Odd Issue: Freezes/Unfreezes When Bumped

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AetherSmyth
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Odd Issue: Freezes/Unfreezes When Bumped

Postby AetherSmyth » Wed Aug 23, 2017 6:51 pm

I recently became the proud owner of a later-version VIC-20 a couple of months ago, but have been having a rather odd issue: the computer has a tendency to freeze when physically moved, jostled, or bumped. What's particularly odd is that if it's bumped it a few more times, it will eventually unfreeze, after which it continues executing exactly where it left off, showing no sign of anything being amiss.

All I can think of is that there might be a loose connection pertaining to the clock signal, but I'm not sure where on the board to look for it. Does anyone here happen to have any thoughts?

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Re: Odd Issue: Freezes/Unfreezes When Bumped

Postby ral-clan » Thu Aug 24, 2017 5:25 am

In this case the first things I would do are:

- reseat all socketed chips (but you said it was a later version so there might not be any)
- clean connectors on all i/o ports
- reflow all solder points on the bottom side of the board.
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joshuadenmark
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Re: Odd Issue: Freezes/Unfreezes When Bumped

Postby joshuadenmark » Thu Aug 24, 2017 6:49 am

- and don't bump your Vic!!! What's a matter with you...
Kind regards, Peter.
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Re: Odd Issue: Freezes/Unfreezes When Bumped

Postby joshuadenmark » Thu Aug 24, 2017 6:50 am

And welcome to our friendly non-bumping forum :D
Kind regards, Peter.
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AetherSmyth
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Re: Odd Issue: Freezes/Unfreezes When Bumped

Postby AetherSmyth » Thu Aug 24, 2017 8:23 am

ral-clan wrote:In this case the first things I would do are:

- reseat all socketed chips (but you said it was a later version so there might not be any)
- clean connectors on all i/o ports
- reflow all solder points on the bottom side of the board.


Thanks, that was my basic plan, I was just hoping I might be able to get a more specific idea on where to start looking.

joshuadenmark wrote:- and don't bump your Vic!!! What's a matter with you...


To be clear, I'm not in the habit of abusing my classic electronics. When I say "bumped", I mean that even the vibration from typing can trigger this.

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ral-clan
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Re: Odd Issue: Freezes/Unfreezes When Bumped

Postby ral-clan » Thu Aug 24, 2017 8:42 am

You might also want to try reseating / the fuse and cleaning contacts on it (not sure if the later VICs have a motherboard glass fuse, though).
Since you mentioned it happened when typing, I would also try swapping the keyboard from another VIC into it if you cannot isolate the problem.
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Re: Odd Issue: Freezes/Unfreezes When Bumped

Postby mrr19121970 » Thu Aug 24, 2017 9:51 am

You can remove the fuse. The vic20cr still works fine without it.

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Re: Odd Issue: Freezes/Unfreezes When Bumped

Postby eslapion » Thu Aug 24, 2017 11:31 am

AFAIK, this is the scientific explanation for what happens with your VIC-20:

When dissimilar metals comes in contact (such as a chip seated in a socket), it sometimes causes a very small voltage to form, a bit like a very low power battery but this battery draws its energy from the chemical reaction between the two metals and this causes them to undergo a form of energy decay. The net result is the formation of a very thin layer of oxidation or other undesirable compounds between these metals which, over time, begin to act as a resistor.

Most resistors will be sensitive to temperature and the restriction to the flow of electricity will usually increase with a rise in temperature. If your VIC warms up enough then this restriction becomes significant enough to cause communication problems between the various ICs of your VIC-20.

Reseating the ICs causes the problematic metals to 'rub' against each other and the offending compound to be partly 'sanded' off, restoring proper communication for a while. However, a more durable solution would be to replace the sockets with newer ones. I suggest you avoid 'machined pins' sockets as they may look high tech but they don't necessarily offer the best contacts.

The best sockets are called 'double leaf' because they make contact with the IC's pins on both sides.

The same can occur with the various I/O ports and the keyboard connector since they too involve dissimilar metals being in contact with each other.

Cleaning the contacts can also help but it is usually next to impossible to clean contacts inside a DIN connector or a DIP socket.

The effect caused by dissimilar metals contact and temperature can be used to measure high temperatures accurately. See: https://en.wikipedia.org/wiki/Thermocouple
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Re: Odd Issue: Freezes/Unfreezes When Bumped

Postby Mike » Thu Aug 24, 2017 2:45 pm

AetherSmyth wrote:Does anyone here happen to have any thoughts?

You can check if there's 'just' something fishy with the IRQ line or if the VIC-20 freezes whole by starting up the following small program, while it's still working:

Code: Select all

1 PRINTX,TI:X=X+1:GOTO1

RUN

Then 'whack' your VIC-20 (in the hope to trigger the error). As long as there's no CPU freeze, the left column will continue to count up. If however the IRQ stops working, TI will also stop incrementing, and the right column won't change anymore.

Please try it out and tell us what happens.

Oh, and a picture of the mainboard would also be handy. :wink:

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Re: Odd Issue: Freezes/Unfreezes When Bumped

Postby AetherSmyth » Sat Aug 26, 2017 12:46 pm

Thanks for the suggestions, folks, I narrowed down the offending chip to the 6522. By my reading of the technical data, that's an I/O chip, so I'm not sure how it was causing the symptoms described, but after removing and reseating the IC I'm having no further issues.

Here's the troublemaker:
2017-08-26 12.27.32.jpg

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joshuadenmark
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Re: Odd Issue: Freezes/Unfreezes When Bumped

Postby joshuadenmark » Sun Aug 27, 2017 11:06 am

Great news, another Vic saved :D
Kind regards, Peter.
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