Black Screen VIC-20, and more issues.

[USA_Joe]

Promised information on the connections to/from chip UD9 (74LS133) on a VIC20CR. The schematics are not very helpful with the connections to and from this chip.

Joe

[Orangeman96]

Joe: I wanted to recommend looking at this if you haven't already done so. -OGM

[USA_Joe]

Thank you! But I already have. Ray's information is my primary "go to" for such things. That's where I got the list of chips that could be involved in the "Black Screen" (actually blank screen) issue.

I received the 74LS133 chips yesterday during a family gathering, so installing one had to wait until today. I haven't tested things yet, but I don't expect the issue to be resolved so easily. Especially when the chip appeared to have shorted out from water incursion.

Joe

[USA_Joe]

I spoke too soon! The VIC-20CR booted immediately! Replacing the 74LS133 at UD9 did fix it!

Unfortunately, this VIC-20CR also fails the diagnostics at the Cassette Port test. I'll have to test it with a datasette instead (which works fine on the other VIC-20CR giving me the same test results). I wonder if that harness piece is bad or dirty. I'm going to burn a copy of the diagnostics that was modified to not stop on a BAD test result and run that as well.

More to investigate. But at least it's booting!

Joe

[Orangeman96]

Wow! A NAND gate that costs less than five bucks was indeed the culprit! Great work, man! -OGM

[USA_Joe]

I got lucky! Working on the cassette/datasette port diagnostic failure now. Using a logic probe, pin 39 on the VIA at UAB3 is held LOW once the cassette diagnostics runs. On my working VIC-20CR, it flashes and goes back to normal. I'm checking the zener diode at CR1 and the transistor at Q3. I'm pretty sure the diode has failed. Just verifying now.

Thank you!

Joe

[USA_Joe]

At this point, I'm waiting for some new diodes to arrive. Thanks to Orangeman96 for the help in identifying what I needed to order and other assistance! Now I just hope it's actually the diode that is the problem with the cassette port not getting power. But learning more and replacing other parts is fine too.

But here's an update on the case, shield, and the cardboard shield. I painted the cardboard shield black since the gray that I have is Rustoleum brand which has metal flakes in it. It's still ugly because of the wrinkles caused by the water damage. I'm also in the process of trying to make a new one from a piece of cardboard that was the backing to a drawing paper tablet.

As you can see, the case came out nice, and the shield is nice and clean as well.

When I finally fix the remaining issues with this VIC-20CR, I'm going to go back and fix my other CR that has a somewhat similar issue with the cassette port.

Thank you!

Joe

[Orangeman96]

Keep going! -OGM

[USA_Joe]

I made my first cardboard insulator by using the cardboard backing of a drawing tablet. I'm no artist by any means, but I think it came out good enough for a first try. Cutting the vent slits took a long time. About two hours overall for cutting it out slowly, and being slowed down by a blade that was getting more dull as I went along.

The black one is the original water damaged one that shrank down due to the water. Luckily, I used a good one from my other VIC-20CR as a template. So the new one is the size it should be.

Just waiting for parts to arrive so that I can repair the cassette/datasette power issue next.

Joe

[Orangeman96]

Wow, Joe: that looks spot on! -OGM

[mathom]

That’s impressive especially for hand cut. I would have tried to get it cut on my Cricut machine. But then again I am stupid dangerous with a knife.

[USA_Joe]

It turns out that it's just a little short on the width, but not enough to where I have to make another one. The template also had a little water or heat damage which in turn shrank it some. I'm just going to insulate the metal mounts on the shield before I connect it.

I'm also dangerous with a cutting blade. I'm surprised I didn't follow my usual practice. 1) Take out cutting tool; 2) Cut yourself; 3) Curse and bleed.

Joe

[JonBrawn]

Always with retro-tech you have to make the blood sacrifice. I wish my hands were steady enough to cut details like that, but alas not. A while back I caught myself just before trying to guide the wobbly tip of the soldering iron to the joint with my index finger. That would have hurt. And been really embarrassing to explain. Good looking job, I hope it's all working now?

[USA_Joe]

The parts arrive today, and hopefully they are the correct ones.

Joe

[USA_Joe]

The system has been fixed. It was a bad Zener Diode.

Please see this link for details: viewtopic.php?t=11043

Thank you!

Joe