RT85A FAQ

Conversion to 70cm

by Mark, VK3BYY
Melbourne, Australia

This page is a collection of information on the AWA RT85A FM mobile transceiver.
Most of this information applies specifically to the UHF low band units that were used by the Royal District Nursing Service, Melbourne, Australia, and found their way into the amateur service around January 2002.
The A version is different to the standard RT85 and the C models.
These instructions assume a reasonable level of technical competance and familiarity with radio tuneup proceedures.
Use this information with care, and always use the correct tools!

WARNINGS:

Do not tune the PA with the tin plate cover removed - safe RF emmision levels may be exceeded.
Do not adjust any ferrite cores during this proceedure - they require additional test equipment and tools, and adjustment should not be necessary in a previously well serviced radio.

Preliminary Checks:

1/. Hook up the unconverted radio. Connect a power supply and UHF power meter (or dummy load and SWR meter) to the antenna socket. Check the radio produces power (10 - 30 Watts) and that the volume, squelch, up and down buttons work. If the radio was programmed for CTCSS or SELCALL operation then the squelch may open only after the SILENT button is pressed (indicated by OPEN lamp lights). If it doesn't work then it's better to debug in this state before changing things!

Fit EPROM:

2/. Disconnect radio and open the top panel (larger lid). All directions are described with the Heatsink facing away from you.

3/. Replace the EPROM with a new version with Amateur frequencies programmed.
(pry it out of the socket with a small flat blade screwdriver under each end of the EPROM, being careful not to bend anything.
Be very careful not to bend the pins when replacing the EPROM).

VCO tuneup:

4/. Remove the coax cable going to the PA board (J366 - top right hand corner).

5/. With a fine screwdriver, screw the receive (L702) and transmit (L107) VCO slugs all the way in (so they bottom) - don't work about breaking these - they are metal.

6/. Connect up the control head, power supply and a dummy load again.

7/. Turn on radio. Select a channel in the middle of the desired frequency range. The radio will beep continuously if the PLL is unlocked. Tuning the VCO should stop the beeping.
Connect a volt meter to Rx VCO test point TP701 (top left corner - large silver pin). It should measure about 3.5 Volts but his is not critical. Now select the lowest frequency channel - check the voltage does not fall below 1.7 Volts. Select the highest frequency. Check the voltage is below 6 volts. If not, adjust L702 for the best compomise around 3.5 Volts.
(The VCO is quite broad and can tune at least 15 MHz).

8/. Repeat step 7, but this time for the transmit VCO adjusting L107 while measuring TP101 (right hand side, mid way between tin cans). Push the microphone PTT while doing this!.

Transmitter Alignment:

WARNING: keep transmissions short to reduce heat damage to PA components while untuned.
Keep an eye on the supply current - try to keep it under 7 Amps.
Watch out for "takeoff" or "spewglies" in the transmitter during tuning (this may sometimes be heard faintly in the loudspeaker).

9/. If you have a suitable connector, connect a power meter where the PA coax was unplugged( J366, top right corner). While pressing PTT, adjust trimmer CV102 with an insulated tuning tool for maximum output (about 500mW - 1.2 W).
If a power meter cannot be connected, just follow the next step and tune CV102 for maximum output.

10/. Connect the PA coax back into J366.
Connect a 10 Amp ampmeter in the main 13.8Volt supply line.
Rotate RV501 & RV502 fully clockwise.

11/. Select a Press PTT and adjust trimmers CV501 then CV504 then CV502 and finally CV503 for maximum output power.
Tweak CV102 if necessary. Repeat adjustments for maximum power. Power out should be around 20 to 25 Watts.
You may find that CV501 & CV504 have little effect on second adjustment - Try detuning CV102 so the output power is around 15 Watts, this will make peaking CV501, CV504, CV502 & CV503 easier. Finally reset CV102 to original position.
With careful adjustment you will find that slight detuning of CV504, CV502 & CV503 results in lower current consumption.
Avoid tuning points where the transmitter "takes off" - observable as spots where power output rises and falls rapidly when rotating trimmers through their peaks.
Check power output and current consumption across all channels. Retune for best compromise if necessary.

12/. Adjust output power with RV502. Set output slightly lower than required as the output power rises with temperature. Either adjust so that power is reduced slightly on all channels (if you want to live dangerously!), or reduce to a more conservative value (say 20 Watts - for a longer PA life!).
Check output across all channels, Current consumption should be under about 5.5 Amps at 20 Watts. Efficiency falls off rapidly at higher power and just creates a lot of heat so keep current under 6 Amps.

13/. Select low power by linking P302 to P303 on back of head. Adjust output to desired low power level with trimmer RV501 (say 3 to 5 Watts). Do not adjust too low as this adjustment changes with temperature and may turn off output completely in cold temperatures.

Receiver Alignment:

14/. Turn over radio, remove lower cover.
Connect suitable UHF signal source (see below).

15/. If this is an unmodified district nursing service radio, start by unscrew (anti-clockwise) all the helical resonator adjustment screws by 2 turns each (L201, L202, L203, L204, L205 & L206 are metal screws on diecast housings). This gets the tuning into the right ball-park for 70cm.

16/. Turn on radio and select mid-frequency channel.
Connect voltmeter to LO buffer between test point pin 4 and pin 1 on connector CM202 (connect located near left hand side. White plastic header with 4 pin positions, pin 2 is the missing pin).
Adjust trimmers CV202 & 203 for maximum reading.

17/. With suitable signal source at input, connect meter between pin 3 and pin1 of connector CM202. Positive goes to pin 3.
The RT85A is relatively narrow banded ~2 MHz and can be tuned for maximum sensitivity if all channels are within this band as follws: Adjust all helicals (L201 to L206) for maximum reading. Reduce signal source level as you go so signal is reasonably noisy when making adjustments. Also adjust CV202 and CV203. Repeat several times.

Maximum bandwidth can be achieved by stagger tuning the helical resonators. Try the following for a reasonable compromise with good sensitivity in the 438-440MHz range (repeater outputs & high end simplex) and acceptable but poorer sensitivity in the 433-435 MHz range (repeater reverse and low simplex band): First select a channel near 439 MHz and adjust L201, L202, L205 & L206 for best sensitivity. Also adjust CV202 & CV203 at this frequency. Then select a channel near 435 MHz and adjust L203 & L204. There is some interaction, so repeat the tuneup cycle for 439 and 435 MHz several times.
The lower frequencies will be a few S-points lower than the higher channels.

18/. Finally connect an antenna and try an on-air test. Receive performance should be similar to most amateur transceivers of a similar vintage.

Suitable signal sources:

Ideally a calibrated synthesised FM signal generator and a SINAD (signal/noise and distortion) meter should be used.
Many amateurs don't have these.
Free running UHF oscillators may drift too quickly to be useful.
A handheld radio set to low power and with a dummy load in the antenna socket may work, but often radiates directly into the mixer of the RT85 making tuning of the pre-amp stages difficult and often very diseptive!.
A better solution is to use a 1 MHz marker generator running through a switched attenuator into the RT85 antenna socket. It's relatively easy and cheap to build this type of test equipment (several designs in ARRL and AR magazines).
Failing all that, a weak off-air signal may suffice, but please don't key up repeaters endlessly while trying to tune your receiver!

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Last updated: 4/3/2002