Commodore 64 Restoration

[1024MAK]

There is a nice write-up on a common crowbar circuit here

Mark

[GaBuZoMeu]

1024MAK wrote:

There is a nice write-up on a common crowbar circuit here

Mark

This is a well written explanation how a crowbar works. I think they missed to say that regular zener diodes have pretty large tolerances. And the schottky diode in the output is almost always unnecessary. Using a bullproof thyristor or triac (some that can deal with 25 amps and more) would be my choice

Perhaps one thing more: there are resettable fuses available e.g. these kind. They work as well but you have to turn off the power supply (to stop the thyristor being conductive) and wait a little until the fuse is cooled down.

[petrus]

GaBuZoMeu wrote:

I wonder why anyone cares about a few cents when going to revive a valuable thing Use caps from Panasonic, they are not cheap nor expensive but reliable.

It isn't a cost consideration. NTE is the brand that is stocked by my local electronics shop, and I prefer to shop locally if the parts are suitable. I don't really know anything about the various capacitor brands, and I was asking to get a sense for what their reputation is as a capacitor brand... if if even has one.

[GaBuZoMeu]

Well, NTE claimed to be highly reliable. I must admit I have never heard of them before

[petrus]

Sadly, I have worked in Sales and Marketing too long and don't really trust company websites. In any event, I've decided this poor computer has waited years to be brought back to life, another week or two for capacitors maybe isn't such a big deal. So I ordered some Nichicons from Mouser. As a side benefit, Mouser also has the 65816, which I've been wanting to play with.

[ArnoldLayne]

petrus wrote:

My board is a 250466

.. which is not very common, so it's always a good idea to put some effort into bringing one of those back to life.

[cb3rob]

the meanwell switched psu's 'advised' by someone else earlier actually are not safe to use on non-grounded wall outlets when connecting the c64 to anything else (disk drives?) that have their own power supply.

also, usually it's the pla and those 2 mos 74ls clone chips (they made their own lousy attempt at producing those) -before- the dram dying first. (some chemistry problem with the chip production itself, not related to voltages or it even being on or not) why the hell all those dram chips used in c64s are also dying in masses is unclear to me. those usually came from quite respectable companies and were not made by mos themselves, and yet every second dead c64 out there nowadays seems to have a few dead dram chips or a dead color ram.
(natural causes of death seem to be mainly the pla, the 2 74ls clones before the dram in as far as they are MOS internal products, and the contacts in the powerswitch but that's rather obvious. all of which also apply to systems that haven't even been turned on in decades, so in all cases it's a chemical aging problem) unnatural causes of death usually are either the 6526's or the sid having been blown up by connecting something to it while it was on with voltage differential or spikes. (in which case it should boot just fine with the 6526'es and sid pulled out. just that sockets are a rare thing in 'factory issue' commodore computers the potted in triangular powerbrick already was a fire hazard when it was new, and nowadays quite a few of them output 8 volts or more instead of 5 volts (basically anything that isn't 5 volt, sometimes also significantly less than 5 volts) some of the 1541's leak power from their psu over the iec bus as well.

anyway, the sheer amount of dead dram chips should be valid grounds to just return them to the manufacturer, even 30 years after they were made as they die by the truckload. youtube is full of c64s with dead ram.

[cb3rob]

plenty of new -properly designed- c64 psu's on the market, if all else fails just go for an era pcb transformer (or rather 2). 9vac and 5vdc --must- come from at least seperate windings. (preferably different transformers as that way you can take a 6v one to provide the 5v limiting heat dissipation on the power regulator and if all else but the rectifier bridge blows up it's still just 5-volt-ish with 50hz noise not doing much damage. also using real transformers nicely isolates it from ac. plenty of 'usb chargers' that have electrocuted people by now. and furthermore, usb chargers rated for 2 amps or more will still only supply 500ma unless instructed to supply more using the usb protocol itself. (if they don't, they're not usb compliant, which raises the question what else they're not compliant to

[cb3rob]

unfortunately the AC is actually -needed- as the c64 uses it to drive the time of day clock in one of the acias (the sid and the vic also require a higher voltage DC rail derived from the AC feed, but no 50hz signal afaik. it's also fed to the userport but who uses that anyway (something from the days of modems that didn't come with their own powersupply, plus you don't want a modem on a bitbanging 6526 in the first place, just put an acia on a cartridge in that case but can't ditch the 9vac power. unfortunately. basically the entire reason for them to have external powerbricks is that so the computer itself would not need seperate approval (for each tiny revision or change of chip suppliers) for each country back in the days when 'approval' for plugging things into the state owned electricity network was still a thing in most places, but it would at least be nice if they had the psu output a single voltage to the computer and do the rest in the computer (or just design the chips so that they all run on a single 5v only rail). why the hell those dram chips all die is beyond me tho. systems with a broken psu have them running fine on 8 volt for decades but systems that have been turned off for 20 years have dead ram.

[cb3rob]

would all those dead dram chips be of a certain specific manufacturer and production years? commodore always just dropped whatever they had in stock into the boards resulting in some of them having the same dram chip of 3 different manufacturers on the same board, factory issue... seems like a good point for a major RMA action (and valid grounds to force them into reproducing those chips as supplies are running low anyway

[cb3rob]

anyway all that broken late 1970s and 1980s stuff is a good indication on 'what not to do' in new systems
so far we have got:
- electrolytic capacitors (just use foil capacitors, who gives a crap about the 20 cents each extra if they last for centuries)
- dram (although some sram also seems to have died that usually was linked to 'voltage abuse' or chemical problems
- not properly isolated connectors to the outside world
- potted in power supplies
- mylar (keyboards, or anywhere else)
- edge connectors, ENIG or otherwise, they just rot away while you look at them, no matter how much gold you put into them.
- non socketed ic's (although use the more expensive gold plated sockets with the small round hole as the other ones tend to 'creep out' over the years, also the other ones turn green when oxidizing and sometimes lose contact)
- cercaps of which the legs can bend over pcb traces (that means you commodore
- cardboard 'rf shielding' (that means you commodore - although on most of the c16 series they did it properly for a change, probably took an atari xl/xe apart to spy on them
- 'nintendo' top loading cartridges (oh wait, also see the edge connectors part
- mechanical drives as a whole but anything with a belt in it in specific.
- switched power supplies (not entirely sure what the goal with those was, but just buying a lump of copper is cheaper and safer and lasts longer)
- rohs (generally, rohs is just crap. 'lead' isn't a 'hazardous substance' lead is a natural element. and it just so happens to do a pretty good job at keeping electronics together so we keep it around. it won't make you die unless you mix it into your food anyway.
- 1970s 'brown' moist-sucking-up pcbs that 'self expand' over the years, breaking contacts. (no idea what they were made of, probably compressed cardboard, but they're quite common in older transmitters and consumer electronics, as well as some cheaper (even cheaper than a c64) computers up to the mid 1980s)
- anything smd ( 1) held in place only by the pcb traces 2) subject to moist getting under the ic and expanding/freezing there 3) hard to repair/replace in the field 4) no need to make things smaller that go into a huge case anyway)
- bga: just horrid. it is simply -physically impossible- to use bga packages -and- expect your product to work 2 years onwards.
- anything glued in place or held in place with double sided tape or sticky tape (apple)
- bromeates either used in chip packages (where their leakage affects the chip die) or outside casing (where it causes discoloring), use some other fire retardant or ceramic chip packages and metal cases.

all those 'youtube repair' videos make very clear who has been deliberately selling broken-by-design crap over the past decades by just running statistics. lol. stuff that previously was only known to repair centers frequently handling product x of manufacturer y, (and apple and such don't like it being exposed one bit they also make very clear how to not design new products.

[cb3rob]

btw as far as i understand, regardless of the brand of elcos, most of them have a version with a higher temperature rating, that will last longer than the 'normal' one. (getting a higher voltage rating over 35V if the original wasn't over 35V doesn't seem to help much in terms of lifespan, in fact, it usually makes them perform worse) but getting the higher temperature rating does give a better quality product. anyway for anything under 10uF there are better options than elcos, that last -forever-. (just that big square colored things in place of elcos don't look very original in old computers

[cb3rob]

check all those tiny ceramic caps too. not for their capacity or dc resistance as that is not a problem with those, but for them being upright and not having crumbled near where the legs come out and actually being in contact with a pcb trace as on most c64's that have been open a couple of times they're actually bent down, lying over other things with their legs.