VIC-20: Need Help with an Issue + Quick Teardown  - Page 1
Forum section: electronics / general

author: sonnytiger
author_rank: Frequent Contributor
timestamp: January 13, 2014, 12:21:53 pm
content: Just as it says in the title, there is something wrong with the text mode colors, orange and blue, purple and green among others. Random characters are one color or another instead of just one like it should.Thanks for any help/watching!

author: synapsis
author_rank: Regular Contributor
timestamp: January 13, 2014, 06:36:03 pm
content: I didn't see the whole video (at work), but does it do the same thing if you hook it up to the Luma/Chroma outputs instead of the RF modulator?I have three VIC-20s here (two working) if you need some signal references or anything.

author: Unknown
author_rank: Unknown
timestamp: January 13, 2014, 07:36:07 pm
content: This might help - it's a short bit of text by a guy called Ray Carlsen who is very well known to the retro computer enthusiasts; in the text he describes common problems you might encounter with a Vic20.http://www.classiccmp.org/dunfield/vic20/v20diag.txtOne thing you must watch out for is the "brick" type power supplies as they are notorious for failing and producing an over-voltage supply which will damage chips on the board if plugged in.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: January 13, 2014, 09:29:36 pm
content: According to the link you sent me, it's a problem with the 74LS04 chip. Do you suggest I make a different power supply?

author: JoeO
author_rank: Frequent Contributor
timestamp: January 14, 2014, 04:09:47 am
content: I know this sounds flakey but I used to locate TTL/LS problem chips by bridging them. While it doesn't always work, just take a known good LS04 and squeeze in the pins a little bit. Press the chip onto the suspected defective LS04 and run the computer. It has located bad chips for me, more than 50% of the times I tried, and it is quick.Of course it isn't sure fire. The LS04 may still be bad even after bridging the chip does not fix the problem.YMMV

author: sonnytiger
author_rank: Frequent Contributor
timestamp: January 14, 2014, 12:35:17 pm
content: That's an interesting way of doing it, though I think the issue is almost certainly the chip I mentioned due to how closely the issue matches with his description. It does sound pretty flaky

author: Unknown
author_rank: Unknown
timestamp: January 14, 2014, 01:40:57 pm
content: Quote from: sonnytiger on January 13, 2014, 09:29:36 pmYes - it does sound like a good place to start!Quote from: sonnytiger on January 13, 2014, 09:29:36 pmNo - I'm only alerting you to what I've discovered whilst researching the C64; I grew up with the Vic20 and then a C64 and never had a problem but other people haven't been so lucky. One day I cut open a "brick" supply to see what was inside; not seen from this angle is a 7805 5v regulator - but that is directly connected to much of the logic on the board; perhaps they used poor quality caps in the later versions to save money. I'm very much a beginner at electronics but even I know that this is not a very good design.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: January 15, 2014, 12:08:07 pm
content: Easy part to replace at least. I think I should be good with the stock power supply, its one of the later model ones (the one that are the same as the C64) plus I don't have any addons or anything other than the datasette so there is a minimum of load on the 5V rail.

author: MrAureliusR
author_rank: Unknown
timestamp: January 22, 2014, 06:11:54 pm
content: I have both a C64 and a VIC-20 and have done lots of work on both. As someone else mentioned before, I'd be happy to measure signals or whatever you need if you need help. I have decent equipment so I can get you fairly accurate readings. If you think the problem is a 74LS04 replace it! Those chips cost pennies. Could be a very easy fix!Let me know if you need any assistanceAlso, check your inbox, I sent you a PM. Cheers!