News flash: I fixed the BIG Marconi signal generator. I found it produced a splat from the RF output amplifier coil assembly on range C, the bottom end of the BC band, a very important range for me. The splat was followed by a burning smell.
I started out by dawing a diagram of this complex turret switch assembly and then measuring voltages on each contact in each position. It soon became evident there was a short on the HT on range C, hence the splat and the smell.
My first reaction was to suspect a faulty cap in that coil assembly or a shorted primary to secondary in the coil. To my relief I soon found it was as simple as a short through the cambric sleeve insutalting one of the tinned copper wire leads from the anode contact to an earth contact. Of course it was not realy that easy, the short was not in the range C coil pack, it was in range G coil pack, why ?. The wire was hard against the earth contact and the sleeve had squished through.
All I have to do now is clean and adjust the contact blocks on some ranges
It is a very complex beast with two hugh turrent coil assemblies that rotate with the range change switch. One assembly is the actual oscillator and the other is an RF output stage that puts the modulation onto the carrier. Unlike most signal gens it can be modulated to 100% and is calibrated. It makes a great AM transmitter for my collection of old radios.
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Written by and Copyright, Phil. Storr © Last updated 14th February 2011