Radio Repair, Maintainence and Modifications — Totseans

Radio Repair, Maintainence and Modifications

DaktologistDaktologist Global Moderator
edited December 2011 in Life
Well the shortwave receiver of mine has been causing problems with me lately among other things. Firstly an intro to how it works.

The radio receiver is a Wadley Loop design which differs from the Superheterodyne type receiver used in most radio receivers (google it). They work by mixing the received signal with an oscillator to create an intermediate frequency or IF. Then the first oscillator signal is mixed with the signal from the harmonic generator before being filtered with a band pass filter then mixing all this with the IF from the first stage. It sounds rather complicated but makes for a rather stable receiver.

Example
Let's say we want to pick up signals from 0 to 30 MHz. We'll divide this into 30 1 MHz bands, and translate them to a band at 44-45 MHz. To convert 0-1 MHz, the first oscillator must be 45 MHz, to convert 1-2 MHz it must be 46 MHz, and so on. Meanwhile, we also mix the first oscillator with harmonics from a 1 MHz crystal and put the result through a 42 MHz filter. Only one harmonic gets through. When the first oscillator is 45 MHz, it is the third harmonic, because 45 - 3 = 42. At 46 MHz, it's the fourth harmonic, and so on. The oscillator does not have to be exactly 45, 46, and so on, only close enough to get through the 42 MHz bandpass filter. Let's say it is 45.1 . Then we get 42.1 from the filter, and 45.1 - 42.1 is still 3. When we mix the high IF with the 42 MHz, we get a band of signals from 3 MHz to 2 MHz, from which we select the desired signal, perhaps with a conventional superheterodyne back-end converting 3-2 MHz to 455 kHz and finally demodulating the signal back to audio.

Anyway the problem was that the first oscillator tuning knob was slipping. It has some slip to prevent one from turning the knob all the way to one end and snapping the linkage to the variable capacitor but it wasn't able to go through its full range of travel. After removing the grub screws to take the slip linkage out and washing it in gasoline to remove the grease that shouldn't have been there in the first place (most likely done by the previous owner) and reassembling and recalibrating the slide rule readout according to the service manual it works as good as new. Older radio receivers are much easier to repair as most of it is mechanical rather than electronic.

When I got it, it still had its original incandescent lamp based back lighting installed although the lamps had burned out. Being the person I am, I couldn't be bothered with bulb replacement so I replaced them with a couple white LEDs wired in series with a 220 ohm resistor since they are on 12 volts. At least now I won't have to worry about light bulbs. I am going to add a C13 socket on the rear rather than the hard wired AC power cable it has on it at some stage since I also use it on battery power and the AC cord pisses me off when I use it this way.

If you have some technical experience, these sort of jobs are easy enough to do and save money in the process.


Pics:

SAM_1521.JPG

As you can see here, this is the tuning unit for the first oscillator. The variable capacitor is the bit that sticks out from the steel backplate.

SAM_1522.JPG

Here you can see the mechanical linkages inside the tuning unit. The brass coupling houses the slip mechanism which was faulty. It passes through to the tuning capacitor and actuates the slide rule readout via the nylon gearing. You can also see the LED back light I added.

SAM_1527.JPG

The tuning knob and readout.

SAM_1529.JPG

The radio working after the work is complete. The readout is accurate now that I recalibrated it.
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