EICO Model 315 Signal Generator
A seven-band kit-built RF generator that servicemen assembled on the kitchen table - and calibrated against the radio stations already coming through the window.
Overview
The Model 315 is a signal generator, not a piece of hi-fi. Keep that in mind when you go hunting for one. Electronic Instrument Co., Inc. of Brooklyn sold it as a build-it-yourself kit. The construction manual reads like a patient instructor standing over your shoulder: study the schematic and pictorial wiring before you start, don't rush, use rosin-core solder, and under no circumstances reach for acid flux. EICO even advised holding a pair of longnose pliers between a component and the joint you're soldering, so the pliers wick heat away before the part cooks. It was written for someone with a soldering iron and an afternoon.
For their effort, builders got a highly stable electron-coupled RF oscillator calibrated across seven bands, covering 75 kc to 150 mc with overlap on each band. The oscillator is electron-coupled to one half of a 7F7 buffer stage for isolation. That keeps the generator's output from yanking the oscillator off frequency as you change attenuator settings. The second half of the same 7F7 runs as an inductive-type audio oscillator producing a 400-cycle sine wave with less than 5% distortion, and that audio signal modulates the RF in the buffer stage. A four-step ladder-type attenuator plus a variable control in the buffer stage's cathode circuit handle output level without loading the oscillator.
A voltage regulator tube lets line voltage wander from 55 to 135 volts without affecting the generator's accuracy. In a 1950s service shop where the wall voltage sagged every time a compressor kicked on, that spec counted. The unit runs on 105 to 120 volts, 50-60 cycles, switched by a bat-handle toggle. EICO's instructions call for a fifteen-minute warm-up before any alignment work. The voltage chart in the manual was taken with a VTVM, chassis as ground, multiplier switch at X1000, attenuator pot at maximum, band switch at F, signal switch at RF.
The front panel carries the band switch, a fine-adjust attenuator, the four-step multiplier, a selector switch that chooses modulated or pure RF (or accepts external modulation), an RF output connector, and a separate audio output connector. EICO's manual is emphatic that you not confuse the two. It is equally emphatic that the coaxial ground lead goes to the receiver's B-minus, not necessarily the chassis.
Key Features
Seven Bands from 75 kc to 150 mc
The band switch selects the RF oscillator range across seven bands with overlap between them, spanning 75 kc at the bottom to 150 mc at the top. The manual's alignment procedure walks through the specific trimmer points: 200 kc on Band A, 550 kc on Band B, 1600 kc on Band C, 5 mc on Band D, 15 mc on Band E, and 50 mc on Band F. The last one comes with a warning - the zero-beat whistle at 50 mc may be somewhat erratic simply because of the frequencies involved. EICO didn't pretend the top band behaved like the bottom one.
Electron-Coupled Oscillator with 7F7 Buffer
The 7F7 does double duty here. One half buffers the RF oscillator for isolation. The other half is the inductive audio oscillator. Because the oscillator is electron-coupled rather than directly loaded, the output level controls - the four-step ladder attenuator and the variable cathode control - can do their jobs without dragging the frequency around with them.
400-Cycle Audio Oscillator, Under 5% Distortion
The internal audio oscillator produces a 400-cycle pure sine wave with less than 5% distortion, used to modulate the RF in the buffer stage. The selector switch also permits modulation from an external source. A dedicated audio output connector feeds the 400-cycle signal out on its own.
Regulated Against Line Voltage Sag
The voltage regulator tube is what separates this from a toy. EICO's documentation states that line voltage variations from 55 to 135 volts won't affect the generator's accuracy. That matters in a service shop running off the same circuit as a tube tester and a soldering iron.
Micro-Cycle Vernier Tuning
EICO's manual spends its warmest words on the vernier dial. For FM receiver alignment, the critical characteristic is the IF bandpass, and the vernier lets you shift frequency accurately to either side of center to map that bandpass. The second use is more interesting. Once a frequency is checked against a known standard and noted, the vernier scale can repeat it to within .025 accuracy, which turns the dial into an individual calibration reference. The manual notes that for best accuracy the center of the scale should be used, though frequency stays accurate when switching between bands.
Four-Step Ladder Attenuator
The coarse output control multiplies the RF signal by x1, x10, x100, or x1000, with a separate fine-adjust attenuator pot handling the in-between settings. The manual's alignment steps set the multiplier to X1000 and the attenuator to maximum when comparing against a second generator, so this was a tool meant to be used alongside other test gear.
Historical Context
EICO released the Model 315 as a kit aimed at servicemen and engineers who preferred to build their own test equipment. Audio (September 1950, p.26) noted the company's decision to release the well-known Model 315, framing it as a natural fit for readers who built their own gear. The construction manual explains that EICO ordered parts from several sources to keep kits in supply, so a resistor marked 470,000 ohms might arrive as 510,000 ohms. That's fine, because most parts carry a 20% tolerance and the circuit is designed around those variations.
The manual also documents the support model of the era. If you couldn't sort out a problem yourself, you could ship the instrument back to the factory, where it would be calibrated and returned to operating condition for $5.00 plus parts, sent prepaid Railway Express and returned Express collect. EICO asked that you include the stock number, quantity, model number, and the serial number from the panel when ordering replacements.
Collectibility & Value
The Model 315 is a kit-built instrument, so condition varies enormously, and the biggest variable is who soldered it. EICO's own troubleshooting section is blunt about this: nearly all cases of trouble in the past had improper wiring as their cause. Before chasing a dead band or a drifting frequency, the manual's first three steps are check all voltages, check individual components for improper wiring or breakdown, and recheck the wiring procedure. Take that order literally.
The manual supplies full voltage and resistance charts for the four tubes - V1 (6C4), V2 (6SL7), V3 (VR150), and V4 (6X5) - taken with a VTVM and chassis ground, with readings expected within 20% or 30%. That documentation is why these units remain restorable. You can check a suspect stage against the chart rather than guess. The resistance chart specifies C9 and C10 shorted to ground for the measurement, so anyone comparing readings without doing that will get confusing numbers.
The construction manual's parts list also notes that some components mount temporarily during assembly. C9 and C10, for instance, are twisted at the lugs before final mounting, so a unit that's been apart and reassembled may not match the pictorial exactly. Expect a kit-built unit to show the marks of its builder.
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