Bosch AL2215CV charger fix

[Dannyx]

Good day folks. This was originally meant to be a post asking for help, but just as I was writing the last few lines, I managed the fix the damn thing, so rather than throwing all that material in the trash, I decided to alter it a bit and turn it into a "how I repaired it" post

My friend asked me to have a look at his Bosch cordless drill charger AL2215CV which went boom. I took it apart and indeed found a blown resistor and transistor. I managed to find a relatively accurate schematic (attached) and got to work: I replaced the main transistor V6 (F3NK80Z), the small one V5 (2n3904), resistors R6 and R7 according to the schematic. Unfortunately, my 60w series lightbulb I always use for testing came on relatively bright and steady, so there was still a fault somewhere...I started double checking everything on that primary and the only things left were the transformer itself and the two optoisolators. There were no shorts on the secondary either, at least not that I could spot on quick inspection, that could cause a high load.

Remember I said the schematic was "relatively" accurate, which means the part numbers in it don't match the ones printed on the board, but the schematic itself does, so I could follow along at least. Make sure you do the same: identify the parts by how they're connected (use your brain ), not based on their numbers ! Also notice what I did for R7 (in the schematic): I had to twist two resistors together to give me 30ohms because I couldn't find such value locally, though it would've probably worked with the next higher/lower value (feel free to comment on this).

Still, no matter how much I tried it wouldn't work: the bulb would still glow brightly, WTF is there left ? I was just about ready to give up and ask for help, but I decided to push on. To make a long story short, the problem was very simple but impossible to spot visually: the pad I pictured up close, dead-center in the last picture got separated from the track. It goes between the "AUX" leg of the transformer and diode V8 and cap C6, so basically that winding was "floating". There's no way you would be able to spot it ! I scraped some of the insulation off and strengthened the bond with some solder and of course it worked perfectly: green LED came on at the front and the bulb went off.

Electronically speaking, what I think was happening is with essentially no "V-AUX" to power the optoisolators, there was nothing "pulling down" the gate of transistor V6 so it was staying on all the time. The way I figured it out was rather accidental: I measured from the "top" of the pad to the next component down the track and got Meg-Ohms instead of a short....I pushed and prodded at my meter's probes thinking there's some sh!t on them or the pads, but no: still no continuity....there you go Cheers guys.

[momaka]

Good job with the repair. Goes to show how sensitive 2-transistor self-oscillating flyback PSUs are when something isn't quite right.

That said, I'm surprised the PSU didn't self-limit. Even with the primary-side auxiliary winding disconnected, the primary switching transistor still shouldn't get stuck On. Once it turns on, current will start to flow through the primary-side main winding and also sense resistor R6. The current through R6 will cause a voltage rise on the Base of transistor V5, which should turn Off the primary switching transistor. Of course, with this operation, the secondary-side output voltage will probably go very high and won't be regulated to what it should be. Nevertheless, a well-designed 2-transistor self-osc. flyback should still self-limit as described above.

So in short, whoever designed this PSU could have done a slightly better job.

[Dannyx]

Originally Posted by momaka

Of course, with this operation, the secondary-side output voltage will probably go very high and won't be regulated to what it should be

Strangely no: I got absolutely nothing on the secondary, since I remember measuring it anyway with the bulb all bright and it was completely dead...

[SoporteHR]

Hi. Dannyx

Good job and thanks for sharing your experience, recently here in the forum they helped me repair a laptop power adapter, and the same thing happened to me, after changing the damaged components it still did not work, in the end and almost by accident, I saw that 2 pad of the tracks were separated and corresponded with the coil of the input filter, therefore there was no voltage in the chopper transformer.

Greetings.

[eccerr0r]

C5 kills the self-limiting deal...

[Dannyx]

Why's that ?

[momaka]

Originally Posted by eccerr0r

C5 kills the self-limiting deal...

I don't know if it's just C5.
C3 may be partially responsible as well. Part of the "feedback" voltage rise from R6 will get coupled through it and slow down the turn Off time on the Gate.

I think it may also be a better idea if R5 was instead replaced with a Zener diode and connected directly to hot ground (to protect the MOSFET Gate better).

Then there is the whole base-driver circuit of U1 and U2 for the smaller transistor...
1) Ideally, there should be a resistor between V5 Base and U1/U2 Emitters to limit current.
2) A cap between hot ground and V8 cathode would provide a more stable voltage to U1/U2 Collectors.
3) The parallel 4007 diode with U1/U2 C-E junctions will likely "eat" some of the turn-On "feedback" coupled through C5.

All in all, it looks like a barely stable design to me. Probably just better to take a Deer/L&C/Allied 2-transistor 5VSB circuit and copy it. They run quite stable after a recap (but they do have a "critical" cap).

[gvb]

To me it is not clear how you combined the two resistors in R7 (on the picture R6) to reach 30 Ohms.
When I check the color code, the first Braun, Red, Black brings 12 Ohms. But how to reach 30 Ohms with the second one? With the colors on the resistor I cannot get 18 Ohms.
So please help.

[Dannyx]

Originally Posted by gvb

To me it is not clear how you combined the two resistors in R7 (on the picture R6) to reach 30 Ohms.
When I check the color code, the first Braun, Red, Black brings 12 Ohms. But how to reach 30 Ohms with the second one? With the colors on the resistor I cannot get 18 Ohms.
So please help.

Now that I look at the pictures myself, the colors indeed make no sense and I can't remember what I did there after so long. As I was reading my original post, I thought I combined two 15ohm resistors in series to get 30ohms (didn't explain it in detail myself), but the colors don't match and they're also different between the two....still, the schematic stands, so do whatever it takes to have 30ohms there.

[llonen]

Looking over the component side of the board it is clear tome that there are still numerous bad solder joints.

[Erdist]

I also have a blown resistor in my Bosch AL2215 CV charger, and hope you guys can help out, because I have to admit that knowledge about electronics is zero. Nevertheless I have to do the repair myself, so please be patient if I ask something obvious.

It looks like the burned out resistor is the same as on Dannyx photo, however on my circuit board (1607502155) it is marked as R5.

According to the schematics that he posted R5 is supposed to be 22k Ohm. However using resistor colour coding on the resistor that he has in that spot it looks like that it is only 22 Ohms. Unfortunately the original resistor is difficult to read too.

Any help is much appreciated!

[Dannyx]

I'd say the resistor should be 30 ohms, as the first band to the right looks orange to me and it also goes with what I was talking about in the thread....it's been a while, so I can't remember exactly unfortunately...

[petehall347]

Originally Posted by Erdist

I also have a blown resistor in my Bosch AL2215 CV charger, and hope you guys can help out, because I have to admit that knowledge about electronics is zero. Nevertheless I have to do the repair myself, so please be patient if I ask something obvious.

It looks like the burned out resistor is the same as on Dannyx photo, however on my circuit board (1607502155) it is marked as R5.

According to the schematics that he posted R5 is supposed to be 22k Ohm. However using resistor colour coding on the resistor that he has in that spot it looks like that it is only 22 Ohms. Unfortunately the original resistor is difficult to read too.

Any help is much appreciated!

there is another burned resistor there above it

[Dannyx]

Yeah, some close-ups of that area would be great (like in the 4th picture of my original post).

[momaka]

One of the diodes in the bridge rectifier looks physically damaged too (possibly lead broken or pulled out.)

[Erdist]

Originally Posted by Dannyx

I'd say the resistor should be 30 ohms, as the first band to the right looks orange to me and it also goes with what I was talking about in the thread....it's been a while, so I can't remember exactly unfortunately...

To be honest they do look orange to me too, both on yours and the factory one too. And having the black band in the middle would suggest to me that they are not kOhm? On the factory resistor the second stripe looks almost completely missing, if it was there in the first place it is hard to tell. Could it be only 3 ohms? Your's has a blue/silver line as well after the black, that I can't decipher. Enlarging from your photo:

[Erdist]

Originally Posted by petehall347

there is another burned resistor there above it

I see which one you are referring to. You've got sharp eyes! Thanks for pointing it out

[Erdist]

Originally Posted by Dannyx

Yeah, some close-ups of that area would be great (like in the 4th picture of my original post).

Attached more photos. Taking out that burned electrode was my inexperienced butcher work.🙄

Thank you gentlemen for your help so far, you guys rock!

[Erdist]

Originally Posted by momaka

One of the diodes in the bridge rectifier looks physically damaged too (possibly lead broken or pulled out.)

I cannot seem to find the one you are referring at. The others look ok to me. Any chance you could highlight it on one of the photos for me? Thanks!

[momaka]

Originally Posted by Erdist

I cannot seem to find the one you are referring at. The others look ok to me. Any chance you could highlight it on one of the photos for me? Thanks!

Sure.
Look in the lower-left corner of this picture:
https://www.badcaps.net/forum/attach...2&d=1611786894
See the 4 diodes between the transformer and input filter choke (located right next to a blue safety Y2 cap too)? One of them (the 2nd one from the bottom up) appears dented and lead possibly pulled out. I would suggest to remove it and make sure it's not reading O/C. The diodes look like they might have ran a bit hot too, so maybe not a bad idea to check them all anyways.