Scope – Plan and Specs

“Back in the day”, scopes that a hobbyist could afford (like the many Heathkits) and even ones that many professionals used (like Dumonts) were likely to be AC coupled only and have their sweep speed adjusted with a pot.  The horizontal oscillators of these old guys were multi-vibrators and sync was achieved by adjusting the horizontal frequency close to some submultiple of the vertical input frequency until the horizontal oscillator “locked”.  Vertical gain was also often adjusted with a pot.  They were capable of accurately displaying the shape of a repetitive waveform, but were not useful for single shot or low duty cycle waveforms.  In addition, they could not accurately measure amplitudes or repetition rates of signals.  They certainly beat the stuffing out of “no scope at all” but they had severe limitations.  New scopes are mostly digital nowadays, but digital scopes have their own limitations and good ones are very expensive.

What I wanted to build was a small analog scope that would be generally useful for troubleshooting around the shop.  I wanted something that I could make copies of for young people who could use them to get a start in this great hobby.  This means that the scope must be built with readily available and reasonably low cost components and a simple design.  Here are the specs I was aiming for:

  • DC coupled vertical amplifiers with a sensitivity of a few tens of millivolts per division to around 10 Volts/div.
  • Optional AC coupling.
  • Calibrated vertical deflection.
  • Calibrated sweep speeds, good linearity.
  • Triggered sweep to allow “zeroing in” on glitches in a waveform.
  • Trigger from vertical signal or AC line
  • Bandwidth – I was willing to take what I could get here while keeping the circuit simple and low cost.  A few MHz would be adequate.
  • AC line power supply built in.
  • Single board with everything but CRT, pots and switches.

That seemed like a tall order, but hey, this is a hobby, not a job.  I was willing to compromise if necessary.

Finding a CRT:
RCA, Sylvania and others used to make a lot of standard numbered electrostatic CRT’s (3BP1A, 5CP1A, etc.) that were widely used in scopes and other devices.  I found quickly that this is no longer true.  The US based general purpose CRT manufacturers have long since stopped production and the old familiar part numbered tubes are now very hard to find.  Just about all scope manufacturers these days use tubes that are specially made for their products.  These can sometimes be found as replacement or surplus parts at high prices or used parts on e-bay.  This did not fit my definition of “readily available”.

I did find one standard CRT that is still available.  The 3RP1A   was widely used in all kinds of test equipment and even some stereo audio equipment and is now made in China.  It is available from tube sellers including Radio Electric Supply  for about $40.00.  This tube looked pretty good as it has good deflection sensitivity and will work with an anode supply of only 1KV, compared to some others that require as much as 4KV.  The tube has a round flat face with a diameter of 3 inches, good enough for a small scope.

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