Prepping the Receiver and Transmitter Radios

Revision 03/12/14


Get a P100/HT50 Manual. Motorola part number 68P81055C50-B for UHF.

Why we kept the cases:

1. It's a handy way to mount the PC board.

2. The battery contacts - the programming interface uses two power sources. The radio side of the interface is powered by 10V from the radio. The TTL (computer) side of the interface is powered by the RS232 port of the programming computer. If you use a standard Motorola programming interface, you have to have the battery contacts of the radio to energize the interface.

Remove the radios from their cases.

1. Release the two screws in the top of the radio (they have little insulating washers, don't lose them).

2. Remove the screws visible in the case when you take the battery off. Keep the the screws, pitch the battery, keep one of the front covers.

3. Slide the radio chassis out of the case. At this point, take a small Phillips screwdriver with a good tip and GENTLY tighten any ittybitty circuit board mounting screws you see. This will prevent odd noises later....the characteristic Motorola portable "flutter" sound.

4. Remove the threaded insert that the antenna used to screw into. Gently unsolder the micro coax from the external antenna jack in the top panel of the radio.  (Don't even consider using the external antenna's too flaky, especially for the exciter.

5. With a handheld ream, open the keyed hole for the threaded antenna insert up to 3/8" round or whatever your chassis mount BNC connector needs. With an Xacto knife, cut away the escutcheon on the radio panel to expose the metallic surface below it, so that the BNC connector has no escutcheon between it and the metallic surface.

6. Install the BNC connector with the ground lug and lockwasher. Orient the ground lug so it is toward the center of the front panel (toward the external antenna jack). Nip off the center solder cup of the connector as short as you can without messing it up. Tin it.

7. Solder the micro coax shield to the ground lug and the center conductor to the BNC solder cup. You will have to bend the ground lug a little to make the spacing work out. DO NOT SHORTEN the coax or re-dress are going to need every millimeter of its length.

8. As you do this, keep in mind that when you re-assemble the radio, that little coax is going to have to be carefully dressed so it does not get pinched or make contact with the positive DC path inside the top of the radio. It's very close.

9. Remove the battery latch entirely. It will be a pain in your backside if you use the standard radio programming interface.

10. For the UHF exciter radio, cut a strip of lightweight copper sheet the same width as the cover over the final output transistor, and about 2" long. This is going to be a heat sink extension/RF chassis bond, we'll install it later.

For the VHF exciter radio, cut a strip of lightweight copper sheet the same width as the grounded leads on the final output transistor.

11. On both radios, tin a small area of the battery contacts somewhere that it does not affect the springiness or operation of the battery spring contact. On both radios, solder a 12" piece of red hook up wire to the + and a 12" piece of black hook up wire to the -. This should be 18 or 20ga, and flexible.

Refer to the photo below for connections to the radio PC board.

12. Notch the cases of the radios at the locations indicated in the photos to provide clearance for wiring to come out when the radios are mounted, and when the programming interface is attached. (If you build dedicated interfaces, you don't have to notch the case on the battery compartment side.)

13. Re-assemble the radios using the two screws in the top panel (these also make the DC to front panel connections) and the screw in the center of the radio  that does not go to the front cover studs. (the front cover is gone). After you tighten down the two top panel screws, undo them again and slide the chassis back out and make sure that you have routed the micro coax correctly and have not pinched, stretched, or made contact with the +DC post. It's a close, close thing.

Try to fire up the radio with a battery....if you have failed to make adequate clearance, you will blow the little fuse on the battery spring contact that looks like a green resistor. If you blow it, you can externally fuse the radio.


The UHF PA transistor shield to which you will solder a strip of copper sheet.

This will thermally connect the heatsink to the repeater chassis, and bond the PC board groundplane to the repeater chassis. You will do this after you have tested the finished mod and have mounted the exciter. Don't do it now.





PC Board Connections

Solder very small wires to these points, use shielded wire for Mic Hi. Leave the wires about 12" long for now.

The discriminator output level is compatible with the levels in the audio/squelch board of the station.

The receiver unmute signal will go low anytime the audio section is enabled, in other words, on either a Carrier or  PL detect. If PL defeat is high, it will be on PL detect only. If PL defeat is low, it will be on carrier detect.

With the radio's squelch control turned all the way to the left to the click stop, the radio will be in PL mode.

The receiver unmute signal will be buffered and inverted with the 2N2222 level shifter to be compatible with the station's CMOS level positive logic.

We'll provide an adjustable pad (the pot) and DC blocking cap to feed into the mic input. +5v electret mic element bias is present on this pin.

PTT will be connected to the "channel element ground" signal in the station. Pull-up is provided by the exciter deck.

bnc.jpg (127466 bytes)
Coax and BNC connector detail - correctly routed to avoid pinching

top-panel.jpg (106695 bytes)
Escutcheon material removed

notch-1.jpg (155812 bytes)
Tinned battery contact points and notch in case-battery side. Polarity markings are in the case.

notch-2.jpg (132165 bytes)
Notch in case - PC board side

top-of-chassis.jpg (127552 bytes)

whole-pc-board.jpg (180245 bytes)

pa-transistor.jpg (118633 bytes)


circuit-points.jpg (130245 bytes)
Connection points on the exciter/receiver PC Board


cosswitch-new.gif (4038 bytes)

You are basically done with prepping the radios.

Options that this article does not cover:

You can decode the channel switching and bring those connections out for using a 4-freq board and a diode matrix or something.......

You can bring out the high/low power switch to use in a low power battery operation mode (bypass the power amp and run the repeater at 4 watts on emergency battery revert power, etc.) by using T/R relays to switch around the PA.

You can bring out the PL disable contact from the radio if you want to be able to defeat Receive PL for some reason.

These are all functions you can "easily" implement using the tone control functions which are inherent to the station with the correct modules, but which are not specifically covered in this modification article.


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