Columbia Data Systems 1600-VP CRT issues

[Coder]

If you have an oscilloscope...

I don't have one but I've been considering getting one for years now. One of my other many hobbies is working on cars and I've wanted to get a scope so I can test sensors and look at CANBUS signals when diagnosing issues. What are the features I should look for when shopping for one? Especially for diagnosing circuits like this? I'd like to stay under $300, but if there's a good reason to move up a little in budget, I might go a little higher. What would you guys recommend?

Let me ask a couple of bonehead questions with regard to the monitor.

1. In a darkened room, with the computer turned on and booted, can you turn up the brightness control so that you can see a raster on the screen?
2. In the same darkened room, can you see the heater at the rear of the CRT glowing?

One suspect pulling things low might be the horizontal output transistor. Certainly worth checking before going after all the capacitors.

I'll try it out this evening. I'd do it now, but I have it completely torn down so I could take high res pictures of the motherboard front and back.

 

[SomeGuy]

The very first post in the thread shows a picture that seems to show a raster, just somewhat collapsed horizontally and vertically.

Semiconductors commonly fail in CRT monitors.

If you haven't already done so, also check for bad solder joints, especially around anything that gets warm.

I actually replaced the caps on my CRT board (except for the large one) hoping it might make some difference with a couple of minor issues, but they made no difference.

Given that both the H and V seem affected, it would make sense that some component shared between both might be an issue.

With a partially collapsed raster, I would still sort of expect to see some distorted video appear on the screen as the machine boots. (the test memory prompt, and the boot disk failure text). If not, and the contrast is turned up, then it could be affecting the video section as well.

 

[Coder]

Here's a video of it with the contrast all the way up:

 

[Chuck(G)]

That's definitely a power supply issue. (shrink in both the X and Y direction). The trick now is to find why your supply voltage isn't up to spec.

 

[SomeGuy]

Yea, at this point I would try powering the CRT board with an external +12v supply. If it comes to life, then focus on the power supply. A problem with +12v would also cause the floppy drive to not spin properly.

 

[SomeGuy]

Just happened to see a Kaypro 16 on eBay with the cover open... that CRT board looks very familiar:

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[Coder]

Just happened to see a Kaypro 16 on eBay with the cover open... that CRT board looks very familiar:

Haha, I happened to see that one this morning too. I would have considered it, but that's complete system money. He doesn't even have the case cover lol. But it's definitely a VERY similar board. I have the schematic for that version of the board from a thread that was on this site.

 

[Coder]

Welp, something went up in smoke and the fuse is blown now. I tried to power it from a 12v 3a power supply I had laying around and must've done something wrong. I haven't taken it back out to see what I burnt up. I may transition to trying to get a usable composite signal and replace the CRT with an LCD, as much as I don't want to.

I think I bit off more than I can chew. I'll probably still play around with the board. I want to further my understanding of circuits, but I think I should start with something simpler. I would like to see the computer boot, however, so I'm going to see what I can do with composite.

I appreciate everyone's help so far!

 

[SomeGuy]

Ouch, I should have mentioned to check and re-check all connections, especially making sure to fully disconnect that purple wire somehow, because that magic smoke loves to come out.

That could have damaged a lot of stuff. But I'd back up and focus on the power supply for the moment.

 

[Coder]

As far as I can tell, the power supply is fine. It gets a solid 12v with the CRT out of the loop. Only with the CRT board connected, does it drop to 8v. Hence all my questions about shorts. Am I wrong and should still suspect the power board? What can I do to test it?

 

[Coder]

I attached my 12v power supply with all other connections except the chassis ground disconnected. I used alligator clamps to connect to the appropriate pads on the CRT board. I felt that was safest. I assumed it didn't need the video signal to see if the CRT itself was working on the power supply and that I would see a solid green screen.

 

[SomeGuy]

Yes, you should absolutely still suspect the power supply. Voltage dropping down when loaded is a common mode of power supply failure. Additionally, your description earlier of how the square image was larger and then got smaller suggests the voltage continued to drop as the power supply heated up. That is a very common failure mode of bad electrolytic capacitors.

As I mentioned earlier you should have seen around 11.90v with the CRT board hooked up. If anything had been shorted, it would have already let a lot of smoke out.

Gator clips can move around a lot. I don't know that you would have seen a raster or not, as I think the computer was providing a sync signal.

The approach I would have taken would be to cover the pin with electrical tape, make sure that pin and only that pin was making no contact with the plug, then temporarily solder some wires to the CRT board for power. Although someone else might have a better idea. Well, fixing the power supply first probably would have been a better idea.

But at any rate, probably just damage to the CRT board then.

(BTW, if you can't fix the CRT board, I could use the parts)

 

[Coder]

I'm taking a break from the VP to clear my head a bit. I think I was a little too eager and I paid for it. I did look the board over and didn't see an obvious burnt components. I'll have to take the board out to examine it closer. If I give up on the CRT board completely, I'll pass the parts along. I don't give up easily though. I get discouraged, put it away for a while, and then come back at it later with my sails full.

Thanks for all the help you guys have given me. I'm sure it's frustrating watching someone you're trying to guide make some poor choices and do unnecessary damage. But I appreciate it a lot. I'm learning and consider it a hard lesson learned. I'll ask more questions when I'm unsure how to proceed from now on, that's for sure.