Which 6.8V Zener Diode for VIC-20CR?

[tlr]

I did more checking today. I do have 15v at CR2 and at the subsequent capacitors at C39 and C38.

When I check the collector (center) pin for Q4 on the board with the issue, I get 15v. On the board that is working fine (which I'm using to compare test results to), I also get 15v.

At R22 I also get 15v on both boards.

Great, that means the circuit is powered! 15v sounds a little high, but as you get it on both boards I suppose it's ok. This voltage is directly dependent on the power brick used.

So that, according to my logic (as rookie as it is) leave the transistor at Q3 and the Zener Diode at CR1. I honestly don't know how to test them.

I agree. See test for Q3 below.

I read your comment: "From the looks of the schematic you should get 6V-ish on the motor pin (under load) when the base to Q3 is not getting current. Measuring on both sides of R22 you should be to see if the controlling signal works and/or if there is a short in Q3." Is "under load" using an actual cassette? Or can I just use the diagnostics cartridge and harness? And which lead from my multimeter goes where when testing. I guess I'm asking for more of a "step by step" due to my low amateur experience when it comes to testing at this component level. Sorry.

I don't have fully step by step as I'm just going from the schematic, but from your deduction above it seems you have a good feel for it.

"under load" means that something is pulling current. I think the harness would do the trick assuming it has a resistor from C-3 to GND.'

When measuring voltages, measure between "GND" (black lead) and the "point desired" (red lead) unless otherwise is mentioned.

How to see if Q3 is bad:
If R20 at the 6522 end is high (not sure 5v-ish?), then the other end of R20, i.e at the base of Q3 should be 0.7v-ish. If not there is probably a short in Q3.

[USA_Joe]

First, thank you very much tlr for all of the information you have provided. Your input, and the input of several others, has been very helpful!

I did replace the Zener Diode this morning, and the cassette/datasette port now passes diagnostics!

Before I desoldered the bad diode, I took a look at the back of the board where I needed to desolder and noticed what appeared to be two intentional solder bridges. I checked my working board for the same solder bridges, and they were there as well. A poor man's trace? Regardless, I'm glad I noticed so that I could make sure my soldering job replicated the two solder bridges. Both of the bridges involve the legs of the Zener Diode.

This recently acquired non-working VIC-20CR is no fully functional. I'll probably be listing this one on ebay after I return from my Hawaii vacation that I start next week.

Thank you all once again!

Joe

[tlr]

Congratulations on getting it working!

I'm glad I could provide some useful information, although your hunch about the zener diode was correct all along.

[mathom]

Congratulations! I’ve been following this thread and very happy you got it sorted. Gives me a little more courage to try something like this in the future.

[USA_Joe]

I'm glad it may help you.

But when I put the entire system back together and tested the keyboard, the SPACEBAR didn't work. I tore down the entire keyboard, cleaned the circuit board, and clean the SPACEBAR contact by rubbing it back and forth on clean paper three times. That fixed it.

Then, I put everything back together and tested the keys again. This time, several keys did not work. This surprised me. But when I looked at the photos that I took of the screen keyboard testing I had done earlier, I failed to see that others didn't work. I only saw the SPACEBAR since it was the last key I pressed. I took photos because I intend to sell this VIC-20CR later. I won't well anything that's not working, unless I list it as not working.

So now I'm tearing down the keyboard again so that I can clean those specific key contacts. What a pain! But, it was my fault for not seeing it earlier. It's a very time consuming process with so many screws, and having to desolder the SHIFT LOCK key switch from the board.

Joe

[USA_Joe]

Finally! Everything works, including the keyboard after cleaning key stems.

Recap of corrected issues:
1) Black (not blank) Screen.
2) Case, shield, and some chips had rust on them.
3) Cardboard insulator for between the board and the shield was damaged by water.
4) Cassette/Datasette didn't pass diagnostics.
5) Several keys on the keyboard didn't work.

Remedies:
1) Black Screen - Bad 74LS133 at UD9. It had rust of it from water damage. Many of the pins on the chip were shorted as result. Lucky find using multimeter. Replaced the chip, and the computer booted. Keyboard was not attached at the time.

2) Rust
A) Case - Cleaned with Apple Cider Vinegar and cleanser. Lots of scrubbing with scrub pad. Also took all of the keys and springs off of the keyboard and cleaned the keys and the keyboard. I did not clean the circuit board at that time since I left the back on.
B) Shield - Soaked the shield overnight in Apple Cider Vinegar, scrubbed with steal wool scrubbing pad, cleaned with WD40, soaked in white vinegar, scrubbed clean with steel wool scrubbing pad, cleaned with window cleaner.
C) Chips - Only the chip at UD9 (74LS133) was damaged by the rust. Another chip had a little rust on it which I cleaned with Isopropyl Alcohol.

3) Cardboard insulator - The cardboard insulator had warped and shrank in size due to water damage. Also had rust on it. Tried to fix it, but ended up making a new one by using the cardboard backing of a large drawing tablet. Used another cardboard insulator from a different VIC-20CR as a template. Cut the cardboard using an Exacto knife. Not perfect, but it works. Since the board of a VIC-20CR attaches to the shield, which is then attached to the case, the cardboard insulator is needed to prevent shorts from the back of the motherboard.

4) Cassette/Datasette - By using the knowledge and input from several knowledgeable people in this forum, watching a YouTube video where I remember that the cassette/datasette port was also failing on a C64, and reading an online article about a similar issue, the Zener Diode at CR1 was targeted for replacement. Again with help from this forum as to what specific diode to order, a new diode was ordered an installed. This resolved the issue. I did not notice the intentional solder bridges stemming from the pins of the diode at CR1, and made sure to restore them.

5) Several keys not working: Only after I had put the system components back together did I test the keyboard. At first I had thought that only the SPACEBAR was having an issue, so I fixed that by taking the keyboard apart so that I could get to the key stems. I cleaned the stem for the SPACEBAR and cleaned the circuit board since it was no easy to do so. Then later I tested the keyboard again and paid attention this time (after looking at test result photos I had taken earlier which showed issues I didn't see) to what was showing up on the screen for all of the keys I had pressed. That's when I noticed several other keys not working. So once again I took apart the keyboard and cleaned the key stems by rubbing the contacts back and forth three times on a clean piece of white paper being careful not to rub too much. This resolved the issue, and the keyboard functioned normally after reassembly.

Here's a photo of the happy patient:

So what's next? I have a "no power" Atari 400 showing up today. Time to venture into a different brand of vintage computers for repair.

Thank you!

Joe

[Orangeman96]

Outstanding work, Joe! -OGM