Dual trace oscilloscope choppy chop?

[eccerr0r]

For those with dual trace oscilloscopes (analog)...

Now I'm not sure how digital scopes work for using dual traces as since they tend to have more than one ADC they could dedicate one per channel for sampling two signals and implicitly are "chop" mode, but for analog scopes there needs to be a stable, high speed switch and oscillator to view two signals at the same time, chopping the signal.

However I was looking at my "old" "cheap" Vu-Data in chop mode and both inputs are set to GND and the time/div knob set to 1µS/div:



(alt mode is clean as expected, no chopping noise of course).

I don't see this on my Tektronix scope (my Tek is 30 years old, my Vu-Data is 35 or so years old so both are relatively the same age). I wonder if this is to be expected in "cheap" scopes or even cheap scopes should not do this?

"cheap" I have to say, because compared to the Tek, this is a "cheap" scope... Then again pretty much all the electrolytic caps in this "cheap" scope are tantalum, save for a very small handful.

[stj]

Re: Dual trace oscilloscope choppy chop?

probably just a transistor, but you will need a schematic.
on the bright side, getting schems for old scopes is much more easy than for modern(ish) digital ones.

is everything soldered btw, or are the transistors in sockets like old telequipment stuff?

[eccerr0r]

Re: Dual trace oscilloscope choppy chop?

It's all soldered, which leads back to possibly a bad capacitor. There are a mixture of plastic DIP and TO-5 can ICs, and transistors are mostly TO-92.

The unfortunate part of this scope is that it almost seemed like a fly-by-night company, and barely any information is around anymore. Google reported almost no hits about the company or the scope.

I probably should make my Tek return the favor and see if the few aluminum electrolytics in this scope are indeed working properly or not...

[eccerr0r]

Re: Dual trace oscilloscope choppy chop?

Oscilloscope porn:



No aluminum electrolytics in sight?

BTW yes you see the primary anode wire in this picture... Yes you do.... I couldn't believe it myself...

Ok the other side picture is coming. I count 5 aluminum electrolytics, 19 tantalum, and the rest are a mix of silver mica and ceramic disc. I see a few glass embedded through hole multilayer ceramic.

The three easily probe-able aluminum electrolytics measure about 2-3 ohms ESR (two 100uF are 2 ohms, third one I can't see is 2.5 ohms) -- these are probably not very good. The main cap is a 1000uF 25V which I can't get to the negative pin due to construction... but if I pierce through the outer insulation, I get a fraction of a ohm - so actually this cap looks like it should be OK. It's an "Extralytic" (seems like a Sprague brand, and the other caps are also Sprague) and has a date code of 1960!

I'm still trying to figure out how to open this thing without massive surgery, this thing meshes together very tightly. Not a quick throw-together scope at all!

[eccerr0r]

Re: Dual trace oscilloscope choppy chop?

Okay, that 100uF cap actually was a 6uF 100V cap that's filtering + and - 63V. So 2 ohms is actually probably not out of the ordinary... Hmm. I don't see anything that looks bad actually.

Other than the fact it looks kind of messy, it works just fine... so nobody else has a scope that has this sort of behavior?

[eccerr0r]

Re: Dual trace oscilloscope choppy chop?

Hmm..speaking of service manuals, anyone just so happen to run across the Vu-Data PS940 series (PS940 PS941 PS942 PS943) service manual?

I've seen the PS935/975 but that seems to have more discrete transistors than mine.

I think I've only seen one "for rent" though I have no pressing need to rent it...