Page 1 of 1

Broken capacitor-looking thing on a 770 DC/DC board...

Posted: Mon Sep 18, 2023 12:10 pm
by solidpro
Hi Everyone

I was repairing a 770 DC board and there are two electrolytic capacitors on there and for some unknown reason, IBM decided to dab some white paste stuff on them.

So after desoldering the legs, pulling one off pulled apart this small black 'bit' that was underneath it, but I can't tell what it is, to replace it.

It looks exactly like a Tantalum capacitor. There are many others on the board. It's marked S48E which I've googled and does appear to be some kind of solid state capacitor but seems to be any number of values.

This is what we left behind on D33:

Image

And this is what an undamaged one looks like:

Image

I did manage to work out what end was positive end, but have no idea what to replace it with - any ideas?

Thanks

Re: Broken capacitor-looking thing on a 770 DC/DC board...

Posted: Mon Sep 18, 2023 12:53 pm
by BillP
It's a diode and K is the cathode end. Surface mounted components have a two or three digit code for identification because there isn't room to print more on the part body. Unfortunately the few characters that fit end up not being unique. The first entry in the table at https://www.s-manuals.com/smd/s4 is a good candidate.

Re: Broken capacitor-looking thing on a 770 DC/DC board...

Posted: Tue Sep 19, 2023 8:46 am
by RealBlackStuff
Looking at that link from BillP, I'd say it's the 5th one: S4 SMA EC10QS04 Nihon Schottky diode.

Re: Broken capacitor-looking thing on a 770 DC/DC board...

Posted: Tue Sep 19, 2023 11:42 am
by solidpro
I don't really know much about diodes, apart from them allowing current to flow one way...

Is it just the rated max voltages and amps that matter?

How about this one? https://uk.farnell.com/multicomp/ss110b ... dp/1861422

Re: Broken capacitor-looking thing on a 770 DC/DC board...

Posted: Tue Sep 19, 2023 12:11 pm
by BillP
RealBlackStuff made a better recommendation than I did so go with what he said.

Schottky diodes can tend to have a larger forward voltage drop when rated for higher reverse voltage. So it is best to match the reverse voltage rating and not go way above it. In this case 40 volts is what you want.

Aside from electrical ratings, the part must fit the space on the board. This link explains the size options: https://en.wikipedia.org/wiki/DO-214

I would measure the size of the diodes on the board to confirm which one fits best. Then you can search for something that meets electrical and physical requirements.

Example search: https://uk.farnell.com/w/search/prl/res ... 040v%20smb

Re: Broken capacitor-looking thing on a 770 DC/DC board...

Posted: Tue Sep 19, 2023 1:29 pm
by SMA
When a component number can be identified on a pcb, then the type of component can be identified by that number.
Component numbers are usually one letter and some (3 or 4) numeric digits.
The letter means:

R - resistor
C - capacitor
D - diode
L - inductor
T - transistor
X - Crystal (XTAL)
F - fuse

I might have forgotten some but they are usually rather easy to guess.

When looking for replacement parts, remember that there is also the option of grapping from another otherwise faulty board.