On.
This resistor is used for current sensing, so higher value means it will give more negative feedback, so the PSU may self-limit the power faster... which might be desirable, given that it likes to blow up. Lower resistor = more power allowed by the PSU.

- Right after that, comes F3NK80Z (V6 on the schematics, V5 on the board)

LOL.
Yeah, looks like the schematics are more or less accurate, but the designators aren't. Not a big deal, I suppose, as long as we say explicitly which part we are talking about so we don't get confused.

- then comes R5 on the schematics with 22k ohm, and it looks like it is 39k in the board. Seems like a significant difference.

Not a big deal either.
39K will give about 12.76V max (with a 240V AC line input) at the Gate of the MOSFET, which is OK, since the F3NK80Z is rated for +/-30V V_gs.

-followed by R7 in the schematics with 30 ohms, and on the board (also damaged) looks like it is 30.1k with 1%.

I don't think 30.1 KOhms makes sense for that position. In fact, I'm not even sure why they even have a resistor there. Most 2-transistor circuits I've seen connect the Gate of the MOSFET directly to the small BJT's Collector. That way, when the BJT turns On, the MOSFET Gate is driven to ground and that makes the MOSFET turn Off. If you use 30.1K, that MOSFET may never turn Off properly or make the PSU work only in self-limiting, thus grossly overshooting voltage on the output... or worse - blow up again. The bands also seem to indicate 30 Ohms from your picture. I see Orange, Black, Brown (should be Black, though), Gold, and Brown again. In a 5-band color code, first 3 bands are the digits, 4th is the multiplier, and 5th is the tolerance. So the above should read 300 with the multiplier being 0.1 (Gold)... i.e. 300 x 0.1 = 30 Ohms. And last band, of course, is for 1% tolerance.

With that said, again I don't understand why they even have that resistor there. Just as a reference, here are a few 2-transistor 5VSB circuits from ATX PSUs, so you can see and compare how similar they are and what they did different:
https://www.badcaps.net/forum/attach...1&d=1578459617
https://www.badcaps.net/forum/attach...0&d=1451693563
https://www.badcaps.net/forum/attach...9&d=1443215464

[Erdist]

Re: Bosch AL2215CV charger fix

Thanks again Momaka for your insights and helping out! I will give the repair a go and see how it goes, hope I can get the required parts. I need that drill going!

[momaka]

Re: Bosch AL2215CV charger fix

Thanks again Momaka for your insights and helping out! I will give the repair a go and see how it goes, hope I can get the required parts. I need that drill going!

No problems.
It really is just a matter of finding the right value resistors... or resistors that you can combine in series or parallel to get the right values.
If you can't find any parts locally, see if there are any electronics repair shops near you and ask if they have any broken PC ATX PSUs or LCD monitor power boards. Chances are, you might find most parts on one. The MOSFET parameters for this PSU are not too critical, just as long as the voltage ratings are more or less the same.

Once you fit all the new parts and are ready to test, make sure to use the series incandescent bulb trick. That way, if something is still bad, it should save most or all of your new parts from getting damaged/burned/blown.