Capacitor interchangeability in old computers - Page 1
Forum section: electronics / beginners

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 11, 2016, 09:32:39 pm
content: HeyI have a few old Commodore computers that exhibit some intermittent glitches that are likely to be caused by out of spec electrolytic capacitors, so I decided to play it safe and replace all the elecs in them... Now as some time has passed since the 80s, there is a much larger selection of affordable caps in the required values and sizes.So I have basically two questions.One is, are solid polymer electrolytics a safe drop-in replacement for old liquid electrolytics? They seem superior in pretty much all areas, except for a higher leakage current...The other, for small values below 10uF, there are some affordable ceramic bead caps available. Is there any reason why I shouldn't replace a 1uF/16V elec with a 1uF/16V bead of a similar size - say aTDK FK?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 11, 2016, 10:47:54 pm
content: Which Commodore computer? They've made many. If it's a C64, the caps are fine, it's the power brick that's just waiting to fail and apply unregulated 8V to the 5V line.You're setting yourself up for a lot of work with the risk of damaging custom chips. Far from playing it safe.You'd be better off telling us what model, what glitches, and why you want to replace capacitors. For the record, I have 3 C64s, 3 128s, several 1541s, a 1571, 1581, 1764, 1351, SFD-1001, etc... and not once has there been a problem with any capacitor on any board.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 12, 2016, 12:16:37 am
content: Quote from: Alex Eisenhut on August 11, 2016, 10:47:54 pmThe power brick is fine for now, replacing that is another thing on the list.And honestly I really don't trust 30 year old electrolytic capacitors that have spent most of this time discharged. Yea it's some work, but I'm doing a dualSID on the c64 and a 64k memory expansion on the c16 anyway, and the 250469 has far, far less electrolytics to worry about than the older boards...As for the glitches, on the c64 (assy 250469) when powering up after a longer period, the screen sometimes starts flashing about once per a second. Switching off and back on fixes the issue.The c16 powers up fine, but when I play sound from BASIC, I have a periodic glitch, almost as if it stopped playing the sound for a moment every second, like to serve an interrupt or something.Since I'm pretty sure this isn't how it should normally sound, my best guess is that it's some kind of crosstalk, maybe due to a dried-up filter cap.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 12, 2016, 01:15:15 pm
content: Anyway, the core question still stands - will a ceramic do well in place of a small elec, and is a solid aluminum polymer a drop-in replacement for an elec of similar value, and what caveats are there...Specifically in the following use cases:- Bypassing digital chips- Filtering power lines- Adding the 9vac power line in the c64 onto 9vdc rectified from it, to gain ~18vdc. (Don't ask me, those Commodore engineers must have had some real good weed, lol.)- Audio ac coupling

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 12, 2016, 11:42:40 pm
content: The per-IC decoupling caps are already ceramic discs, or those little yellow sausages. Take one apart, you'll find a leadless ceramic cap with leads attached...You won't find many small value electrolytics in a 64.Some ceramic caps derate their capacitance a lot above 0V.Anyways, since you seem set on doing this, you can simply get a new electrolytic and use alligator clips to put the new cap in parallel with the old one and see if it makes a difference.A 64 only draws about 1 amp at worst case, the capacitors simply aren't stressed that much. They're fine. I have no idea why you want to change capacitors like this without doing some troubleshooting first.Did you measure any voltages or waveforms?Why do you think a problem that is solved by "Switching off and back on fixes the issue." has anything to do with capacitors?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 12, 2016, 11:51:53 pm
content: Oh yeah, forgot there's also a C16 here, that computer has the TED chip that cooks itself to death because Commodore saved 25 cents by not putting a heat sink on it.The TED chip generates the display ... and the sound.Put a heat sink on it to help it out a bit. New capacitors won't change a thing to how hot TED runs. And there's no replacement for it, it's strictly a Commodore ASIC, and no one has made a VHDL model of a TED yet.Ah, the Commodore 16/264 line... Only Commodore could replace a serial disk interface with an 8 bit parallel interface and yet only go 4 times faster...