[TenTec] OMNI VI mixer

[John - G3JAG]

Hams stop at nothing ....!!!

Every once in a while I see an e-mail which causes my eyebrows to head
for my hair line. But if I had the nerve, I'd do what you did Bill !!
Except I'd pop in one of my MCI RAY-3's... I have had a lonely CP-643
here for years, and I have a German circuit (re-published - with errors
- in Ham Radio some years back) which uses it with the RAY-3, but 
because Crystalonics went out of business, I never followed thru and
used the thing. Maybe I should check their web site. For sure their UK
sales agent no longer exists. 

The key question is whether its worth the effort. I guess that for the
ham who wants to see the full benefit of spending lots of cash on INRAD
filters, the answer is "MAYBE". The benefit in IMD performance would
presumably come entirely from proper termination of the mixer. At
present, the termination relies on paralled JFETS which may or may not
be well matched. The single CP-643 is in theory capable of doing a
better job. But would the improvement be any help in working 3C0R or
T31T through the European racket ???? MAYBE ...

(No prizes for the first guy to suggest buying a bigger antenna
instead. Now that is a good idea, but the bigger the antenna, the
bigger the strength of the QRM and the better the RX needs to be).

As with all non-standard modifications, this kind of hacking is (a) a
matter of choice, and (b) you are on your own if it does not work !! I
suggest getting a spare rig just in case..

Yes, the OMNI-V has passband tuning, although the only time I use it is
if I switch in the TT 1.8 kHz filter to try to improve SSB copy in
heavy QRM. Otherwise its of little practical use. I could live without
it. I have several SBL-1 mixers in my 45 year old junk pile. I never
thought about the mixer in that area of the OMNI-V.

Bill, you have given us some interesting ideas. Thank you.

73 John

On 27-Sep-99 Bill Cook wrote:
> 
> 
> 
> John - G3JAG wrote:
> 
>> I just downloaded a couple of the Synergy spec sheets, out of
>> curiosity. They give enough info for me to stick my neck out and
>> pontificate, although I do not have an Omni-VI. My Omni-V appears to
>> use exactly the same mixer  -- unless someone out there knows
>> better.
>>
>> There should be no problem - the Synergy CMP 212 is probably a safe
>> bet.
>> It appears to have all the right characteristics. I cannot believe
>> that
>> they made a version of it with a special pin-out, just for TT -
>> these
>> packaged mixers all have the same pin-out almost regardless of
>> manufacturer. Ham Radio even published a generic PC board layout
>> based on this common pin-out.
>>
>> Unless the TT mixer really is unique, the Mini Circuits RAY-3 is a
>> very
>> safe bet. I found one for a few bucks at Dayton (its a $50+ piece
>> here
>> in UK) and I'd have no hesitation in dropping it into my Omni-V if I
>> thought the mixer had gone out. The SRA1-H may be another option. 
>> The
>> key figures to look for are the local oscillator level (17 dBM or
>> higher) and the frequency range,  which needs to go low enough, like
>> down to 0.025MHz (most go high enough). The regular SBL-1 type of
>> device simply does not have the local oscillator signal handling
>> capability.
>>
> 
>   John  et al,
> 
> I have two MCI SRA-3H and decided to try one in my OMNI VI  (upg3). 
> It
> works good.  I cannot tell the difference from the original Synergy
> mixer.
> I am terminating the mixer with a Crystalonics power fet CP 643
> (which
> provides 50 ohms resistive at 30ma).  Crystalonics is back in
> business,
> they have a web site, and they sell in small quantities (expensive
> though).
> 
> I am not sure if the Omni V has Pass Band Tuning.  However the 1st
> mixer on
> this board is a discreet diode/xfmr DBM.  I removed these parts and
> mounted
> a MCI LRMS-1 (low priced surface mount plastic pkg DBM/LO +7) on the
> underside of the pc bd .  The receiver has less noise and maybe
> better IMD.
> It is difficult to quantify without serious lab tests.
> 
> 73  Bill  N4WC
> 
> 
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----------------------------------
E-Mail: John - G3JAG <patents@dx0man.prestel.co.uk>
Date: 27-Sep-99
Time: 23:49:50

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