INTRODUCTION

The Wavemeter Class C is a device used for setting the receive or transmit frequency of any wireless set. The manual specifically mentions Wireless Sets Number 1, 2, 3, 7, 9, and 11. The frequency range covered is 1360 Kcs to 7.5 Mcs, in 3 ranges. It is a small device measuring 16 by 10.5 by 9 inches, and weighs 28 pounds. It is in a metal case, with a vibrator power supply inside. There internal vibrator power supply and the whole unit can run from 6 volts at 0.9 amps from a large battery. Alternatively an HT battery (100 volts) and filament battery (6 volts) can be used.

Picture 1: Front View

MECHANICAL DESIGN

The wavemeter is in an aluminium metal box, with a fabric shoulder strap. The front cover is for protection, and hinges up, after opening the latch. Inside the front cover is the circuit, parts list, and working instructions. To the left is a separate compartment, also with a cover and latch. Behind this is the vibrator power supply, and a spare valve in small case. There is a calibration correction chart here as well. Also in this box, is a 2 pin cable to allow the connection to a 6 volt battery. The box is splash proof, and has a gasket around the wavemeter front panel.

Picture 2: Case

There is a panel in the back, which can be removed to change the valve (if required). There are 3 holes in the top to allow access to the trimmers for the 3 ranges. The design appears to be an old style, as the electronics is mounted on a fibre board, which is on standoffs inside the metal case. Most of the wiring and components are mounted with nuts and bolts. The wiring is plated copper wire covered in spaghetti and bolted to the components. There is a complicated tuning dial and gear train behind the front panel.

The controls are very simple. There is an ON and OFF switch which controls the 6 volt input through a 2 pin plug. There is a central dial window, which can be removed to change the 2 dial lights. Below this is the nameplate, and a small white area for notes, to allow the recording of special frequencies. At the right is the 3 position range switch, and the large tuning knob. Below this is a socket for connecting headphones.

Picture 3: Inside View

ELECTRICAL

The wavemeter uses one valve, an ARTP1 triode and pentode. The triode is connected as a Colpitts oscillator, using a tuned circuit. This has 3 ranges, 1360 - 2235 Kcs, 2220 - 4170 Kcs, and 4135 - 7510 Kcs. The frequency is continuously tuned by a fixed capacitor and a coil with and adjustable wiper. This gives a fairly linear frequency range. There is a capacitor for each range and there is a trimmer to calibrate the frequency on each range. The capacitor is formed by a network of ceramic and mica capacitors. The temperature coefficients of these are negative and positive, so temperature effects are nearly cancelled out. The pentode is wired as a mixer, and connected to the aerial as input. The oscillator is also coupled to it by a capacitor. There is a transformer in the plate that connects to the headphones.

When checking a receiver, there is sufficient coupling through the wavemeter wiring, to radiate through the aerial. The aerial is the insulated handle on the front panel. The dial reading indicated the receiver frequency.

The pentode part of the valve is used as a mixer, and any input from the aerial handle, is mixed with the triode oscillator. This generates a beat note which can be heard in the headphones, and when tuned to zero beat, the frequency can be read from the dial. In this way a transmitter frequency can be determined.

There is a correction chart inside the case that shows any differences from the dial reading.

Picture 4: Correction Chart

The 6 volts enters the unit through the 2 pin plug and the ON/OFF switch. It goes directly to the 2 dial lamps. It also goes to the valve filament through a dropping resistor. It goes to the vibrator power supply, which generates 100 volts DC. The vibrator is in its own box, with a 4 pin connector. It can be quickly unplugged and changed. There is a circuit for it on the side of the vibrator power supply case. The vibrator is synchronous, so no rectifier valve is required. There is a choke filter inside the vibrator box, to prevent any noise reaching the wavemeter valve. If the vibrator is unplugged, and the wavemeter is run from batteries, a 5.5 ohm resistor is required to be used, to reduce the filament voltage. This is contained in a plug, and is normally clipped into the lid in a special holder.

Picture 5: Plug

Picture 6: Wavemeter Circuit

RESTORATION

The wavemeter was in a poor condition, with flaking paint and parts missing. The knobs and trimmers were sourced from RM in the UK (thank you). When fitted and wired in, the wavemeter was powered from a filament voltage source. The voltage was slowly increased until there was 2 volts across the valve filament. The HT was then connected, and this was increased slowly, up to the 100 volts required. There was a signal displayed on the oscilloscope. The frequency was measured and found to be higher on all ranges. I added capacitors across each trimmer, until the frequency was within the trimmer capacitance range, and then adjusted the trimmer.

Picture 7: Missing Trimmers

The knobs were fitted. The front panel was repainted.

POWER SUPPLY

The power supply had 2 capacitors changed in it. I replaced them with 8 micro farad 350 volt electrolytics. The vibrator was opened and the contacts were cleaned. All was reassembled and 6 volts DC was applied. It generated 80 volts, but after a few minutes, this voltage began to drop. The capacitor C4A across the vibrator contacts was replaced and the voltage was now 110 volts DC and did not sag oven time.

Picture 8: Vibrator Unit

Picture 9: Vibrator Inside

Picture 10: Vibrator Circuit

MODIFICATION

A wavemeter is not required nowadays. We can check the frequency of a receiver with an accurate signal generator. We can check the frequency of a transmitter with an accurate receiver or frequency counter or spectrum analyser. Often the receiver or transmitter has a phased lock loop to generate the frequency, and this is crystal controlled, so is accurate.

The wavemeter was modified to use it as a short range transmitter. This is for testing valve receivers, and for producing some program material that is not normally available from an off-air broadcast. The frequency range is from the top end of the broadcast band from 1360 Kcs to short wave frequencies up to 7.5 Mcs.

The oscillator was not altered. The mixer was changed to a modulator. The output transformer secondary was removed from the plate circuit, and an RFC connected to replace it. The aerial capacitor was removed from the grid and connected to the plate. The grid was connected through a 0.001 microfarad capacitor to the oscillator plate. The transformer secondary was connected to the screen grid through a 0.001 microfarad capacitor.

A program source from a speaker (CD player, tape recorder, or radio) was plugged into the phones output socket. This was the modulating signal, and applied to the suppressor grid of the valve. The plate was connected to the aerial though the existing capacitor, to conduct the signal to the outside of the case. The RFC was to provide some RF isolation from the HT supply. Extra signal from the oscillator was applied to the grid of the valve, even though there was some from the adjacent oscillator in the same valve envelope.

Picture 11: Modifications

Picture 12: Modified Circuit.

CONCLUSION

The wavemeter was a useful device for checking receivers and transmitters. It can be easily modified to produce a modulated transmitter, for testing receivers. These modifications provide a tunable signal with about 50 % modulation. The modifications are simple and reversible.

REFERENCES

Signal Training Volume III, Pamphlet No. 29, Wavemeter, Class C, No. 1, July 24, 1940

Copyright
Ray Robinson

INDEX