EICO Model 250 AC VTVM & Amplifier
A laboratory-grade bench meter that doubles as a wide-band amplifier, and reads down to 100 microvolts.
Overview
The Model 250 is a high-sensitivity, wide-band AC VTVM and amplifier. EICO's own manual calls it a bench instrument built to laboratory accuracy and quality. The headline number is the bottom of the range: 100 microvolts, full scale, on the most sensitive setting. Twelve voltage ranges run from 1 millivolt up to 300 volts RMS. A function selector on the panel flips the whole instrument into a wide-band amplifier with 60 dB of gain on the 1 mV range and a maximum output of 5 volts RMS.
The circuit is more interesting than the spec sheet suggests. EICO used a frame-grid triode cathode follower at the input, followed by a frequency-compensated input attenuator and a cathode-circuit attenuator. Two high-gain frame-grid pentodes do the amplification work, which keeps the stage count low. Fewer stages means less phase shift and better stability, a real concern in a meter this sensitive. The two-stage R-C coupled amplifier and the full-bridge meter circuit sit inside one overall feedback loop. The power supply is regulated, with DC-biased filaments and a hum-adjust pot. At 1 mV full scale, heater hum is the enemy, and EICO clearly knew it.
Accuracy is specified at ±3% of full scale. Because the tolerance is a percentage of full scale rather than of the reading, the meter is at its best near the top of the dial. The range intervals are arranged so that any voltage above one-third of a range can be read on the upper two-thirds of the scale, so you're never stuck squinting at the bottom of the meter. Frequency response is quoted as ±0 dB from 10 cycles to 600 kc in voltmeter mode, and +0/-3 dB from 8 cycles to 800 kc as an amplifier. Input impedance is 10 megohms shunted by 15 mmf. Output impedance in amplifier mode is 5K ohms, and the manual notes that whatever you feed should present an input impedance of not less than 5K.
The decibel scale is based on 0 dB = 1 mW in 600 ohms, with 10 dB between ranges, spanning -80 to +52 dB across the twelve positions. The inner ring of numerals on the range dial gives the cumulative correction for each position. The manual also has a nomograph for correcting dBm readings into load impedances other than 600 ohms. It walks through a worked example: a -6 dBm reading on the 10V range into a 200-ohm load corrects to 19 dBm, which the milliwatt table puts at 79.43 mW.
One quirk: on the four lowest ranges, the pointer will slam to the right-hand stop if the input terminals aren't connected to anything. EICO's manual calls this a normal effect of stray pickup on an instrument with this much gain and input impedance, and it has no effect on accuracy once the leads are across a source. The manual also warns that non-sinusoidal waveforms won't read accurately. The instrument responds to the average value of the wave but is calibrated to read RMS of a sine. Second-harmonic distortion of 20% pushes readings up to 2% low; 50% pushes them up to 10% low. Third-harmonic distortion of 10% swings readings up to 4% either way, depending on phase. There's no fixed correction because the error depends on the phase relationship between fundamental and harmonic.
Physically, the Model 250 is compact: 8½ by 5¾ by 7 inches, 7 pounds. The tube complement is two 6EJ7/EF184 frame-grid pentodes, one 6FY5/EC97 frame-grid triode, a 6X4 full-wave rectifier, and an OA2 voltage regulator. Power draw is 15 watts from a 105-125 VAC, 60/50 cycle line. The manual is emphatic that it will not operate from DC and should never be connected to a DC line.
Two calibration adjustments are documented, and both require pulling the case. Trimmer C1 handles attenuator frequency compensation. Feed 600 kc into the input, set the range to .3V, adjust for full scale, then switch to 1V and tweak C1 for exactly .3 volt on the meter. Calibration itself is done with potentiometer R14 in the amplifier feedback loop, accessible through a plug button on the left side of the case, using the AC line voltage as a known reference, ideally checked against an accurate instrument first. EICO's manual includes the standard warning that when measuring line voltage there's a 50% chance the chassis ends up connected to the hot side of the line.
Factory repair service for the Model 250 carried a minimum labor and handling fee of $7.00 for out-of-warranty units, with parts extra. In-warranty factory-wired units had no labor fee if they complied with the warranty terms. In-warranty kit units still incurred the $7.00 minimum. The warranty itself ran 90 days from purchase and required the registration card to be returned within 10 days. Units wired with acid-core solder, modified, or not built to instructions would be returned unserviced.
For anyone hunting one today, the practical notes are simple. The 10-megohm input impedance makes it a useful bench meter for high-impedance circuits, and the amplifier mode turns it into something more than a voltmeter. The tube complement is all common types, and the regulated supply with DC filaments shows a design that took noise seriously. The catch is the same as with any VTVM of this vintage: calibration drift, tired electrolytics, and the need for a proper alignment if you want the ±3% spec to mean anything. Get one that's been gone through, or budget for the work.
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