ВЭФ 12 (Astrad VEF 12 Project) - including a transistor tutorial

[Nowhere-Man]

Mark did an article on how to repair them. Recently I learned how to test AF117's and identify the leads. I found one defective. I could probably use my restored 1930's battery eliminator to zap it.

[Crackle]

I am not sure 180 volts from an Ekco battery eliminator will do the transistor any good.
I have only "zapped" these AF11x transistors with a 30v bench supply. I hasten to add I have had 100% success on at least a couple of dozen transistors.

Mike

[Keithstv]

I've cleared the whiskers in AF117s using a 100uF capacitor charged to about 20V and discharged between the screen lead and the commoned emitter, base and collector.

Keith

[pwdrive]

I found the ARRL books ok, the cathode tap Hartley is easy enough to build, the cathode voltage is in phase with the grid voltage, that's what's needed for oscillation to occur, in phase feedback, no voltage gain with the cathode follower but there is current gain, the tapped tank coil is in effect an auto transformer.

Lawrence.

NM, since posting the above, I've been trying to find a more in depth explanation for the series fed Hartley oscillator, managed to find one today, scroll down to Fig. 213 for the schematic, a written explanation is given below that, it's for a transistor job but the analogy is basically the same for a valve job where the cathode is connected to a tap on the tank inductor:

http://www.rfcafe.com/references/electri...1-2-20.htm

Lawrence.

[Nowhere-Man]

OK Lawrence, thanks. I spent a good hour maybe Tuesday struggling over that circuit. One thing I noticed is as the signal lifts the cathode grid voltage, the anode cathode voltage should fall (or vice versa) but the signa
on the anode should be in phase with the anode drop. However much simpler to just say the tap controls the grid cathode drop and cathode anode impedance. Mostly I found tickler circuits in actual design.
I hope to translate the Russian circuit descriptions and data for the VEF and I'm forced to learn the terminology such as oscillation is the Russian verb for wobble or swing and thus used for oscillation.

[pwdrive]

The voltages at the grid and the cathode are both in phase, the voltage at the anode is of opposite phase to the voltages on the grid and the cathode.

Lawrence.

[Nowhere-Man]

Yes, that is indeed the case. What I refer to is the signal. Let me clarify my point: We have the centre tapped coil with the cathode at the tap. Each end of the transformer alternates + and -. Let's say a positive signal hits the grid (negative goes to anode, because each end of the coil is fixed to grid and anode). On the positive pulse, electron flow surges and the higher milliamp value times the circuit load reactance (or resistance) causes a larger drop on the load. So the anode cathode voltage drops. As you said, the grid cathode voltage rose so the voltages are out of phase. Here we agree and there's no confusion. My point is simply that the AC signal voltages are in phase with those drops. For example, as the anode cathode voltage drops, the signal phase to the anode will be - as the grid signal was +.
This extra part may or may not be of significance. The book doesn't mention the signal phase much.
Just to clarify to anyone confused. If a triode has a signal between grid and cathode, the bias on the grid will decrease or increase with the alternating signal. If current rises, the milliamp value is higher so multiplied by the anode load resistor drops more voltage across the load (leaving less voltage between anode and cathode). A - grid signal decreases current so less is dropped on the load and the anode cathode voltage rises. I bet if the load resistor was a bulb, the bright flashes would tell you its voltage got higher.
Always best to check I got this correct - hopefully so.

[pwdrive]

Best not to confuse the grid signal (AC) with the grid bias (DC) even though the former can result in the latter.

Lawrence.

[Nowhere-Man]

I sometimes wonder if the discoveries came before the explanations. The very early crystal radios were made by soldiers in the war. I imagine there was a lot of trial and error to get things to work. Later theoretical explanations developed. The main thing is the tap can create oscillation and it wouldn't surprise me if that was discovered by trial and error too.

[Terry]

The very early crystal radios were made by soldiers in the war. 

Which war? There was no public broadcasting as early as WWI - and eavesdropping on army radio traffic would not have been popular with the authorities!

Once 2MT started up - followed, of course, by 2LO at al - it was an entirely different ball game.

During WWII, servicemen did experiment with simple radios, though many of them did not use 'crystals', in the accepted sense, at all but relied on anything that could be made to work. For example, rusty razor blades, of which there was no shortage, were popular for detectors I believe ...