NMS8280 recapping

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By gianlucarenzi

Supporter (5)

gianlucarenzi's picture

01-11-2020, 20:23

Can you show me which jumper?

By SjaaQ

Master (170)

SjaaQ's picture

01-11-2020, 20:31

I've got a 1F supercapacitor installed in my NMS8280 instead of the battery, it hold the RTC data for at least 3 months. That is the longest I kept it off since it was installed. I would recommend it to everybody who needs a battery replacement.

By SjaaQ

Master (170)

SjaaQ's picture

01-11-2020, 20:44

I think this one:

Haven't done this myself, but was also wondering which jumper was referred to.

By Meits

Scribe (6209)

Meits's picture

01-11-2020, 20:48

I never did it myself, but here it's done.

By Meits

Scribe (6209)

Meits's picture

01-11-2020, 20:50

@SjaaQ, that jumper indeed. Before the four diodes next to it were replaced by bigger ones and placed as high as possible. With that jumper you harvest the same effect.

By Wild_Penguin

Hero (623)

Wild_Penguin's picture

01-11-2020, 23:03

So, gianlucarenzi : did you get it working?

A fellow MSXer here who owns an NMS8255. I've never changed any capacitors on it, so changes are, despite them being old, yours work just fine. My NMS8255 is alive and fine (last time I checked, which was not that long time ago, but I have less time to play with my MSXes than I'd like to).

I did have problems with the PSU, though. The thread and the story is somewhere on this forum. But the problem was not with the caps - it was with the +5V regulator. In the end, I sourced a replacement and also changed the rectifier diodes to larger ones. No caps were changed, not even on the PSU.

What I would do, is this: disconnect the power connector from the motherboard (your PSU should now not be connected to anywhere on the MSX). Power on the PSU and measure the voltage lines. They should be somewhat higher (as there is no load) as their nominal voltages are - but not totally out of whack (But how much? I don't know!). There should be no smoke coming out of anywhere. If this test seems OK, power off and reconnect the connector to the motherboard.

Now you are ready to just try to power on the computer. Measure the voltages while it is powered on to be sure (start with +5V, as that is the most important one). The voltages should be very close to their nominal values (I believe the normal spec is +-5%, maybe even +-10%, but usually it is much closer than that; i.e. at max/min you could see something like 4.5 ... 5.5V and 10.8 ... 13.2V, but probably will see something a lot closer - most importantly, it should be stable). Keep the machine switched on, and measure these voltages after it's been powered on for, say, 30 minutes. I believe some broken regulators will start gradually to output a wronger voltage when they warm up (and will output the correct one when cold). You might even want to just leave a meter connected to the +5V, and use the floppy drives. That will stress the +5V line, and it should not fluctuate (at least not by much, and not out of spec). IIRC I saw no fluctuations whatsoever when I used my floppy drives.

But then, A BIG FAT WARNING/DISCLAIMER: I'm not an electrician, but a self-though hobbyist!

(I know electricians lurk on this forum, but how often they pay a visit - I don't know. They will correct me if they come here by chance and see any stupid things in my writing).

Cheers!

p.s. IMHO I've seen way too often recommendations to change capacitors in some old hardware "just because", even to some working piece of old hardware, and sometimes just because of some random problem - without doing any kind of diagnosis as to what the actual problem is. I think it should only be done in case the caps are known to be the problem. It's the same thing with any old hardware failure; don't do random "repairs" in case the problem hasn't been diagnosed, yet in case of no problems. My opinion about capacitors: don't change them. Although - theoretically - they are not 100% hermetically sealed, they do not evaporate as much as some people believe. Nothing will last forever, and often something else - RAM, ROM chips etc. - will fail before the caps. How long can they last, is an interesting question. It seems they can last at least over 30 years, at least in some cases. Perhaps 50 years? 100 years? Only time will tell Wink

By Meits

Scribe (6209)

Meits's picture

02-11-2020, 02:19

I asked the guy who did that cr2032 battery.
Diode D130 (this is with an NMS8250. Check if this diode does the same on an NMS8280) can just be removed.
There will be voltagedrop, but the clock will work just fine at 2V.

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