The EICO Models 232 and 249 are Peak-to-Peak VTVMs (Vacuum Tube Voltmeters) designed for electronic measurements, particularly in TV servicing, radio servicing, and industrial applications.
Function Description
The core function of these instruments is to measure various electrical parameters:
- Peak-to-Peak AC Voltage: They feature a full-wave, high-frequency rectifier circuit that measures the peak-to-peak voltage of complex and sine waveforms, even in the presence of DC.
- RMS AC Voltage: They can read the RMS voltage of sine waves on a separate scale.
- DC Voltage: They measure DC voltage values.
- Resistance: They measure resistance values.
The instruments offer a zero-center indication feature, which is useful for discriminator alignment and bias measurements, allowing observation of both positive and negative voltage excursions without resetting the function selector.
Important Technical Specifications
DC VOLTMETER:
- Ranges: 0 to 1.5, 5, 15, 50, 150, 500, 1500 volts.
- Input Resistance: 11 MΩ (megohms).
- Accuracy: ±3% of full scale or better.
- Range Extension: Can be extended to 30,000 volts with accessory High Voltage Probe HVP-1 or HVP-2 (1090 MΩ multiplier resistor recommended).
- RF Voltage Measurement: Up to 250 Mc with accessory RF Probe Model PRF-11.
AC VOLTMETER:
- RMS Values of Sine Waves: 0 to 1.5 volts on a separate LOW-AC scale; 0 to 5, 15, 50, 150, 500, 1500 volts on other scales.
- Peak-to-Peak Values of Sine & Complex Waves: 0 to 4 volts on a separate LOW-AC scale; 0 to 14, 42, 140, 420, 1400, 4200 volts on other scales.
- Input Resistance & Capacity: 1 MΩ, shunted by 60 mmf (approx.).
- Accuracy: ±5% of full scale or better.
- Frequency Response: 30 cps to 3 Mc (for a source Z 100 Ω or less).
- RF Voltage Measurement: Up to 250 Mc (accuracy ±10%) with RF Probe PRF-11.
OHMMETER:
- Ranges: 0 to 1000 Megohms in 7 ranges (RX1, RX10, RX100, RX1000, RX10K, RX100K, RX1Meg).
- Center Scale: 10 ohms on the RX1 range.
TUBE COMPLEMENT:
- 1-6AL5 twin-diode (full-wave peak-to-peak rectifier).
- 1-12AU7 twin-triode (vacuum-tube balanced bridge circuit).
POWER SUPPLY:
- Transformer-operated selenium rectifier.
- 1.5 volt flashlight cell.
- Power Requirements: 105-125 volts AC, 50-60 cps; drain: 5 watts.
PHYSICAL CHARACTERISTICS:
- Model 232: 8 1/2"h, 5"w, 5"d; Weight: 7 pounds. (4 1/2" meter)
- Model 249: 8 1/2"h, 13"w, 5"d; Weight: 9 pounds. (7 1/2" meter)
- Case: Steel cabinet, grey wrinkle finish; satin-aluminum panel, deep-etched and rub-proof.
- Included: UNI-PROBE (combined isolating and direct probe, used for all functions and ranges) and ground lead.
Usage Features
- Consistent Accuracy: Seven non-skip ranges on all functions provide a uniform 3 to 1 scale ratio between adjacent ranges, minimizing reading errors.
- High Input Resistance (DC): 11 megohms prevents loading error while maintaining instrument stability.
- Zero Adjustment: Unaffected by changing function or range.
- Separate Scale for Low AC Voltage: Improves precision for small AC voltage measurements.
- Electronic Protection: Guards against meter burn-out under ordinary overloads.
- Calibration Controls: All calibration controls are accessible without cabinet removal.
- UNI-PROBE: A single probe for all functions simplifies operation.
- Portability vs. Bench Use: Model 232 is compact for portability, while Model 249 offers a larger meter and longer scales for bench use, minimizing reading errors and allowing for greater working distance.
- Resistance Measurement Above 1000 Megohms: An external DC voltage source (20-500 volts) can be used to obtain measurable pointer deflection for very high resistance values.
- Applications: Suitable for maintaining and servicing television receivers, FM-AM and communication receivers, transmitters, audio equipment, and pulsed electronic and electro-mechanical equipment. Specific applications include oscillator grid-bias measurement, AVC-voltage measurements, DC supply voltage measurements, bias cell voltage measurements, detection of gassy tubes, and output indication for alignment.
- DB Measurements: While there is no dedicated dB scale, AC voltage readings can be converted to dB using a provided chart and formula, with corrections for resistive load.
Maintenance Features
Preliminary Adjustments:
- Connecting the UNI-PROBE and ground cable.
- Plugging into AC power (105-125 volts AC, 50-60 cps).
- Warming up the instrument (several minutes to 2 hours for calibration, 36 hours for maximum stability).
- Setting mechanical zero and adjusting the ZERO ADJ. control.
- Adjusting OHMS ADJ. for full-scale deflection on the RX10 range with leads separated.
- Checking AC Volts zero reading.
Calibration:
- General: Instruments in kit form require calibration before use. Factory-wired instruments are pre-calibrated. Recalibration is necessary if accuracy changes over time or after parts replacement.
- Mechanical Zero: Adjust the slotted screw beneath the meter face to set the pointer to the left-hand zero when power is off.
- AC Voltmeter Balance Adjustment: Set FUNCTION to "AC VOLTS", RANGE to "1500 V", UNI-PROBE to "AC-OHMS". Short the UNI-PROBE to the Ground Cable and use the ZERO ADJ. control to set the pointer to the left-hand zero. With leads shorted, reset RANGE to "1.5 V" and use AC Balance potentiometer R27 to reset the pointer to the left-hand zero.
- AC Voltmeter Calibration: Set FUNCTION to "AC VOLTS", RANGE to "150 V", UNI-PROBE to "AC-OHMS". Connect UNI-PROBE and Ground Cable across a 117 volts AC supply. Adjust AC Calibration potentiometer R32 until the meter reads 117 RMS volts. For maximum accuracy, apply exactly 150 volts (60 cps) on the "150 V" range (or 50 volts on the "50 V" range) and adjust R32 for exact full-scale deflection.
- DC Voltmeter Calibration: Set FUNCTION to "+ DC VOLTS", RANGE to "5 V", UNI-PROBE to "DC". Connect UNI-PROBE and Ground Cable across two fresh flashlight cells in series (3.10 volts). Adjust DC Calibration potentiometer R31 until a 3.10 volt reading is obtained.
- Ohmmeter Calibration: No separate calibration is required.
- Accessibility: Calibration controls (R27, R31, R32) are accessible by temporarily removing plug-buttons on the side of the cabinet.
Tube Replacement:
- New tubes may require re-zeroing when switching DC voltage ranges unless aged by operating for 36 hours. Calibration should be checked after replacement.
Battery Replacement:
- Exhausted batteries should be removed promptly to prevent damage from chemicals. A weak battery will be indicated in the trouble-shooting section.
Trouble-Shooting Guide:
- A comprehensive guide helps localize faulty operation.
- Instrument fails to operate; tubes do not light: Check line cord, switch section S1D, and transformer T1.
- Tubes light; ZERO ADJ. does not work; meter pointer doesn't move or bangs: Check operating voltages on bridge tube V1 (B+ or B-), V1 circuit, V1 tube, switch section S1-C, meter movement, and ZERO ADJ. control R22.
- Meter pointer sticks: Indicates a cracked or broken jewel bearing in the meter; requires repair or replacement of the meter movement.
- Intermittent operation on all functions: Check probe/cable connections, wiper contact on S1B/S1C, bridge circuit wiring, V1 tube, and CR1.
- Instrument fails on OHMS; works on AC/DC volts: Check battery, wiper contact on S2C, "OHMS" contact (terminal 7) on S1B, resistor network (R5-R11), and "Ohms" contact on S1C (terminal 1).
- OHMS ADJ. fails to give infinity setting: Check battery, OHMS ADJ. potentiometer R30, and V1 tube.
- Inaccurate resistance readings: Check resistors in network around S2C, excessive leakage in "ohms" circuit (switch wafers S2C/S1B, humidity), shorted wiring, or faulty high-resistance connections.
- Instrument fails on AC voltage; works on OHMS/DC volts: Check V2 tube, AC volts contacts on S1A/S1B/S1C/S2B, wiper contact on S2D, V2 circuitry (C1, C2, R29 open; C3 shorted).
- Meter pointer moves off zero when ranges are changed: Check AC Balance potentiometer R27 and R28.
- Inaccurate AC voltage readings; OHMS/DC volts normal: Check contacts/wipers or leakage in S2B, V2 tube, C2/C3, and R29. For specific ranges (500/1500V or 1.5V), check contacts 10/11 on S2D, R2/R3/R4, or V2.
- Instrument fails on +VOLTS or -VOLTS; works on OHMS/AC volts: Check UNI-PROBE resistor, DC voltage contacts on S2A/S1B/S1C.
- Inaccurate +VOLTS or -VOLTS readings; AC/resistance readings correct: Check R31, UNI-PROBE switch (shorted), and resistances in network around S2A.
EICO Repair Service:
- If troubleshooting fails, instruments can be returned to EICO for repair at a charge of $5.00 plus parts. Kit-built instruments should refer to the EICO servicing policy in their construction book. The UNI-PROBE should be included when returning the instrument for repair.
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