Oscilloscope under construction. - Page 1
Forum section: electronics / projects

author: GK
author_rank: Super Contributor
timestamp: September 10, 2016, 07:15:59 am
content: Six transistors and two diodes. I'm currently assembling the (deflection) amplifier board.Thepreliminaryfinal schematic:The P.U.T. injection-locked horizontal timebase/sweep-generator synchronised to a sinewave:This is the closest semiconductor equivalent to a 884 thyratron sweep circuit that I could come up with.The inspiration:

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2016, 07:37:12 am
content: Fascinating as always from you Glen.Clever use of a PUT.Expected BW ?

author: zapta
author_rank: Super Contributor
timestamp: September 10, 2016, 02:15:56 pm
content: Quote from: tautech on September 10, 2016, 07:37:12 amAC only, right?

author: GK
author_rank: Super Contributor
timestamp: September 11, 2016, 12:52:49 am
content: Quote from: tautech on September 10, 2016, 07:37:12 amAbout 100kHz or so.The amplifier board with the long-tail pairs is actually my second deflection amplifier design. The original board had a pair of single-ended amplifiers driving only one deflection plate of each pair as per the Philips TA115. That made this a 4-transistor 'scope. My original aim was to replicate the TA115 in solid state (3 transistors). However the CRT I choose, as it transpires, doesn't work well with single-ended deflection plate drive. The deflection plates act to a degree as electron beam accelerators and if the mean voltage between a deflection plate pair does not remain constant with ac signal swing (as it does with differential drive) the net electron beam acceleration potential will be modulated in accordance. Deflection sensitivity is inversely proportional to the acceleration potential, so the single-ended drive results in significant deflection non-linearity - I was getting about 25%. Imagine an AM envelope display of an unmodulated carrier. The vertical deflection sensitivity at the left-most extremity of the CRTs display was ~25% less than at the right most, so the displayed envelope became trapezoidal shaped. I liked the ultra simplicity, but I decided that I couldn't live with such horrible non-linearity, so I re-designed the amplifier board for differential drive and the total transistor count grew by two to six.I also couldn't make do without the emitter-follower buffer for the sweep generator. The capacitor charging current must be kept below (by a safe margin) the PUTs valley current, otherwise the PUT, once triggered, won't turn off again. The input impedance of the BJT horizontal deflection amplifier could not be (practically) made high enough to make loading of the capacitor charging circuit acceptable. So I'm afraid that, as far as the active device count goes in this application, tubes still reign supreme.Also, unfortunately, the PUT appears to have gone the way of the tunnel diode. AFAIK the last manufacturer of the 2N6027 was ON Semi and they've now stopped production. I got five examples from a Jaycar store which still had stock.

author: Cyberdragon
author_rank: Super Contributor
timestamp: September 11, 2016, 01:29:56 am
content: Quote from: GK on September 11, 2016, 12:52:49 amWhat you talking about willis?! We don't need ON Semiconductors!http://www.intertexelectronics.com/Transistor-2N6027-Unijunction-P11164.aspxhttp://www.talonix.com/shop/item.aspx?itemid=3467

author: GK
author_rank: Super Contributor
timestamp: September 11, 2016, 02:13:45 am
content: Those are just distributors with stock, not manufacturers. Though Central Semiconductor Corp is still actively producing the 6027/6028, so it hasn't quite gone the way of the tunnel diode just yet:https://www.centralsemi.com/paraSearch/parametricSearch_v1_1.phpListed under "thyristors".

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2016, 05:02:09 am
content: And Mouser carries the Central Semiconductor 2N6027 and 2N6028 PUTs at a reasonable price.

author: ali6x944
author_rank: Frequent Contributor
timestamp: September 11, 2016, 07:43:38 am
content: Good job GK, this is fascinating...How with a hand full of parts u could build a working scope,Man this is just fascinating.... keep it up m8 I wish u best of luck

author: StillTrying
author_rank: Super Contributor
timestamp: September 11, 2016, 06:03:58 pm
content: In a junk box, I've still got the one I built at about 14 years old, as I had no idea what I was doing it's unique!

author: Berni
author_rank: Super Contributor
timestamp: September 12, 2016, 05:51:07 am
content: Neat oldschool project there.Its quite amazing how much can be done with only a few transistors. Today nobody cares since semiconductors are as cheep as passives unless its a fancy special part of a fancy IC.

author: oldway
author_rank: Super Contributor
timestamp: September 12, 2016, 07:53:48 am
content: I think the real fun would be to make a true TA155 with 902 crt, 6J7's and 884 thyratron.The dg7-32 crt is more modern and not intended to work with asymetric plate drive.It is also able to work up to about 3Mhz and it is a pity to use it for only 100 Khz bandwith.Why not make a 3Mhz project ?Here, some schematics of a simple MBLE oscilloscope BEM16...

author: GK
author_rank: Super Contributor
timestamp: September 12, 2016, 12:18:20 pm
content: Quote from: oldway on September 12, 2016, 07:53:48 amI own and have refurbished my TA155 and the 902's deflection non-linearity really isn't that far behind the DG7-32 with only one plate of each pair driven. I have over thirty NOS electrostatic CRTs ranging 1" to 5" dia. including half a dozen DG7-32s. This isn't the first scope project and it won't be the last.

author: GK
author_rank: Super Contributor
timestamp: September 12, 2016, 12:20:49 pm
content: I got the amplifier board installed and mostly wired this evening. I'll finish it off tomorrow or the next evening.

author: Circlotron
author_rank: Super Contributor
timestamp: September 12, 2016, 12:34:40 pm
content: Quote from: GK on September 10, 2016, 07:15:59 amThe appearance reminds me of this one that I built many moons ago.

author: GK
author_rank: Super Contributor
timestamp: September 14, 2016, 01:42:51 pm
content: Quote from: Circlotron on September 12, 2016, 12:34:40 pmHmm, don't have that issue of RT&H but I think I know the design. Does it use four 6BX6 pentodes in the vertical amplifier?

author: GK
author_rank: Super Contributor
timestamp: September 14, 2016, 01:48:05 pm
content: Soldered the final connection this evening. Now all I have to do is finish cable-tying and drill holes for half a dozen P-clamps to secure/tidy up the rest of the wiring. But anyway, its electrically finished and 100% operational.

author: GK
author_rank: Super Contributor
timestamp: September 14, 2016, 01:56:24 pm
content: I could bang on about just how good the two-transistor injection-locked linear sweep generator (time base) works in practice, but I think a video is better. I used a triangle wave for the vertical input signal to better show the deflection/display/sweep linearity. The time base is synced predominantly by adjusting the time base coarse and fine frequency controls so that the timebase frequency is an exact(ish) divisible fraction of the input signal frequency. The time base will also lock to signals that are lower in frequency (half, quarter, etc).I challenge anyone to design a significantly simpler solid-state time base!http://youtu.be/ZSPjTfhK25A

author: setq
author_rank: Frequent Contributor
timestamp: September 14, 2016, 01:57:27 pm
content: 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.

author: rstofer
author_rank: Super Contributor
timestamp: September 14, 2016, 03:48:11 pm
content: That is a magnificent project. I built something much less capable back in the late '50s from plans in the ARRL Handbook. MUCH less capable but with vacuum tubes...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...The panel is two layers of clear acrylic sheet with a printed sheet sandwiched between:ETA: Having to drill so many precisely spaced holes was the motivation for building a CNC mill. Kind of a twofer project.

author: ajb
author_rank: Super Contributor
timestamp: September 14, 2016, 04:15:42 pm
content: Do you buy or make that aluminum bar with the tapped holes you use to frame the chassis? That'd be handy to have around, if it can be bought cheaply ready-made.

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2016, 07:42:27 pm
content: Quote from: rstofer on September 14, 2016, 03:48:11 pmMy 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.

author: TheBay
author_rank: Unknown
timestamp: September 14, 2016, 07:53:41 pm
content: Amazing bit of work there, nothing better than seeing something you built do something

author: timb
author_rank: Super Contributor
timestamp: September 15, 2016, 12:03:49 am
content: Quote from: David Hess on September 14, 2016, 07:42:27 pmI use inkjet transparency film for this. Print the design out, tape it on and use it as a guide on the drill press.

author: bitseeker
author_rank: Super Contributor
timestamp: September 15, 2016, 12:26:53 am
content: @GK: Awesome scope and amazing how few components were needed.@rstofer: That's a lot of holes to drill. The acrylic sandwich looks great.

author: timb
author_rank: Super Contributor
timestamp: September 15, 2016, 12:58:53 am
content: Quote from: David Hess on September 14, 2016, 07:42:27 pmI use inkjet transparency film for this. Print the design out, tape it on and use it as a guide on the drill press.

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2016, 03:39:10 am
content: Quote from: timb on September 15, 2016, 12:03:49 amI 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.

author: GK
author_rank: Super Contributor
timestamp: September 15, 2016, 08:09:55 am
content: Quote from: ajb on September 14, 2016, 04:15:42 pmNo, 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.

author: GK
author_rank: Super Contributor
timestamp: September 15, 2016, 08:17:39 am
content: Quote from: rstofer on September 14, 2016, 03:48:11 pmMine 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.

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2016, 08:39:32 am
content: 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.

author: GK
author_rank: Super Contributor
timestamp: September 15, 2016, 08:58:09 am
content: Quote from: David Hess on September 15, 2016, 08:39:32 amWould 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.

author: GK
author_rank: Super Contributor
timestamp: September 15, 2016, 09:03:58 am
content: Quote from: setq on September 14, 2016, 01:57:27 pmAbout 35 transistors and twice as many diodes in a 321A though......

author: GK
author_rank: Super Contributor
timestamp: September 21, 2016, 10:44:47 am
content: 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.

author: bitseeker
author_rank: Super Contributor
timestamp: September 21, 2016, 09:38:35 pm
content: That'll make a nice portable scope.

author: minion
author_rank: Contributor
timestamp: September 22, 2016, 12:24:35 am
content: Very nice, post your progres

author: Berni
author_rank: Super Contributor
timestamp: September 22, 2016, 06:08:14 am
content: Quote from: bitseeker on September 21, 2016, 09:38:35 pmI 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?

author: David Hess
author_rank: Super Contributor
timestamp: September 22, 2016, 06:19:56 am
content: Quote from: Berni on September 22, 2016, 06:08:14 amTheTektronix SC501is a contender. I think the Tektronix 200 series portables were larger.

author: Berni
author_rank: Super Contributor
timestamp: September 22, 2016, 06:38:45 am
content: Quote from: David Hess on September 22, 2016, 06:19:56 amOh 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.

author: setq
author_rank: Frequent Contributor
timestamp: September 22, 2016, 06:47:57 am
content: 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.

author: dom0
author_rank: Super Contributor
timestamp: September 22, 2016, 07:41:40 am
content: These are magnetically deflected, but they could still be used...

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 22, 2016, 08:41:17 am
content: Hi,A little of OT, this guy built his own CRT 3mm in diameter !!http://www.sparkbangbuzz.com/crt/crt6.htmMany 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

author: GK
author_rank: Super Contributor
timestamp: September 22, 2016, 01:13:22 pm
content: 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........

author: David Hess
author_rank: Super Contributor
timestamp: September 22, 2016, 04:17:00 pm
content: Quote from: dom0 on September 22, 2016, 07:41:40 amYou 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.Quote from: GK on September 22, 2016, 01:13:22 pmYou have basically duplicated the design of the earlyTektronix 1A1which 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?

author: bitseeker
author_rank: Super Contributor
timestamp: September 22, 2016, 07:14:19 pm
content: Hmm, Google Glass -> tiny scope!

author: GK
author_rank: Super Contributor
timestamp: September 22, 2016, 11:44:02 pm
content: Quote from: David Hess on September 22, 2016, 04:17:00 pmUnfortunately 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.

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2016, 02:58:08 am
content: Quote from: GK on September 22, 2016, 11:44:02 pmDoh! I should have taken a closer look at the schematic.

author: JPortici
author_rank: Super Contributor
timestamp: September 23, 2016, 10:21:40 am
content: Quote from: bitseeker on September 22, 2016, 07:14:19 pmadd averydirectional mic.Look at the waveform :