Component value ID

[David Aurora]

Following on from https://www.eevblog.com/forum/general-chat/when-things-just-fall-to-shit/...

Just wondering if anyone might be able to take a guess at what value this choke might have once been (or at least a suitable value to put in there now).

The circuit seems to more or less follow figure 5 in http://www.active-semi.com/Data/DataSheet/ActivePSR/ACT30_Released_Data-Sheet.pdf?phpMyAdmin=2557f460eb916599c56f453e620ef76a, but I don't really know where to begin guessing a value here. The manufacturer refuses to tell me any part values or provide a schematic so I'm left to work it out for myself. In the example in that data sheet, it would be L1. R1 looks like it copped a beating too (it's directly under L1 on the board) so it's hard to read, but I THINK it's 3k3 (haven't gone through and started measuring anything yet). C1 & C2 are 10uF and 4.7uF respectively. The IC is the ACT30AHT so it's the one listed with a current limit of 400mA, and the transistor is the 13003 mentioned in the data sheet. There's a touch of a silver band on the bit left at the bottom so I'd guess it wasn't the most critical tolerance part. I haven't really done anything with switch mode power supplies before, but I guess that's just an RF filter, so it probably wouldn't be a huge deal if it's not the exact same value as the original?

Any ideas would be much appreciated, cheers

[HLA-27b]

Can't you replace the entire power supply? Or hack it to work with a wall wart?

[David Aurora]

Can't you replace the entire power supply? Or hack it to work with a wall wart?

Not really, no. If it was mine I would. Actually, it would never be mine, because there's no way I would have bought this piece of garbage 

The company isn't going to send out a new power supply to anyone but one of their authorised repairers, and the owner wants me to fix it because he doesn't have a lot of faith in getting a reliable bit of gear back in any reasonable amount of time from the authorised repairer. Plus, I don't really see the point of binning a board that probably only needs a few bucks worth of parts. Until I can test more, I'm working on a worst case scenario of replacing the RF choke, the caps around it, the resistor paralleled with it and maybe the transistor (I haven't tested it yet, but I see a very faint mark on it that looks worth investigating). Probably 15 minutes work and $5 bucks parts to replace all of that

[TerminalJack505]

C1, C2 and L1 in that schematic form a CLC filter (aka Pi-section filter.)  This is a low-pass filter.  Since you already know the values of C1 and C2, the value of L1 should be selected so that the ripple factor is a reasonable value.

You can find the formulas online.

[David Aurora]

Thanks dude. Yeah I could see it was a filter but I haven't really played with inductor based filters before, and I don't know much about SMPS's either so I had no idea of even a starting point haha. I get the concept, but haven't played with the stuff in practice so values and formulas aren't something I know off the top of my head yet. All the formulas I found seemed to work on the assumption that you know something about the circuit it's feeding, which I don't.

But, I just got an email from the importer telling me it should be a 10uH choke. So I guess I can now use that as a starting point to do some learning and measurements and file this stuff away in my brain for future reference.