[HBR] Re: GB> RSGB One-Tube Keyer: UPDATE

[Mike McCarty]

EP Swynar wrote:
> Good Day All,
> 
> Well, I've finished construction of the 12AT7 RSGB electronic
> keyer...and the darn thing refuses to work!

How disappointing.

> http://www.flickr.com/photos/15888272@N05/?saved=1
> 
> The oscillator just won't...OSCILLATE for me, that is (that's the

Amplifiers oscillate, oscillators don't.

> first triode section of the 12AT7 in the schematic). I'm going to
> re-check the wiring yet again, and maybe swap in different
> components...but at this stage, I'm dead in the water...
> 
> Too bad, because it was really beginning to look like something...!
> 
> Does anyone with any past experience with such devices maybe have any
> words of wisdom they might share with me in my quest for a cure...?
> There was a "one-and-a-half" tube version of this very keyer featured
> for years in the early 60's editions of the ARRL's HANDBOOK --- maybe
> some of you guys who built it back then have some tips.

You haven't given us anything to go on. I've printed the schematic,
and it's pretty straightforward. V1a is arranged as a simple
slightly modified Hartley oscillator, and V1b is a voltage comparator
with adjustable threshold.

An oscillator needs three[*] things to work...

Source of DC power
Power Gain at intended frequency of operation
In phase feedback at intended frequency of operation

If you've got these, it'll work. So, you are missing one or more
of the above.

Be sure to use proper HT procedure (one hand in back pocket, etc.).

Let's start with power.
Do you have AC power to the primary?
Do you have Heater power to the tube? Correct voltage? Does the
tube get hot?
What is the voltage across the filter cap? Is it proper polarity?
(Did you connect up the diode the right way?)
Does T1 show continuity?
Set "speed" to fastest (lowest resistance) and "weight" to "most"
	(minimum)
Set "ratio" to midpoint.
Close the key.
Using B- (top of filter cap) as reference, what are the voltages
you read on
	V1a grid (2) (should be 0V)
	V1a cathode (3)
	V1a plate (1)
	V1b grid (7)
	V1b cathode (8)
	V1b plate (6)

Did you use a polarized cap for the 0.22uF? That's not permissible
in this circuit.

> 
> Many thanks in advance...!
> 
> ~73~ Eddy VE3CUI - VE3XZ

[*] To work PROPERLY, it also needs no in phase feedback at unintended
frequencies where the gain is higher than it is at the intended
frequency of operation.

Mike
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