Wednesday, 16 December 2009

IF Amplifier

Now we have some IF Transformers (OK - they're not as good as commercial items but the price was right and I learned again some tiny fraction of the wisdom of the ancients) we can go ahead and make the IF Amplifier stage.

Here's the schematic (modified from the original on Harry sm0vpo's website)...

It went together very easily - and I could confirm operation by applying a signal to the primary of the first IFT (via a 470 kOhm resistor) and looking at the response at the secondary of the second IFT. All very interesting, but it would be much more fun to connect up the detector and LISTEN to the response...

I wired it all up, only to discover that the BFO was not working. It used to work - you know it did because I showed you pictures. What had gone wrong?

After some messing about I discovered that the problem was quite a subtle one...

Although the BFO was working fine before I added the IF Amplifier stage (a single EF80 pentode), the additional valve pulled down the heater voltage slightly. This drop in heater voltage stopped the BFO - I could get it to run again by dropping the HT supply to around 180V (where it would start and run) but if I pulled the HT back to the usual 230V, it stopped again. Similarly, if I lifted the heater volts for a few seconds, it would start, but stop again when I returned it to nominal value - interesting!

Swapping the 12AT7 with another from the "junk" box produced an oscillator that would run robustly.

Vincent 2e0zvl came up with what sounds to me a credible explanation; cathode - heater leakage. The (Colpitts) oscillator circuit runs with quite a high impedance cathode network, which could be swamped with such leakage between cathode and the heater (effectively at ground).

OK - now it's running I ought to be able to hear a 456kHz input to the IF amp modulated down to AF. Yes - sure enough - it is there, a nice tone which moves with the BFO control as expected and which allows me to hear the "selectivity" of the system by changing the RF sig gen frequency around 455 kHz. Fine - until I take the 'scope probe off the BFO oscillator output, when it goes dead again...

Time to make a fool of myself - it took me ages to realize that the 'scope probe was changing the tuning of the BFO to such extent that removing it took the demodulated AF tone outside the passband of the IF (and/or my ears and/or the AF amp/speaker combination). It was still working fine - it just needed tuning without a probe hanging on it. FOOL!

It is a sort of laziness which I guess many of us are guilty of - I am so used to throwing a buffer stage on an oscillator that I wouldn't dream a 'scope probe would disrupt things - FOOL! A buffer in a modern rig might be just another transistor, soldered in place in a matter of seconds. But a buffer in this rig would mean another valve, more heater current, metalwork, space, time - forget it!

The greater part of the work - and the fun - in this project has been in learning exactly this kind of thing. Stuff that was common, everyday practice to the old masters, now waiting to be re-discovered by fools like me!

Here then, finally, is the project to date with two homebrew "IFTs" proudly sitting on the side of the chassis...

The screening cans are made from baked bean cans - it is difficult to find cans without "corrugated" walls nowadays as I discovered walking around Tesco's at the weekend - why is that (rhetorical question - please don't write).

Here's another view (just can't help myself)...

The IFT toward the front of the chassis is the IF Amplifier's load, whilst that to the rear loads the mixer - I wanted to leave the area in the "middle" of the chassis clear for the VFO's tuning arrangements.

OK - next it is the VFO - but I don't know if that will happen immediately. The festive season and "Winter Sports" might push this project onto the back-burner for a while.

...-.- de m0xpd

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