Front View
The AEROVOX Model 75 is a Capacity and a Resistance measuring bridge. It was built by AEROVOX in about 1932. It is supplied in a wooden box with instructions in the lid. It is a portable device and runs from 110 volts AC. It is 11 inches wide 8 inches deep and 8 inches high. It weighs 10 pounds. It is intended to measure capacitance in the range 100 pico Farads (100 pF) to 100 micro Farads (100 uF), and resistance in range 10 ohms to 1 million ohms. The device can also perform other functions.
Front View
The device is in a wooden box, with a handle and a closed lid. When the box is opened, the power cord can be removed and plugged into a 110 volt AC socket. There are 3 valves along the left hand side. The top one is a triode (type 45) used for the high voltage power supply. The middle valve (type 1V) is a single diode used for the normal DC power supply for the VTV. The bottom valve (type 6C8) is a dual triode, used for the meter amplifier and meter rectifier.
The Radiola Wireless Corporation manufactured radio receivers in 1922, and then changed their name to AEROVOX in 1924. They made capacitors and were located in New Bedford, MA (USA). In 1973, AVX became the parent company when assets of AEROVOX were sold. PCB chemicals were banned in 1977, due to cancer fears (Polychlorinated Biphenyl). AEROVOX was a major user of PCB in capacitance manufacture. In 1997 PCB was found in the environment around the factory. They went bankrupt in 2001. They were purchased in 2019 by Cornell Dublier Electronics. They produced the magazine AEROVOX RESEARCH WORKER from 1927 till 1967 which published papers on various electronic items, with a focus on capacitors and resistors.
CONTROLS
Along the top are several terminals, used to access the internal parts of the device. The terminals at the right are for the unknown component, so that the bridge can determine its characteristics. It has a RED and a GREEN terminal, is labeled as X, and is connected to the bridge circuit. The terminals at the left allow access to the metering circuits. In the center is a BLACK terminal which is the COMMON and is labeled as – (minus). Next to this (going left) is a RED terminal which is labeled +V. This provides an input to the volt meter. It also provides the output voltage from the high voltage power supply. The next terminal is GREEN, labeled +MA, and this can be used as an amp meter input. The next terminal left, is another GREEN terminal, labeled +MV, and this allows direct access to the meter. The last terminal at the left, is a BLACK terminal, and this allows access to the Vacuum Tube Voltmeter, labeled HIGH VTV.
Controls
Below this at the left is the actual indicating meter. Next to this are 2 controls, a meter ZERO ADJUSTMENT control, and a press button for INSULATION RESISTANCE. At the far right is the bridge control, labeled 1 to 10. It has a pilot light underneath it, showing the word AEROVOX.
Along the bottom are 4 knobs. The left knob is the function control, labeled METER RANGE. This has 6 positions anti clock wise, operated with the mains power turned OFF. It has 5 positions clock wise, operated with the power ON. The next knob has a 3 position switch in the center, labeled POLARIZING VOLTAGE, selecting the 3 high voltage ranges. The outer knob sets the high voltage required. The center right knob is labeled %POWER FACTOR and is graduated 0 to 50, and labeled 60 cycles. When turned completely clock wise, it operates a switch that selects the INSULATION RESISTANCE. At the right is the BRIDGE RANGE SWITCH that selects 6 positions for capacitance values, and 5 positions for resistance values. Note that the resistance values use the letter M for 1000 ohms, whereas nowadays we would use the letter K for kilo ohms.
DESIGN
The device is basically a bridge to determine capacitance and resistance. It can be considered as 4 separate parts. There is a Wien bridge, a Vacuum Tube Voltmeter (VTV), a high voltage power supply, and a metering circuit. These are described separately, starting with the simplest section.
Circuit
METERING CIRCUIT
The metering circuit is independent of the main instrument. It does not require the device to be turned ON. It is a 3 range voltmeter, and is accessible from the +V terminal and the Common terminal. The METER RANGE switch selects the 3 ranges, 60, 300 and 600 volts DC. The +MV terminal can be used as a 60 milli-volt range as well, when the METER RANGE switch is set to 60MV. The 3 voltage ranges are also used by the high voltage power supply, to show the voltage.
The metering circuit is also a 2 range ammeter, and is accessible from the +MA terminal and the COMMON terminal. The METER RANGE switch selects the 2 ranges, 6 and 60 milli-amps DC. The +MV terminal can be used as a 1 milli-amp range as well, when the METER RANGE switch is set to 60MV. The COMMON or Earth terminal, is connected to the instrument chassis. The 2 current ranges are also used by the high voltage power supply, to show the capacitor leakage.
The chassis is not connected to mains earth, so the instrument can float, and voltage and current measurements are independent of the earth connection. There is a capacitor between one of the mains wires and the chassis.
Meter Circuit
Also, the meter is switched in to show the output of the VTV amplifier. The VTV is used on its own, or as an indicator for the balance of the bridge, or the insulation resistance.
RESTORATION
The voltage ranges were working but out of tolerance. The 3 multiplier resistance were measured and found to be high in value. They were replaced with new resistors and the voltages ranges were now accurate. The current ranges were also inaccurate. One of the shunt resistances was replaced and now the current ranges were correct.
VARIABLE POWER SUPPLY
The Variable Power Supply is available at the +V and COMMON terminals. When the METER RANGE switch is set to PV (POLARIZING VOLTAGE), then any voltage between 15 and 600 volts DC can be accessed. The mains is switched ON, for this function. A transformer supplies an AC voltage to a triode which serves as a grid controlled rectifier. The bias on the grid of the valve (type 45) is adjustable and sets the output voltage. The output has minimal filtering, as this is intended for the bridge measuring circuit, and so some AC ripple is required for this function. If you wish to use this voltage externally, then some extra filtering may need to be added. There is a 10K ohm resistor is series with the output to limit the output current to 50 milli-amps maximum. The POLARIZING VOLTAGE range is selected with a switch that selects 3 taps on the transformer and also selects the volt meter range. The ranges are 60, 300, and 600 volts DC. This also has a variable control and can set the POLARIZING VOLTAGE. There is an additional 10K ohm resistor across the meter to adjust it for voltage accuracy. The high voltage power supply is used in all the capacitance measurements, and the ripple on the DC is used in the resistance measurements.
Variable High Voltage Power Supply
RESTORATION
The 45 valve was missing so a new one was inserted. The 2 capacitors were replaced. The circuit then operated properly.
VTV
The VTV (Vacuum Tube Voltmeter) is used to provide a high impedance measurement for the device. It uses a double triode valve (type 6C8), with the first triode as an amplifier. The second triode is a grid leak detector. This is selected by setting the METER RANGE switch to VTV. The POWER FACTOR control should be at zero. The input is connected to the VTV terminal and the EARTH terminal on the front panel, and the input impedance is about 4 meg ohms. The meter reads backwards with the zero on the right hand side. This position can be set with the ZERO ADJUSTMENT knob.
There is a separate power supply for the VTV. This consists of a half wave rectifier (valve type 1V) and some output filtering, to supply a filtered 180 volts DC for the VTV.
VTV Circuit
Testing the VTV Meter
There is a 5 volt PP 1 khz signal connected which gives a reading of 10. The scale is exponential, with zero on the right hand side, and narrowing to the left.
Scale
The meter scale shows 0-600, 0-300, and 0-60 at the top. This is used for the voltage readings, in volts. The scale 0-60, and 0-6 shows the current flowing in milliamps. The next scale down, infinity to 350, shows the insulation resistance, in meg ohms, with the output voltage set at 500 volts. The bottom scale, 20-0, in RED shows the VTV reading.
RESTORATION
The coupling capacitor was replaced and R10 was double the correct value so it was replaced as well.
BRIDGE
The manual describes several different types of bridges, and discusses the advantages and limitations of them. The ones described are the WIEN bridge circuit, the HAY bridge, the OWEN bridge, the SCHERING bridge, the RESONANCE bridge, the MAXWELL bridge, and the ANDERSON bridge. The WIEN bridge is used in this device. The high voltage power supply has ripple on the DC supply, and this is used to operate the bridge.
RESISTANCE MEASUREMENT
The resistance bridge is supplied with the AC ripple, with the DC being blocked by a capacitor. The unknown resistor is placed at X (the 2 front panel terminals). The variable resistor is used to balance the bridge. The lower 2 arms of the bridge have switched resistors in them for the 5 resistance ranges. The 2 center points of each of the bridge legs are connected to a transformer. The transformer supplies the VTM to show the voltage on the meter. The bridge is balanced for the highest reading. When the balance point is reached, the voltage across the bridge is at a minimum, and this causes the VTV circuit to make the meter move to the right, towards full scale. The ZERO ADJUST knob can be used to change the VTV gain, and keep the meter needle near the center of the scale.
Resistance Bridge
Resistance Bridge Ripple Waveform
The waveform is 50 hertz, is symmetrical about the center axis (zero), and is 300 volts peak to peak.
CAPACITANCE MEASUREMENT
The capacitance bridge is supplied with power, with the DC and the ripple being both the supplied. The unknown capacitor is placed at X (the 2 front panel terminals). There is a switched capacitor (actually two) in the other upper arm. The lower 2 arms of the bridge have a switched and a variable resistor in them for the 6 capacitance ranges. The 2 center points of each of the bridge legs are connected to a transformer. The transformer supplies the VTV to show the voltage on the meter. The bridge is balanced for the highest reading. When the bridge is balanced, the POWER FACTOR control can be moved from 0 towards 50. If the meter reading increases, the power factor can be read from the scale. The power factor is at 50 hertz. Most capacitors how a zero power factor. Note that a high voltage is applied to the unknown capacitor under test. This can be up to 600 volts, so it is wise to turn the mains OFF, when fitting the capacitor for testing.
The 2 resistors connected to earth, are used as shunts, and the meter can be switched across them. This will show the capacitor leakage, when the switch is set to L60 or L6, and the reading is in milli amps. This function is also useful for re forming electrolytic capacitors.
Capacitance Bridge
Capacitance Bridge Ripple Waveform
The waveform is 50 hertz, the bottom axis is zero, and is 200 volts DC, with 75 volts peak to peak of ripple on top of it.
RESTORATION
The resistance bridge was tested and this worked on the 4 lowest ranges, but highest range did not work. The capacitance bridge was tested, and this worked on 4 of the 6 ranges. This was traced to one of the resistors that had changed its value from 100K ohms to 330K ohms. It was replaced and all the ranges now worked. All the potentiometers appeared to be in good condition. All the switches appeared to function properly.
The resistors in the bridge circuit are actually preset potentiometers. There are 6 of them and they can adjusted to perform a calibration of this device. The manual offered no explanation of their function in the bridge. There was also no method of calibration discussed so they were not adjusted. The calibration of the scale was good, as the unknowns measured were close to the indicated values on the scale.
All the capacitors were replaced. They were unusual values of up to 1 micro Farad at 650 volts. I had to order these, and they were much smaller, so the clamps were bent to hold the smaller diameter components. They are yellow in the photographs.
Internal View
Bottom View
INSULATION RESISTANCE
This device can also measure the insulation resistance of a capacitor. This may be necessary for some capacitor uses. It measures this by using a high voltage and a high resistance in series with the capacitor. The value is displayed using the VTV on the meter.
To measure the insulation resistance, connect the unknown to the X terminals. Turn the device ON. The standard voltage setting for insulation resistance is 500 volts, so set the voltage on the PV switch setting. Turn the POWER FACTOR switch to INSULATION resistance. Turn the switch to VTV. Set the meter to infinity with the ZERO ADJUSTMENT control. Press the switch DEPRESS FOR INSULATION RESISTANCE and the meter will move. Sometimes this may be violent, from scale end to FSD (Full Scale Deflection), before it settles down. This because of the charge on the capacitor. The leakage through the unknown, and the 2 resistors R16 and R17, is applied to the VTV amplifier. They add up to 7 meg ohms. The insulation resistance value is displayed on the meter. The purpose of the press switch is to protect the meter, while the unknown is charging.
Insulation Resistance
CASE
The wooden box was missing the bottom, and had seen a hard life. The top closing lid was in good condition. The hinges and catch were rusty. The top handle was missing.
The hinges and catch were removed. A bottom was made of wood and attached. It was stained with wood stain, painted with clear satin finish. The hinges were wire brushed and sprayed with a clear lacquer.
Case
MANUAL
The manual was downloaded from the internet. It was mid way between an operator’s manual and a workshop manual. It did not have a clear detailed explanation of how the device actually worked, and how to calibrate it.
CONCLUSION
Modern bridges show a greater resistance and capacitance range. Most modern bridges show inductance as well. One of the advantages of this bridge, is that it will measure the capacitance, at the rated operating voltage. I have been caught before, when a modern bridge showed the correct capacitance, and an ohm meter showed no leakage. These were all tested at a low voltage. However, the capacitor was faulty at high voltage. If I had used this bridge, it would have shown that the capacitor was indeed faulty, at the high operating voltage. I have previously used a bench power supply, a multi meter, and a potentiometer to reform capacitors. This bridge can be used to do this easily, all in one box. This bridge is useful.
REFERENCES
https://bama.edebris.com/download/aerovox/75/aerovox%20model%2075%20manual.pdf
https://www.radiomuseum.org/r/aerovoxcor_capacitance_and_resistan.html
https://www.worldradiohistory.com/Aerovox_Researcher.htm
https://www.southcoasttoday.com/story/news/2011/03/31/aerovox-history/50143616007/
Copyright
Ray Robinson