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SOME NOTES REGARDING THE  MODIFICATIONS TO RT85 by VK3BKR and VK3TLW
by Steve VK2KFJ
When I modified my two RT85's to 6 metres, one of the things I noticed was I was not getting the 25w o/p from the PA
as expected, I kept checking & re-checking the mods, but it just didnt seem right, I was only getting about 20w
from the two units, I continued using them, but noticed they were getting VERY hot when tx'ing and the current drain
was higher than expected, when compared to the power output measured from the PA.

Eventually one of my two RT85's failed one day, upon inspection I found badly burnt solder joints in the filter section
of the PA, then checking the other unit, it also had burn marks in the same vicinity.

After checking & re-checking, I was convinced that the original article was wrong, something was missed, so I re-wound the
coils in the LPF and Rx filter to what I thought they should be and bingo, I now get 28w o/p, current drain was back to
where it should be and it ran much cooler, I then adjusted the o/p down to 25w and they both been working great over
the past few years.

What I need to do is document the changes here on this page to compliment the original article that Roger & Mark wrote.




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MORE MODIFICATIONS TO RT85 
By Paul VK3TGX
Recently when trying to finally get my RT85 on air, I observed the noise on tx audio fault that has been the subject of
previous mods from Ian, (my rig was totally unmodified) However I didn't like cutting the ribbon cable as that meant 2
cords running from the rig to the head, a bit messy I thought.  So I prodded around in the head and found that putting
a 100nf capacitor from pin 6 to pin 4 of U7, LF351 op amp provided a 90% plus fix to that problem.
Subjectively it seemed to work a bit better that Graeme VK3XTA's rig that has had U7 removed and the ribbon cut to
include some shielded audio cable.

Another mod tried by Ian was to connect the mic directly to the rig bypassing the head altogether.  This seems to have
the same level of success, however PTT control is read by the micro processor from the head - the PTT line from a
directly connected mic has to be routed back up the ribbon cable to the remote head!  (there are spare wires)

Later on I plan to completely replace the ribbon cable with some shielded data cable, earthed directly to the rig and
the head case to provide maximum shielding.  This should servery reduce the crud radiated into other rigs in one's shack.

Another quick mod would be to fit some large ferrite beads to the power cable, etc to try and stop any crud at these points.


Addendum to the above:
by Steve VK2KFJ
1. don't use too long a piece of ribbon.
2. chop the ribbon close to the head, add some IDC header and socket, this way you can swap heads easily.
3. if you do item 2. then with your main length of ribbon, add some home brew shielding, stretch out the ribbon cable,
get some Alfoil and cut some peices so you can fold over the ribbon cable, not too many layers or it wont be flexible,
then use some clear plastic packing tape (the rolls about 10cm diam, 4cm wide) to hold the foil to the ribbon,
then at each end, cut and bare some thin plastic, multi-strand wire , the bare section should be 5-10cm stick this section
under the Alfoil at each end of the ribbon cable, leaving an exposed section on plastic coated wire, attach some suitable
lugs that can be screwed to the head and body of the RT85.
4. alternative to shielding the ribon cable itself, if you running the ribbon in a car, ie. base is in boot, head up at dashboard,
then run the ribbon under the car's floor carpet, throw some strips of Alfoil over the ribbon, so the ribbon resides
between the cars metal floorpan and the Alfoil, or make use of laminated plastic sheets with pieces of Alfoil enclosed and cut
the finished laminated sheets as required.
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