Overhaul of a Class D Wavemeter No.I

 I was reminded that I'd had a Class D Wavemeter for more years than I care to remember, but I haven't touched it since the early 1960s and a recent discussion about the thing spurred me to open it up and see if it still worked.



 Below are views of the chassis. This is the MkII version. This model was very popular during the 1950s and 60s because (a) the GPO required all Radio Amateurs to possess such a thing (or its equivalent) and (b) this type was very cheap to buy.





 Before I tackled it I connected its battery cable to a DC power supply set to 6 volts, but even with the current limiter set to 3 amps the voltage couldn't be maintained so I removed the case to discover the reason. First I noticed the vibrator was missing and a glance at the internal wiring showed it had been modified to run from 6.3 volts AC instead of 6 volts DC.

I connected the cable to 6.3V AC and switched it on. The dial lamp came on but the HT transformer input measured only 5.7 volts and the winding was running warm. Switching off I disconnected the secondary and checked the load resistance. I found a short-circuit. It could be a faulty metal rectifier, but I spotted a condenser jammed vertically against the rectifier so decided it was probably the culprit as it was wired across the rectifier. Sure enough the condenser measured 0.1 ohm.


 This is C30 which was wired across the HT transformer secondary winding and jammed vertically against the rectifier W1A (right).

Why not across the transformer tags where it would have been more accessible?


 The next step was to see if the wavemeter worked and at the same time replace any bad components. I refitted the transformer secondary wires which need scraping to remove oxidation which was preventing soldering. Turning on the power I checked the input to the transformer primary and found it was the same as the power supply output voltage. The transformer secondary measured 188 volts AC and the rectified HT around 150 volts, but as the voltage was moving up and down a few volts all was probably not well. Sure enough, after plugging in a pair of headphones I could hear a loud hum which disappeared if I shunted a new capacitor across the HT rail. Positioned at the rear of the chassis were two rather scruffy condensers and on them I could read the word "electrolytic". I snipped the pair out and tested them.

   From the evidence of melted wax, the two condensers have been pretty hot in the dim distant past and checking them with my ESR meter yielded the results below. Ideal candidates for stuffing with modern capacitors.

 At this point I examined the "Working Instructions" as something wasn't quite right. Where the spare vibrator should have been fitted there was a coil and next to the coil it was obvious the spare valve carried wiring and there were components fitted which although looked to be the same vintage as the wavemeter were not included in the manual.

Pause for thought....

I'll just go through the operation of the unmodified wavemeter. In all operations you must know within a say 50 Kc/s the frequency you need to measure and also that dials of receivers and transmitters are reasonably accurate.

The Mc/s position of the wavechange switch results in a set of 1Mc/s harmonics which can be used to check the Mc/s markings on a receiver dial. With a little skill you can determine whether a received broadcast signal corresponds correctly with a dial reading or an image due to excessive signal at the aerial.

Other than the Mc/s feature which uses a 1Mc/s crystal in the same housing as a 100Kc/s crystal there are two other main parts to the wavemeter: firstly a VFO which tunes either 3400Kc/s to 3500Kc/s or 6100Kc/s to 6200Kc/s selectable by a wavechange switch and secondly a crystal controlled oscillator which produces a set of 100Kc/s harmonics.

The two signals, the VFO and the crystal oscillator are mixed together in a triode hexode valve to provide a set of frequencies used to place tunable markers on a receiver dial.

If you want to measure the frequency of a broadcast within the range of the wavemeter (1900 to 8000Kc/s) you initially check the zero marking on the VFO dial so that it beats with the 100Kc/s crystal (using the wavemeter headphones), then tune the VFO until it beats with the received broadcast. Reading the VFO dial, say 45Kc/s, means the received broadcast is 45Kc/s higher than the lower Mc/s receiver dial marking. However you can't say if the receiver dial is correct unless you know roughly what the broadcast frequency purports to be. If for example the receiver dial indicates something over 7Mc/s and the wavemeter indicates 45Kc/s you can say the broadcast is 7045Kc/s. However, if the receiver is out of adjustment and the true frequency is over 8Mc/s then the broadcast would be 8045Kc/s. If you tune the receiver from 1900Kc/s upwards you'll hear markers at 1945, 2045, 2145 etc up to the limit of the 100Kc/s crystal harmonics.

This means that the wavemeter is of use only if you know the frequency of the signal to be measured.

You can also use the wavemeter to measure the frequency of a transmitted signal. Again you would need to know the approximate frequency. This feature is used by listening to a beat note in the headphones.

A primary purpose of the wavemeter might be to set up a net of stations using receivers and transmitters whose dials are reasonably accurate. An instruction might be given to listen out on 4555Kc/s. Each potential member of the net would use their wavemeter to set their receiver to 4555Kc/s. The spec of this wavemeter is give or take a couple of Kc/s and this would be within the passband of the receivers in the net.

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