Teardown: 1950s Heathkit vacuum tube oscilloscope

Do you have experience with vacuum tubes? Help us analyze a Heathkit oscilloscope schematic and compare it to what AI told me.

I bought a vintage Heathkit oscilloscope at a swapfest just to study it. Now I need your help. The oscilloscope is a Heathkit model IO-10 (Figure 1). It’s from the late 1950s, built with vacuum tubes, it has no circuit boards or BNC connectors. Instead, it uses banana jacks and hand-soldered components. After all, it came as a kit.

Not having experience with vacuum tubes, I asked Google Gemini and Microsoft CoPilot to describe the circuit operation. Compare those to the circuit description that Heathkit provided.

Hand-assembled components

Lacking a PCB means all the oscilloscope’s components are “sky wired,” making it look like a proverbial “rat’s nest” shown in Figure 2. Resistors and capacitors connect directly to the vacuum tube sockets under the chassis.

Available online, the OI-10 manual contains a schematic and assembly instructions that provide detailed hand-drawn figures of each part. Assembly drawings show where and how to connect each component with step-by-step instructions. Anyone building this instrument must take great care to make sure everything is connected properly, especially considering the high voltages at play here. You don’t want any short circuits. The assembly work is not for the faint of heart.

Figure 3 shows the CRT with two 1200 V capacitors. Shown in Figure 4, the iron-core transformer with its windings makes up the bulk of the oscilloscope’s weight.

Circuit diagram

Figure 5 shows the circuit schematic, which diagrams the power supply, sweep generator, vertical amplifier, and horizontal amplifier circuits. Having never worked on vacuum-tube circuits, I uploaded the schematic to Google Gemini AI and Microsoft Copilot, asking each, using identical words, to analyze the circuit in 400 to 600 words. Compare the manual’s circuit description (see below the images) to those generated by AI. Is the AI assessment accurate? What would you change? What would you add to either description?

It’s alive, sort of

I dared to apply AC mains power and turn on the oscilloscope. Fortunately, it didn’t send off smoke nor catch fire. The tubes glowed except for V8, a 1V2 tube that’s part of the CRT’s 1100 V high-voltage power supply circuit. The tube could have failed. The video below shows the tubes lighting up.

Because some of the circuits came to life (Figure 6), I made some voltage measurements on the power circuits. Most voltages were higher than documented in the schematic but not unreasonably high. Figure 7 highlights the measurements. Figure 8 shows the resistors where I made the measurements.

Buying a replacement 1V2 tube could bring the CRT to life. I found some on eBay and I’m confident that I would get one at the swapfest, shown in Figure 9.

Google Gemini vacuum tube oscilloscope circuit description

This schematic for the Heathkit IO-10 oscilloscope is a classic example of a vacuum tube-based electronic test instrument. It’s composed of five main sections that work together to visually display electrical signals on a cathode ray tube (CRT). The sections are the Vertical Amplifier, Horizontal Amplifier, Sweep Generator, Power Supply, and the CRT itself.

The Vertical Amplifier
The Vertical Amplifier processes the signal being measured. The input signal, applied to the VERTICAL jacks, is first sent to the V1 and V2 tubes. These tubes are configured as a differential amplifier. A differential amplifier amplifies the difference between two input signals, which in this case helps reject common-mode noise, improving the signal-to-noise ratio. The signal is then sent to a high-gain amplifier stage. The vertical gain control adjusts the sensitivity of the amplifier, allowing for the display of signals of different magnitudes. This amplified signal is then passed to the vertical deflection plates of the CRT (V7). The voltage difference across these plates causes the electron beam to move up or down, tracing the vertical component of the waveform.

The Horizontal Amplifier and Sweep Generator
The Horizontal Amplifier is responsible for moving the electron beam horizontally across the screen. In most cases, the oscilloscope is used to display a signal over time. To achieve this, the Sweep Generator produces a sawtooth waveform. This waveform’s voltage increases linearly over time, then quickly drops back to its starting value. The linear increase moves the electron beam steadily from left to right across the screen, while the rapid drop-off returns the beam to the left side to begin the next sweep.

The sweep frequency control determines how quickly the beam sweeps across the screen, which corresponds to the time base of the oscilloscope. The sweep generator’s sawtooth output is fed into the Horizontal Amplifier, which is another differential amplifier stage using tubes V3 and V4. This amplifier increases the amplitude of the sawtooth wave and applies it to the horizontal deflection plates of the CRT. The amplified voltage drives the electron beam across the screen. The horizontal gain control adjusts the sweep’s width. For special applications, an external signal can be applied to the HORIZONTAL jacks, bypassing the sweep generator and allowing the user to display an X-Y plot, such as a Lissajous figure, where one signal is on the vertical axis and the other is on the horizontal.

The Power Supply
The Power Supply is a critical, but often overlooked, part of the circuit. It converts the incoming AC line voltage into the various DC voltages needed for the oscilloscope’s operation. The schematic shows a transformer-based power supply, which is standard for vacuum tube equipment. It has multiple secondary windings that provide power for different parts of the circuit. One winding supplies the high-voltage DC needed to accelerate the electron beam inside the CRT. This high voltage ensures the beam has enough energy to strike the phosphorescent screen, creating a bright and visible trace. Other windings supply lower DC voltages to the tube heaters (filaments), which are necessary to emit electrons, and to the plate and screen grids of the various amplifier and generator tubes. Diodes and capacitors are used to rectify and filter the AC voltage into a smooth DC voltage.

The Cathode Ray Tube (CRT)
At the heart of the oscilloscope is the CRT (V7). It’s a special vacuum tube with an electron gun at one end and a phosphorescent screen at the other. The electron gun generates a focused beam of electrons. This beam passes through two pairs of deflection plates: the vertical and horizontal plates. The voltages from the vertical and horizontal amplifiers are applied to these plates, creating electric fields that steer the electron beam. When the beam strikes the phosphorescent screen, it emits light, creating a bright spot. By rapidly moving this spot across the screen, the oscilloscope traces the waveform of the input signal. Controls for intensity and focus adjust the brightness and sharpness of the trace, while position controls adjust the beam’s starting point on the screen.

Microsoft copilot vacuum tube oscilloscope circuit description

The Heathkit OI-IO oscilloscope is a classic analog instrument designed to visualize electrical signals by displaying their voltage over time on a cathode ray tube (CRT). Its schematic reveals a well-structured circuit divided into five main functional blocks: Vertical Amplifier, Sweep Generator, Horizontal Amplifier, CRT Section, and Power Supply. Each plays a critical role in signal acquisition, processing, and display.

Vertical Amplifier
The vertical amplifier processes the input signal and controls the vertical deflection of the CRT beam. The signal enters through the vertical input and passes through multiple amplification stages using vacuum tubes (e.g., V2A–V2D). These stages increase the signal’s amplitude to a level suitable for driving the vertical deflection plates of the CRT. This vertical movement represents the voltage level of the input signal at each moment in time.

• Purpose: Amplify the input signal for vertical deflection.
• Key Components: Vacuum tubes, coupling capacitors, biasing resistors.

Sweep Generator
This section generates a time base signal—typically a sawtooth waveform—that moves the electron beam horizontally across the screen. It uses vacuum tubes (V4A–V4D) to produce a linear ramp voltage. The sweep rate can be adjusted to match the frequency of the input signal, allowing for stable waveform display.

• Purpose: Create a horizontal time base for signal visualization.
• Key Components: Multivibrator or relaxation oscillator circuits, timing capacitors, sweep rate control.

Horizontal Amplifier
The sawtooth waveform from the sweep generator is fed into the horizontal amplifier, which boosts the signal to a level sufficient to drive the horizontal deflection plates of the CRT. This ensures the beam moves steadily from left to right, forming the time axis of the waveform.

• Purpose: Amplify the sweep signal for horizontal deflection.
• Key Components: Vacuum tubes (V5A–V5D), coupling networks.

CRT Section
At the heart of the oscilloscope is the CRT (V7, type 3RP1), which converts electrical signals into visible traces. The CRT has vertical and horizontal deflection plates that receive signals from the respective amplifiers. An electron beam emitted from the cathode is accelerated toward the phosphorescent screen. The deflection plates steer the beam, tracing the waveform based on the input signal and time base.

• Purpose: Visually display the waveform.
• Key Components: CRT tube, deflection plates, high-voltage anode supply.

Power Supply
The power supply provides all necessary operating voltages for the oscilloscope. It includes a transformer, rectifier tubes (V8 IV2), and filter capacitors. The transformer steps down the AC mains voltage, which is then rectified and filtered to produce stable DC voltages for the amplifiers and CRT.

• Purpose: Deliver regulated DC voltages to all circuit sections.
• Key Components: Transformer, rectifiers, filter capacitors.

Summary
The Heathkit OI-IO oscilloscope operates by amplifying an input signal, generating a time base, and using a CRT to display the waveform. The vertical amplifier controls signal amplitude, the sweep generator and horizontal amplifier manage timing, and the CRT renders the signal visually. The power supply ensures all components receive the correct voltages. This design exemplifies mid-20th-century analog electronics and remains a valuable educational tool for understanding oscilloscope fundamentals.
Would you like a simplified block diagram or a walkthrough of how to troubleshoot one of these sections?

Heathkit circuit description from the manual

Amplifier Circuit
Since the vertical and horizontal amplifier circuits are identical, only the vertical circuit will be discussed.
A signal applied to the VERTICAL input terminals is coupled to the grid of V12A via the frequency compensated attenuator network. Capacitor C1 acts as a blocking capacitor in the AC positions of the VERTICAL switch.

Tube V12A is an input cathode follower; a zero DC reference is established in the V12A cathode circuit by adjusting the Vertical Balance control. The signal from V12A is coupled through Vertical Gain control R8 to V1A and V1B, the push-pull driver stage. (Common mode signals are rejected by the gain of V1A to V1B.) From V1A and V1B the signal is routed via phase inverter grids R15 and R16, to push-pull output stage V2A and V2B. Here the signal is amplified and applied to the vertical deflection plates of the CR tube, where it provides balanced deflection of the electron beam.

Simultaneously, the push-pull output of the horizontal amplifier is applied to the horizontal plates of the CR tube, creating a complete pattern on the CR tube face.

Vertical centering is accomplished by adjusting control R13 which changes the bias at the grid of V1A. The effect of this change is coupled through the push-pull driver and output stages to the CR tube.

Sweep Generator
The horizontal sweep waveform is created by V4A and V4B, the sweep multivibrator. The sweep timing capacitor that is switched into the cathode circuit of V4B at the beginning of the Fine Frequency control determines the horizontal sweep rate. The horizontal sweep waveform is applied to the grid of V3B, the horizontal amplifier. The output of V3B and the neon lamp, the horizontal sweep waveform, is used to provide the reference for the center of the sawtooth waveform.

V3A, the sync cathode follower, receives either an EXternal SYNChronizing signal from the binding post or an INTernal SYNChronizing signal from the vertical amplifier. The synchronizing signal is applied to the sweep multivibrator.
If the linear output of the sweep generator is not desired for a special application, a signal of any shape may be applied to the HORizontal INput terminals. This external signal will then be amplified in the horizontal channel and applied to the horizontal plates of the CR tube.