Converting the Midland LMR70 or AWA RT85 to 53 MHz.

Introduction.

This report describes modifying a Midland LMR70-076B to 53 MHz amateur. The AWA RT85 low band is very similar to these transceivers, but is not the same. I have based the modifications presented here on Roger Baker and Mark Detering's report. I have assumed that anyone attempting these modifications has a copy of the circuits and is familiar with this type of equipment. I do not have a manual for the 70-076B, but the circuit of the RT85 is close enough for the purpose.

This is not a newby project. You should be competent at soldering, including surface mount components. You should also have access to a DVM, a frequency counter, an accurate power meter, a dummy load, and a weak signal source – not an off air signal. A power operated solder sucker (desoldering station) will make the work much easier and board damage less likely. An experienced RF person may be able to get away with less, but this depends on the persons skill.

Before commencing it is a good idea to check out the transceiver on its commercial frequency into a dummy load. Quite a few of the ex commercials I have run across come complete with faults, its nice to know that they are working before you commence !

Programming.

Programming the transceiver requires an eprom programmer. The eprom is soldered on to a small circuit board that plugs into the synthesizer board, originally a purpose built programmer was used, but these are a rare item today, if you have one, use it.

The old way Desolder the 2716 eprom from the circuit board, be careful doing this as 2716's are getting hard to obtain ! I use a power desoldering tool to do this – it makes the job much less painful.

You can either resolder the eprom back onto the board or place a socket onto the board, the trick here is to find a really low profile socket, if you can find one. You can use a standard socket, but again it takes some fiddling. To use a standard socket, fit the socket to the board and then carefully desolder the two header connectors. Replace the header sockets but with a gap between the board and the socket, a piece of veroboard makes a suitable spacer. When the eprom board is fitted with an eprom and plugged into the synthesiser board it should “feel” like the connectors have mated correctly. If you are going to use the transceiver mobile I suggest that you place some packing material between the eprom board and radio outer cover to prevent the board working loose.
The New way Remove the two socket headers first. Cut a 30mm piece of heavy 1.6mm copper wire, solder to all pins of one of the socket headers, melt the lot and pull off the socket header. Do the other one. Now you have access to the eprom to do the same trick again, remove the eprom, one side at a time. Solder suck all the holes. Refit the socket headers with a piece of vero-board under them. Fit a low profile eprom socket, done. No Vacuum Desoldering Tool needed. - Alan

The Synthesizer Board.

RX (main) VCO, add 15pF to C709.

TX (offset) VCO, add 15pF to C137, located under VCO cover on the track side of the board. Some transceivers may require more.

RX buffer amp, add 5.6pF to primary of L709

TX buffer, remove L114 and keep for use on the PA board; replace with 7.5 turns of 0.5mm wire, same diameter former.

You should be able to apply power with the eprom board in, and the RX LO output and TX exciter output disconnected. Use the correct alignment tool – the ferrite slugs are easily broken. Connect a frequency counter to the RX LO output J365 and a DVM to TP701. Carefully adjust L702, the the RX (main) VCO, so that the synthesizer locks. Set it such that the DVM reads about 4.5 V mid channel. The counter should now read the RX frequency plus 21.4 MHz.

Connect the counter to the TX exciter output J366 and the DVM to TP101. Key the transmitter and adjust L107, the TX (offset) VCO for lock and about 4.5 V mid channel. The counter should read the TX frequency.

The Receiver Board.

Front end.
Add 22pF to C202, C205, and C212.
Short C210.
Add 1.5pF to C203 and C211.

Add 8.2pF to C201, C206, and C213.

LO tuned buffer.

Add 5.6pF to C220 and C223.

Receiver tune up is straight forward. Monitor CM202 pin 4 with a DVM and peak L209 and L210 on a mid frequency channel. A typical reading is 0.4 V. A common problem with these transceivers is lack of LO injection if they are not modified and tuned correctly.

On a mid frequency channel (53.250) feed a signal in from a signal generator. Monitor CM202 pin 3 with a DVM and peak L201 to L205 for maximum while remaining in the linear range. An alternative method is to use a Sinad meter or tune for maximum quieting. When correctly aligned the sensitivity is typically 0.35uV for 12db sinad over the range 52.5 to 54 MHz.

The PA Board.

Remove L501 and keep; replace with original L114 from the synthesizer board.
Remove L503 and keep; replace with original L501.
Remove L507; replace with original L503.
Remove L513 and keep; replace with 2.5 turns 1.25mm wire, same diameter former.
Remove original L512; replace with original L513.

Remove L515, L516, L517, L518 and L519; replace with 6.5 turns 0.63mm wire, same diameter former. Since these coils will not be reused, use sidecuters to cut them in half and then remove the pieces by heating the joint from the underside and carefully removing the part coil from the top.

Add 33pF to CV501 and CV502 on the underside of the board.
Add 100pF between B and E of Q502 on the component side of the board as per the photo.
Add 1000pF between B and E of Q503 on the component side of the board as per the photo.
Add 1.5pF to the RF sensor circuit (C554) on the underside of the board as per the photo. On the RT85 this capacitor is already fitted.

Adding the capacitors B-E on the driver and PA transistor is required to stabilize the amplifier. It is necessary to replace the coils in the LPF to get the second harmonic output of the transmitter down to an acceptable level.

Having made the modifications, reassemble the PA and apply power. Terminate the output in a good dummy load with a power meter – not an antenna ! Set RV502 fully clockwise for maximum power out. Key the transmitter and adjust for maximum power starting from the output end and working back towards the synthesizer. Don't forget CV102 on the synthesizer board. After modification and tuning it is normally possible to get 50W out of the transmitter with no sign of instability, do not run it at this power for long – there is not enough heatsink on the PA. When you have maximum power out adjust RV502 for no more than 30 Watts out.