The finished RT-70 modification ready for installation, Its small size and minima! power consumption make it ideal for mobile use, but for serious work you wilt need an "afterburner. "

In recent months a large number of RT-70 transceivers have appeared on the surplus markei ai prices from $15 to $25* These transceivers arc designed for ihe frequency band from 47 to 58,4 Mll^ and therefore completely straddle the 6 meter ham band. To find a surplus transceiver that doesn't require extensive changcs yq operate on a ham band, and is simultaneously cheap, is a real achievement that many hams would take advantage of if they knew for sure what the conversion entailed.

The RT-70 is an FM transceiver covering the entire 6 meter band in continuous tuning, and transmitting at 500 mW output The receiver section is a very good unit as is and will operate alongside the best of them without modification. The transmitter section is equally good, but several peripheral modifications will make it more convenient to use*

The first thing that is noticed when an RT-70 is

Left side and bottom view of the RT-70\ showing the "Tank, Field, Veh" switch, SWL on the left sidet in the "Veh'*position. Look closelyt just below the crystal Y-!02t to see the regulator chip installation as described in the text.

first fired up with its original microphone is that the audio quality is unbelievably lousy in that it won't modulate at all. Apparently years ago the carbon buttons were good but they have now deteriorated* because even the "new" buttons that I bought for 3 for S2.00t still packed in a hermetically sealed package, failed to remedy the situation. Fortunately this problem is easily solved. A standard telephone handset can be used as the miaophone by placing a resistor across the button to decrease the sensitivity. Without the resistor you can stand across the room from the handset and have enough audio to modulate the transmitter. The resistor value has to be individually selected according to the handset used. Mine required a 27 Ohm resistor. If you can get a handset with push-button in the palm you Have a fine TR switch buik-in.

Since some of the RT-70s being sold come without schematics, I will include partial schematics to illustrate the hook-ups. Fig. 1 shows the audio plug and connections to the handsel. Place the resistor inside the handset on the terminals of the carbon button itself. Later in this article, i will show how to add a switch to turn off the earphone in the handset for speaker operation. Note that in most handsets the ground lead from microphone, earphone and TR switch are common compter™

and a single wire. This wire can connect to either pin H, pin E or pin B as all are grounded inside the set.

Finding audio plugs for the TR-70 is not as hard as finding power plugs; therefore, I have elected to modify the power input to use a piece of five conductor cable that can have any type of connector you Mke

mounted on the outboard end. The modification is easy and works fine- Take a 5/16" drill and drill out the center of the present connector. Push the end of your 5 conductoi cabie ihrough this hole, lie a knot in it and connect the five wires as follows (see Fig.2): Connect the red one (if you have a red one) to pin J for the 90 V dc input. Next, connect the black one (again, note in parentheses above} to pin D for the ground. The remaining three wires are connected to pins B, F and A. But, prior to soldering to pin B, remove the wire already on that pin and place it on pin F along with the fourth wire in the cable. Now with the addition of a Fairchild 7806 regulator chip which will be described later, you can use the set with any voltage from 6 to 1 5 V dc. The Fairchild 7806 regulator chip is a 1 Amp 6 V regulator, all in one three head package with a mounting tab. The mounting tab is in most cases identical electrically to the center lead. Check your unit first, and if this is the ease, the center lead can be cut off. Next, looking at the bottom of the chassis, you will see jl mounted on the right side with two screws. Remove the nut on the screw roward the front of the chassis. Take a knife and scrape the protective coating from the chassis in a circle about 1/2" diameter around the screw. Now place the tab of the regulator over this screw so that the center lead of the regulator is pointing directly at the chassis mounting nut that holds the front pane! on. This center lead may touch the nut, but no harm is done as this lead is grounded. The other two leads must be bent away from the chassis to avoid contact. Now replace the nut and tighten the regulator in place. This mounting will act as both ground and heatsink for the chip.

Now, using the diagram in Fig. 3, connect the inpui lead of the regulator to pin B on the power connector and the output lead to pin F of the power connector.

Now in operation, if you have a 6 V filament supply it can be connected to pin F for


Fig. L Audio plug connections.

Left side and top view of the RT 70, showing the antenna connector on the end panel\ and most of the tuning adjustments for the transmitter.

the cable conductor connected Eo pin F), while a filament supply of 8 to 15 V should be connected to pin B. Do not try to run a 6 V input through pin B and thus the regulator, as there is a 2 V drop in the chip which will result in a 4 V filament voltage, and the set will not operate on 4 volts. Also, do not connect both a 12 V and a 6 V supply simultaneously: You may blow the chip. And a final caution; Do not use ac filament voltages. The chip doesn't like ac, and neither do the fifamems, Ac ¡ust wilt not work, so don \ try it.

This regulator is an important addition, as the filament voltage is very critical for proper operation, A 5.5 V filament will cause a reduction of transmit power of 50%!

There is a switch, S101, on the left side of the receiver chassis. This switch is primarily used to compensate power supplies to hold the current drain constant during both transmit and receive modes. If correct drain is a problemJ put it in the "field1" position and then use a very well regulated 90 V supply capable of regulating from 30 mA to 80 mA. If on the other hand you have a good supply of 90 mA at anything from 90 V to 120 V dc, you can put the switch in the "Vcb*" posiiiun and use a dropping resistor lo drop the voltage to 90 V. I used a surplus Sorensen power supply with an output of +107 V and a dropping resistor of 200 Ohms. The dropping resistor is calculated using the formulas ¡n fable 1, and then adjusted to get what you want using a voltmeter.

t finally used a 200 Ohm 3 Watt resistor. Since the switch SI01 is in J1Veh" position ihe current remains nearly constant antf the drop on the resistor remains constant.

The finaf three modifications ! will describe are primarily convenience mods and not absolutely needed. The set will now run having made the modifications already described. Remember, FM on 6 meters is allowed only on the upper portion of the band You can set the "preset"1 stops to mark the band edges, or use one of them for the Eocal frequency.

If you use the set mobile, you will need a dial light. The one in the set is a 331 and is useless. Remove the bulb and replace it with a 3^8; then, in order to get the correct voltage on the tube, "short13 out R-206- R-206 is mounted on the back wafer of the "Cal, Ant" switch, 5*202, on the front panel. The resistor can be identified by its code, as it is an 82 Ohm resistor. Merely solder a piece of wire across it and your dial light is ready.

This modification is a matter of preference, but if you are to make the next modification, this one precedes it. The present antenna connection is on the front panel. I like mine on the back, so 1 moved it back. Remove Ihe BNC connector and mount it on the chassis on the right side just ¿bovc V1-1, in the aluminum web that supports the chassis mounting screws (see Fig. 4). Then drill a 3/8" hole in the cover so that when the set is in the cover the BNC antenna connector protrudes through the back. Now, using a miniature 50 Ohm coax such as RG-188A/U, connect the new antenna connector to the fee dt h r ough originally connected to the front panel po*ej=r CC KWECTOP

po*ej=r CC KWECTOP


Fig. 2. Power cable connections.

R * E/l, where I \s The current required to operate The unit (approximately ,079 Amps) and E is the voltage of the power supplv less90 V,


Fig. 2. Power cable connections.

Table h connection. The shield should be grounded at both ends and placed in the channel runway along the housing of C-10.

Now, using the hole the original antenna connector came out of, put a singfe pole, single throw toggle switch in the front panel. Remove the red wire from pin A on the audio connector and connect it to one o! the switch contacts; connect the oiher contact to pin A. Now you can disable the earphone in the handsel to allou more volume in the speaker for monitoring when you are across the room, or for use when others in the car or shack want to hear both sides of the conversation, without blowing your ears off with the handset!

There are two further modifications that are possible on the RT-70. However, these are specialty items, and f won't go into great detail on them.

First, since this is an FM

transmitter, a ,4cheap and dirty" amplifier can be placed on the output using a ~IR switch and1 rf power transistor to achieve 5 Watts or so in one stage.

Secondly, the RT-70 is ideally designed to use on a repeater. Since the transmit frequency is the sum of the variable local oscillator in the receiver and a 15 MHz transmitter oscillator, a change ¡n the frequency of the 15 MHz oscillator allows the transmitter to be separately tuned. To operate on a repeater, it is only necessary to changc the crystal in the 15 MHz oscillato rf doubler (the crystal is 7.5 MHz bear this in mind when calculating the transmit/receiver offset). Thus you will have a transmitter that tracks the receiver, offset from the receiver by a previously determined frequency difference.

Final notes: All these modifications work equally as well on the RT-70 as on the RT 70A, but, in selecting the unit, try to get an RT-7QA. They usually arc newer and in better condition. Second, don't pay extra for the I MHz crystal in the calibrator unless it is real cheap. Some places con you for $5.00 and you don't really need it at any price! It is nice for tuning up, but not at $5.00. And finally, try to get a manual. They are worth the price if available. The manual describes a complete tuning and maintenance procedure, as well as the equipment. It is one of the best equipment manuals I have ever seen.

if you can think of any other mods that would be helpfu! I would like to hear from you, and I will keep a file on any "new" mods that ( will trade for your suggestions if you send an SASE, ■


Fig. 4. New antenna connector location.



Fig. 4. New antenna connector location.


Fig. J. Addition of regulator.




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