Simpson 438 Colorscope Oscilloscope

[gui_tarzan]

A friend gave me this, said his dad used it a long time ago but he didn't know anything about the condition. It's super-clean inside but I'm sure after sitting for at least 30 years the cap cans need replacing before I fire it up. I could probably do it on the variac I suppose.

Does anyone know anything about this particular unit? I know it's not actually a color screen but it was free and I'll use it if it's any good.

[Enzo]

https://www.radiomuseum.org/r/simpso...lorscope.html#

[gui_tarzan]

Thanks, I found that. I was just wondering if anyone here had experience with it.

[dmeek]

It's similar to my first scope, my Dad's Eico 460. I used it for my organ/synth experiments for a few years but it had serious
limitations like:

AC coupling only - can't show DC levels
Lots of drift - constantly having to adjust horizontal and vertical position
Little to no calibration - hard to make measurements of time or amplitude
No triggered sweep - hard to get a stable display
Low bandwidth - 300 KHz is barely enough even for audio

My second scope was an HP 130B. Still all tube, built like a tank, but still only 300 KHz bandwidth
My third scope I bought new. A Telequipment D31A 10Mhz dual trace. That was a great scope. I miss it (snifff),
but I eventually needed more bandwidth for digital circuits.

[Enzo]

It is like a cheap guitar. It works and you can play it, just not as good a thing as a more serious guitar. dmeek's story sums it up.

[gui_tarzan]

Well that's a bummer. Thanks for the info.

[Enzo]

You can do a ton with it. Make it work, learn about scopes. Like a early guitar, playing it for a year teaches you what you might want to look for in a better guitar. I like most expect a triggered scope for its stable screen, but if you have a sine wave up there, and it slowly drifts across the screen because it is not triggered, you still can easily see it is a sine wave and whether it has clipped peaks or whatever. There is nothing wrong with something being basic, just realize that it is and as a wise man said, "You've got to know (its) limitations."

[dmeek]

I agree It's still useful. I would prefer it over one of those mini lcd digital "oscilloscopes" you see on ebay

[gui_tarzan]

Funny you should mention that. I got one of those and it is cool for about five minutes. I'll bring this up slow on the variac and see if it works.

[gui_tarzan]

After a while on the variac, bringing up voltage slowly, it appears to be alive. Now I need to get a probe and go through the manual service instructions to see if it will cooperate.

[gui_tarzan]

Well that fizzled. I turned it on today, again on the variac, let it run for a while and noticed the screen had gone dark. Something shut down thermally inside because I couldn't hear the transformer humming either but the fuse is still good. So I shut it off, turned it back on and the image started to come up. Then it went dark again and I could smell something hot. Shut it down, pulled the cover and I don't see anything physically burned but something was obviously on its way to doing so. I plugged into the LBL and both the indicator light and the 100w lightbulb came on dim. After a few minutes I shut down, plugged it into the non-limited outlet and turned it on. After about a minute it started smelling hot again. I don't know if this is worth putting any diagnostic time into or not.

[dmeek]

A simple test you could try is to turn it on with the 2 5Y3s removed. If something is still burning it's probably the transformer and not worth fixing. If it seems OK then it could be the filter caps C35 2X .5/1500V or C32 3X 10/450.
I like working on test equipment. It's usually an educational experience if nothing else.

[gui_tarzan]

It almost looks like the top rectifier is going to the CRT section and the other is a standard circuit. This CRT stuff is not something I'm familiar with. I'm not totally sure what's going on here to be honest. I pulled the 5Y3s out and it isn't heating up now. But that .5/1500 cap can is a monster. Does it need to be that big physically or can I use alternatives there? Again, it appears to be on the CRT side. They're over $60 at the parts houses.

[dmeek]

You can use 2 of these, about $5 for both. https://www.digikey.com/en/products/...hwBUcBpEAXQF8g
If you can power up with V9 installed and only V8 removed. then it is likely those caps. Be careful, there is 1200 or more Volts in that area.

[gui_tarzan]

I let it run for about an hour without V8 in and nothing got hot so I know it's on that side of the circuit at least. I'll get a couple of those caps, I don't have any rated that high voltage-wise.

That is a strange circuit. I know it's not an amp but it looks like the rectifier is wired backward.