"Commodore 64 powers on but..."

[motrucker]

I never said "the power supply will cook everything on the board". However, if the regulator goes out, it can sure take out all of the RAM chips, and maybe an IC or two. The following is a quote of Ray Carlen's site;

 "The worst and most destructive PS failure is caused by a bad regulator that shorts out internally so its input voltage (more than 11 volts) is passed directly to its output! Most semiconductors (transistors and IC's) inside the computer run at 5 volts DC and can withstand a minor voltage overload without damage... but some cannot. The eight RAM chips in a C64 are especially vulnerable to over-voltage damage. Those IC's have an "absolute maximum rating" of 5.5 volts and are quickly destroyed above that maximum level. I've seen several CBM bricks that always worked normally when cold but would output excessive voltage when warm. Those supplies went on to damage several users computers because the owners didn't realize the fault was repeatable. Any PS brick failure means it must be replaced, preferably with something better if available."

I have worked with Commodore equipment since about 1982, and have seen more than a few C-64s suffer this fate. I live in the U.S., so all of 64s I have worked on were using North American type power supplies. These epoxy filled beasts are downright dangerous to use today, period.

 

 

[CaptPKelly]

Motrucker: I wasn't pointing fingers at anyone here. I'm trying to point out to the non-technical people that a smoked power supply and a black screen does not automatically mean their C64 was completely fried. This is a mistake many have made and all you have to do is read the comments on YT videos dealing with the issue to know that a lot of people (with no technical knowledge) believe (without evidence) that this has happened to them.

 

Now it's time for a retraction on my part:

Because of this discussion I have spent the last week pouring over schematics and data sheets of various components on both the C64 mother board and the external PSU. Understand that much of the information on this circuitry has only become available in the last ten years or so since the resurgence of the popularity of the C64 and it's "siblings". Also understand that I have not concerned myself with the circuitry of the C64 in 20 some years. In the 80's, unless you had a Dremel tool and the time and patience to dig it out, the only thing you could do was assume what was buried inside the PSU.

 

What I have found out:

The original power supply did NOT use an IC, 5 volt regulator as I always assumed. It used a PNP transistor configured as a "C" class amplifier - essentially a voltage regulator. The output voltage is set by a 10 watt 220 ohm resistor. It is an extremely simple circuit with no protection at all other than a 5 amp fuse on the input side, also encased in the epoxy. The problem with this dangerously over simplified regulator is the 220 ohm resistor. When it fails, while it's burning towards failure, it can cause the SK3052P to regulate to a higher and higher voltage. This won't last long, a few seconds perhaps, but it could put out close to 7 to 9 volts on the 5V. line. Although the PSU was designed to at least adequately meet the power demands of the mother board, a cartridge plugged in to the system could increase this demand on the PSU to near maximum producing extra heat causing the resistor to eventually fail.

The other thing I discovered is that I was mistaken over a particular marking on the C64 schematic and a "warning" notation that further confuses the matter - all concerning 5Vcc, 5Vc, 5Vcan, and +5V. This was something I misinterpreted 25 years ago and never discovered my mistake. As I said, my concerns on the C64 circuitry was shelved in 1988.

The bottom line is that the 5V from the PSU actually does feed most of the TTL ICs on the board, and although these old TTL ICs are somewhat tolerant of higher current, they do not withstand an increase in voltage - even momentary especially those in the 74LSxxx series.

If you still use an original power supply, then it's actually a good idea to invest in a (reliable and tested) "Saver" power cable.

Edited November 21, 2017 by CaptPKelly

[motrucker]

I am sorry my posts were so cut & pasted together, but right now my arthritis is so bad I can't hardly type at all.

C-64 users are lucky these days, that they can get modern versions of the PLA (Look on "http://www.melon64.com/forum/index.php?sid=61ef32d5a71cf5672737f51f3279982a")and even the 8701 replacement for good prices.

The C-64 hasn't had it this good in years.

I personally use a 1764 PSU on my C-64, so don't worry about this aspect of the computer. But these are extremely hard to find these days.

Edited November 24, 2017 by motrucker