Heathkit EC-1 — Volume 2 — Hardware & theory of operation

Power supply architecture, 6U8-based op-amp topology, nine amplifier modules, metering and control circuitry, and stage-by-stage schematic walk — grounding all later volumes in measured hardware reality

About this Volume

This volume dissects the Heathkit EC-1 Educational Analog Computer at the circuit level, providing the depth of technical understanding required for confident restoration, calibration, and fault diagnosis. It covers every major subsystem from the mains transformer through the nine vacuum-tube operational amplifiers to the front-panel meter and control logic. The reader should be comfortable with vacuum-tube circuit theory, DC power supply design, and the feedback-amplifier analysis found in texts such as Korn and Korn, Electronic Analog Computers (McGraw-Hill, 1956).

Cross-references to other volumes in this series are as follows:

Vol 1 — introduction, history, and acquisition guidance
Vol 3 — patch panel, computing elements, and sign-convention
Vol 4 — acquisition, restoration, alignment, and fault-finding procedures
Vol 5 — programming methodology and problem scaling
Vol 6 — sample programs and illustrative problems

All component designators cited in this volume follow the Heathkit Assembly Manual schematic numbering. Where the manual does not assign a designator, the part is described by location and function. Voltages are DC unless annotated “AC”.

Danger — The EC-1 operates with internal rails of approximately +300 V and −150 V with respect to chassis ground. These potentials are lethal. No work should be performed inside the chassis with power applied. The filter capacitors retain charge after power-down; wait at least 60 seconds and verify with an insulated meter probe before touching internal components. See Vol 4 for a safe power-down and discharge procedure.

System Block Diagram

The EC-1 can be understood as five functional blocks connected by a common signal ground and distributed power buses.

┌─────────────────────────────────────────────────────────────────────────────┐
│                          AC MAINS  105–125 V / 100 W                        │
└────────────────────────────┬────────────────────────────────────────────────┘
                             │
                    ┌────────▼────────┐
                    │  POWER SUPPLY   │
                    │                 │
                    │ +300 V @ 25 mA  │◄─── electronically regulated
                    │ −150 V @ 40 mA  │◄─── OA2 VR tube regulated (150 V)
                    │ +100 V (×3 IC)  │◄─── OB2 VR tube regulated (108 V), front-panel pots
                    │  6.3 VAC heater │◄─── transformer secondary, unregulated
                    └────────┬────────┘
                             │
           ┌─────────────────┼─────────────────────────────────────┐
           │   FIVE CHASSIS BUS BARS (run full chassis length)      │
           │  +300 V  │  −150 V  │  +IC  │  −IC  │  6.3 VAC heater │
           └──────────┼──────────┼───────┼───────┼─────────────────┘
                      │          │       │       │
          ┌───────────▼──────────▼───────▼───────▼──────────────────┐
          │         NINE IDENTICAL DC OP-AMP MODULES                 │
          │  Amp 1 ─ Amp 2 ─ Amp 3 ─ Amp 4 ─ Amp 5 ─ ...─ Amp 9   │
          │  Each: 6U8 triode/pentode, balance pot, socket strip      │
          └──────────────────────┬───────────────────────────────────┘
                                 │ ±60 V signal outputs
          ┌──────────────────────▼───────────────────────────────────┐
          │               FRONT-PANEL PATCH SYSTEM                   │
          │  27 × two-pin sockets (R/C plug-ins)                     │
          │  9 × amplifier output binding posts (RED)                │
          │  9 × amplifier summing-junction posts (BLACK)            │
          │  5 × coefficient potentiometers                          │
          │  3 × initial-condition (IC) power supply binding posts   │
          │  4 × relay contact binding posts                         │
          └──────────────────────┬───────────────────────────────────┘
                                 │
          ┌──────────────────────▼───────────────────────────────────┐
          │          METERING & CONTROL CIRCUITRY                    │
          │  50-0-50 µA meter movement                               │
          │  METER FUNCTION switch (Amp select / IC monitors)        │
          │  METER RANGE switch (×1 / ×10)                           │
          │  OPERATION switch (RESET / OPERATE / REPETITIVE)         │
          │  12BH7 multivibrator repetitive oscillator (0.1–15 cps)  │
          │  6AQ5 relay driver + 4× relay contacts                   │
          │  Oscilloscope output jacks                               │
          └──────────────────────────────────────────────────────────┘

The signal ground is the aluminum front panel and chassis, tied to one side of the transformer secondary. No floating grounds exist in the EC-1 design. The ±60 V computing signal range occupies the low-impedance output of each op-amp and is referenced entirely to this single chassis ground, which eliminates common-mode interference between amplifiers.

Power Supply — +300 V and −150 V Rails, Heater String, Turn-On Sequencing

3.1 Transformer and Rectifiers

The mains transformer provides several secondaries:

Secondary	Nominal Voltage	Load	Purpose
HV winding 1	~350 V AC	Rectifier B+ string	+300 V regulated supply
HV winding 2	~200 V AC	Rectifier bias string	−150 V regulated supply
IC winding	~120 V AC (×3)	Three separate rectifiers	Initial-condition ±100 V supplies
Heater	6.3 V AC	All 9× 6U8 heater pairs + other tubes	Filament heating

Danger — Both HV secondaries carry voltages capable of delivering a fatal shock. The transformer secondary voltages rise to full value within one AC half-cycle of power application. Capacitors on these rails charge to peak voltage (~495 V peak on the B+ winding) before regulation clamps the rail. The chassis provides no inherent interlock; power must be physically disconnected before opening the cabinet.

Early production EC-1 units used selenium rectifier stacks (potted multi-plate assemblies) for the HV rectification. Selenium rectifiers have a characteristic failure mode in which the stack develops high forward resistance and emits a pungent burned odor; they present a possible fire hazard. Any unit still using selenium devices should have them replaced with 1N4007 silicon diodes (400 V PIV, 1 A) before restoration. The physical selenium package may be retained for cosmetic purposes if desired.

Later production units used silicon diodes from the factory.

3.2 +300 V Electronically Regulated Supply

The +300 V supply is the primary computing rail. The Heathkit circuit uses a conventional electronic voltage regulator — a series-pass vacuum-tube stage controlled by an error amplifier (6BH6) that compares a divided sample of the output against a fixed reference. The OA2 and OB2 VR tubes serve their respective separate supply rails (−150 V and IC supplies); they are not part of the +300 V regulation path.

HV Secondary
     │
  [Full-wave
   rectifier]──[C_filter]──┬──[R_series]──[V_pass tube plate]──► +300 V out
                           │                     │
                           │              [V_pass tube grid]
                           │                     │
                           └──[6BH6 error amp]───┘
                                  │
                                 GND

The 6BH6 driver stage provides the error-amplifier function for the electronic regulator, comparing a divided sample of the output against a reference to control the series-pass tube. The regulated output is adjustable and the circuit provides clean, low-ripple regulation.

The pass tube grid is controlled by the error amplifier; any tendency for the output to rise causes the pass tube to conduct less, reducing the output — classic series-pass regulation.

The regulated output is adjustable from approximately +250 V to +350 V via a front-panel pot labeled HIGH VOLTAGE. The design center for analog computation is +300 V, at which point the amplifier gain and the ±60 V output swing are correctly scaled. Deviating from +300 V changes the effective gain of each op-amp and shifts the usable output range.

Supply specifications:

Parameter	Value
Nominal output	+300 V DC
Adjustment range	+250 to +350 V
Maximum continuous load	25 mA
Ripple at full load (typical)	< 5 mV rms
Regulation tube	Series-pass triode (6BH6 as driver stage)
Reference elements	6BH6 error amplifier + series-pass regulator
Filter capacitor(s)	Electrolytic, original spec 40 µF / 450 V

3.3 −150 V OA2 Regulated Supply

The negative supply provides the cathode bias and one plate supply rail for the op-amp stages. It is regulated by a single OA2 VR tube operating in its glow-discharge region; the OA2 regulates at 150 V. The −150 V rail draws up to 40 mA at full complement of nine amplifiers operating.

The distinctive purple-blue corona glow visible through the ventilation slots of a running EC-1 comes from these VR tubes. The hue is a spectroscopic signature of neon (OA2) or argon (OB2) fill gas excited by the regulation discharge current — typically 5–15 mA per tube.

3.4 Initial-Condition (IC) Supplies

Three separate IC supplies each derive from an isolated transformer secondary; each is regulated by an OB2 VR tube (108 V regulation). Each IC supply produces approximately ±100 V. The front panel provides binding posts for each IC supply at the +IC and −IC jacks adjacent to each supply’s panel pot. The operator sets the desired initial condition voltage (0 to ±60 V at the patch point) using the panel potentiometer; the op-amp input network referenced to the IC rail charges the integrating capacitor to this voltage during the RESET phase.

The IC supplies are not regulated because absolute accuracy is unnecessary — only repeatability matters, and the main op-amp feedback loop tracks out small IC supply variations during OPERATE mode.

3.5 Heater Supply and Warm-Up Sequencing

The 6.3 V AC heater winding energizes all tube heaters simultaneously with AC power application. The EC-1 has no warm-up delay interlock; the HV rail rises immediately while the cathodes are still cold. This is not ideal tube practice but was acceptable for the EC-1’s intended service life and cost target. The operator is instructed to allow a few minutes of warm-up before running problems.

During a restoration, it is best practice to verify that the heater winding delivers 6.0–6.6 V AC under load (heaters are series-parallel wired). An out-of-tolerance heater supply indicates a shorted heater or marginal transformer.

Heater current draw estimate (all tubes operating):

Tube	Qty	Heater V	Heater mA each	Total mA
6U8	9	6.3 V	450 mA	4,050 mA
12BH7	1	6.3 V	600 mA	600 mA
6AQ5	1	6.3 V	450 mA	450 mA
6BH6	1	6.3 V	300 mA	300 mA
OA2	1	6.3 V	300 mA	300 mA
OB2	1	6.3 V	300 mA	300 mA
Total	14	—	—	~6.0 A

The 6.3 V winding must be rated for at least 7 A; the Heathkit transformer is wound to this margin.

The Vacuum-Tube DC Operational Amplifier — Topology, Gain, Drift, Balance

4.1 Topology Overview

The EC-1 operational amplifier is a two-stage, direct-coupled, inverting DC amplifier using a single 6U8 envelope. The 6U8 is a compactron-era miniature tube containing one triode section and one pentode section in the same glass envelope. Heathkit exploits both sections in a single gain chain.

                      +300 V
                         │
                    [Pentode Plate Load]
                         │
INPUT ──[Ri]──►[SJ]──►[Pentode Grid]──►[Pentode]──►[Triode Grid]──►[Triode]──► OUTPUT
               │                                                         │
               └─────────────────[Rf or Cf feedback]────────────────────┘
                                                                         │
                                                                       −150 V
                                                                    (cathode bias)

First stage (pentode): The pentode section provides the bulk of the open-loop voltage gain — typically 150–300× at the operating point. The pentode grid is the summing junction (SJ), the virtual-ground node of the amplifier. High pentode transconductance (gm ≈ 5 mA/V for the 6U8A) gives low output impedance from this stage.
Second stage (triode): The triode section acts as a common-cathode voltage amplifier and phase inverter, adding an additional gain stage and providing sufficient output current drive to swing ±60 V into the patch-panel impedances. The triode plate serves as the amplifier output.
Overall open-loop gain (A): Approximately 1,000 at DC, falling at higher frequencies. The −1 dB bandwidth is 600 Hz, limiting the machine to low-frequency simulation work.

The summing junction is held at virtual ground by the deep negative feedback. For any practical computation, the summing junction sits within a few millivolts of chassis ground potential. This is a critical property: it allows multiple input resistors to be paralleled at the junction without interaction between sources.

4.2 Gain and Error Analysis

With open-loop gain A ≈ 1,000, the closed-loop gain error relative to the ideal (infinite-A) case is:

Closed-loop gain (actual) = (Rf/Ri) × [A/(A+1+Rf/Ri)]

For typical values Rf = 1 MΩ and Ri = 1 MΩ (unity inversion), with A = 1,000:

Actual gain ≈ 0.999 × ideal gain  →  0.1% gain error

For Rf/Ri = 10 (scaling by 10), the error rises to approximately 1%. This is consistent with the ±1% resistor tolerance of problem-element resistors and is the dominant accuracy limit of the machine rather than the amplifier itself.

Note — The EC-1 Operational Manual (p. 7) confirms that ratios Rf/Ri greater than 100 introduce unacceptable inaccuracies because the denominator term [A/(A + 1 + Rf/Ri)] departs significantly from unity. Problem setups should be scaled to keep all resistor ratios at 10:1 or less.

4.3 DC Drift and Balance

DC operational amplifiers are plagued by input offset voltage — any asymmetry in the input stage appears as an error voltage at the summing junction, indistinguishable from a real input. Sources of offset in the 6U8 amplifier include:

Source	Typical Magnitude	Notes
Pentode grid-cathode contact potential	0.5–2 V equivalent	Varies with tube age and cathode chemistry
Power supply noise coupling	Depends on bypassing	More significant on +300 V than −150 V
Heater–cathode leakage	Increases with age	Primary failure mode in old 6U8s
Resistor mismatch in plate loads	< 0.1 V typical	Stable once resistors are in-tolerance
Thermal EMF at solder joints	< 1 mV	Usually negligible

The amplifier balance pot (3 kΩ, one per amplifier, mounted along the bottom of the front panel) corrects for input offset. It applies a small differential voltage to the pentode control grid through a resistive bridge. Proper balance procedure:

Short the summing junction to ground (no input connections).
Switch METER FUNCTION to the amplifier’s output.
Adjust the balance pot until the meter reads exactly zero.
Restore connections and run the problem.

The balance adjustment must be repeated after tube substitution, after extended warm-up, or whenever the ambient temperature changes significantly. Carbon-composition balance pots are susceptible to wiper contamination and should be replaced with wire-wound or cermet types during restoration.

4.4 Frequency Response

The EC-1 specifies −1 dB at 600 Hz. The roll-off arises primarily from:

Stray capacitance at the summing junction — the physical node at the patch panel accumulates capacitance from nine parallel amplifier input sockets, banana binding posts, and patch-cord distributed capacitance. Total stray at the SJ is estimated at 50–200 pF.
Miller effect on the pentode plate:** The plate-grid capacitance (Cgp ≈ 0.01 pF for the 6U8 pentode, very low by design) contributes less than the external strays.
Output stage bandwidth — the triode section introduces an additional pole at several kHz.

For practical purposes, the EC-1 is suitable for simulations requiring time constants from approximately 1 ms (1 µF × 1 kΩ) to thousands of seconds. The 600 Hz bandwidth is not a limitation for mechanical, thermal, or biological differential equation problems, which constitute nearly all educational use cases.

The Nine Amplifiers — Identical Modules, How Configured

5.1 Physical Arrangement

The nine amplifier modules are arranged in a single row across the top of the front panel. Each module occupies an identical vertical slice of the front panel and chassis, giving the EC-1 its distinctive appearance.

Chassis interior with all nine amplifier modules visible, tubes in sockets, wiring harness to front panel binding posts.

Each module consists of:

One 9-pin miniature tube socket (for the 6U8) mounted on the chassis floor
Two binding post pairs on the front panel (one red OUTPUT, one black AMPLIFIER INPUT)
One or two two-pin patch sockets immediately adjacent to the binding posts for problem-element plug-ins
One balance potentiometer along the bottom row of the front panel
Wiring harness connecting socket pins to binding posts and bus bars

5.2 Electrical Configuration per Module

Every module is wired identically. The front panel connections seen by the operator are:

Front Panel Element	Internal Node	Function
AMPLIFIER INPUT (black)	Summing junction (pentode grid via Ri_internal)	Signal input(s) arrive via problem-element resistors patched here
AMPLIFIER OUTPUT (red)	Triode plate	±60 V output; drives problem-element feedback Rf/Cf and next stage inputs
Two-pin socket (left)	Paralleled across INPUT and OUTPUT	Accepts resistor or capacitor plug for feedback element Rf or Cf
Two-pin socket (right)	Same paralleled topology	Second plug position for dual-element feedback configurations
Balance pot	Pentode grid bias bridge	Nulls DC offset

The same physical socket pair that accepts a 1 MΩ feedback resistor (for inverter/summer operation) also accepts a 1 µF capacitor (for integrator operation). This is the fundamental versatility of the analog computer: the function of each amplifier is determined entirely by the passive elements plugged in — not by any internal switching.

5.3 Amplifier Function Configuration Table

Plug-in elements	Feedback element	Result
Ri = 1 MΩ (input socket)	Rf = 1 MΩ (feedback socket)	Inverter, gain = −1
Ri = 1 MΩ (input socket)	Rf = 10 MΩ (feedback socket)	Inverting amplifier, gain = −10
Ri₁, Ri₂, Ri₃ (multiple inputs)	Rf = 1 MΩ	Weighted summer: eo = −(Rf/Ri₁·e₁ + Rf/Ri₂·e₂ + …)
Ri = 1 MΩ (input socket)	Cf = 1 µF (feedback socket)	Integrator: eo = −(1/RC)∫ei dt = −(1 s⁻¹)∫ei dt
Ri = 100 kΩ (input socket)	Cf = 1 µF	Integrator with τ = 0.1 s
Ci (input socket)	Rf = 1 MΩ	Differentiator (noise-amplifying; avoid in practice)
Coefficient pot + Ri	Rf	Scaler with variable coefficient 0–1

Note — Heathkit specifies 1 MΩ as the standard resistor value for problem elements and 1 µF for standard capacitors. These values set the integration time constant RC = 1 second, convenient for human observation. Non-standard values are used for time scaling and for coefficient adjustment. Vol 3 covers problem scaling in detail.

5.4 Amplifier Module Tube Data

The 6U8 (also marketed as ECF82 in European nomenclature) is a medium-mu triode / sharp-cutoff pentode in a 9-pin miniature (Noval) envelope.

Parameter	Triode Section	Pentode Section
Heater	6.3 V, 450 mA	(shared envelope)
Max plate voltage	300 V	300 V
Typical plate voltage (EC-1)	~150 V	~250 V
Max grid voltage	—	−55 V (G1)
Transconductance (gm)	3.1 mA/V	5.0 mA/V
Amplification factor (µ)	19	—
Plate resistance (rp)	6.1 kΩ	300 kΩ (pentode)
Max dissipation	1.8 W (plate)	3.3 W (plate)
Socket	9-pin miniature	(same envelope)

The 6U8 was widely manufactured by RCA, Sylvania, GE, Mullard, and Philips. All brands are electrically interchangeable in the EC-1. NOS (new old stock) and used-tested specimens are readily available from Antique Electronic Supply, VIVA Tubes, and similar vendors (see Vol 4 for sourcing guidance).

Metering and Mode/Control Circuitry

6.1 Meter Movement

The EC-1 uses a 50-0-50 µA D’Arsonval meter movement with a center-zero scale. The meter is protected by series resistors that scale the deflection to the EC-1’s ±60 V output range.

With the METER RANGE switch at ×1:

Full-scale deflection (50 µA) corresponds to ±50 V at the selected amplifier output.
Sensitivity: approximately 1 µA / V → 1 division per volt on a 50-division scale.

With METER RANGE at ×10:

Scale is multiplied; full deflection corresponds to ±5 V, providing 10× sensitivity for balance adjustment and low-level signal monitoring.

6.2 METER FUNCTION Switch

A multi-position rotary switch routes the desired signal to the meter. Positions include:

Switch Position	Signal Monitored
AMP 1 – AMP 9	Output of the selected operational amplifier
IC-1, IC-2, IC-3	Voltage at each initial-condition power supply output (set by panel pot)
OFF	Meter disconnected

The AMPLIFIER OUTPUT terminals used for metering are the same nodes connected to the amplifier output binding posts — reading the meter does not load the amplifier separately.

6.3 OPERATION Mode Switch

The three-position OPERATION switch governs the computation cycle:

Position	Action	Internal state
RESET	All relay contacts closed; integrating capacitors discharged to IC voltages	Relay driver energized; amplifiers are hold mode
OPERATE	Relay contacts open; computation begins	Relay driver de-energized; normal amplifier operation
REPETITIVE	Multivibrator drives relay at set rate	Relay cycles between RESET and OPERATE automatically

Note — During RESET, the relay contacts connect each integrator’s feedback capacitor directly to the relevant IC supply voltage through a 100 kΩ series resistor. The capacitor charges to the IC voltage within a few time constants (typically < 100 ms). Switching to OPERATE releases the capacitor; integration proceeds from the preset initial condition.

6.4 Relay and Relay Driver

The EC-1 uses a single relay with four sets of contacts (4PST). The relay coil is energized by the 6AQ5 pentode, which receives its gate signal from the OPERATION switch.

The four contact sets are:

Shorting contacts across each integrating feedback capacitor (for RESET)
Contacts that connect the IC supply voltages to the integrator input networks (for RESET)
Output-disconnect contacts isolating the oscilloscope output during RESET transient
A spare contact set available to the operator via front-panel RELAY CONTACTS binding posts (usable as a logic input for nonlinear simulations such as the bouncing-ball problem)

The 6AQ5 is rated at 4.5 W plate dissipation and can comfortably drive the relay coil current (typically 50–100 mA at 12–24 V DC for the Heathkit relay used).

6.5 Repetitive Oscillator (12BH7 Multivibrator)

The repetitive-operation oscillator is an astable multivibrator built around the 12BH7 dual triode. The two triode sections cross-couple through RC networks to produce a square-wave output that alternately triggers the relay driver between RESET and OPERATE states.

Oscillator timing:

Control	Effect
REPETITION RATE pot (front panel)	Adjusts RC time constant; range 0.1 to 15 Hz (0.1 to 15 cps per original specification)
REPETITION RATE at minimum	~67 ms per half-cycle (15 Hz) — useful for oscilloscope display
REPETITION RATE at maximum	~10 s per half-cycle (0.1 Hz) — slow visual observation

At repetitive rates of 5–15 Hz, the output waveform repeats fast enough to produce a stable trace on a DC-coupled oscilloscope in X-Y mode, allowing continuous real-time observation of the solution.

The 12BH7 is a robust tube with good service life; failures are uncommon. When the multivibrator does fail, the symptom is loss of repetitive operation with normal single-OPERATE function intact. Replacement is straightforward (see Vol 4).

6.6 Oscilloscope Output Jacks

Two binding posts labeled SCOPE OUTPUT X and SCOPE OUTPUT Y (or equivalent labeling depending on production date) provide buffered access to the amplifier outputs for external oscilloscope connection. These jacks are protected by series resistors (typically 1 kΩ) to prevent oscilloscope input damage if the operator connects a scope probe to a high-impedance source.

The EC-1 Operational Manual specifically recommends the Heathkit IO-10 (3-inch CRT scope), OR-1, or OP-1 oscilloscopes as period-correct accessories. Any DC-coupled oscilloscope set to the 20 V/div or 50 V/div range is electrically compatible.

Danger — The EC-1 amplifier outputs reach ±60 V under normal operating conditions. Connecting an oscilloscope with input sensitivity settings below 5 V/div risks overdriving the oscilloscope input stage and may cause damage. Always begin with 20 V/div or greater and reduce sensitivity only after confirming signal amplitude.

Schematic Walk by Stage with Component Callouts

7.1 Reading the Schematic

The full schematic (Heathkit EC1 Schematic.pdf, one sheet) is dense but logically partitioned. From left to right it presents: AC input, transformer, rectifiers, regulator pass tube, VR tube stack, IC supplies, the nine amplifier sub-circuits (drawn identically for Amps 1–9 as a collapsed array), then the meter, relay, and multivibrator circuits.

Schematic sheet thumbnail showing full EC-1 circuit — power supply at left, nine amplifier columns center, meter/relay/oscillator at right.

7.2 AC Input Stage

Fuse F1 (3 A, slow-blow) protects the primary. Capacitor C_line (0.01–0.05 µF, rated 600 V) across the mains provides RF bypass and reduces radio-frequency emissions. The power switch (front panel) interrupts the primary; both legs are switched together.

The transformer primary is tapped for 105–125 V, with the tap chosen at assembly to match the local supply. Most US units are set to the 117 V tap.

7.3 High-Voltage Rectifier and Filter Stage

The HV secondary feeds a full-wave rectifier (two rectifier diodes, original selenium or replacement silicon). The rectified DC appears across the main filter capacitor bank. Original specification is 40 µF at 450 V working.

Danger — The filter capacitors on the +300 V rail charge to peak rectified voltage (~490 V peak from a 350 V AC secondary) before the regulator clamps the rail. Even with mains disconnected, these capacitors retain charge sufficient for a lethal shock. Electrolytic capacitors that have not been reformed for years should be treated as potentially explosive as well as electrically hazardous. Follow the slow-reform procedure in Vol 4 before applying full mains voltage to an unrestored unit.

After the main filter, the voltage passes through the series-pass regulator tube (triode connection, driven by a 6BH6 driver stage). The regulated output is metered by a string of dropping resistors to the front-panel HIGH VOLTAGE control pot, which sets the regulation point. The feedback error amplifier (6BH6) compares the pot wiper voltage with a reference and adjusts the pass tube grid bias accordingly.

7.4 Negative Supply (−150 V) Regulator Stage

The negative supply rectifier feeds a simpler regulation circuit. A single OA2 VR tube is connected between the raw negative rail and ground; it strikes at its characteristic breakdown voltage and regulates at 150 V, clamping the negative rail to −150 V through a dropping resistor network.

The filter capacitor on the −150 V rail is typically 50–100 µF at 200 V (note: this is a negative rail, so the capacitor’s positive terminal is at ground, negative terminal is at −150 V).

7.5 One Amplifier Module (Representative — all nine identical)

The 6U8 tube socket wiring in one amplifier module follows this signal path:

Pin 9 (Pentode G1 = control grid) ── summing junction ── input socket(s), balance bridge
Pin 5 (Pentode plate) ─────────────── plate load R (~100 kΩ) ── to +300 V
Pin 7 (Pentode G2 = screen) ──────── screen dropping resistor ── to +300 V (~150 V at G2)
Pin 6 (Pentode G3/suppressor) ────── internal to cathode
Pin 8 (Pentode cathode) ─────────── cathode resistor ── to −150 V

Pin 2 (Triode grid) ─────────────── pentode plate (direct-coupled)
Pin 1 (Triode plate) ─────────────── plate load R ── to +300 V → AMPLIFIER OUTPUT binding post
Pin 3 (Triode cathode) ───────────── cathode resistor ── to −150 V

Heater: Pins 4 & 5 (shared with pentode envelope at Pin 5) ── 6.3 V AC bus bars

The balance pot (3 kΩ) sits between a positive reference tap and ground, with the wiper feeding a small DC correction voltage to the pentode control grid through a high-value series resistor (~1 MΩ). The correction range is ±several volts at the wiper, attenuated to ±tens of millivolts at the grid after the 1:100 resistor divider — sufficient to null the typical tube offset.

Plate load resistors are 1% metal-film types (original Heathkit used carbon-film with a 5% tolerance; upgrading to 1% metal film significantly improves channel-to-channel tracking). Typical values are 100–220 kΩ for the pentode plate and 47–100 kΩ for the triode plate.

Screen decoupling is provided by an individual bypass capacitor on each module’s screen supply rail. Loss of this capacitor causes instability and high-frequency oscillation in the affected module.

7.6 Front Panel and Patch Board Wiring

From the tube socket, wire runs to a strip of 3-lug terminal strips along the chassis floor below the front panel. From there, individual wires connect to:

The red OUTPUT binding post (triode plate)
The black INPUT binding post (summing junction / pentode G1)
The two-pin plug sockets immediately adjacent to each amplifier’s binding posts

The two-pin plug sockets are wired in parallel between the INPUT and OUTPUT posts. This means a resistor plugged into either socket appears directly between summing junction and amplifier output — i.e., as the feedback element Rf. Additional resistors at the INPUT post proper (banana jacks) appear as input resistors Ri. This topology allows the operator to combine input elements (from binding posts) with feedback elements (from plug sockets) in any combination.

7.7 Meter, Relay, and Multivibrator Stage

The METER FUNCTION wafer switch (12-position) feeds the selected amplifier output (or IC voltage) through a precision attenuator to the 50-0-50 µA movement. Two resistor values corresponding to the ×1 and ×10 RANGE positions complete the attenuator.

The 6AQ5 relay driver has its grid tied to the OPERATION switch. When the switch is at OPERATE, the 6AQ5 grid is biased to cut-off; the relay is de-energized and contacts are in the compute position. At RESET, the 6AQ5 conducts through the relay coil; the coil current pulls the relay armature and connects all RESET paths.

The 12BH7 multivibrator consists of the two triode sections cross-coupled:

Triode A plate load resistor (~100 kΩ) coupling through a timing capacitor (~0.1–1 µF, sets the cycle period) to Triode B grid.
Triode B plate load coupling through its timing capacitor to Triode A grid.
The REPETITION RATE front-panel pot adjusts the timing capacitor’s charging resistor in series, varying frequency from 0.1 to 15 Hz.

When the OPERATION switch is at REPETITIVE, the multivibrator output is connected to the 6AQ5 relay driver grid, overriding the direct OPERATE/RESET control.

7.8 Initial Condition Supply Detail

Each of the three IC supplies uses an isolated transformer secondary feeding a full-wave rectifier and a simple RC filter. The rectified voltage is approximately ±100 V (relative to IC supply ground, which is also chassis ground). The front-panel IC pot (linear taper, 10 kΩ) forms a voltage divider across the ±100 V span; the wiper provides the desired IC voltage at the binding post. A series resistor (100 kΩ) prevents excessive current if the IC binding post is accidentally shorted to another potential.

Key IC supply specifications:

Parameter	Value
Unloaded output voltage	±100 V
Pot range	0 to ±60 V at wiper (full travel)
Series protection resistor	100 kΩ
Filter capacitor	10–25 µF / 150 V
Ripple at full IC load	< 1 V (acceptable; IC is set during RESET, not compute)

7.9 Power Bus Distribution

Five bus bars run the full length of the chassis from left (power supply board) to right (amplifier 9). Each bus bar is a tinned-copper wire or strip clamped to insulating standoffs. The five buses are:

Bus	Voltage	Connected to
B+	+300 V	Pentode and triode plate loads, screen dropping resistors
B−	−150 V	Pentode and triode cathode resistors
IC+	+IC (variable)	IC supply positive rail
IC−	−IC (variable)	IC supply negative rail
H	6.3 V AC	All heater pins

The five-bus architecture is responsible for the EC-1’s relatively clean inter-channel rejection. All nine amplifiers share the same regulated rails with local bypass capacitors on each module. In the original design, bypass capacitors are 0.01 µF ceramic types across each power pin to ground; these degrade with age but rarely fail completely.

Known Failure Points, Ageing Patterns, and Forward Reference to Vol 4

8.1 Electrolytic Capacitor Failure

Probability: High. Consequence: Severe.

Electrolytic capacitors manufactured in the 1960s use an aluminium-oxide dielectric formed on the anode foil by electrolytic oxidation. After 50+ years of storage, the oxide layer partially dissolves back into the electrolyte, and the capacitors exhibit dramatically reduced capacitance, elevated ESR, and high leakage current.

The failure spectrum in the EC-1:

Location	Failure Mode	Symptom
Main HV filter cap (+300 V rail)	Low C, high ESR, leakage	Excessive ripple, low +300 V under load, possible catastrophic failure
Negative supply filter cap	Low C, high leakage	Oscillation, instability, −150 V sag
IC supply filter caps	Low C	IC voltage error during RESET
Screen bypass caps (per amplifier)	Open or short	Instability in one or all amplifiers
Multivibrator timing caps	Value drift	Repetition rate error
Coupling caps (if any)	Open	Loss of stage

Danger — A shorted or leaky main filter capacitor on the +300 V rail can produce sustained fault currents through the power supply wiring. In a worst case, the series-pass regulator tube fails short, applying full raw-rectified voltage (~490 V peak) to the +300 V bus and potentially destroying all nine 6U8 amplifier tubes simultaneously. Always verify capacitor health before first power-up of an unrestored unit. See Vol 4 for the variac reform procedure.

Recommended action at restoration: Replace all electrolytic capacitors with modern types of equal or higher voltage rating. Use the original capacitor sleeves (slit open, remove old can, slide new cap inside) to preserve cosmetic appearance if desired. AES (tubesandmore.com) and Digi-Key are the primary sources.

8.2 Balance Potentiometer Degradation

Probability: Very High. Consequence: Moderate.

The nine front-panel balance potentiometers are 3 kΩ wire-wound or carbon-track types. After decades, the wiper tracks develop oxide layers and intermittent contact. The symptom is an inability to null one or more amplifiers — the meter refuses to go to zero despite pot adjustment, or the null point shifts with mechanical vibration.

Replacement with 3 kΩ cermet or wire-wound pots (knob-shaft style for convenience) is standard practice. See Vol 4.

8.3 6U8 Tube Ageing

Probability: Moderate. Consequence: Moderate.

The 6U8 is a relatively robust tube. Primary failure modes:

Mode	Detection	Notes
Cathode depletion	Low emission on tube tester	Gradual onset; amplifier gain falls, offset worsens
Heater–cathode leakage	Hum on amplifier output	60 Hz modulation of output; more common in aged tubes
Gassy tube	Erratic behavior, blue glow in envelope	Gas ions cause positive grid current; amplifier oscillates
Open heater	Cold tube, no output	Usually immediately obvious

The EC-1 uses 9 × 6U8, plus one each of 12BH7, 6AQ5, OA2, OB2, 6BH6 — all of which are readily available as NOS or pull-tested. A full tube replacement set costs $40–$80 at time of writing.

8.4 Selenium Rectifier Replacement

Probability: Present in pre-~1965 units. Consequence: High if unreplaced.

Selenium rectifiers present two hazards: electrical (high forward drop and elevated leakage causing incorrect supply voltages) and chemical (burning selenium produces toxic selenium dioxide fumes). All selenium rectifiers must be replaced with 1N4007 silicon diodes during restoration regardless of apparent condition.

8.5 Carbon Resistor Drift

Probability: Moderate (up to 30% drift in some resistors). Consequence: Low to Moderate.

Carbon-composition resistors of 1960s manufacture are notorious for drifting high with age. In the EC-1 signal path, this most affects:

Pentode plate load resistors (shifts op-amp operating point, degrades gain)
Balance bridge resistors (shifts null range)
IC supply series resistors (minor effect, but shifts IC accuracy)

Test all resistors in situ (power off, one lead lifted) against the schematic value. Replace any resistor measuring more than ±5% of nominal. 1% metal-film replacements are preferred.

8.6 Patch-Board Binding Post Corrosion

Probability: Very High. Consequence: Low (connection quality).

All front-panel banana binding posts tarnish and develop oxidized contact surfaces. The symptom is intermittent connections and high contact resistance (measurable as noise on amplifier outputs when flexing a patch cord). A complete set of replacement red and black banana binding posts (approximately 60–80 posts total) costs under $30 and dramatically improves reliability and appearance.

8.7 Two-Pin Plug Socket Integrity

Probability: Low. Consequence: Moderate if failed.

The 27 brown two-pin sockets for problem-element plug-ins are phenolic wafer sockets with brass-spring contacts. These rarely fail but can lose spring tension if repeatedly forced with oversized plug stems. Replacement sockets are not available as direct replacements; machined reproductions from Delrin rod stock are the only option (see Vol 4).

8.8 Known Failure Table: Triage Summary

Symptom	Most Likely Cause	Second Possibility	See
All amplifiers unstable/oscillating	+300 V excessive (regulator failure)	Shorted screen bypass cap (multiple)	Vol 4 §4.3
One amplifier cannot be balanced	Weak/gassy 6U8	Bad balance pot	Vol 4 §4.4
One amplifier no output	Dead 6U8 (open plate or heater)	Wiring break at binding post	Vol 4 §4.4
60 Hz hum on all outputs	Heater–cathode leakage (multiple 6U8)	IC supply coupling into amps	Vol 4 §4.5
Relay does not cycle in REPETITIVE	Bad 12BH7	Dried timing capacitor	Vol 4 §4.6
Relay stays in RESET	6AQ5 plate-to-cathode short	Relay coil winding open	Vol 4 §4.7
OA2/OB2 does not glow	VR tube gone	Dropping resistor open	Vol 4 §4.3
+300 V reads −200 or erratic	HV filter cap shorted/leaky	Rectifier reverse-leakage	Vol 4 §4.2
IC voltage wrong by fixed offset	IC pot dirty	IC filter cap leaky	Vol 4 §4.8
Meter pegged on all positions	+300 V on meter circuit	Meter shunt resistor open	Vol 4 §4.9

Reference Tables and Component Data

Complete Tube Complement

Tube	Qty	Location	Function	Heater	Availability
6U8 (ECF82)	9	Amplifier 1–9 sockets	DC operational amplifier (triode + pentode)	6.3 V / 450 mA	Good — multiple NOS sources
12BH7	1	Multivibrator socket	Astable multivibrator, repetitive oscillator	12.6 V / 300 mA (or 6.3 V / 600 mA)	Excellent — widely stocked
6AQ5 (EL90)	1	Relay driver socket	Relay driver pentode	6.3 V / 450 mA	Excellent — widely stocked
OA2	1	VR tube socket 1	Voltage reference / −150 V regulation (150 V)	6.3 V / 300 mA	Good
OB2	1	VR tube socket 2	Voltage reference 108 V	6.3 V / 300 mA	Good
6BH6	1	Regulator driver	Error amplifier / driver for pass tube	6.3 V / 300 mA	Good

Note — Some EC-1 units may substitute a 6AU6 for the 6BH6 in the regulator driver position; both are sharp-cutoff pentodes with similar parameters. Verify against the specific unit’s schematic revision.

Key Passive Components (Power Supply)

Designator	Value	Voltage	Function	Restore with
C_main_HV	40 µF	450 V	+300 V rail filter	47 µF / 450 V axial electrolytic
C_neg	47–50 µF	200 V	−150 V rail filter	47 µF / 200 V (note polarity: neg terminal at −150 V)
C_ic1-3	10–25 µF	150 V	IC supply filters (×3)	22 µF / 150 V
C_screen (×9)	0.01–0.1 µF	400 V	Pentode screen bypass per amp	0.1 µF / 400 V ceramic or film
R_plate_pent	100–220 kΩ	—	Pentode plate load	1%, metal film, 1/2 W
R_plate_tri	47–100 kΩ	—	Triode plate load	1%, metal film, 1/2 W
R_balance (×9)	3 kΩ	—	Balance pot per amplifier	3 kΩ wirewound or cermet with shaft
R_ic_series (×3)	100 kΩ	—	IC output protection resistor	100 kΩ, 1/2 W

Front Panel Control Summary

Control	Type	Range	Function
HIGH VOLTAGE	Panel pot	+250 – +350 V	Sets +300 V rail level
METER FUNCTION	12-pos rotary	AMP 1–9, IC 1–3, OFF	Selects meter input
METER RANGE	2-pos toggle	×1, ×10	Sets meter sensitivity
OPERATION	3-pos toggle	RESET / OPERATE / REPETITIVE	Controls compute cycle
REPETITION RATE	Panel pot	0.1 – 15 Hz	Sets multivibrator frequency
BALANCE (×9)	Panel pot (bottom row)	—	Nulls each amplifier offset
COEFF POT 1–5	Panel pot	0.00 – 1.00	Multiplies signal by set fraction
IC POT 1–3	Panel pot	−60 V – +60 V	Sets initial condition voltage

System Performance Specifications

Parameter	Specified Value	Source
Amplifier open-loop gain	~1,000	EC-1 Operational Manual
Output voltage range	±60 V	EC-1 Operational Manual
Maximum output current	0.7 mA per amplifier	EC-1 Operational Manual
−1 dB bandwidth	600 Hz	EC-1 Operational Manual
Number of op-amps	9 (identical)	Assembly Manual
Coefficient potentiometers	5	Front panel
Patch sockets	27 × two-pin	Assembly Manual
+300 V supply range	+250 to +350 V	Research Guide §3
+300 V supply current	25 mA max	Research Guide §3
−150 V supply current	40 mA max	Research Guide §3
Meter movement	50-0-50 µA	Research Guide §3
Repetition rate	0.1–15 cps	Research Guide §3
Relay contacts	4 sets	Research Guide §3
Mains	105–125 V AC, 50/60 Hz	Assembly Manual
Power consumption	100 W	Assembly Manual / front panel
Dimensions (chassis)	19¾” W × 11½” H × 15” D	Research Guide §1
Weight (shipping)	~43 lbs	Research Guide §1

Visual Reference

Chassis Interior — Running Condition

EC-1 chassis interior with nine 6U8 tubes installed, power supply components at left, relay and control circuitry at right.

The photograph above shows the interior of a restored, operating EC-1. The nine 6U8 tube envelopes are visible in a row across the top of the chassis. The OA2/OB2 VR tubes — when illuminated — glow with characteristic purple-blue light through the chassis ventilation slots. The large cylindrical components to the left are the main HV filter capacitors and transformer.

Chassis Interior — After Restoration

EC-1 chassis after full restoration showing new wiring harness, replaced capacitors, and reconditioned tube sockets.

Resistor and Capacitor Problem-Element Plugs

Resistor and diode plug-in elements for EC-1 two-pin sockets — shows the plug body style and component leads.

The two-pin plug-in format is unique to the EC-1. Each plug consists of a molded plastic (or machined Delrin) body with two brass pins spaced to fit the front-panel sockets. The resistor or capacitor is soldered between the pin ends and the component body sits in the plug body. Plug bodies hold the component physically and route its electrical connections to the socket contacts.

Supplementary SVG Diagrams

Power Supply Regulation Tree

Single Operational Amplifier — Functional Signal Flow

Open-loop gain A ≈ 1,000 · Closed-loop: eₒ = −(Rf/Ri)·eᵢ for inverter; eₒ = −(1/RiCf)∫eᵢ dt for integrator

Amplifier Mode Selection — Decision Flowchart

Operator goal for this amplifier?
        │
        ├─── Negate (invert) a signal ────► Rf = Ri (e.g., 1MΩ / 1MΩ) → gain = -1
        │
        ├─── Multiply by constant > 1 ───► Rf > Ri (e.g., 10MΩ / 1MΩ) → gain = -10
        │                                   (keep ratio ≤ 100 to stay in accuracy spec)
        │
        ├─── Sum multiple signals ────────► Multiple Ri each to SJ, one Rf → weighted sum
        │
        ├─── Integrate with respect to t ► Cf = 1µF in feedback, Ri = 1MΩ → τ = 1 sec
        │                                   faster: Ri = 100kΩ → τ = 0.1 sec
        │
        ├─── Multiply by 0–1 coefficient ► Use front-panel coefficient pot upstream of Ri
        │                                   (pot × signal feeds amplifier input)
        │
        └─── Initial condition? ──────────► Connect IC supply to SJ via 100kΩ during RESET
                                            Capacitor Cf charges to IC voltage
                                            OPERATE releases cap; integration starts from IC

Heathkit EC-1 — Volume 2 — Hardware & Theory of Operation. Cross-reference: see Vol 1 for history and acquisition; Vol 3 for patch panel and computing elements; Vol 4 for acquisition, restoration, alignment, and fault-finding; Vol 5 for programming methodology; Vol 6 for sample programs.