Simpson 438 Colorscope Oscilloscope

[dmeek]

The CRT needs a negative voltage (maybe somewhere around -1300V)

[gui_tarzan]

That would explain it.

[The Dude]

And, a note for those unfamiliar with CRT's: The CRT holds that voltage (a charge) for a long time. Be sure to discharge it when messing with the anode wire. It can give you quite a jolt! Don't ask me how I know.

[Enzo]

A CRT usually has a conductive coating on the outside of the glass, and also a conductive coating in the inside of the glass. It is this inner coating that the third anode cup connects to on the side of the tube. The two coatings on either side of the glass constitute a capacitor, which can remain charged quite a while.

In a scope the voltage is usually something like 1200-3000 volts On a color picture tube in a TV set or computer monitor (Or a Pac Man arcade game and similar)mit runs at about 30kV. That is 30,000 volts. Grab that while it is charged, and it is an eye opener.

[dmeek]

You don't even have to grab it. Just get your fingers too close and it will jump.

[The Dude]

OK, so I'll tell the quick story (just a temporary diversion). Apologies to the OP.

When I was in tech school, I was removing a CRT anode cap. My hand slipped down the screwdriver and touched the metal shaft at the same time it contacted the anode clip. I got about 30kv and was already shaking. Your first reaction is to jump back as fast as you can to get away from the jolt. When I did that, I hit both elbow funny bones on the file cabinet behind me. So I had two spasming elbows to go along with the shock. I'll say this: You only have to do that ONE time to learn respect for CRT's.

[gui_tarzan]

Yep, that much I do know. I learned that when I used to take tubes out of old TVs. Someone warned me about that so I stuck a long screwdriver under the rubber cup and heard a loud zap. Then I found out they were in the tens of thousands of volts and people have been thrown across a room by it. The caps are on the way, here's hoping that's the worst of it and I'll have a usable scope.

[gui_tarzan]

Got the caps today. Woohoo! We all had to go into the office today which is a 45+ minute drive each way for me and I'm tired. Maybe I'll put them in tomorrow.

[gui_tarzan]

Well I'm getting -1400v at the new caps (and a new 47K 2w FP resistor between them) and I have an image. It's different from the previous image, this is just a matrix of lines. But it's progress I suppose.

Yes it looks like it's upside down because it is. No matter where I twist the dials the best image I get is what you see.

[dmeek]

Probably the other filter capacitors are open.

[gui_tarzan]

I checked them all, no open connections. But that doesn't mean they're not out of spec by any means. Surprisingly all of the oem cans look great, no bulges anywhere.

On the other hand, the part that actually processes the signal is separate from the CRT circuit so that narrows it down a bit. I almost need another scope to troubleshoot this one.

[Enzo]

That screen shot looks like sweep from top to bottom, which might mean your CRT has been rotated in its mount.

[gui_tarzan]

Started testing tubes since it's not giving out that burning smell now. 6BK7A is good but the first 12AT7 I put on my tester shoved the needle back the wrong way. Tested the second half and it was good. Replaced it and now we're making some progress.

[Enzo]

If you have another scope, you might scope the power supplies in this one.

[gui_tarzan]

Just that $30 cheapie from Amazon that is basically worthless.

The Simpson has a 1v P-P test port in the front and when I put my probe on that it shows a nice sine wave. The picture above is the cheapie 1k signal generator. I think I'm getting close.