[Heath] More on HW-101 VFO

[Bob G3OOU]

Hi Ken

In the days when reverse biased base emitter junctions were used as 
zener diodes I recall 5.5volts was a typical result. This is supported 
by the 2N3393 data sheet which says 5v max Veb.

These days I use low noise LDO regulator ICs like the LP2950/51 (DIP) or 
MIC5205 (SMT) which are much better than zeners and not expensive.

On a related oscillator topic I did some work on a Vackar VFO using a 
J310 which although extremely stable in frequency would occasionally 
fail when run from a 9v DC supply. I reduced the supply to 6v and no 
more failures. I assumed that the reverse gate source voltage of -5V was 
being exceeded but did not do any further tests. The MPF105 Vgs max is 
stated as -8V.

The Vackar circuit and test results are shown on my technical web site.

73

Bob

Bob F Burns G3OOU, G-QRP 6907, @BobFBurns
Crystal Palace Radio & Electronics Club: http://cprec.btck.co.uk/
Technical web site: www.qsl.net/g3oou

On 06/03/2018 01:47, Kenneth G. Gordon wrote:
> Thanks for the advice, folks.
>
> The subject VFO is defective. Output disappears after the VFO is well-warmed up.
>
> A "spare" VFO, substituted for the suspect one works perfectly and makes the rig work as it
> should.
>
> Measuring the voltage at the emitter of the BJT "zener", I get 11.8 VDC in the good VFO. I
> have not yet checked that in the suspect one but will asap.
>
> Does anyone know, for certain, what that voltage should be? Or to put the question another
> way, does anyone know, for certain, what REAL Zener diode one would use to replace the
> BJT emitter-base junction?
>
> Ken W7EKB
>
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