Reverse-engineering the waveform generator in a 1969 breadboard

https://www.righto.com/2022/03/reverse-engineering-waveform-generator.html

by picture • 4 years ago

100 27 4 years ago

kens6• 4 years ago

Author here for all your questions about obsolete oscillator circuitry :-)

AdamH12113• 4 years ago

Nice write-up! Question about the (excellent) schematic PDF -- in the pulse trigger section, the emitter of Q3 seems to be tied to the top of R14, but I don't think that can be right? Is it supposed to be connected to ground?

kens6• 4 years ago

You should expect about 90% accuracy from the schematic :-) I didn't check the transistor part numbers, so I kind of guessed at NPN vs PNP. Most likely it's a PNP transistor. But the pinout could also be wrong.

adrian_b• 4 years ago

Even if I do not have any question, because I happen to be familiar with this kind of circuits, I just want to thank you for your work resulting in one more interesting article.

I suppose that there must be many like me, who normally would not feel the need to comment your articles, but who nonetheless enjoy very much reading them.

A long time ago, when I was young, I have worked with many devices and circuits similar to those reverse-engineered by you in this series of articles. Unfortunately at that time it was tedious and expensive to make photographic records of such technologies and such work, so now I am sad that I do not have records and the memories fade away.

Your articles fill this void and I am grateful for them.

tgflynn• 4 years ago

I'm curious how you were able to source the parts to replace the burned out transistors and other components.

kens6• 4 years ago

CuriousMarc has lots of old transistors, so he found some that worked as replacements.

myself248• 4 years ago

That sine wave shaping network is blowing my mind! I've always wondered how function generators do that, and I knew filtering would only work at one frequency.

The diode-resistor network is all sorts of elegant. Using twelve resistors of carefully chosen values though, tolerance issues must be tricky. I'd imagine laser-trimmed networks being a good fit for that topology.

Is that likely used in monolithic function generator chips like the XR2206?

kens6• 4 years ago

I looked at the XR2206 datasheet. It has a "sine shaper", but from the schematic it isn't a resistor-diode network. It's not immediately obvious to me how that sine shaper works.

https://www.sparkfun.com/datasheets/Kits/XR2206_104_020808.p...

parksy2• 4 years ago

There's a group in my city who meet every month to generate wacky noises from all sorts of things. With this thing's built-in generator and layout I feel like you could build a primitive step sequencer with those switches and dials and deliver a godlike performance of audio and engineering prowess.

Even better if you can voltage control the frequency. Then if you can introduce a delay in the oscillator output, filter it, then feed back into the variable resistor(?) that controls its frequency, you have the basis of a mini-FM synth - if you can get it to stay stable in the normal human hearing range of course...

Nothing really to add on the engineering side, just seeing how it looks, I want to hear it making some noise, although it's not its intended purpose.

Nice work on figuring out how to get it going again!

earthscienceman• 4 years ago

Which city and how do I join something near me? Denver?!

parksy2• 4 years ago

Our local one is https://www.noizemaschin.com/ - they're based in London and Perth Australia, it's kind of a hybrid maker space / underground scene. Good folk from all walks of life.

Not sure about Denver but I did some searches for experimental music, perhaps jump on the local subreddit as it seems there's a small noise music scene but I wasn't able to pinpoint anything specific.

scarecrowbob• 4 years ago

You might take a peek at the Colorado Modular Synth Society:

https://www.facebook.com/groups/1216704935132398

dnos• 4 years ago

Yes, I was thinking the same. I am super curious about what functionality in this could be used for sound generation.

It even looks like a full synthesizer. When I first saw the pictures, I thought it was some weird version of an EMS VCS 3 synth — which coincidentally is also from 1969!

gzalo• 4 years ago

Am I the only one that thinks that $1300 is absurdly high? acording to radioshack catalogs, in 1968 you could get an rf signal generator for just 30 bucks, and an oscilloscope for 130 bucks, so why whould you spend that much in this breadboard? it doesnt seem like it would cost that much!

bragr3• 4 years ago

Zooming in on the unit, the range knob on the function generator goes from 1hz to 1Mhz and the pulse generator 100ms to 100ns. I looked in a 1968 catalog a didn't see anything with close to those capabilities. https://radioshackcatalogs.com/flipbook/1968_radioshack_cata...

TheOtherHobbes• 4 years ago

This HP catalog from 1960 has some comparable items - probably better engineered.

The breadboard on this unit probably at least doubled the price.

Still seems quite spendy though.

http://hparchive.com/Catalogs/HP-Catalog-1960.pdf

kens6• 4 years ago

I think that the signal generator is closer to HP test equipment quality than Radio Shack quality, which may explain the price. I was expected a simple circuit, but when I looked inside it turned out to be very complex.

Here's the brochure for the breadboard with specs and the price: https://archive.org/details/TNM_Elite_1_2_3_dynamic_breadboa...

amelius• 4 years ago

You can't get very high signal integrity on a breadboard.

formerly_proven• 4 years ago

The sine-wave shaper appears to be inspired by the similar circuit in the HP 3300A Function Generator, introduced in 1965. The schematic below shows the HP 3300A's sine-wave shaper; the breadboard's network is similar. The resistances are carefully chosen to achieve the sine wave.

In some Philips FGs they had sine shaping ASICs (presumably with a ton of taps and good matching) and managed to get distortion down to 0.1 % iirc (which for the 70s or early 80s would have meant that you don't really need a proper RC oscillator any more for testing audio amplifiers).

ChuckMcM• 4 years ago

Lovely. And I really like the design aesthetic of the knobs and switches (similar to analog computers which I also find pretty fascinating). I've seen the variable voltage regular to change output amplitude trick in circuits where that signal is low impedance and designed to have a lot of drive current. One was a pulse generator that provided the signal for an animatronic robot, and the pulses were basically used directly to fire the solenoids on the pneumatics. Saved having a transistor on every solenoid.

bragr3• 4 years ago

I love the hand hewn charm of these old boards. Now even homemade boards are designed in software so everything is very straight and regular, but with old boards you can tell everything was done by hand from the design to the layout to the assembly. Don't get me wrong, it's great that you can cad something up, send the files off, and get perfect boards in 2 days, but these old boards are the electronic equivalent to a charming, rustic, cabin in the woods.

thecloud• 4 years ago

If you buy a guitar pedal from a small-ish boutique company, you might just get a little box with a lot of charm that you can open up and peek at some beautiful hand assembled/soldered circuits :)

Gigachad• 4 years ago

It's the only way they can justify charging $400 for $10 worth of electronics.

gotaquestion• 4 years ago

That sine wave shaper is fascinating. Typically OpAmp-101 circuits integrate the triangle wave, but this is a poor approximation of a sine wave (since it is a quadratic), unless the output is filtered (but then the filter needs to change with the frequency, which is complex). The diode/resistor ladder is very interesting.

jacquesm• 4 years ago

What a gorgeous board with the exposed ground plane. The parts all look like they could have been taken out of my parts bin in 1977 or thereabouts. I wonder what shape the breadboard is in, time can't have been nice to those contacts, likely lots of them will have an oxide layer on it. And great that you guys got it to work again.

kens6• 4 years ago

Yes, some of the switches were oxidized or broken, but those are easy problems to fix.

Reverse-engineering the waveform generator in a 1969 breadboard

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