Reverse-engineering the waveform generator in a 1969 breadboard

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Reverse-engineering the waveform generator in a 1969 breadboard

How arduous may well presumably also or no longer or no longer it’s to fix a classic solderless breadboard that does no longer moderately work?
The “elite 2 circuit make test machine” underneath blended a solderless breadboard with some supporting circuitry: energy affords,
a waveform generator, a pulse generator, switches, and lights.
CuriousMarc chanced on one in every of those breadboards on eBay, however the characteristic generator didn’t work, so we arrangement out to repair it.

The E&L Instruments elite 2 solderless breadboard has a variety of supporting circuitry.

The E&L Instruments elite 2 solderless breadboard has a selection of supporting circuitry.

I figured that the waveform and pulse generators would be easy circuits, however they develop into implemented with
a board crammed fleshy of system, including over 40 transistors.
I reverse-engineered the circuitry and chanced on some titillating circuits inside, including
op-amps implemented from discrete transistors.
This complexity presumably explains the shockingly high sign of this breadboard:
$1300 in 1969 (identical to $10,000 in most up-to-date greenbacks).1

The circuit board for the function/pulse generator is crammed full of components. The upper part holds the waveform circuitry while the lower part holds the pulse generator.

The circuit board for the characteristic/pulse generator is crammed fleshy of system. The upper part holds the waveform circuitry whereas the lower part holds the pulse generator.

The waveform generator

The breadboard has a waveform generator that produces triangle, sq., and sine waves over a huge frequency fluctuate, up to
1 megahertz.
These waveforms are generated via a complex circuit that prices and discharges an integrator to plot the triangle
wave.
A comparator turns the triangle wave into a sq. wave.
Lastly, a sine-wave shaping network produces a sine wave from the triangle wave.

Triangle-wave generator

The oscillator’s frequency is chosen by resistors and capacitors.
The sooner the capacitor prices via the resistor, the increased the frequency. Thus, rising the capacitance and resistance
slows the oscillation.
The frequency fluctuate knob turns a classic wafer rotary switch to come to a decision one in every of seven totally different resistors and capacitors, allowing frequencies over a nice selection from
1 Hertz to 1 megahertz.
A potentiometer adjusts the frequency inside the fluctuate to provide the steady desired frequency.

The triangle wave is generated from an op-amp integrator circuit, using the arrangement in which underneath.
This circuit makes use of the capacitor to mix the enter voltage, producing the output voltage.
The consequence’s that the sq. wave enter will improve or decreases the output linearly, yielding a triangle wave.
(The op-amp keeps that correct aspect of R1 at ground, so the most up-to-date that prices the capacitor is proportional to the enter voltage.
In distinction, in a easy R-C charging circuit, the capacitor prices exponentially; the charging most up-to-date drops as the capacitor’s
voltage will improve.)

A simplified op-amp integrator, based on image by Rutujadeshpande, CC BY-SA 3.0.

To fetch an oscillator, the loads of portion is a comparator to provide the sq. wave enter to the integrator.
The comparator reverses course on the tip and backside of the triangle wave, as
shown underneath.
When the enter to the integrator is determined, the integrator output climbs linearly.
This output is fed into the comparator, along with a restrict stage (purple dots).
When the output exceeds the better restrict at “A”, the comparator output becomes adverse.
For the rationale that comparator output is extinct as the integrator enter, the integrator now discharges and the signal drops.
When the signal drops underneath the restrict at B, the comparator switches on and the course of repeats.
A hysteresis circuit adjustments the comparator stage (dotted purple line) at A and B, environment the better and lower limits of the triangle wave.2

The triangle-wave generation process.

The triangle-wave generation course of.

The comparator is a 710HC, a easy differential comparator integrated circuit launched by Fairchild spherical 1965.
Right here’s one in every of honest two integrated circuits on the board.3
The IC is packaged in an 8-pin spherical metallic can.
This package deal used to be total on the time for analog integrated circuits, as the metallic can supplied shielding.

The 710HC comparator is packaged in a round metal case.

The 710HC comparator is packaged in a spherical metallic case.

Op-amp

The op-amp is a key ingredient of the integrator.
Even though integrated-circuit op-amps date reduction to 1963,
this board builds op-amps out of discrete system.
The integrator op-amp consists of seven transistors, along with a bunch of resistors and capacitors, as shown underneath.
The center of the op-amp is the differential pair (Q30 and Q32), a ragged analog circuit.
A mounted most up-to-date is fed into the differential pair transistors.
If one transistor has a rather of increased enter than the loads of, that transistor turns on and most of the most up-to-date will battle via that transistor.
Thus, the differential pair amplifies the adaptation between the inputs, the principle characteristic of an op-amp.
Further amplification is supplied by Q33, whereas Q34 and Q35 buffer the dual outputs.

Implementation of an op-amp in the waveform generator.

Implementation of an op-amp within the waveform generator.

The output amplifier for the waveform circuit makes use of 1 other discrete op-amp. This one has two phases of differential pairs for
further amplification, adopted by energy transistors to plot a high-most up-to-date output.
But another op-amp circuit is extinct within the sine-wave shaper, mentioned underneath.

Sine-wave shaper

The board makes use of a phenomenal methodology to generate sine waves: it synthesizes a sine wave from the triangle wave.
Namely, a resistor-diode network shapes the sine wave using piecewise-linear segments.
The speculation is to make use of diodes as switches that suggested as the signal stage crosses varied functions.
This adds resistance into the circuit, altering the slope.
The consequence’s a sine wave with less than 1% distortion.

This diagram explains the sine-wave shaping network. From HP Journal, Nov 1965.

This arrangement explains the sine-wave shaping network. From HP Journal, Nov 1965.

The sine-wave shaper appears inspired by the an identical circuit within the HP 3300A Characteristic Generator, launched in 1965.
The schematic underneath exhibits the HP 3300A’s sine-wave shaper; the breadboard’s network is an identical.
The resistances are reasonably chosen to attain the sine wave.
Identical resistor-diode networks were also extinct in analog laptop programs to put into effect arbitrary functions, most frequently with user-adjustable
resistances to commerce the characteristic.

The sine-shaping circuit from the HP 3300A is very similar to the circuit in the breadboard. The resistor-diode <a class=network is highlighted. The surrounding circuitry biases the network and amplifies the output. From the Service Manual Fig 6-2.” height=”419″ src=”https://static.righto.com/images/breadboard/diode-network-w700.png” title=”The sine-shaping circuit from the HP 3300A is very similar to the circuit in the breadboard. The resistor-diode network is highlighted. The surrounding circuitry biases the network and amplifies the output. From the Service Manual Fig 6-2.” width=”700″>

The sine-shaping circuit from the HP 3300A is extremely identical to the circuit within the breadboard. The resistor-diode network is highlighted. The surrounding circuitry biases the network and amplifies the output. From the Provider Handbook Fig 6-2.

Pulse generator

The heartbeat generator produces pulses from 100 ns to 100 ms extensive.
These pulses may well presumably also additionally be precipitated by the waveform generator, an external trigger enter, or a “one-shot” pushbutton.
The heartbeat width is managed by a switch-selectable resistor-capacitor network.

The most abnormal part of the pulse generator is how the output circuit adjusts the pulse amplitude.
As an alternate of simply adjusting the amplification, the circuit adjustments the voltage that powers the output amplifier.
This variable voltage is produced by an LM305 voltage regulator IC, adjusted by the amplitude knob on the breadboard.
A four-transistor circuit produces the matching adverse voltage.4
These voltages energy a moderately complex output stage with two circuits.
One circuit produces determined pulses, whereas the loads of produces adverse pulses.

The LM305A integrated circuit is in an old-fashioned metal can.

The LM305A integrated circuit is in an frail-long-established metallic can.

Conclusion

A prototyping breadboard may well presumably also seem treasure a easy product, however all the pieces becomes extra refined when constructed with 1969 technology.
This breadboard comprises a precision waveform generator and energy affords, designed for high accuracy, so it used to be nearly treasure
having test equipment integrated.
Nonetheless these aspects came at a steep sign, identical to $10,000 on the recent time.

After I reverse-engineered the board,5 CuriousMarc extinct the schematic to fix the issues.
The breadboard grew to change into out to be in execrable shape with a broken wire, a bunch of execrable transistors, and a failed bridge rectifier
within the facility supply.6
It be unclear why the board had so many problems, bigger than you’d request from age by myself.
Perhaps the facility supply over-voltaged the system at some level?
My fleshy schematic for the board is right here.7

Some of the bad transistors that needed to be replaced.

A pair of of the execrable transistors that wanted to derive replaced.

Prepare me on Twitter @kenshirriff for extra posts. I also beget an RSS feed.

Notes and references

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Vanic
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Vanic

β€œSimplicity, patience, compassion.
These three are your greatest treasures.
Simple in actions and thoughts, you return to the source of being.
Patient with both friends and enemies,
you accord with the way things are.
Compassionate toward yourself,
you reconcile all beings in the world.”
― Lao Tzu, Tao Te Ching