Update on the $80 Oscilloscope

Finally got the intensity problem fixed on the new (used) scope. It was a couple of bad diodes and some supporting components. I tried to get the originals, but that was not going to happen in any decent time frame.






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The Fix.

Trusty ol' 1N4004 diodes don't have the fancy low-capacitance, high-switching-speed of the original diodes, but they have more than enough reverse voltage and power handling. . . Hmmm. . . would they work? YES! The scope is all calibrated now, and works like a charm. Now that the brightness works I can see what the whole Inten'D thing is for (my old Heathkit scope didn't have delayed triggering). Pretty cool!




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Summary: $80 + a little elbow grease = 35 MHz Dual Trace Scope

The $80 price includes the shipping cost!

Model: JDR Instruments 3500

Found and downloaded a free manual (images scanned into a PDF). It was needed for the schematics to fix the intensity problem, but it also included the calibration procedures.

The guy who sold it was a bit of a sleaze bag (imo). He said, directly, that he wasn't aware of any problems, though he took a picture of it with the traces showing pulses. To me, that means he would have fiddled with the focus and intensity controls and seen that the intensity control had no effect.

So, it had a problem with the intensity that turned out to be two different diodes in the intensity circuit (go figure).

Now I've got a scope that probably cost >$1K new, and would cost $200-500 used in good working condition today. Not too shabby.

-djr