[Heath] HW-101 VFO - more yet

[ChrisIwata]

The J310 will probably work, but isn't a near replacement for the 
MPF105.   The J310 has a much higher Idss, so the VFO will draw more 
juice.  It also has a higher transductance.

The MPF105 is no more, but I did find a shortened datasheet.

A nearer replacement might be the  MPF102 or 2N3819 that everybody seems 
to like.  Personally, their tolerances are way too wide:  From one batch 
to another they are very different.  I prefer the 2N5486 which has 
tighter specs.

Truth be told, almost any JFET will probably work

Chris, KL7DM


On 3/6/2018 2:50 PM, Kenneth G. Gordon wrote:
> OK. Several discoveries:
>
> 1) BOTH BJT "zeners" in both VFOs are open.
>
> 2) Removing the "defective" VFO from the rig, removing the open BJT, and replacing it with
> a junk-box old GE 9.1 V 1 watt Zener (all I had), I measure 8.89 VDC where the manual
> says one should measure 8.7 VDC.
>
> 3) The defective VFO works just fine....as long as I have a DVM test lead connected to the
> JFET's source-lead at C-945... from this I draw the conclusion that the higher voltage from
> the open BJT has caused the JFET to deteriorate to the point it needs more feedback in
> order to work. Possible? Maybe...
>
> I'm going to order a few LM336-5 and LM336-9 precision Zeners from Mouser today, and a
> couple of J310 JFETs.
>
> It is kinda tight in the completed VFO, but I manage.
>
> More later,
>
> Ken W7EKB
>
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