21-08-2012, 05:16 PM
I've had a look at the switches and it's not promising. Despite what the Schematic shows there are a full set of wires on the Impedance Selector switch, so that's no good. There are spares on the Power Range switch as far as I can tell, but that's buried behind the transformer and it would be a major dismantling job to get at it.
While I was at it I thought I'd better make sure that the Calibration is at least reasonable. I shorted out the duff switch position, fed it with 1kHz from my J-2 and set the Output to give a reading of 100mW on the Marconi. An AVO 8 across the Input showed 0.9Vrms. Assuming that the Marconi Input Impedance really is 8R then that shows an error of 1.25% - well within what I can measure or trust. So far so good, but what is the Input Impedance of the Marconi? I put an R-Box - a very good one - between the J-2 and the Marconi and set it to show half power on the Marconi, while keeping the Output of the J-2 constant - it does vary with Load. The answer was 5R!
Anyone care to explain that? Clearly the Input Impedance is not purely Resistive because of the Transformer, but I didn't expect it to be that far out. I'll try it gain at a lower frequency.
Alan
While I was at it I thought I'd better make sure that the Calibration is at least reasonable. I shorted out the duff switch position, fed it with 1kHz from my J-2 and set the Output to give a reading of 100mW on the Marconi. An AVO 8 across the Input showed 0.9Vrms. Assuming that the Marconi Input Impedance really is 8R then that shows an error of 1.25% - well within what I can measure or trust. So far so good, but what is the Input Impedance of the Marconi? I put an R-Box - a very good one - between the J-2 and the Marconi and set it to show half power on the Marconi, while keeping the Output of the J-2 constant - it does vary with Load. The answer was 5R!
Anyone care to explain that? Clearly the Input Impedance is not purely Resistive because of the Transformer, but I didn't expect it to be that far out. I'll try it gain at a lower frequency.
Alan
