VOGONS

[SOLVED] XT mobo with all bypass/decoupling ceramic capacitors missing

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First post, by analog_programmer

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Hi, fellow vogons forum members,

The board is JUKO ST-12 like this one. For unknown reasons the previous owner of the board cut off with pliers all the small ceramic bypass capacitors, leaving none on the PCB. Otherwise, the board is in very good condition, boots normally and count all the 640 kB of RAM installed. I want to restore the board to it's factory condition, but I don't know the value of all cut small ceramic capacitors - they look completely identical in the photos on TRW site.

Can anyone help me with this problem?

Marked missing caps - about 35 pcs (the picture is taken from i-net for illustration, as my board is still drying from the cleaning bath):

The attachment XT board missing caps.jpg is no longer available

P.S. There's also one missing electrolytic capacitor (C57), but it seems like it was never soldered on the PCB. I think, its value identical to all the rest electrolytic capacitors - KoShin 47 uF / 25 V / 85 °C. I'm not sure if it has to be Low ESR or regular electrolytic capacitor.

Last edited by analog_programmer on 2025-09-12, 09:31. Edited 1 time in total.

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Reply 1 of 4, by keropi

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it is safe to assume for almost all of these that they are 100nf - the ones next to chips are sure to be that
to make sure just check they are indeed on 5v + GND , if yes I would personally use 100nf for everything

about the electrolytics, normal general purpose one will be fine - this is an old board 😉

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Reply 2 of 4, by analog_programmer

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keropi wrote on 2025-09-11, 10:54:

it is safe to assume for almost all of these that they are 100nf - the ones next to chips are sure to be that
to make sure just check they are indeed on 5v + GND , if yes I would personally use 100nf for everything

about the electrolytics, normal general purpose one will be fine - this is an old board 😉

Thanks for the input.

I've checked the through holes for al of the missing ceramic caps. They're 5 V and Gnd, so - bypass caps. If this may help, on one of the i-net pictures I see markings like "10 42"/"42 10" or "10 4z"/"4z 10".

As for the missing electrolytic cap - most likely it has to be a general purpose 85 °C electrolytic cap. But on some i-net pictures these are tantalum caps.

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Reply 3 of 4, by tehsiggi

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104 marking means 100nF which would fit exactly.
You can go with 100nF ceramics for all of them.

For the electrolytic: Check the other ones on the board (at least on the picture there are more). A general purpose, normal ESR one should do just fine.

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Reply 4 of 4, by analog_programmer

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tehsiggi wrote on 2025-09-11, 11:13:

104 marking means 100nF which would fit exactly.
You can go with 100nF ceramics for all of them.

For the electrolytic: Check the other ones on the board (at least on the picture there are more). A general purpose, normal ESR one should do just fine.

Thanks for the confirmation. So, "10 4z" (split on two rows) is like "104Z"and all of the missing ceramic caps are 100 nF (and not 10 nF as I first thought).

All the electrolytic caps soldered on the board are identical - 12 pieces KoShin 47 uF / 25 V / 80 °C, so I may replace these too.

The word Idiot refers to a person with many ideas, especially stupid and harmful ideas.
This world goes south since everything's run by financiers and economists.
This isn't voice chat, yet some people overusing online communications talk and hear voices.

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