3.3V to 5V Level Adjustment

Only just after I’d written last month’s post about an X-Y-Z display for an HUD, the customer asked for a spot of extra help with his new playtoy.

Uh oh - image not centred and way too big...

Uh oh – image not centred and way too big…

The Oscilloclock Deflection Board currently assumes X and Y input ranges of 0-5V, centred on 2.5V. However, the customer was programming an Arduino-based controller board with analogue output from 0-3.3V. Applying this directly of course didn’t break anything, but sure did make it hard to centre on screen! Would there be a quick way to adjust voltage levels?

Another issue was that the gain in the current-revision Deflection Board is hard-wired, and the image was not the right scale to just fit the screen. The gain could be changed via a single resistor per channel, but would there be an easier, more flexible way?

YES on both accounts!

A Quick and Dirty Level Adjuster

A bit of research and doodling the next day, followed by an even quicker breadboard test in the evening, and the basic level/gain adjuster circuit below was born!

Very simple circuit for a 3.3V to 5V level adjuster

Very simple circuit for a 3.3V to 5V level adjuster

... breadboarded up and ready for testing!

… breadboarded up and ready for testing!

Testing it out

Avid readers will recall that I lose no opportunity to flex my old equipments’ muscles. Let’s see what gets powered up this time!

First, I used an NF DF-193A to generate a nice sine wave centred on 1.62V (approximately 3.3V / 2) with 1V amplitude. (NF Corporation is a niche Japanese manufacturer of high-performance test equipment. The quality (and secondhand pricing!) approaches that of HP and Tektronix.)

Simple 3.3V to 5V analog level adjuster - Test setup

Next, I checked  the output of the circuit on a slightly less venerable and arguably uninteresting HP 54615B 500MHz oscilloscope. The output was a nice 1V P-P sine wave, and slight adjustment to the 130K resistor allowed it to centre nicely on 2.5V.

Simple 3.3V to 5V analog level adjuster - Leveled Output

And the gain adjustment? Changing the value of the 22K feedback resistor worked a charm! However, it also affected the offset level, as shown below.

Simple 3.3V to 5V analog level adjuster - Offset effect on Gain

Changing gain (1.03 to 0.98) impacted the offset (2.443V to 2.648V)

This is certainly not my ideal circuit, because the two adjustments interact with each other. Ah well, I did say that it was quick and dirty…

Op amp selection

One thing to be careful with here is to choose a rail-to-rail op amp. The hapless customer initially tried this out with an LM358, and obtained an awfully shaky and unstable output. It turns out that this chip is a particularly poor performer for this application on a single 5V supply, because its input range is limited up to only Vdd-3V. Even the 2.5V centre area would be unstable!

Other op-amp selection tips: Choose a lower-bandwidth device (say <50MHz) to avoid oscillation without worrying about layout and external damping. Also, choose a device that is stable at (and below) unity gain!

Good enough? Maybe…

This circuit probably got the customer out of a tight spot, but I really do need to make a revised Deflection Board with a proper on-board level and gain adjustment feature. Without the unwanted interaction…

Yet another task on the ever-growing list!

Heads Up!!

Recently I received a most intriguing request: I was asked to build a self-contained, super-bright X-Y display unit with 3-inch CRT, for use in an “HUD“. Hmm…

Holographic Utterance Device?
Horizontally Unstable Doohickie?

Fortunately, I didn’t need to guess any further. As I was once an avid flight simulator enthusiast, I quickly hit upon the correct meaning: Head-Up Display. This is a mechanism that overlays instrumentation or map data onto the view looking forward from the cockpit, so that the pilot doesn’t have to look down to see this information.

HUD in an F-18 aircraft. Source: AC Aviation Life

HUD in an F-18 aircraft. Source: AC Aviation Life

Wikipedia has a great introduction to HUDs and their history, but Mike’s Flight Deck has the definitive tome for flight simulator enthusiasts who want to actually build an HUD. According to Mike, the system employs various optical paraphernalia, but at the heart of the mechanism is what lies closest to my own heart – a CRT Display!

Oscilloclock 3-inch X-Y-Z display, custom-built for an HUD

An Oscilloclock 3-inch X-Y-Z display unit, optimized for use in an HUD

Key Requirement: Brightness

The CRT display used in an HUD needs to be very, very bright. The image loses its intensity as it traverses through collimators and other optical devices, and it needs to be visible even in daylight conditions. To achieve this, HUD displays used in real aircraft employ specialized CRTs with very high accelerating voltages (10-20kV).

Adding in the resilience requirements demanded of aircraft equipment, these displays go for tens of thousands of dollars, and are not easily available on the retail market. Maintenance is also prohibitive. Ultra-bright thin display technology may well replace the CRT in the near future (if not already), but such displays may be even more expensive for hobbyists to procure.

My customer needed a lower-cost, easily maintainable alternative. He had read my 3″ VGA Display article, and noted that I love to eke every spare lumen out of my CRTs. Would an Oscilloclock display be bright enough? Possibly – so I took up the challenge!

Control Requirements

The customer was building his own control board, that would generate the symbology using vector graphics (as opposed to a raster-scanned image like in the VGA display). This was great news – a typical vector image will easily appear brighter than a raster image, since the beam is turned on nearly 100% of the time.

So what was needed was a standard Oscilloclock X-Y Display, optimized for deflection bandwidth and brightness. Digital blanking (Z axis) would be required as well, just in case his controller could not move the beam rapidly enough between segments to avoid a visible trace.

Board Enhancements

This was a perfect time to put in place some enhancements that had been on the backlog for more than a year! The new Oscilloclock Power Board rev 2.1x offers the following:

  • Maximum cathode to deflection voltage of 2175V (max 3675V if isolated blanking is not required)
  • Maximum accelerator voltage of 3525V for PDA type CRTs
  • TTL/CMOS compatible digital blanking (grid modulation), safely isolated at 2.2kV continuous working voltage, 250-350ns end-to-end propagation delay
  • Onboard blanking amplifier – external “CRT Board” no longer required (but can optionally be used for longer CRT harnesses)
  • Support for “Deflection Blanking” CRTs (see treatise here)
  • Dim/Bright and Power Off inputs
  • Temperature-sensitive fan controller with Failure and Overtemp safety features
  • CRT rotation coil supply (+/-5V)
  • CRT heater soft start / inrush current limiting

And the Oscilloclock Deflection Board rev1.3x now sports:

  • Precision deflection amplifier capable of driving +/- 550V with 0.1% linearity
  • 0-5V analog X and Y inputs, gain adjustable via single resistor per channel
  • Onboard 2.5V reference output

But which CRT??

With brightness being such a key requirement, a PDA (post-deflection acceleration) CRT seemed the ideal choice. However, new-old-stock PDA CRTs are not exactly plentiful on the market, and one goal of this project was to reduce maintenance costs associated with CRT replacement. The customer wondered if the more ubiquitous non-PDA type 3RP1A might be bright enough?

I thought it might, if I powered it at 2.2kV. This is the safe continuous working voltage limit of my isolated blanking circuit, and is still within the CRT’s maximum specification of 2.7kV. (If blanking were not required, we could have maxed it out – but then the deflection amplifier would need enhancement to drive even more than +/- 550V. My 3″ VGA Display article examines the relationship between deflection sensitivity and acceleration voltage.)

The 3RP1A was a great choice, and I optimized the assembly for that CRT.

HUD Display Assembly - complete and configured for 3RP1A

HUD Display Assembly – complete and configured for 3RP1A

3KP1(F) – an alternative to 3RP1A?

I got to thinking again about maintenance. The customer might operate his simulator for hours each day. He will set the intensity quite high. The vector graphics symbology might be fairly static on the screen. These usage factors will naturally and unavoidably lead to either the dreaded SCREEN BURN-IN, or at least CRT BURN-OUT!

What if the customer had a stable supply of used CRTs that were so cheap that he would be happy to (literally) burn through them?

Oscilloclock labs had just the thing! I regularly procure used 3KP1(F) CRTs in great condition. Electrical characteristics are similar enough to the 3RP1A. Size is not.

3KP1(F) is longer than the 3RP1A

3KP1(F) is longer than the 3RP1A

But what about performance? Below is a qualitative look at a 3KP1(F) against a 3RP1 (I didn’t have a flat-faced 3RP1A in the lab at the time). At the macro scale, the difference isn’t hugely obvious.

HUD Display Assembly - 3KP1(F) vs 3RP1 - macro

Comparing the 3KP1(F) (left) to the 3RP1 (right)

But what about brightness and line resolution at smaller scales? Let’s examine line resolution:

Comparing line-width resolution - 3KP1(F) vs 3RP1

Comparing line-width resolution – 3KP1(F) vs 3RP1

At this scale, the 3RP1 does appear more performant.

I configured the board to support both 3RP1A and 3KP1(F). The battle will be won when the customer tests both tubes in the actual installation. Stay tuned!

What’s Next?

Raster graphics is mediocre. Vector graphics is good. But Circle Graphics is simply stunning! Could I reprogram the Oscilloclock Controller Board to interface to a flight sim API, and display HUD imagery and symbology in silky curved figures? I could! I would! Well, okay, I might… a project for another rainy day.

Imagine your HUD with these silky smooth characters!

Imagine your HUD with these silky smooth characters!

Like what you see?

Are you building an HUD and need a 3″ display? Are you interested in a Circle Graphics based HUD controller? Or are you looking for a nice standalone X-Y(-Z) display for your fancy new thing-a-me-bob? Contact me – and perhaps Oscilloclock.com will take up yet another challenge!

A Humpty Dumpty CRT

Humpty Dumpty sat on a wall…
Humpty Dumpty had a great fall…

…and so the great nursery rhyme goes! But here at Oscilloclock labs, we’re not talking about an egg (which, one theory goes, represented the defeated King Richard III). We’re talking about a beautiful old CRT, savagely shaken and shattered during international shipping. What a waste. But oh, what a great chance to see the insides close-up!

This broken CRT missed its chance to live again in a nice VectorClock

This broken CRT missed its chance to live again in a nice VectorClock

Looking down the barrel. Imagine you are a phosphor molecule, with projectiles from this gun hitting you at the speed of light!

Looking down the barrel. Imagine you are a phosphor molecule, with projectiles from this gun hitting you at the speed of light!

The Guts of a CRT

Many reference materials explain the bits and pieces of a CRT’s electron gun assembly, something along the lines below.

CRT - Typical Electron Gun

But here we have such an assembly exposed, up-front and personal!

Broken CRT assembly

Special Treatise – Deflection Blanking

This CRT is a bit unusual – it has a deflection blanking electrode. What on earth does this do?

Well, in most CRT applications, the beam is blanked by applying a negative voltage to the grid, which literally shuts off the beam. This is all fine and dandy, but there is one VERY annoying thing: the grid is actually at a very negative potential with respect to the deflection plates – often up to 2kV ! Translating a blanking pulse from a low-voltage microcontroller onto such a dangerous potential is tricky. Doing it at high frequencies is even more of a challenge. (Of course, the latest-revision Oscilloclock Power Board solves the challenge, offering fully isolated blanking up to 2.2kV at multi-MHz bandwidth.)

The deflection blanking electrode, on the other hand, blanks the beam by bending it so much that it hits the electrode wall. This seems like a bit of a waste – the beam is always on, and much energy is dissipated in the electrode. BUT this electrode is at roughly the same potential as the other deflection plates!! O happy day! There are no high voltages involved, so direct drive at very high frequencies directly from the microcontroller or low-level amplifier is possible.

For a more precise treatise, I defer to G. N. Patchett, who writes thus:

CRT - Blanking 1

CRT - Blanking 2

Aren’t CRTs dangerous?

Before you take your own CRT and hack it apart, a few words of caution. DO NOT TRY THIS AT HOME until you have at least read this, and please do be careful!

1. High Vacuum

CRTs are devoid of air, to allow electrons to flow readily, just like in space. This means they will implode. The electron gun might just get sucked through the screen and shoot into you, causing massive injury or death. One tried-and-proven way to safely break a CRT is to wrap it in several layers of towels or rugs, go outside, and whack it with a long board.

Or, as was my case, simply ship it to someone with very little packaging!

2. Broken Glass

Not much to say here. Don’t cut yourself!

3. Leaded Front Glass

Some CRTs, particularly those driven at 10kV and above, employ leaded front glass in order to stop harmful X-rays. Some people may feel this lead is a problem, dangerous in some way. But remember that this is the same material as leaded crystal, used in glassware for aeons – and even today. You can dispose of this glass in the same way that you would a broken crystal wine glass, without impacting your conscience. (Unless you have hundreds of CRTs, in which case – please use a commercial disposals company!)

4. Poisonous Phosphor

Some kinds of phosphorous materials used in CRTs are quite poisonous, and can be absorbed through the skin. Do not touch the phosphor or attempt to re-use it in mad-scientist experiments without extreme caution – always use gloves and goggles when working near these chemicals.


I have many superb books describing the glorious guts of cathode-ray tubes, but one of my very favourites, and the source of the extracts above, is The Cathode-Ray Oscilloscope and its Use by G. N. Patchett. This recently saved me literally tens of hours in a frustrating week of failures using an unknown type tube from China, as it was the only book in my library that helped me understand maximum-scan limitations in non-spiral PDA type CRTs.

The Cathode-Ray Oscilloscope and its Use

Toshiba Transformed

I believe in reincarnation. Every vintage device sporting a CRT deserves to live again, to be loved again, to lift someone’s spirits again. And in 2014, this beautiful Toshiba ST-1248D received its chance, born again as a suave Oscilloclock!

Toshiba ST-1248D Oscilloclock

See this in HD, and find more exciting videos on my YouTube channel

Manufactured sometime in the mid to late 1950’s, the ST-1248D was extremely well-designed and assembled, compared to other compact models available on the domestic Japanese market at that time. The engineers considered both function and form – latched panels on the side and back, delicately laced wiring, and a relatively spacious interior conducive to heat removal and circuit reliability. But the delightful brass bezel is what really makes this one of the most beautiful Oscilloclock conversions ever.

Toshiba ST-1248D - Brass bezel

The new owner’s corporate logo, being quite curvaceous, was terrifically appropriate for rendering in Circle Graphics! Even using the Oscilloclock Figure Creator, it took a good deal of trial-and-error to accurately portray the different line widths and shades of ‘grey’ required. But not a bad result!

Conversion highlights

In a previous post, I mentioned there are several general approaches to converting an oscilloscope. For a unit as venerable as this Toshiba, though, the only choice was to bypass all existing circuitry with a full set of latest-revision Oscilloclock boards. This allows for:

  • Super-bright, sizzingly clear trace
  • Ultra-linear deflection, rock-solid stability
  • 10W nominal power consumption!
260+ individual components. Lovingly hand-mounted by yours truly!

260+ individual components. Lovingly hand-mounted by yours truly!

Naturally, the original circuitry is left almost entirely intact, with only the HV supply deliberately disabled. This allows the owner to turn on the original power switch and enjoy the delicious warm glow of lit valves!

Toshiba ST-1248D Oscilloclock - Tubes

Toshiba ST-1248D Oscilloclock - Underneath

Under the Oscilloclock! Try to spot where the original HV supply has been disabled…

At this point you must agree that the ST-1248D is a cute, compact little unit. But due to the size, it wasn’t possible to mount all five Oscilloclock boards facing outwards for easy access. So, three boards got the short stick, and had to be tucked neatly inside. Intensity, Focus and Astigmatism trimpots are therefore mounted vertically to allow in-situ adjustment:

Toshiba ST-1248D Oscilloclock - Adjustments

Cables are also sufficiently long to allow the boards to be pulled out for maintenance and servicing.

Toshiba ST-1248D Oscilloclock - Boards pulled out

12m of 3kV melt-proof silicone wire!

Oscilloclocks need just one control. Where to install it? The Toshiba’s Sync switch was in just the right spot. But because this was a stacked switch + potentiometer control, with two concentric knobs, it was quite a challenge. To keep the external appearance unchanged, I removed the front wafer section and rear potentiometer, and installed the rotary encoder – allowing full re-use of the original shaft and mounting assembly. Invasive? Yes, but also fully reversible…

Toshiba ST-1248D Oscilloclock - Control installation

The only irreversible change in this conversion is the attachment of the GPS socket to the rear access panel. Hopefully a forgivable sin!

Toshiba ST-1248D Oscilloclock - GPS connector

Spare CRT

The original CRT in this Toshiba was defunct, so I replaced it with an unused, new-old-stock CRT from the lab, and provided a nice 3-month guarantee. But beyond that, how long might a tube dragged from 40-year hibernation survive the torture of modern-day life?

Well, Oscilloclocks have several software and hardware features to avoid CRT burn-in and generally extend the life of the unit:

  • Hourly XY shift setting
  • Auto clock face change setting
  • Auto power-off setting
  • Soft-start power supply
  • Series-lamp inrush current limiter

But even with these life-saving features, there’s really no telling. So I supplied a second CRT as a spare. And this tube even has a pedigree!

Toshiba 3KP1(F) Original Inspection Certificate 1965

The spare CRT actually has a pedigree – an original inspection certificate from 20 January 1965!

Like what you see?

Oscilloscopes and other vintage CRT equipment are remarkably varied in size, shape, and ambience. There’s one out there that’s just right for your office, bedroom, restaurant or shack! Contact me if you’re interested in a custom reincarnation.

Oscilloclock in a Book!

Now here’s a familiar picture!

Oscilloclock Heathkit OR-1 in Chuck Penson's Heathkit Test Equipment Products book 1

It seems that the Heathkit OR-1 is a very rare oscilloscope nowadays, and Chuck Penson reached out to me for a photo to put into his latest book, Heathkit Test Equipment Products. This is a very well-written, well-researched treasure trove of data about the most iconic kit manufacturer of its time. Highly recommended!

Chuck Penson's latest book - superbly authoritative

Chuck Penson’s latest book – superbly authoritative

And of course here is the Oscilloclock Heathkit OR-1 again, in all its glory:

Heathkit OR-1 Oscilloclock

VectorClock Reloaded!


Fresh from Oscilloclock Labs – a new VectorClock creation, commissioned for the office of a world-famous film and television director:

Tek 520 VectorClock - S/N 002 (image published with permission of the owner)

Tek 520 VectorClock – S/N 002 (image published with permission of the owner)

This unit is based on an original Tektronix 520 vectorscope, which is the predecessor of the 520A that was used in the first VectorClock, described here. This custom conversion employs several key enhancements, and performance has never been better!

Be sure to check out videos on my YouTube channel.

Robbie’s Place

What do you do on a mundane business trip to London?

Why, shopping, of course! But if you were the humble proprietor of Oscilloclock.com, you would do much more than that… You would seek to expand your vintage electronic empire!

And so it was that I found myself hunting old electronic devices on Portobello Road one fine Saturday morning. Unfortunately, the game there was far and few between; only two relatively mediocre valve radios, in even more mediocre shape, at far more than mediocre prices…

Fortunately, my colleague was going to save the day. “Pop out to Cambridge for a visit – I’ve seen a few antique shops here,” said he. And after the hour train journey and a wee but of walking, we stumbled onto a veritable gold mine – Robbie’s.

Continue reading

VGA display… On a 3″ scope tube!

Yes, you’ve all thrown away your lunky old CRT monitors, in favour of sleek ultra-thin LCD displays. And, you thought you’d never see another one again…

But this CRT display has a twist! It’s round. It’s small at just 3 inches diameter. And it’s awfully cute.

Oscilloclock 3-inch CRT VGA Display Assembly - overview

Last year, I was approached by a dedicated flight simulation enthusiast, who needed a radar indicator to use in a fighter cockpit replica. The indicator should employ a CRT, for the most realistic look. Could Oscilloclock design and construct such a display?

It didn’t take much convincing! Diverging only temporarily from building clocks, I took up the challenge to create my first raster-scan CRT display unit. In the ensuing months, difficulties sprang forth from every direction in the project, but ultimately I was able to avoid a diraster (sic) and deliver a functional assembly:

See more related videos on my YouTube channel

The Setup

The key component of this setup is a new prototype VGA Board that converts a VGA signal into analogue X and Y outputs. Both analogue intensity and binary blanking outputs are provided.

Oscilloclock VGA Board prototype

Oscilloclock VGA Board prototype

The X and Y outputs drive an Oscilloclock Deflection Board, while the binary blanking output drives the blanking amplifier in a CRT Board.

Oscilloclock Deflection Board - modified for ultra-linear HV output

Deflection Board – modified for ultra-linear HV output

CRT Board - heavily modified for improved frequency response

CRT Board – modified for improved frequency response

Blanking isolation, heater, and HV supplies are provided by a Power Board.

Power Board - with improved optocoupler

Power Board – with improved optocoupler

It all looks so easy! But noooo. Astute readers will recall from other posts that every Oscilloclock project involves sleepless slumbers, horrific hair-pulling, and forgotten family members. Let’s see what caused me grief this time…

Continue reading

From the Archives – a 400-LED Oscilloscope

Long, long ago… In a workshop far away…

Recently, I’ve seen quite a few search hits and even an enquiry regarding the 400-LED dual-trace oscilloscope that I briefly mentioned on my History page. With renewed enthusiasm therefore, let’s take a trip down history lane and see what I was doing back in 1990!

A compact dual-trace 1MHz DC scope - what more could a high school kid want?

A compact dual-trace 1MHz DC scope – what more could a high school kid want?

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Tek 520A VectorClock!

Television broadcasting has switched from analog to digital - and if you’ve got a nice HD TV, you’ll be loving it!

But with that transition came the death of an entire breed of equipment – the Vectorscope.

Tektronix 1420 Vectorscope

Just to be clear, these are not monitors for playing ancient video games using vector graphics!!  No, the Vectorscope is (was) used to give a delightful view of the ‘vectors’ inside an NTSC or PAL video signal, describing the color components of the signal.

If you were lucky enough to be a TV broadcast technician, you’d use your Vectorscope all the time to check your vectors’ amplitudes and phase. You would even give your vectors names like ‘Jack’ and ‘Jill’, and check up on their relationships daily, just as any responsible guardian would!

But above all, you would marvel every single day at the beautiful hardware you were using, and the complex circuitry involved. Take a look at my Tektronix 526 Vectorscope, which has oodles of delicious tubes to heat my shop on a nice winter’s day:

Tektronix 526 Vectorscope

Well, it all went digital and there is no longer any need for analog color signal analysis. But dry your tears… There is something even better:

Announcing the Tek 520A VectorClock

This lovely Oscilloclock reincarnation of a Tektronix 520A, sold at Maker Faire Tokyo 2013, allows its new owner to forever relive the magic of NTSC, PAL and SECAM analog color.

Tektronix 520A VectorClock - brilliant blend of the old and new!

Tektronix 520A VectorClock – brilliant blend of the old and new!

See more related videos on my YouTube channel

The Tektronix 520A has a stunning built-in array of lights for illuminating the CRT graticules. By simply removing the bezel and external graticule, the Tek 520A morphs into a deliciously moody timepiece!

Tek 520A VectorClock - Glorious Glow

Normally, I shun CRTs with built-in graticules. Their lines detract dreadfully from an Oscilloclock image. But here! The Tek 520A’s internal vectorscope graticule is round! What better way to accentuate a Circle Graphics driven display?

Silky smooth Circle Graphics on steroids!

Silky smooth Circle Graphics on steroids!

Under the Cover…

The Tek 520A is solid-state. It can be left on 24 hours a day and not fail for many years. This makes it a perfect match for my Maximum Re-use + Minimum Invasion policy: nearly all existing circuits – HV power supply, deflection amplifiers, blanking – are put to use, with just a few (reversible) tweaks.

Tek 520A VectorClock - Maximum re-use, Minimum invasion

The Oscilloclock Power Board is mounted neatly next to its own dedicated low voltage supply. A small relay board can be seen below, for controlling the Tek’s main power unit. All cabling is HV-tolerant and neatly fastened with high-temperature cable ties.

Tek 520A VectorClock - Control Board mount and cabling

Of the more interesting reversible ‘tweaks’ needed for this retrofit, here we see a delightful little trimpot pretending to be a transistor. Quite an act, I would say!

Tek 520A VectorClock - an unorthodox transistor replacement

Like what you see?

If you love big, looming Vectorscopes and need to have one put to good use in your living room, Contact me. And be sure to subscribe from the front page, to track all the other exotic experiments and unique timepieces targeted for 2014!

Credits to [Quinn] in Canada, for providing the initial inspiration for the Tek 520A VectorClock project!