IBM powersupply resistor?

[frank]

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

 

On Saturday, 11 November 2017 16:04:34 UTC, frank wrote:

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

they knew it could fail


NT

 

[frank]

tabbypurr@gmail.com wrote:

they knew it could fail

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

 

[lsmartino]

El domingo, 12 de noviembre de 2017, 7:31:39 (UTC-4), frank escribiĂł:

tabbypurr@gmail.com wrote:

they knew it could fail

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

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 no resistor present, as soon the cap shorted, you would have had blown traces inside the PSU, rendering the printer inoperable at least. The way the PSU was designed allowed for the cap to be damaged without causing a fire risk or destroying the PSU.

 

[Mr. Man-wai Chang]

On 12/11/2017 12:04 AM, frank wrote:

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....

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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[frank]

lsmartino <luismartino76@gmail.com> wrote:

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 no resistor present, as soon the cap shorted, you would have had blown
traces inside the PSU, rendering the printer inoperable at least. The way
the PSU was designed allowed for the cap to be damaged without causing a
fire risk or destroying the PSU.

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