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Motherboard capacitors - which to replace?

Posted: Thu Dec 21, 2017 8:35 am
by kodak80
OK, I have now had fun replacing capacitors on my ZX Spectrum boards (x3) and changing these to composite video. This was my learning ground and all are now done and working. I have also replaced the capacitors on 2 ST PSUs and they work well. Still working up to replacing my Falcon NVRAM.

Whilst Exxos has the replacement for the 4700uf 16v capacitor in his store. I was looking at my STE motherboard and there are quite a few capacitors on it. I am wondering which ones can be replaced or to put it another way, which ones should NOT be replaced? I also have some STFMs so same question for this motherboard as well? :D

Re: Motherboard capacitors - which to replace?

Posted: Thu Dec 21, 2017 9:45 am
by exxos
As far as I got is the video re-cap kit on the STFM..

Any electrolytics should be replaced. I did wonder about designing small adapter pcbs for all caps (kinda like my 4,700 kit) but using SMT caps.. but its a lot of kits to mash up... plus I doubt there will be many users still in the next 30 years anyway :lol:

Re: Motherboard capacitors - which to replace?

Posted: Thu Dec 21, 2017 9:59 am
by kodak80
As there was a 1uf timing capacitor on the PSU that you recommend NOT be changed, I thought that there may be similar ones on the motherboard that if changed may cause issues?

Anyone already re-capped all capacitors on their motherboards?

Re: Motherboard capacitors - which to replace?

Posted: Thu Dec 21, 2017 10:13 am
by exxos
kodak80 wrote:
Thu Dec 21, 2017 9:59 am
As there was a 1uf timing capacitor on the PSU that you recommend NOT be changed, I thought that there may be similar ones on the motherboard that if changed may cause issues?
Its a good point.. I haven't of course looked into every capacitor on the motherboard, but I do not believe it would cause any harm anywhere..

As for the PSU, changing the capacitor resulting timing differences within the switch mode control loop. As a capacitor basically poor quality, it actually works really well with the value it actually is. If it was replaced, it would have to be replaced with same "spec" capacitor, which would then make it a little pointless changing it in the first place.

I doubt there is such timing related capacitors on the motherboard, other than the reset circuit. I think mostly coupling or smoothing capacitors along the motherboard to changing them the better ones is a good thing.

I was thinking more of ceramic capacitors in particular relating to the audio circuit, as electrolytic cause a lot of hiss, and ceramics do not. It is also possible they may work better in the video circuit is well. But it is all something I simply do not have time to try out :(

Re: Motherboard capacitors - which to replace?

Posted: Thu Dec 21, 2017 10:55 am
by mikro
kodak80 wrote:
Thu Dec 21, 2017 8:35 am
OK, I have now had fun replacing capacitors on my ZX Spectrum boards (x3) and changing these to composite video. This was my learning ground and all are now done and working. I have also replaced the capacitors on 2 ST PSUs and they work well. Still working up to replacing my Falcon NVRAM.
:thumbup: Great job, Rob! I hope our Atari sessions have been at least partially useful in motivating you to learn this kind of skills! I'm afraid that while I'm in Tassie you will become ten times better at soldering than I am now. :D (funny, I've learned how to solder in QLD, too ;-)).

Re: Motherboard capacitors - which to replace?

Posted: Sat Dec 30, 2017 11:23 pm
by gazak28
Hi Mikro,
Both Rob and I have both had to learn how to do soldering, when a certain person says they are going to do it and lets you down. ;)
I should have taken up your offer, more fool me.
But on the good side Rob has shown me how to change the caps on the Falcon PSU, and I have replaced the NVRAM on both Falcons :D and both working good.
Will practice more soldering skills, when I look at some of the STE boards I have laying around that need fixing or upgrades.
Until next time happy holidaying in the Apple Isle.

Gary

Re: Motherboard capacitors - which to replace?

Posted: Fri Jul 13, 2018 11:33 am
by tzok
I have recapped a few STs and never found a faulty capacitor on its motherboard. Only one have slightly corroded aluminum cup, but still was well in specs according to the capacity, ESR and leakage. If they are sealed well (and apparently they are) and not put to excessive heat, they should be ok for the next 30 years... For sure it will do more harm to replace them with a modern low cost capacitors, and hand-soldered without a proper thermal profile.

Re: Motherboard capacitors - which to replace?

Posted: Fri Jul 13, 2018 12:06 pm
by exxos
It's not so simple to simply test with ESR meter to see if caps are OK or not. You only get a indication, not a straight forward good or bad test. I tested some a while ago, if you heat them up (like in PSU's next to heatsink) value increases a lot and ESR falls to near zero. Surely not ? Testing newer caps, ESR and value remains a lot more stable regardless of temperature. I need to do more tests, though if the value & ESR changes a lot, I would say the dielectric is starting to break down causing the "thinner" layers and higher values. Ironically you can end up with a 30 year old capacitor which can appear to have a better ESR than a modern one.

In my old job we found capacitors which gave a higher value than expected would not work in power supplies powering lamp arrays. The lamps would flicker like no capacitor was even there. Mix in that capacitors have 20% tolerance, and such capacitors can still be within that tolerance and still be faulty.

So sure, old caps may work fine for a long time, depends on a lot of factors. I need to do more work on this, but no time to build up testing circuits for it all at the moment. Though only proper testing can say if cap is good or not, meters are no good for accurate readings (unfortunately).