6502.org

Hi.

A friend brought me today a C64 and a C128 in unknown conditions with no power supply. There is no reason why they shouldn't work, but when I wanted to power them up I found out that the thing needs 9V along with 5V, so since I didn't have such a supply I stopped poking and left it aside.
Why do this system need a 9V supply anyway?

Also I was wondering for what could I use such machines?

You need a power supply.

The 9V~ is used to create various DC voltages and is essential. The C64 can be used for games, controlling peripherals, program in assembly and much more. The CPU is the 6510, which is very similar to the 6502. It has an additional 6-bit I/O port addressable at $0000 and $0001. The graphics (VIC-II) and sound (SID) are remarkable and ahead of its time. The C64 scene is very much alive.

The C128 is in essence a C64, a C128 (with double RAM and better BASIC) and a Z80 CP/M compatible computer.

Mario:

I can read a schematic as well as the next guy, but I wouldn't use the one posted from the SAMS manual for anything except scratch paper. Depending on that schematic being an accurate representation of the power regulation circuits will certainly require the C64 to be repaired.

There must be another source for the schematics of the power conversion circuits for C64. Perhaps there's a user group dedicated to it, and one of their members can supply a schematic for these circuits from the original Commodore documentation.

I see on the C64 schematic that the video uses 12V. The AC input apparently also was the timebase for the time-of-day clock.

I have quite a few C64 links at http://wilsonminesco.com/links.html#brands.

I have a feeling the drawing Marco posted is intended as a block diagram. In that sense perhaps it's adequate, but I agree you'll need to find a better reference if you intend to repair or reproduce the power supply.

- Jeff

Michael, I’d like to see myself as a C64 enthusiast but I see your point. The diagram is more intended for a global overview. I’ll include a schematic from the C64 service manual from march 1992. It shows the internal power supply and what the 9V~ is used for. The power “brick” itself consists of a transformer with 2 x 9V~. One 9V~ is used to create a stable 5V=, and the other goes directly to the power connector.

The C64 power supply isn’t the most reliable and should be replaced with something better. I for myself use an ATX power supply for the 5V= (with a 2A fuse) and mounted a new 1 x 9V~ 2,7A transformer.

A good place to go would be:
http://www.lemon64.com/forum/ English
http://www.forum64.de/wbb3/ German

The complete Service manual can be found here.
Among AL LOT of other documentation.
http://www.bombjack.org/commodore

Marco:

Thanks for taking the time to find and post the schematic in the previous post for the C64 power circuits. Those schematics and the accompanying description are certainly much better. I will have to file away for a future project the simple AC boost circuit included that is used to provide the power to the +12Vdc linear regulator.

I think that Dajgoro will be able to use the new schematics you posted to service his newly acquired treasures. Your recommendation regarding of using an ATX (+5Vdc) and a 9Vac supply to replace the C64 power brick has merit.

I sort of remembered that you mentioned your interest in these computers. I followed your link to your web pages, and found an interesting collection of projects. Although I can read Spanish (my native language as a child), and French and the other romance languages with some difficulty and sometimes frequent consultation with a dictionary, I am essentially lost when faced with German/Dutch.

Hi.

My idea is to try to use just a single ATX power supply.
Seems like I just need to convert the 12V into 9V.
How precise does the 9V supply have to be, would it work if I just stacked a bunch of diodes in series and connected it to the 12V supply?

Dajgoro:

Unless it is your intention to bypass and replace the power regulators in the C64, I think that lordbubsy's recommendation is your best path forward. Applying 9Vdc instead of 9Vac will not correctly power the C64's internal regulators.

There is a concept embedded in the power circuits shown in the last schematics that is worth studying. The 9Vac power signal is rectified by a full-wave rectifier CR4. The output of CR4 is called +9Vdc(unregulated), and it feeds the 7805 linear voltage regulator and another diode, CR6. The cathode of CR6 feeds into the anode of CR5 and to a dc blocking 470 microfarad capacitor. The unregulated +9Vdc signal passing through CR6 biases the capacitor so that on the next positive half cycle of the 9Vac power signal, the additional charge plus the power signal is rectified by CR5. The result is a signal at the input of the 7812 linear voltage regulator that is greater than +12Vdc.

This voltage pumping circuit achieves two goals. First, it keeps the voltage at the input of the +5V 7805 linear regulator to a level that doesn't cause too much power to be dissipated in that linear voltage regulator. Second, it raises the voltage input to the +12V 7812 linear regulator so that it can regulate its output properly while not being subjected to too high an input voltage. Power dissipation across linear regulators should always be a concern, and applying high input voltages is the primary manner that most linear regulators are damaged by users.

I think that the power regulation arrangement you see in the C64 is a good example of an economic design. It is not overly complex, and makes effective use of a single transformer to provide both +12Vdc and +5Vdc without subjecting the +5Vdc linear regulator an input voltage that would lead to excessive power dissipation.

All that said, unless you are willing to completely scrap the circuits in the C64, your best approach to powering your C64 and C128 is to follow lordbubsy's recommendation to supply the 9Vac from a standard wall wort.

Note that the C-64 (and C-128) requires the 9 volt AC input for the aforementioned charge pump circuit, and as an input to the two CIAs. The CIA depends on the power line frequency to run the time of day clock. This is another case of "don't make it complicated." All the C-64 needs is 5 volts DC and 9 volts AC to operate.

Now the question is where am I going to find a 9V transformer...
I can order one custom made, or, wait for ages until I find one somewhere.

Power line frequency to drive the clock!? That means that it goes slower/faster depending where you live.
Over here the power grid runs on 50Hz, so I am guessing the clock will be slower?

I know they exist but can't offer any suggestions on a source.
There's a flag bit in the 6526 that tells the device what frequency is being used. The default at power on or reset is 60 Hz, but can be changed to 50 Hz by setting the flag bit.

Also, there's a memory location in the C-64 that tells the kernel if the unit is using an NTSC or PAL version of the VIC-II. Prior to using the TOD clock, you would interrogate that flag and if it indicates your C-64 is a PAL unit, you would tell the 6526 to assume 50 Hz power. I no longer recall all the details, but a perusal of Mapping the Commodore 64 should get you up to speed.

Hi Dajgoro,
I wouldn’t recommend making extensive alterations to the C64 main board just to get rid of the 9V~.

Keep in mind that the CPU 6510, VIC-II 6569, SID 6581, the two CIA’s 6526 and the PLA aren’t produced anymore. So why take any risk.
If I recall right, you can’t / won’t buy on eBay? However, if you’re searching for a replacement, a 1A (10VA) will suffice, especially if you don’t use a cassette tape drive you could get a 500mA (5VA) one.

The 9V~ of the C64 power brick can be reused, it comes directly from the transformer, which almost never dies. You only have to take care of 5V=, at least 1.7A.


/off topic
Yes, there are a lot of projects / peripherals for the Commodore 64. Anything in particular? My favorites are:

SD2IEC
http://www.c64-wiki.com/index.php/SD2IEC
Based on an ATMega644 and an SD-Card, it simulates a Commodore 1541 diskdrive. It’s open source and easy to build. It could also be used for homebrew 6502 systems.

EasyFlash(3)
http://skoe.de/easyflash/doku.php
A modern Cartridge providing 1MB of flash memory. Also open source. Version 1 is DIL 2.4mm pitch and version 3 is TQFP100 0.5mm pitch.

S-Video hack
This is a less common hack to provide a very good S-Video signal, it involves removing the HF-Modulator and provides a new circuit for delivering chroma and luma separately from the VIC-II to the CRT monitor.

If all else fails, this item could be made to work. It has a 10 volt, 10 VA output, but I don't believe the slightly higher voltage should be a problem. If it is, you could use a simple resistance divider to get it down to 9 volts.