IBM powersupply resistor?

Hi all, I dismantled an old IBM laser printer, it was hopelessly broken on various internal gears. I salvaged the power supply, it's a nice switching unit with a few different outputs, however the usual RIFA X2 capacitor smoked badly at one time. In this particular unit, the X2 capacitor have a small 1/4W series resistor that was completely burnt with the shorting X2 capacitor. I don't know why they put this resistor in series with the capacitor, maybe to prevent a much bigger explosion event when the capacitor would short? Since I've seen way too many RIFAs letting the smoke out and nothing else was really damaged after that, I think there might be another reason for putting that resistor in series with the X2 cap. The resistor has no external coating anymore, and it measures 1k5 ohms. I'm unsure what value it had in the beginning and the power supply still works without it, but in this way the X2 filter is disconnected of course. Any good guess about the original value of that resistor? I don't think it matters much, but I'm always reluctant to make a substitution for something I don't fully understand. Capacitor was a standard 100nF X2. For what is worth the power supply part no. (IBM) is 1039701 and manufactured by Salcomp of Finland with part no. 7765. What do you think?

Frank IZ8DWF

Reply to
frank
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they knew it could fail

NT

Reply to
tabbypurr

I'm dubious: they knew it could fail and they made another part fail with it? So two failed parts instead of just one? Does that make sense?

Frank IZ8DWF

Reply to
frank

it?

Of course it makes sense.

The resistor probably was used like a fuse, so when the capacitor shorted, the resistor opened interrupting the flow of electricity to the cap. With n o resistor present, as soon the cap shorted, you would have had blown trace s inside the PSU, rendering the printer inoperable at least. The way the PS U was designed allowed for the cap to be damaged without causing a fire ris k or destroying the PSU.

Reply to
lsmartino

You talking about those big mainframe or mini-computer printers right?

Don't you have IBM dealers in your city? Call them! Email IBM headquarter if needed. :)

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Reply to
Mr. Man-wai Chang

Well, not quite. The capacitor still burned completely, the PCB was mostly carbonized under both the capacitor and the resistor. The resistor measures now 1k5 ohms, I don't know what was its initial value, but it could only limit the current and burn itself too. Not too clever as a fuse. If I'd expect an X2 rated capacitor to catch fire, I'd chose a different brand of X2 capacitors. They are not supposed to start burning if used under their voltage limit.

Frank

Reply to
frank

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