Hi

Can the Atari console CPU 6502C be used as a replacement for a standard NMOS 6502/A/B? I understand I have to pull HALT (pin 35) high, but is there anything else I need to think about? The only pinout I have found shows R/W on pin 36 instead of pin 34, but apart from that it looks the same.

As you say the only difference is the halt input that allows the video system to grab the bus when it needs it, and the small pinout change. So you would have no trouble using it. From memory I think it runs at around 1.5 MHz too.

Just to update: I tried this - soldering pin 34 to pin 36 and pulling pin 35 high through a 10k resistor to pin 8, then replaced the 6502 in a Vic-20 with the modified Sally chip. The Vic-20 runs at 1.1MHz.

It did work, but the booting is unstable at times (and requires a hard reset if it fails). That may be due to the old socket or me forcing the 6502 out of it.

Thanks for letting us know! If it works sometimes, then it's logically OK, and if it's unreliable, then yes, that sounds like electrical level trouble. (Or mechanical.)

I guess so. I will try to clean the socket and re-solder its pins. In my experience, solders tend to develop tiny cracks that can cause OC or poor contacts. At least my 1084 got it after a few years.

Other possible long-term problems are tin whiskers, and long term damage from flux residue.

Tin whiskers do not happen with conventional leaded solder.

Cracks in solder over time are due to improper soldering technique.

VIC-20s are not normally succeptible to either. Did it work prior? It may have bad RAM.

Re-seating the chip certainly helped. It boots every time unless I plug in some experiment on the cartridge port. But that is not really a problem.

One thing I have found out is that this chip can supply a lot more current on its address pins than a 6502A. E.g. forcing down the A13 line is easier with the original chip than with the Atari one.

Not that it matters, but interesting anyway.