C64 Power Supply, monitoring lower AC voltages - Page 1
Forum section: electronics / projects

author: DHeth
author_rank: Newbie
timestamp: October 30, 2016, 05:08:41 am
content: Hello,I inherited an old Commodore 64, No power supply or hookups but it does have the 1541 Drive. After looking around on e-bay and finding only overpriced used & worn out vintage power supplies I decided to build my own. After searching around I happened upon Ray Carlsen's websitehttp://personalpages.tds.net/~rcarlsen/cbm.htmland he does a great job explaining the specs of a C64 power supply . I've gathered most of the parts I need to build it. a 9-0-9 18V transformer, LM350 adjustable Stepdown regulator and an old computer PSU housing for an enclosure. Also built one of Ray's "Computer saver" moduleshttp://personalpages.tds.net/~rcarlsen/cbm/c64/SAVER/saver%20schematic.jpg.One thing I decided to add were some Panel Volt Meters to monitor both "Rails" of the Power Supply.I found one easily enough for the 5V DC Rail:http://www.digikey.com/product-detail/en/adafruit-industries-llc/705/1528-1149-ND/5353609They are all over the place and relatively inexpensive.I'm having a harder time finding one for the 9V AC Rail.My question is does anybody know of a Panel Mount Style Voltmeter for lower voltage ranges, all of them I find are for 80+ Volts or prohibitively expensive for a hobby project.Thanks!

author: obiwanjacobi
author_rank: Super Contributor
timestamp: October 30, 2016, 06:24:17 am
content: ebay?like:http://www.ebay.com/itm/Digital-Voltage-Meter-Panel-DC-2-5V-to-30V-voltmeter-5V-9V-12V-24V-with-RED-LED-/252610848362

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 30, 2016, 01:59:51 pm
content: Commodore sold millions of C64s and no one needed to monitor the power supply rails. Neither does anyone of the billions of PCs and whatnot.By the time you'd notice at a human level that the voltage is off, the damage would be done.You're letting anxiety guide your design.Simply get a better power supply than Commodore made then use it and forget it.

author: .::Rizwan::.
author_rank: Newbie
timestamp: October 30, 2016, 06:04:58 pm
content: Very well said Alex.Sent from my iPhone using Tapatalk

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 30, 2016, 10:33:33 pm
content: A few other things, a single 18V center tap transformer won't do the job, you need aSEPARATE FLOATINGwinding for the 9VAC.If you're going to build a supply with a center tap winding, with one side to make the 5V and the other side for the 9VAC, you will do the opposite of what you intend: you will fry the 64.The other thing is, the 9VAC *never* fails, it's a simple transformer winding straight to the computer. There's nothing to monitor. It's fused, then rectified and regulated inside the 64, and passed out the user port.You could simply buy a Commodore brick supply and only use its 9VAC source and wire in a wonderfully cheap and safe 5V 2A wall wart to a 7 pin DIN connector. Problem solved, it's cheap, efficient, and a lot less bulky than a PC power supply case.

author: @rt
author_rank: Super Contributor
timestamp: November 01, 2016, 06:32:16 am
content: You’d be better of making something that looks at the current consumption,since in almost every case, a chip fails close to short and gets hot, an old capacitor across the supply ESR is too low, etc.

author: Crystal-JU-206
author_rank: Newbie
timestamp: November 01, 2016, 08:49:57 am
content: I think it will be helpful to me.

author: CJay
author_rank: Super Contributor
timestamp: November 01, 2016, 09:55:44 am
content: Quote from: Alex Eisenhut on October 30, 2016, 01:59:51 pmAnd if you're desperately worried about popping some of the more difficult to source parts, get a suitable fuse and crowbar on the 5V rail.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 06, 2016, 05:38:57 pm
content: The great paradox is that in any case, the 64 will cook itself to death quite nicely whether you use a shitty power brick or a lab-grade NIST-traceable power supply.Case in point: I just repaired a 64 that had a bad 906114 PLA: my own 64 running on my home-brew super-supply. Those PLAs will eventually fail even when operated correctly. Or even, apparently, when unpowered: there's some kind of chemical reaction happening at the die level because of bad formulation of the passivation. Bond wire fails, there you go; dead 64.So again, get a good enough power supply and buy spare parts, swap out the linear regulators on the mobo with switchers, put heat sinks as required, replace the HMOS parts with the newer CMOS versions.There's no need for crowbars, displays, monitors, current limiters, sense lines, etc. You can't add reliability that wasn't there in the first place.

author: CJay
author_rank: Super Contributor
timestamp: November 07, 2016, 11:08:13 am
content: You can't prevent old components eventually 'just' dying, that's true, but what you can do is prevent them being killed by overvoltage, it's a trivial thing to add a crowbar to a PSU for peace of mind.Those chips were either plastic encapsulated OTP PROM or fusible link PROM, both types of device are known to eventually corrupt themselves and it's not a problem that's restricted to Commodore parts, I was replacing dozens of that sort of chip on IBM 3270 cards back in the 80s every month for instance.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 07, 2016, 12:41:22 pm
content: No, some PLAs really did have a defect with the passivation.And how are you getting more "peace of mind" from a crowbar than from a properly rated power supply in the first place? Crowbars are last-resort imprecise catastrophic devices. Fuses react too slowly in any case. The old joke of the "transistor blowing up to protect the fuse"...And the price for some of these "power saver" devices is just absurd.A nice modern 5V power supply will have all the protection you need built right in.But by all means, Rube Goldberg away.

author: CJay
author_rank: Super Contributor
timestamp: November 07, 2016, 01:42:07 pm
content: Quote from: Alex Eisenhut on November 07, 2016, 12:41:22 pmOf course, well designed modern power supplies never go overvoltage so a crowbar is a complete waste of parts.Except when they do.Yes, a crowbar is a brute force, last resort circuit, by sheer coincidence they're often 'built right in' to nice, modern power supplies.Go figure....I never doubted or questioned the problem with passivation on the PLA either, my own experience would back up that they're not the most reliable of parts and I certainly replaced many in C64s when the machines weren't long out of warranty (I set up and ran a home computer repair department back when the C64 was new) I just said that they're known to go corrupt but that applying higher than rated voltage to one is a good way to kill them quicker.As it is with most electronic devices.Now if you want to carry on with the snarky comments, feel free.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 08, 2016, 03:10:04 am
content: Quote from: CJay on November 07, 2016, 01:42:07 pmThe 1541 drive's power supply has no crowbar circuitry and the 1541 is reliable as far as the power supply goes. The 128DCR has a switching power supply with no crowbar and the 128DCR has no particular history of power supply failures.Interpret that any way you please.