AWA RT-85 (E-band) Conversion Mods for 6 metre FM

Created 13th November, 2002 last updated 6th January, 2014

Modifications to an AWA Carphone model RT-85 to convert its operation from commercial VHF low band (70-85 MHz) to 6 metres HAM band
There are several versions of the RT-85, for high Band VHF and UHF, this page primarily covers the 60 channel low Band VHF,
25 watt output ( E band model)
This is transceiver is the later, EPROM programmable version of the AWA RT-80 series.
The RT-80 and RT-85 models were manufactured by Hitachi Denshi Ltd in Japan.

The RT-85 is characterised as a mobile base unit, approx 25cm L, 20cm W, 6cm D, with heatsink at one end, two IDC
connectors at the other end, one 26 pin for remote head connection a 20 pin connector, a BNC antenna socket and 13.8V DC power lead.
The remote Head, includes twin 7 segment displays for channel indication and several buttons for channels selection, scan facilities,
plus Volume and Mute controls, TX & RX indication LEDS.

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Converting RT85 to 6 metre band, articles have been done over the years by VK4TCS, VK3BKR & VK3TLW, VK2EK and VK2DOT.
VK2DOT Midland/RT85 document is probably the most recent and covers those mods done by predecessors.

Follow the links to these RT-85 pages:


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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.  The other articles by VK4TCS, VK2EK and VK2DOT  have the  four LPF coils re-wound to 6.5T.


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 Issues encountered with RT85 on 6metre band
One thing I found annoying is the image rejection in the receiver, the RT85E has 21.4 Mhz IF, High Side Injection,
so you will be able to hear a signal eminating at 2 x 21.4 Mhz = 42.8 MHz above your receive signal, so in a place like Sydney,
where there is a 95.3 MHZ FM station, that the image of say 52.500 is smack on 95.3, although not as bad at 52.500,
I find that 52.525 gets a bit of bleedthrough from 95.3, since the 95.3 FM broadcast station is wide band,
Now you would think that the Receiver front end would be detuned enough up near 95 MHz, but in fact the image signal is getting
in via the power and/or ribbon cable leads, not via the coax connector, so I had to add some ferrite cores to both the 13.8v
power cable and to the ribbon cable from the remote head to reduce the chance of bleedthrough  and it only happens when mobile in the
vicinity of the 95.3 Mhz signal, i.e. a few kms of the Artarmon, or from home where I look directly at the TV tower that has the
95.3 FM broadcast. Most mobile activity is non-existant to the interference.

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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.
5. replace the two tag pins for the speaker connection by soldering a 3.5mm speaker socket (available at Jaycar) to you can
plus a standard 3.5mm speaker plug into it.

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NOTE: only licensed Amateur Radio operators can operate this modified equipment on assigned Amateur frequencies/bands.

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