Motherboard capacitors - which to replace?

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Dlfrsilver
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Re: Motherboard capacitors - which to replace?

Post by Dlfrsilver »

Steve wrote: Fri May 27, 2022 12:21 am
Dlfrsilver wrote: Thu May 26, 2022 11:18 pm Exxos, if you read me, i have a question toward the 2 big capacitors on the ST/falcon motherboards.

What are exactly their roles ?

Thanks :)
I'm not Exxos of course but I have a good analogy. Think of those large caps as big reservoirs (of electricity of course, rather than liquid) they sit directly between the PSU and the motherboard. This ensures a safe and steady flow of current to the board at all times even if the PSU fluctuates. Also if the board suddenly requires more current (perhaps a drive access or something) it will drain the reservoir a little first rather than putting excessive load on the PSU, so it works both ways.
Thanks for that insight. What would be the risk if they fail (drying ?) ?
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Re: Motherboard capacitors - which to replace?

Post by Steve »

Less protection for the motherboard components, less protection for the psu... bad all round. Somethings gonna give.
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exxos
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Re: Motherboard capacitors - which to replace?

Post by exxos »

I'll do a switchmode round up when I get chance. But when caps are failing, they allow high voltage spikes onto the PSU rails. I showed this on a falcon PSU. I also think its why videls and ram fails in machines. You can also end up with high ripple as I also showed on my PSU page. Like having 5vdc with 1v ac on top of it. Its not going to he doing the system any good when the voltage is bouncing up and down all over the place.

It's why PSUs shutdown without a load as the spikes are tripping the over voltage protection. At least Atari have a resistor on the output to keep things more stable. A lot of PSUs don't and will literally explode without a load on them.

A lot of people claim their system works fine without recapping. It may, it also may not. It depends on many factors and nobody is really qualified to say either way. But they probably never change the oil in their car engine as it "works fine" as well. If you care about something you look after it. It's not hard to do, otherwise it's just being lazy.
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Dlfrsilver
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Re: Motherboard capacitors - which to replace?

Post by Dlfrsilver »

exxos wrote: Sat May 28, 2022 9:51 am I'll do a switchmode round up when I get chance. But when caps are failing, they allow high voltage spikes onto the PSU rails. I showed this on a falcon PSU. I also think its why videls and ram fails in machines. You can also end up with high ripple as I also showed on my PSU page. Like having 5vdc with 1v ac on top of it. Its not going to he doing the system any good when the voltage is bouncing up and down all over the place.

It's why PSUs shutdown without a load as the spikes are tripping the over voltage protection. At least Atari have a resistor on the output to keep things more stable. A lot of PSUs don't and will literally explode without a load on them.

A lot of people claim their system works fine without recapping. It may, it also may not. It depends on many factors and nobody is really qualified to say either way. But they probably never change the oil in their car engine as it "works fine" as well. If you care about something you look after it. It's not hard to do, otherwise it's just being lazy.
Thanks for this explanation. In fact, from the informations you and Steve gave, it appears that in fact the 2 big caps on the motherboard must be replaced, and not only those in the PSU. I understand now why some users report having the RAM failing with 'no reason', as well as in some case the TOS rom dying without warning.

PSU and MB caps are working hand in hand, and if only those in the PSU are replaced and the others not, the components don't have anymore any protection.

Just for the record, i know a guy who bought your PSU, but he did not replaced the caps on his motherboard, he went killing his TOS ROMS, and on another ST he tried the PSU on, he killed the RAMs !

Everything has an explanation now. Those who say that their machine are working simply forgot that it does for the moment, but when the failing will be acted, the machine will simply die. The way Atari used the capacitors means that it's toying with fire not doing anything. and that in a few years, some of them will cry with their machines dead, shame.
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Re: Motherboard capacitors - which to replace?

Post by exxos »

Dlfrsilver wrote: Sat May 28, 2022 5:04 pm Thanks for this explanation. In fact, from the informations you and Steve gave, it appears that in fact the 2 big caps on the motherboard must be replaced, and not only those in the PSU. I understand now why some users report having the RAM failing with 'no reason', as well as in some case the TOS rom dying without warning.
Bad PSU's will indeed kill things which otherwise are "working". Its not the only cause of failure either.

Dlfrsilver wrote: Sat May 28, 2022 5:04 pm Just for the record, i know a guy who bought your PSU, but he did not replaced the caps on his motherboard, he went killing his TOS ROMS, and on another ST he tried the PSU on, he killed the RAMs !
That would be quite a accomplishment to kill a board with a new PSU. Man-handling the boards can break the old solder joints "killing" the board. There are many posts about all this. A lot of people think they can do stuff and break stuff themselves. Always easier to blame someone else, such as people do. I never had any ROM's fail on my machines, unless plugging them in backwards, or bad sockets, or simply bad solder joints. Its all common on these old machines. They need servicing properly if their life is to continue.
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Dlfrsilver
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Re: Motherboard capacitors - which to replace?

Post by Dlfrsilver »

exxos wrote: Sat May 28, 2022 5:14 pm That would be quite a accomplishment to kill a board with a new PSU. Man-handling the boards can break the old solder joints "killing" the board. There are many posts about all this. A lot of people think they can do stuff and break stuff themselves. Always easier to blame someone else, such as people do. I never had any ROM's fail on my machines, unless plugging them in backwards, or bad sockets, or simply bad solder joints. Its all common on these old machines. They need servicing properly if their life is to continue.
the new PSU is not the root cause in fact. he did not replaced the 2 dead big caps on the motherboard before installing your PSU. Since your PSU operates with normal values and current (i know you test them before sending ;) ), when the 2 big caps get the current full flavour while they are dead, components get shot. It's a very bad idea to service a machine by half, he learned it the hard way.
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Cosmic Puppet
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Re: Motherboard capacitors - which to replace?

Post by Cosmic Puppet »

I thought I'd check through the photos of my old crashy ST that I sold. I took pictures before I sold it (which turned out to be useful with upgrading my new machine)

And sure enough you can see that the plastic on one of the caps has gone from the top:

20190525_204811.jpg
20190525_204811.jpg (576.79 KiB) Viewed 1776 times
20190525_204811zoom.jpg
20190525_204811zoom.jpg (230.31 KiB) Viewed 1776 times

These supplies get crazy hot and I'm glad I took advice to get an Exxos supply. Now my Marpet RAM chips are hotter than my PSU (when I say 'hot' I mean slightly warm after hours of use.)
US Atari 520 STFM (C070523-001 REV.D1) , Exxos PSU, 4MB Marpet Upgrade, Exxos 6 Chip TOS 1.04 USA, EmuTOS 20211020, Ultrasatan, PP Driver, Original Floppy Disk Drive, NEC Multisync EA193Mi Monitor, Star LC-10c Printer, iMP C100110-001 DMA.
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Re: Motherboard capacitors - which to replace?

Post by exxos »

Cosmic Puppet wrote: Sat May 28, 2022 10:10 pm These supplies get crazy hot and I'm glad I took advice to get an Exxos supply. Now my Marpet RAM chips are hotter than my PSU (when I say 'hot' I mean slightly warm after hours of use.)
Yep. The good old days of leaving the floppy on top of the PSU for a couple hours to get the labels off :lol:

Yeah caps being cooked next to a hit heatsink for 30 years. Caps look fine, carry one :lol:

Had similar in my old job. The old timers were replacing the 85c caps for 105 as they got cooked and failed. Was a discussion about that. I ordered 125c caps just because :lol:
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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Cosmic Puppet
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Re: Motherboard capacitors - which to replace?

Post by Cosmic Puppet »


Yep. The good old days of leaving the floppy on top of the PSU for a couple hours to get the labels off :lol:

Yeah caps being cooked next to a hit heatsink for 30 years. Caps look fine, carry one :lol:

Had similar in my old job. The old timers were replacing the 85c caps for 105 as they got cooked and failed. Was a discussion about that. I ordered 125c caps just because :lol:
Maybe I should've kept my old PSU to keep warm this winter :lol:

p.s. Those quotes got me again. :comp:
US Atari 520 STFM (C070523-001 REV.D1) , Exxos PSU, 4MB Marpet Upgrade, Exxos 6 Chip TOS 1.04 USA, EmuTOS 20211020, Ultrasatan, PP Driver, Original Floppy Disk Drive, NEC Multisync EA193Mi Monitor, Star LC-10c Printer, iMP C100110-001 DMA.
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Re: Motherboard capacitors - which to replace?

Post by rubber_jonnie »

If you want to replace any capacitors, then for me there are 2 simple rules:
  1. Replace with the same value and voltage rating, though a higher voltage rating should do no harm.
  2. Replace only one at a time and review the outcome before moving on. Changing many things can give you undesirable results and you don't know what item you changed is the cause.
As I said on another thread, I have yet to replace any mainboard caps on any of my STs, but I have recapped many PSUs.
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