C41 ceramic capacitor replacement

[Winfried David Cachet]

Hi,

 

I broke a C41 ceramic capacitor 1uf 25v by replacing a RAM chip, possible to replace it by an electrolytic 1uf 50v ?

Edited August 30, 2020 by Winfried David Cachet

[thorfdbg]

No. Electrolytic capacititors have a parasite inductivity that is too high to deliver the required current quickly. BTW, are you sure that this is 1uF and not 0.1uF?

[Winfried David Cachet]

Not sure, shooted from officiel service manual, page 32 (see attachment).

 

.1uf = 0,1uf ? 🤨

 

 

[xrbrevin]

a tantalum one should be ok

[www.atarimania.com]

Yes, .1 or 0.1 would be 0,1 in a number of European countries.

[thorfdbg]

Yes, so it is really 0.1uF, as expected, and no, a tantalium capacitor is not the right solution, for exactly the same reason. The purpose of these capacitors is to buffer up small splikes in the power supply that arise if the RAM pulls power when it is addressed. This requires relatively large amounts of current in relatively short time, and this requires a relatively low parasite inductivity. Neither electrolyte nor tantalium capacitors offer this. Ceramic ones do, and it should not be a problem to find a suitable 0.1uF = 100nF ceramic capacitor.

[Winfried David Cachet]

Ok thanks.

 

This kind of capacitor should be ok ?

 

 

Edited August 30, 2020 by Winfried David Cachet

[+DrVenkman]

2 minutes ago, Winfried David Cachet said:

Ok thanks.

 

This kind of capacitor should be ok ?

 

 

Yes. They’re super cheap - if you expect to do repair work on machines in the future, buy an assortment of a couple hundred capacitors in a few different common values and you’ll be set for years. 

[1050]

Known as bypass capacitors and/or decoupling capacitors, their use is common. One for every chip is the generic rule with some high performance people up sizing the capacity at the same time. Your 104 examples would then be 204 or 304 if you cared to double or triple that actual capacity shunting the noise spikes to ground plane locally. Atari commonly used a chip capacitor inside a glass case with axial leads mounted to the captured chip capacitor inside = insanity on some level.

 

http://docs.eao.hawaii.edu/JCMT/i/012_HARPB/localOscillator/Manufacturers/Phytec/AppNote/Atmel/PLD/doc0484.pdf

 

This note primarily aimed at PLD devices like PAL or GAL, but please note - EVERY chip deserves the upgrades suggested in this application note.

 

This one a little newer, is on about static ram chips.

http://ww1.microchip.com/downloads/en/AppNotes/Atmel-8580-TPM-Power-Supply-Decoupling-ApplicationNote.pdf