Oscilloscope under construction.

[David Hess]

ETA:  Having to drill so many precisely spaced holes was the motivation for building a CNC mill.  Kind of a twofer project.

My solution for that has been to print out a 1:1 punch out diagram, tape it to the front panel, and use an automatic center punch before drilling.

I use inkjet transparency film for this. Print the design out, tape it on and use it as a guide on the drill press.

I save the transparency film for the reverse printed front panel and use cheap paper for the 1:1 punch out diagram.  Center punching prevents the drill from wandering.

[GK]

Do you buy or make that aluminum bar with the tapped holes you use to frame the chassis?

No, those lengths are all cut, drilled and thread-tapped by hand. It's 10mm X 10mm square solid. Not very expensive though, my last 16m (4 X 4m lengths) cost less than $40.

[GK]

I would certainly be interested in how the front panel was built.  It seems that panels are the bane of all hobby projects.  So, I'll share mine...

Mine is done very similar. The front panel layout is done in my PCB package (Protel/Altium); I use one layer for the mechanical outline, two for the controls (back and front) and one for the legend. The latter is printed and then laminated. My inkjet does A3. The mechanical outline serves as a drilling template. The front of the CRO is covered by a 3mm-thick clear acrylic sheet.

[David Hess]

The design is simple enough but I still would have included two more transistors for cascodes in the vertical amplifier to push the vertical bandwidth as much as possible.

[GK]

The design is simple enough but I still would have included two more transistors for cascodes in the vertical amplifier to push the vertical bandwidth as much as possible.

Would barely make a difference. There is ~50pF of equivalent Miller input capacitance (gain [~25] x Cob [~2pF]) which corners with a max of 25k (sensitivity pot at 50%). The second major pole is with the collector resistance and the deflection plate capacitance (including the wiring to the plates). At a rough estimation the latter is at least equal if not dominant.

Any substantial bandwidth improvement would require cascoding in combination with a reduction in collector resistance and that would mean dissipation not manageable without heat-sinking.

Additionally, lowering the collector resistance would require a commensurate reduction in the emitter resistance (to maintain the same voltage gain), proportionally lowering the input impedance and the loading of the 100k Sensitivity potentiometers and thus the net input impedance. Of course an emitter-follower buffer for each Sensitivity pot. would get around that issue, but then that and all of the above would morph the design into something it wasn't intended to be.


 

[GK]

Interesting thread and nice work!

I've got a long term plan to build a more modern clone of the Textronix 321A (as described here: http://www.helo.de/helo/tek/portable/321a/321a_e.htm ). It was one of the first purely solid state scopes and only contains a few transistors which roughly replaced similar functions as the previous revisions with vacuum tubes. That managed a good 6MHz bandwidth and I reckon you can do better with newer parts.

Replying to the above, Tektronix produced miller integrator based timebases. Check the SM for the Tek 321A. It's a very nice design.

About 35 transistors and twice as many diodes in a 321A though......

[GK]

Here is another oscilloscope. I designed and made this over 17 years ago and the circuit boards have been sitting in my junk box ever since. This evening I resurrected the boards by replacing/fudging replacements for the various components that have been salvaged from them over the years, found a couple of suitable power transformers and wired it all up as originally intended. About 14 years ago the junk box spent a winter in the rain, which is why the boards are corroded through the conformable coat. Though whoopie, it still works! 

The CRT is a 1" model, part #913. The time base is of the injection-locked sort, utilizing a 555 timer in conjunction with a constant current source to deliver a linear sweep waveform for the horizontal deflection. And wow, I even implemented retrace blanking, so that the retrace isn't visible no matter how far up you turn the brightness control.

I was still a bit wet behind the ears when I designed this, but I've posted the circuit diagram below anyway. For the sake of nostalgia I'm going to amend/improve the overall design, layout new boards and build it into a proper/finished instrument enclosure.

[bitseeker]

That'll make a nice portable scope.

[minion]

Very nice, post your progres

[Berni]

That'll make a nice portable scope.

I was thinking the same thing. If one was to redesign the PCB with modern small SMD parts it could be made to fit a case not much bigger than the CRT itself. It wouldn't be a scope watch but you could claim it to be a pocket scope.

Is there a record for the smallest CRT scope yet?

[David Hess]

Is there a record for the smallest CRT scope yet?

The Tektronix SC501 is a contender.  I think the Tektronix 200 series portables were larger.

[Berni]

The Tektronix SC501 is a contender.  I think the Tektronix 200 series portables were larger.

Oh that is so cute, but i take it this is a plugin module for one of those multi instrument cases.

Tek probably also has a good contender for the largest scope with there huge heavy vacuum tube packed mainframe scopes.

[setq]

I wonder if it's possible to build a scope with one of those 1980s-1990s CRT based video camera viewfinders. That would possibly be the smallest CRT available.

[dom0]

These are magnetically deflected, but they could still be used...

[Jay_Diddy_B]

Hi,

A little of OT, this guy built his own CRT 3mm in diameter !!

http://www.sparkbangbuzz.com/crt/crt6.htm

Many years ago the 3BP1 was the CRT that most of the DIY scope used. 3 was three inches diameter, P1 is the phosphor if I remember correctly.

Regards,

Jay_Diddy_B

[GK]

The slightly redesigned vertical amplifier. DC-coupled this time, will have 1M DC input resistance. Rather than 1/2/5 steps the 1M input attenuator will have decade steps only. The "variable gain" knob accordingly gives a ~11:1 maximum reduction ratio rather than ~2.5:1 to cover the middle. The vertical amplifier Will be calibrated in volts/FSD (full-scale deflection) rather than volt/div(cm), as this little CRT isn't really suited to a graticule. For example on the most sensitive decade (no attenuation) the sensitivity is 50mV/FSD. 50mV peak will deflect a centered trace to the outer edge of the CRT -  12.7mm (100mVpp returning the full 1" peak-to-peak deflection). I kept the Hi-z input stage symmetrical to minimize DC drift. The "unused" half of the twin triode temperature compensates for the DC offset and drift of the other. The input stage is protected and pretty much (should be) blow-up proof.

Yes, I had to design a tube into it . Might use a couple more 12AT7s as series-pass devices in the regulated HV power supply........

[David Hess]

These are magnetically deflected, but they could still be used...

You beat me to it.  Some oscilloscopes did indeed use horizontal magnetic deflection.  I suspect you would have to turn a video CRT 90 degrees to make the most of the potential magnetic deflection bandwidth.

...

I kept the Hi-z input stage symmetrical to minimize DC drift. The "unused" half of the twin triode temperature compensates for the DC offset and drift of the other. The input stage is protected and pretty much (should be) blow-up proof.

Yes, I had to design a tube into it . Might use a couple more 12AT7s as series-pass devices in the regulated HV power supply........

You have basically duplicated the design of the early Tektronix 1A1 which used 7586 Nuvistor cathode followers for the input stage.  Using tubes for the input stage has the advantage of massive overload tolerance compared to FETs.  Nuvistors held on for many years in applications like front end RF amplifiers for this reason and I remember someone building Tesla coils mentioning this as an important consideration if you wanted to use an oscilloscope around them.

Since you already have most of the circuit for driving the vertical deflection plates differentially, why not do so to double the deflection sensitivity?

[bitseeker]

Hmm, Google Glass -> tiny scope!

[GK]

Since you already have most of the circuit for driving the vertical deflection plates differentially, why not do so to double the deflection sensitivity?

Unfortunately the 913 doesn't give you access to both plates. One plate of each pair is internally strapped to Anode#2. Not enough pins on the 8-pin base.
 

[David Hess]

Since you already have most of the circuit for driving the vertical deflection plates differentially, why not do so to double the deflection sensitivity?

Unfortunately the 913 doesn't give you access to both plates. One plate of each pair is internally strapped to Anode#2. Not enough pins on the 8-pin base.

Doh!  I should have taken a closer look at the schematic.

[JPortici]

Hmm, Google Glass -> tiny scope!

add a very directional mic.
Look at the waveform :