πΊπΈ Commodore VIC-20 Refurbish: Part 4 (65C22 & more troubleshooting)
23So, planned for this episode to be pretty quick - show the difference replacing the 6522 VIAs with 65C22 versions can make. Buuuuuuut there's still some instability with the machine so this took a bit longer.
Ended up replacing all of the electrolytic capacitors. This did seem to help - the machine still has instability but seems to recover.
I like using the bench power supply because I can see that when the problem occurs, the current draw also drops so it is like some chips are powering off ... I'm wondering if the 6560 VIC has some problems?
The machine has probably been untouched for 30-odd years and now I've been poking, prodding and powering on/off. This is good, though, because if I re-sell the machine I want to be sure the buyer is getting something reliable.
There is also some issue with the replacement character ROM ... after a while the text becomes corrupted. Interesting.
Anyway, ended up replacing the 6522's with W65C22N's ... it's quite important that the N-variant is used (NMOS compatible) and not the S-variant. The N-variants are drop-in compatible.
And what a difference they make ... they essentially run at ambient temperature.
The summary, so far, is that we've gone from 1.17A (stock VIC-20) down to 940mA. This means a lot less waste heat floating around the board and also means you could get away with a 1A power supply instead of 2A ... we still have some way to go though, especially if we want to use a Datasette.
Still to be done:
[1] Replace the 6502 with a 65C02. Will need a daughterboard designed by @thebyteattic:
https://www.thebyteattic.com/p/6502-adapter.html
[2] Replace the BASIC ROM - reduce heat & current draw. Also, possible to switch to another language, like Forth ... ?
[3] Maybe replace the 74LS245's with 74HCT245s - reduce heat/current draw
[4] Replace the internal 2114 SRAM chips with a 6224, also giving a 3KByte expansion, i.e. the 8KByte 6224 will handle BLK0. Maybe also replace the 512byte (1024 x 4bit) colour RAM?
[5] And, very finally, improve the darn video output: improve the composite video output, convert to S-video (separate chroma & luma) ... ?