Missing Characters From a Commodore 64 Character Set

You most likely will not be able to fix this unless you were already familiar with the operations of a Commodore 64. Even if you were familiar, you have some very obsolete parts that made a Commodore 64. Finding the critical parts may be a challenge. Actually in computing hardware terms this is more like antique than obsolete. If my memory serves me correctly, all of the parts are through hole parts so technically it is possible for a novice to replace parts. I suspect that the Read Only Memory (ROM) of the operating system has been left stagnant for so long that individual bits have randomly changed and that the system will not boot into a mode that keyboard entries will be accepted.

But all is not lost, Google can be your friend. You can even buy a new one.

GA

I agree fully.

If it was a ROM and there is a friendly person out there who can copy his and put it in a new (old!) ROM of the same type, then it may work again....

I am guessing here, but its possible that the ROM contains all the boot code and control storage etc as well as the character set....

Look here for some free diagrams:-

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/index.html

http://www.oldsoftware.com/history.html

I hope this helps.

Don't forget Google!!

The screen memory consisted of a two byres for each character space character = color. the CRT memory may be bad or slow or the cmos holding data may be slow, Check for a battery!!!

It was the power supply that was causing the problem. I am powering it with a 12v lead acid battery and a 7.2v lithium ion battery. It doesn't work with switching power supplies.

Now there's a bogus solution. The C64 was designed long before switching supplies were common to power a computer. I'd be very surprised if a British C64 used a switching supply.

You definitely have a very inquisitive mind. I wish you the best in learning the fundamentals of electronics. But I'm getting very tired of you not providing us with accurate information about what you are doing. The C64 was powered by an external dual output voltage transformer that produced two unregulated DC voltages. All regulation was handled on the C64 motherboard. You never thought to mention that you fabricated your own power source connection. I will not guess what you have or have not done in your projects.

I also have a ZX Spectrum but I am currently fixing the keyboard membrane for it. It was cracked.

I had a Spectrum years ago and I hated the keyboard. So I bough a cheap mechanical switch Keyboard for a few Marks (before Euros!) and traced the Spectrum keyboard PCB out, cut through most of the traces on the new Keyboard and then wired it up as a replica and ran it parallel to the original one, worked really well for some years.

Did it also for a friend as well, cost almost nothing to do....

I kept the connecting cable down to about 12" in length and got no problems....

Commodore 64 CPU clock was near 1Mhz, low enough to get interference from a badly filtered switching supply, and age deteriorated electrolytic caps with high ESR can't help much. If you're sure voltage is within specs, let it work for a couple of hours to warm-up. If not totally wasted, electrolytics get back some of the functionality they lost, after a long period of no use. An EMI filter and some distance apart from supply could also help. S.M.

Can you tell if this is dead pixels on the screen, versus characters missing from the (ASCII?) set?

Definitely not dead pixels.

Fonzie finger thumping on the components and connections not working either? I Used to love a game on the 64 where you would build neighborhood and roadways then race around causing destruction.. seemed so advanced at the time.. thanks for the memories.. it was also another reason "floppy discs!" became a bit of an expletive for me..

You should disassemble it and check to see the connectors and chips are all correctly seated and also check for corrosion on the pins or sockets of any of the removable chips. Ideally carefully remove and then replace removable chips one at a time to ensure the connections are clean.

Not sure if the commodore 64 has removable chips and connectors (like other computers of its vintage), but it is definitely something to look at and could potentially be the problem in this case (assuming you haven't damaged the computer with your home-built power supply).

I have not damaged it. The chips are all soldered on. The only problem that I am now having is that the tape drive just will not work.

The only problem that I am now having is

Did you find the problem causing the missing characters?

the tape drive just will not work

And the symptoms would be.....? Have you tried a standard tape head cleaner kit or is it completely dead.

I have cleaned the heads with isopropyl and I have tested all the connections. The tape will play and pause but it is like no data is being sent or recieved.

Play the tape on a normal tape recorder (I am assuming that its a Musicassette), data can be heard and is very distinctive.....

If the tape has been sitting around a long time, you may be suffering from "print through" which may make the data unreadable. Though it will sound OK to you.....very little you can do to fix it....

My first thought would be corroded or oxidized pins on the ROM chips.

Try pulling out any socketed chips and re-seating them. This may wipe some of the oxidized metal away and give a better connection. Pull one out at a time and replace it in EXACTLY the same orientation as you pulled it. Do not mix them up or move them around and do not bend any pins.

If you have any knowledge of soldering, look on the bottom of the PC board for cold solder joints or corrosion. Repair them if you are able.