Omni VI 2nd order IMD Mod

[H. L. Serra]

Del- I think there are two separate Tentec mods you all are discussing
here, both involving only a few parts. The first is the mod to effect
better second and third order IMD rejection, a quick and dirty version of
Rohde's suggestion in QST to change all the front end switching diodes to
PIN diodes, which have a higher resistance to rectification of strong
signals. Tentec's answer to this is to switch out the resistors which set
the switching voltage through the exisitng Tentec diodes. Their engineer
told me that raising that voltage on the existing diodes substantially
increases the reluctance of the diodes to rectification-- it's provable
according to a formula. The second mod also uses just a few parts, and is
meant to increase the resistance of the product detector to IMD from two
or more strong signals LEGITIMATELY within the passband of the OMNI. G4PKT
or someone produced a daughterboard to fit right on the OMNI product
detector board, and which contains a higher order product detector than
the one on the OMNI, which is bypassed. I installed the daughterboard and
it does work better than the old one-- even with strong signals within the
filter passband. For example, on CQ Alley on 20M and 40M during CW tests,
you can hear all the signals separately, even weaker ones. Tentec claims
that their mod using a couple of parts works just as well at no expense
(the daughterboard is about $80-90, and installation takes a few hours). I
really cannot judge how well their mod works, since I only have the G4PKT
daughterboard. 73, Larry N6AZE

On Thu, 28 Mar 1996, Del Seay wrote:

> John Morris wrote:
> > 
> > Would appreciate hearing from anyone who ever installed the subject
> > modification in their Omni VI.
> > 
> > This mod involved changing 2 resistors on the BPF/Front End board and was
> > supposed to improve 2nd order IMD from 40 dBm to 60dBm.However, TenTec
> > advised that "precious few hams in the U.S. would gain anything from
> > improved 2nd order IMD".
> > 
> > If you installed the mod:
> > 
> > 1. Degree of difficulty?
> > 2. Any noticeable improvement or difference?
> > 
> > TKS es 73, John K5OTI
> 
> Hi John. I did the change and in my case, it was worth the effort.
> It only takes a few minutes (Typical of Ten-Tec repairs) and the
> problem with ringing on receive disappeared. I assume the gain of
> the rf-amp is slightly lowered in the change. I see no difference
> in sensitivity and noise floor, so did not cause any unwanted
> changes that I can see, and certainly made the receiver much more
> pleasant to listen to. I cannot attest to the improvement in
> imd numbers, don't have the equpment to properly measure.
> However, Ten-Tec states it is their version of the mod suggested by
> the 'G' guys which changed out the balanced mixer. I doub't that it
> would compete with a better balanced mixer, but who knows?
> Anyway - It is certainly worth the few minutes of your time.
> 73  de KL7HF
>