Sunday, 27 December 2009

Mixer

The worst excesses of Christmas being over, I returned today to the Valve Receiver Project (Blogs passim)...

the last stage to be constructed is the Mixer - a modified version of the schematic on Harry sm0vpo's site is shown below...



I had a slight panic on looking at the schematic - for I realized that I had mis-read Harry's original instructions and not noticed that the valve is a HEPTODE (not a Pentode).

I had a rummage through the junk boxes and came up with only two heptodes - one an ECH84 which looked as if it had been to hell and back and the other an EH90, which looked as if it never had been used. Fate made the choice for me, as I had already drilled the chassis for a B9A valve base (as used by the ECH84) whilst the little EH90 has a Miniature 7-pin base. For the record, here's the pin-out for the ECH84...



This valve is similar to the ECH81 which Harry used - another Heptode/Triode, the Triode section of which I'm ignoring.

The mixer stage is simple to build - here's a shot of the ECH84 doing its stuff...



Readers may recall that I needed to find a twin-gang capacitor to use as local oscillator tuning AND first stage resonant grid circuit - the best I could do was the ugly triple-gang unit seen below. Actually I like it - it looks appropriately "old-fashioned" and clunky.



Whilst I was in metal-working mode I fitted the remaining controls to the "front" - an RF gain potentiometer (extreme left) and what will eventually be the band-change switch...



The receiver to date is seen in all its (limited) glory in the next photo...



The good new is - it works! It doesn't work very well (yet) but I can hear CW on the 80m band - which was my target all along.

Next steps will be adding trimmer caps to align the two tunable circuits and investigating the low sensitivity. For now, I'm just going to bask in the warm glow of thermionic radio reception!

...-.- de m0xpd

No comments:

Post a comment