Thursday, 26 September 2013

Spreading the spectrum

The 10m WSPR beacon still causes me headaches. After a lot of twiddling, and much technical advice from Ken G8BEQ (Thanks Ken!) I realised a mistake i'd made with the bias setting of the doubler, and now once again have some sort of signal from it. The modulator however, continues to give trouble. There seems to be no carrier balance at all, all the 28MHz LO in all comes out, AF input or not. So, I have added a preset resistor to try and null the carrier.

Before I can sensibly try it though, I need to get around the problem of observing the spectrum properly on the spectrum analyser. Thanks to Bob G3OOU, I now know how to set the analyser up for this, although it shows that the machine is rather drifty! (either that or the 2955 is!). It seems that since the machines minimum span is 100kHz, I need to really give it some welly with a modulation frequency if at least 10kHz to be able to make the spectrum spread out over a few divisions. Now I know this, I can give the WSPR transceiver 10 or 15kHz of AF drive and see what the output of the modulator really is doing.

One further thing that Ken has advised me with, is the standing current of the PA section. In the last post I mentioned it was getting too hot even with no drive, well, it seems its dissipating nearly a Watt! This is supposed to be a 200mW output PA! Ken has given me some new bias values to try which bring the standing current down to a mere 20mA. But the PA has to wait, the modulation needs sorting first!

I still have my doubts about either the transformer or the diodes. Its the National Hamfest tomorrow, so if im able to go, i'll get some proper OA91 or 1N34s!

Rang Geoff at Castle Electronics today, my FT-857D is repaired and ready. RF preamp transistor, set of IF filters and a few PIN diodes needed changing. Learnt an interesting trick from Geoff on using both antenna ports to see if the front end streering diodes are leaky. Should have the rig back mid week (being sent to work) so finally back on air HF mobile!

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