Birth and Rebirth of a 1961 DIY Oscilloscope

[ccktek]

I describe here my 1961 construction of an oscilloscope from scratch and its recent resurrection.

58 years ago my home electronics “laboratory” consisted of a Heathkit VTVM and a home-made one-tube AM band signal generator.  I craved an oscilloscope, but as a high school student my funds were limited.  The cheapest Heathkit scope sold for about $65, the equivalent of several hundred of today’s dollars.  I had little choice but to construct a scope from scratch.

The first step was to find an appropriate schematic.  The design had to be simple but useful – more than just a power supply and CRT with inputs connected directly to the deflection plates.  The diagram that I found and its source are long since lost to history, but I've determined that around 1950 several manufacturers sold inexpensive scopes of very similar design; schematics are currently available online.  The tube lineups were usually 5Y3-5Y3-6SJ7-6SJ7-884 plus the CRT, usually a 5-incher.  The Heathkit model O-2 was one of these, and its circuit is nearly identical to the one that I evidently used.  The 5Y3s are rectifiers in the low and high voltage power supplies, the 6SJ7 pentodes are single stage AC-coupled horizontal and vertical amplifiers, and the 884 is a gas discharge sweep oscillator.  I used a 5W4 rather than a 5Y3 for one of the rectifiers, probably because I didn’t have two 5Y3s but did have one 5Y3 and one 5W4. 

My parts collection was extensive, the product of cannibalization of many defunct radios and other electronic equipment and even a TV.  I was able to construct the scope with only parts on hand plus a few components found locally in St. Louis at Gateway Electronics, a surplus dealer still in business today.  The frame of the case is from a WW2 BC-375 transmitter tuning unit.  The 3-inch 3AP1 CRT was chosen because it fits in the case and because it has a 2.5-volt filament, the voltage of the filament winding of one of two power transformers that I used in the power supply, and especially because Gateway happened to have one.  I connected the high voltage windings of the two transformers in series to yield both -1200 and +350 volts.

I hadn’t yet adopted the practice of constructing projects in blocks, testing each block before proceeding to the next.  Completely wired, the chance of the oscilloscope working on the first try was nearly zero.  Indeed, it did not work, but at least the screen displayed a fuzzy green cattywampus band that moved spontaneously around the screen.  And there was no smoke.  After I did lots of tracing and made some wiring corrections, the scope did work reasonably well. 

Two modifications were made early on.  First, significant 60Hz hum was superimposed on the trace, caused by magnetic effect from the transformers on the CRT electron beam.  I removed one of the transformers and extended it a couple of feet away from the scope on long connecting leads.  Second, the bandwidth was abysmal, worse even than the 60KHz or so specified for this design.  This problem was caused by parasitic capacitance to the grounded case of the bathtub capacitor I’d used to couple the high impedance output of the 6SJ7 to the vertical deflection plates.  Simply replacing the bathtub with an axial lead capacitor and rerouting it and associated wiring away from the chassis improved bandwidth significantly.  This change is evident in the photos; apparently I never got around to removing the disconnected bathtub.  I didn’t make the same change to the horizontal circuit but should have; doing so probably would have made the sweep flyback less conspicuous.

A more advanced modification came a couple of years later.  Wanting still more bandwidth and also more sensitivity, I replaced the 6SJ7 vertical amplifier with a 6C4 triode and a 6U8 triode-pentode.  The 6C4 cathode follower at the input drove the 6U8 triode section which drove the 6U8 pentode section.  With a low resistance vertical gain potentiometer now at the cathode of the 6C4, bandwidth was no longer dependent on gain setting as it had been with an uncompensated high impedance pot at the grid of the 6SJ7.  Of course the 6C4 was then subject to overload, but its 60 volts p-p input capability was enough for almost anything.  To maximize bandwidth I kept the resistance of the plate load resistors of the 6U8 sections fairly low and used small peaking capacitors across the cathode bias resistors.  I mounted ceramic coil forms for 6U8 plate circuit peaking that could have extended the bandwidth still more but postponed winding the coils indefinitely.   

As limited as this scope was, it was infinitely better than none at all, and I used it regularly until about a decade later when it was boxed up and didn’t see the light of day until 2019.

When I opened the old carton and dug out the oscilloscope I was surprised at its sorry condition.  The case was dented and filthy.  Several disconnected wires led to where the intensity pot once had been.  Several knobs were missing.  Inside the case my early wiring practices were revealed.  The hookup wire used throughout, even for the 1200 volt wiring, might have been bell wire from a hardware store.  Many small components floated with only one lead connected to terminals – sort of a vacuum tube era version of more modern dead bug prototyping.



I replaced the intensity control pot and powered up the scope very slowly with a Variac while monitoring the filter electrolytics’ voltages and watching their ripple on an oscilloscope.  After about an hour the +350 and -1200 volt supplies were operating normally with very little ripple and no significant heating of the filter caps, which are now at least 60 years old – they were far from new when I installed them.

Too late I noticed that the front panel was getting quite warm around the new intensity control, which burned out with a quiet pop inside its case (it was one of those sealed Allen-Bradleys).  One side of this control is connected to the -1200 volt supply and the other side to the CRT cathode and filament.  A near-short had developed from the 2.5 volt filament winding to ground in one of the power transformers.  I was fortunate to obtain an appropriate filament transformer online, wired it in, and tried again.  This time a thick trace appeared on the screen, but the focus and intensity controls were ineffective.  Yet another low resistance path to ground had developed, this time within the focus control, possibly through a carbon track caused by arcing.  Once the focus pot was replaced and many intermittent switch contacts and potentiometer wipers were cleaned, the scope actually worked.

It seems amazing that no capacitor failure prevented the oscilloscope from working.  However, the sweep was very non-linear on two ranges, more so than expected in the inherently nonlinear gas discharge sweep oscillator.  The rate-determining caps for these two ranges had high ESRs; replacing them returned the sweep operation to normal. 

I resisted the urge to make improvements, even simple ones, as I serviced the scope, preferring to keep its character as original as possible.  I did, however, tweak the cathode peaking caps of the two 6U8 stages of the vertical amplifier individually, now an easy procedure with modern test equipment, for optimum square wave response.  Bandwidth is 330KHz.  Vertical sensitivity is 70mv/inch.

[Vgkid]

That is very cool, and a neat story to go with it. Good job as a kid back in the day.

[bob91343]

Yes very cool.  Labeling of the controls might be a nice touch.

So next you add a delaying sweep, graticule illumination, voltage and frequency calibrator, and increase the bandwidth to 100 MHz.  Well, maybe not.

A true hobbyist.

[L_Euler]

Nice work for sure.  I like the rebuild of the cabinet.  Do you have sheet metal tools in your shop, and if so, did you build those as well?  I'm giving thought to a Grizzly 3 in 1 for just such an occasion.  Yea, I know 3 in 1 master at none.

[rsjsouza]

That is a true work of art. I love the story and the looks of it.

I still have several of my late father's projects he made when we were kids - mostly electronic games and noise machines that gave us hours of entertainment. I also have some that we made together.

[ccktek]

Thanks to all.  L_Euler, the case is original; I had removed it for the initial pre-restoration photo.  As I recall I simply bent aluminum sheet metal around pieces of 2X4 clamped in a vise.

[KE5FX]

Needs tunnel-diode triggering!  Come on, you know you want to.

[ccktek]

Needs tunnel-diode triggering!  Come on, you know you want to.

True, but some urges are best left un-indulged.

[GreyWoolfe]

Nice scope and story.  The best part for me is seeing that Gateway Electronics is still in business after all these years.  So many of these brick and mortar stores have closed.  Make sure you keep giving them patronage so they don't go away.

[duak]

Way cool!  That brings back memories.

I built a 'scope in 74-75 from a conglomeration of new, surplus and used parts all packaged in an innovative hybrid sheet metal and wood case.  The handle was from an old reel-to-reel tape recorder.  The CRT was vintage Eisenhower era NOS in the original box from a surplus shop - the owner didn't even know he had it.  The power transformer was from some old Telco line equipment as were the industrial vacuum tubes, HV caps and rectifiers.  I used transistors for the low level, sweep and triggering with tubes for the CRT plate drivers.  I think I was able to get maybe 100 kHz vertical bandwidth - probably no need for a tunnel diode trigger.  It did have triggered sweep based on the Telequipment D54 design.  I couldn't implement proper DC blanking though so there was a problem with possibly burning the phosphor on the left side in normal trigger mode.

I started rebuilding it in a new case but never finished because I bought a Tek 2215 when they first came out.  I've been thinking of at least repurposing the old CRT as a vector display clock.

[bsfeechannel]

Hats off.

[tggzzz]

Neat; good for you, both then and now.

I recently resurrected something simpler from 45 years ago: a new-fangled digital clock with LEDs. It still works, but is no longer so eyecatching, so I diddled the 50Hz input and turned it into a Vetinari clock.

FFI about the clock and the vintage processes used in making electronics back then, see https://entertaininghacks.wordpress.com/category/digital-clock/

[ccktek]

Nice scope and story.  The best part for me is seeing that Gateway Electronics is still in business after all these years.  So many of these brick and mortar stores have closed.  Make sure you keep giving them patronage so they don't go away.

I haven't lived in St. Louis for decades, so my patronage to them will have to be via their mail order service.

I can find nothing about Gateway’s history on the internet, but here’s what I know.  Gateway Electronics was a young electronics gearhead’s dream.  In the early 60s they were in a little storefront on Delmar Blvd.  The store was owned and run by Stu and Lou, Stu the business type, Lou the EE type.  Stu and Lou were always very generous and patient with their clientele, especially us neophytes.  The main front room had neatly categorized small components on display and also larger items such as test equipment under several glass-topped counters.  A darker back room had lots of bigger equipment such as war surplus gear scattered around higgledy-piggledy.  On the floor in one corner was a big pile of defunct car radios that looked as if they’d been dumped there from a front end loader.  That room contained the real bargains.

The St. Louis store has moved at least twice since that time. 

During the 1970s I lived in Denver, and Gateway followed me there!  For several years they had a branch in Denver, and, I believe, another elsewhere, possibly San Diego.  Those branches eventually closed.

I and others would love to see photos and more history from the early St. Louis Gateway Electronics days (possibly on the General Chat board?), should any St. Louis EEVblog readers have access to such items.

[ccktek]

For anyone interested in St. Louis electronics history:  Gateway Electronics has closed, probably permanently.  I have been made aware of obituaries of Stu and Lou.  Excerpts follow.


Stuart Bartfeld grew up in University City and served in the Missouri Air National Guard. He attended the University of Missouri at Columbia and Washington University.

Mr. Bartfeld worked as a radio officer for St. Louis County before he approached Lou Elkins, his childhood friend, about opening a business.

In 1960, Bartfeld and Elkins opened Gateway Electronics Corp. in St. Louis, where they sold military surplus and electronic equipment.

Bartfeld and Elkins started out buying old military surplus electronic equipment and using parts to build ham radios. Soon, ham radio enthusiasts flocked to the Delmar Boulevard shop to buy parts. The store later moved from its original location on Delmar to Vinita Park.

Bartfeld, whose ham radio name was W0IGU, became president of the company while Elkins served as vice president. They later opened similar stores in Denver, Houston and San Diego.

In 1999, Bartfeld retired and moved to San Diego but remained active in the business.
--St. Louis Post-Dispatch; July 12, 2001


Elkins, Alfred Louis on Monday, June 19, 2006.
Mr. Elkins was the founder and co owner for over 46 years of Gateway Electronics with his life long friend, the late Stuart Bartfeld. Lou was a ham radio enthusiast and computer wizard. He had a quiet, patient demeanor and a surprising dry wit.