[Heath] More on HW-101 VFO

[Kenneth G. Gordon]

On 6 Mar 2018 at 9:56, Bob G3OOU wrote:

> 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.

That is more in line with what I thought it should be. I wonder why I am measuring 11.8 VDC 
between the emitter and ground in my "good" VFO? Perhaps that BJT is open. Or perhaps 
there is RF at that point which is effecting my DVM. Maybe I'll measure that point again 
through an RF choke just to see...

> 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.

Good thought. I have some LDO regulators here.
 
> 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.

OK. Hmmm.... I wonder if something like a 2N5524 might work more safely there? The 
problem, in my mind, is that the VFOs have worked correctly and very well for many years, 
so I don't really think I can improve on them. The original designers weren't stupid.

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

Well, I've always thought the Vacker circuit was excellent. The Franklin is pretty good too, 
although harder to implement.

Thanks, Bob.

Ken W7EKB

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus