LC475 PSU RIFA Capacitor Replacement

[VMSZealot]

My LC475 has a dead PSU (I tested it briefly with the lower rated PSU from my LCIII and it worked fine - so it's definitely the PSU and not the logic board). There are no odd smells, no scorch marks, and no leaking or bulging caps - the PSU board looks as good as new. The fuse is fine - so I jumped to a conclusion. The RIFA caps. There are a pair - both the same - KNB1530, 0.22uF, 250V AC, X2 (date stamped, as if it mattered, June '93).

I've removed both of them - neither of them seem to test good (but that could be a limitation of my multimeter), and one has a faint crack across the bottom (so I feel reasonably sure that they have either popped or are about to - although neither are scorched).

I can't find these parts online anymore from my usual sources (I'm in the UK, and I use Mouser, RS Electronics and Farnell for preference), but Farnell seems to be close to the right spec. Does anyone have any experience of these caps? Which would you recommend I buy and why?

This is the list from Farnell - List o' Parts

 

[Phipli]

I jumped to a conclusion. The RIFA caps. There are a pair - both the same - KNB1530, 0.22uF, 250V AC, X2 (date stamped, as if it mattered, June '93).

"RIFA" caps don't usually stop the PSU from working when they fail, and if they do, you'd know about it because they'd be a dead short over the mains.

Sorry to say that this isn’t likely to be your issue

 

[joshc]

As phipli said, these are unlikely to be causing your PSU to not work. I assume you've already recapped the whole PSU?

 

[VMSZealot]

Nope. As I say, the PSU looks pristine. I had a RIFA failure on an Acorn - and the computer definitely didn't work (mind you, that RIFA failure didn't so much have a crack as a blast radius). No leaking or bulging caps. Is there a diagnostic guide to help me get this PSU working again?

 

[Phipli]

Nope. As I say, the PSU looks pristine. I had a RIFA failure on an Acorn - and the computer definitely didn't work (mind you, that RIFA failure didn't so much have a crack as a blast radius). No leaking or bulging caps. Is there a diagnostic guide to help me get this PSU working again?

I've had four BBC Micros fill a room with white smoke as the safety caps have failed, and they all kept running.

They stop if the safety cap becomes a short and blows something, usually a fuse, but by design, the safety caps are meant to fail open, and they are placed between the live and neutral, or neutral and ground, so are not in line with the rest of the circuit. We're not making it up.

 

[Phipli]

I've had four BBC Micros fill a room with white smoke as the safety caps have failed, and they all kept running.

Not all at once, but two of them in one day.

 

[VMSZealot]

Do you know of a diagnostic guide for the LC475 PSU?

 

[Phipli]

Do you know of a diagnostic guide for the LC475 PSU?

I'm not aware of any, there are a number of variants of these power supplies sadly. Possibly at least three brands and two power ratings. I've not paid too much attention. I have an ASTEC and a TDK.

 

[3lectr1cPPC]

No bulging caps doesn’t mean they aren’t bad. All LC pizza box PSUs need to be recapped at this point. Plus these usually don’t bulge they leak from the bottom. You’ll probably find leakage underneath them after desoldering even if you can’t see any now. And even if you don’t they’re likely bad.

 

[joshc]

Nope. As I say, the PSU looks pristine. I had a RIFA failure on an Acorn - and the computer definitely didn't work (mind you, that RIFA failure didn't so much have a crack as a blast radius). No leaking or bulging caps. Is there a diagnostic guide to help me get this PSU working again?

Not leaking/bulging doesn't mean much - those caps are still really old and will need to be replaced. Pretty sure a recap of your PSU should get it going again if its as clean as you say.

 

[Phipli]

Pretty sure a recap of your PSU should get it going again if its as clean as you say.

Sadly these ones frequently don't come back to life with a recap

 

[joshc]

Sadly these ones frequently don't come back to life with a recap

Most of mine did. TDK, Astec and Dynacomp. Recapped about 10 or 11 in the end. The ones that didn't work were the really corroded ones. I think OP's is worth a recap if they board is as clean as they say it is.

 

[Phipli]

Most of mine did. TDK, Astec and Dynacomp. Recapped about 10 or 11 in the end. The ones that didn't work were the really corroded ones. I think OP's is worth a recap if they board is as clean as they say it is.

Oh wow, thats good news. Bolle had quite a bad failure rate with his. Partly why I was planning to just swap the guts out on mine.

 

[joshc]

Yeah, guess it can vary. There are good options for replacing with modern innards now so I would do that in future.

 

[VMSZealot]

Okay. I guess it’s a full recap. Which is a bit of a pest. What modern options are there? I don’t want an external PSU dangling out of my LC.

 

[Phipli]

Okay. I guess it’s a full recap. Which is a bit of a pest. What modern options are there? I don’t want an external PSU dangling out of my LC.

The MeanWell RPT-60A fits in the PSU case very well. Someone even made a 3D printed adapter for some brand enclosures. I did custom aluminium fettling for the one I converted.

Replacement Power Supply in Quadra 605

I found that the original adapter that AEChadwick designed didn't seem to be suitable for TDK PSUs. Or at least, I couldn't see how to make it work. I designed a new version (my first 3D printed part that I actually designed - woo!) that addresses that, attached for anyone else to use and...

68kmla.org

 

[slipperygrey]

Sadly these ones frequently don't come back to life with a recap

I've also worked on a few of these now, and I agree that with the really leaky ones a simple recap won't be enough, as such, because you need to clean up all the cap goo. I usually resort to desoldering all the components in the affected area then cleaning them and the PCB thoroughly before putting them back. One thing that I've seen happening is that some component legs and through holes get so corroded by the goo that unwanted resistance gets introduced which causes a malfunction of the circuit. A fiber glass pencil is handy for scrubbing getting legs nice and shiny again.

Just my 2 cents!