[TenTec] Omni VI D4-D5

[Dr. Gerald N. Johnson]

On 8/18/2010 5:55 AM, Gordon Gibson wrote:
> Hi Tentec'ers
>
> 1) I recently had a problem with D4&  D5 on the low pass filter board
> (81592) on my Omni VI (circa Nov92) which in the manual  are 1N4007
>
> a previous owner had replaced one of the original diodes with a pair
> of MPN3700 in series with a 22pf to earth

The two diodes are the RF switch for the Auxiliary receiver input or 
output. The diodes are operated as PIN diodes though 1N4007 are not the 
greatest PIN diodes. I suspect yours may be counterfit and ordinary 
lower voltage rectifier diodes that won't have the PIN construction and 
that makes them more lossy at RF. The MPN3700 is a PIN diode designed 
for RF switching and so has less less when open or closed.

The only reason 1N4007 have the PIN construction is that it makes the 
1KV inverse voltage rating easier to achieve and that makes it used for 
RF switching in cheap circuits, but there is nothing to force makers to 
meet the ratings with the intrinsic layer between the P and N layers to 
or to keep the junction area small. You could try 1N4007 from a major 
supplier, not generic or put the MPN 3700 back. My manual shows a 22 pf 
capacitor is proper from the common anodes of D4 and D5 to earth.

You could also check to see if any resistors feeding DC to those diodes 
have been changed, the true PIN diode would need less current for decent 
coupling than the 1N4007.
>
> However  in my wisdom i decided to put it back as in the book to a
> pair of 1N4007's this works no problem, however i now find that my S
> meter readings are about 2-3 S Points down from when it last worked
> correctly!
>
> The 1N4007's seem to be lossy on receive so I wonder if anyone can
> recommend replacements
>
> 2) Thanks Carl N4PY for your chip to update my Paragon1 to 1.9, It
> works  superb  its  great to have band stacking on the Paragon.
>
> 73 de Gordon G3ZFZ
>
> gordon at gibson174.plus.com
>
>
73, Jerry, K0CQ