VOGONS


My Fox II blew a cap!

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First post, by root42

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I have never powered it up so far, so I guess it was faulty...? It was an extremely loud bang.

I can replace the 10uF tantalum, but the question is if itwas the faulty part. I can replace all the caps near the power connector if needed.

Any idea how to troubleshoot this?

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Reply 1 of 25, by Deksor

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Yeah that's how tantalum caps die after a while. Replacing them is a good idea.

I have an MFM Western Digital HDD in my 8088 and some caps were making short circuit, preventing the whole computer to power up. Once I desoldered them, a third one exploded. Then I decided to replace all of them and the HDD was fine.

So check the continuity of the other tantalum caps too, they might be shorting

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Reply 2 of 25, by root42

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Ok. That I can measure.

And shall I replace with modern Tantalum? Or rather electrolytics? Or other kinds?

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Reply 3 of 25, by root42

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Electronics stackexchabge has a nice writeup about tantalum capacitors:

https://electronics.stackexchange.com/questio … =google_rich_qa

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Reply 4 of 25, by root42

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Deksor wrote:

Yeah that's how tantalum caps die after a while. Replacing them is a good idea.

I have an MFM Western Digital HDD in my 8088 and some caps were making short circuit, preventing the whole computer to power up. Once I desoldered them, a third one exploded. Then I decided to replace all of them and the HDD was fine.

So check the continuity of the other tantalum caps too, they might be shorting

Can I use MLCC capacitors of the same rating as a replacement? Is that advisable?

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Reply 5 of 25, by Tiido

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MLCC will have to be high voltage (over 25V) or very high capacitance (100µF+) if you want it to still have say 10 or 47µF of capacitance left at the specified voltage. Ceramics lose a lot of capacitance when having a DC over them. High voltage and capacitance ceramics are also expensive so you're much better off using normal low ESR e-caps.

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Reply 6 of 25, by root42

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Tiido wrote:

MLCC will have to be high voltage (over 25V) or very high capacitance (100µF+) if you want it to still have say 10 or 47µF of capacitance left at the specified voltage. Ceramics lose a lot of capacitance when having a DC over them. High voltage and capacitance ceramics are also expensive so you're much better off using normal low ESR e-caps.

Ok, got it. By e-caps you mean electrolytic capacitors, right? I will order a set of good quality 10uF 25V ones then.

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Reply 7 of 25, by root42

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Also: I measured all caps. Most have a resistance of 900 Ohms. Quite a few are shorted completely. Those also beep in continuous testing mode.

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Reply 8 of 25, by root42

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root42 wrote:
Tiido wrote:

MLCC will have to be high voltage (over 25V) or very high capacitance (100µF+) if you want it to still have say 10 or 47µF of capacitance left at the specified voltage. Ceramics lose a lot of capacitance when having a DC over them. High voltage and capacitance ceramics are also expensive so you're much better off using normal low ESR e-caps.

Ok, got it. By e-caps you mean electrolytic capacitors, right? I will order a set of good quality 10uF 25V ones then.

I am eyeing these as a replacement:

https://www.mouser.de/ProductDetail/Nic ... %2fA%3d%3d

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Reply 9 of 25, by Tiido

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Those will be good and will last long. Buy a nice amount to get bulk discounts ~

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 10 of 25, by Deksor

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I'd say, replace ALL of them since there are many dead ones, who knows how long the remaining ones will still last ? Better buy all of them in a row

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Reply 11 of 25, by root42

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Deksor wrote:

I'd say, replace ALL of them since there are many dead ones, who knows how long the remaining ones will still last ? Better buy all of them in a row

That's what I plan to do. However they are coming off very hard. Any desoldering tips?

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Reply 12 of 25, by Tiido

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They are in a multilayer board which will suck away heat from the tip. A normal conical tip will be highly useless unless you can boost the temperature to high levels (i.e 380ºC or even more). Applying some flux will help a bit too.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 13 of 25, by root42

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Tiido wrote:

They are in a multilayer board which will suck away heat from the tip. A normal conical tip will be highly useless unless you can boost the temperature to high levels (i.e 380ºC or even more). Applying some flux will help a bit too.

Ok, I can do both. I noticed that I can hardly get the solder to melt. I use a 60W iron. I can crank it up a bit. Also I have a flux pen ready.

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Reply 14 of 25, by Deksor

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Catch the cap on the other side while you're melting one solder joint and pull it so the leg go a bit, then melt the other solder and pull the cap to the other side and repeat until it's gone 😉

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Reply 15 of 25, by root42

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The Nichicon replacement caps arrived! Now cross your fingers that nothing else blew. Those caps looks really nice! 10uF, 25V, replacing the 30 year old 10uF, 16V tantalums.

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Reply 16 of 25, by root42

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So I now replaced the 10uF caps on the mainboard. I think I did an okay job, and I double checked all solder joints. The board won't boot. I get four beeps when I have RAM installed, and 3 beeps when not. The four beeps seem to be the system timer error. I have never encountered such a problem.

Any ideas and hints what this means and how to fix it? I guess that means some IC on the board is fried? Which one and how can I test this?

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Reply 17 of 25, by root42

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Additional note: I tried different RAM modules, but I think they were broken or incompatible and also gave three beeps.

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Reply 18 of 25, by root42

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Update: I double checked, and found out I forget to recap C54, which was unpopulated after my decapping. 😀 However, I still get the system timer failure. So I am still scratching my head... How can I debug this, which IC is the problem, and can I test it?

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Reply 19 of 25, by quicknick

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SW2 must be set to OFF position, otherwise the board is expecting the RAM to be in the DIP sockets not the SIMM ones. I'm attaching the manual in case you don't have it.
I have this board, it is very picky about the SIMMs it accepts. With most of them it seems to work fine, however if left idle for a few minutes (under DOS) it stops responding. Only way to solve this problem was to use older, 9-chip modules...

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