Introduction:
For repeater use will require a custom-build chassis. You are required to know the micor radio inside and out. This is especially true if you are converting a high range (150.8-174 MHz) transmitter to a low range for the Amateur band. Repeater building experience is needed as well. Mistakes can cause hours of frustration. If this is your first time seek help from an experienced person to avoid problems and equipment damage.
Repeater usage
The micro radio is a very rugged, heavy mobile radio unit (draw unit, if you will). in the 1970's, OEM design was to work with an "accessory group" consisting of a power cable, speaker, control head and microphone. The basic ordering option was 1, 2, 3, or 4-frequency channels controlled with the head. The radio has several sections inside; transmitter deck, receiver deck and control board (in the center). For this project will be using the transmitter section. The others can be discarded or taken apart for parts or other projects.For repeater use will be a single frequency operation and the transmitter section will be pull and mounted on a 2RU 19" rack panel. The appropriate controls then can be installed on this panel, eliminating the need for separate cables and controls. This will clean up the usual clutter in a repeater cabinet or rack. This unit is set up for negative ground. This arrangement is similar to the "compa" station unified (or non-unified) chassis, without the control circuitry. An example shown is the first custom built transmitter pre-1990 started the compact package, being serial #1.
The second example shown here, being serial 2 had some changes-improvements. For starters, the front panel is now white for poorly lighted sites. Notice there's a switch function to "lock on" the PTT line for transmitter testing under load. This ON-OFF-ON switch was sometimes misleading. The intention was center was "OFF" to disable the PTT line for maintenance. However, if the switch was turn down thinking that's a "disable", whereas, actually leaving the transmitter keyed indefinitely. More on this situation is discussed below.
The rear...........
Each side for #2.
The PTT maintenance control was improved by changing back the "enable switch" to a standard ON-ON (up and down, no center position). A second switch was added with a "lock-on" function. Both switches have to be in the up (ON) position for this maintenance feature to work. An LED was added that flashes red to remind the technician of this (temporary) maintenance state. Note the changes on the front panel; taken while in actual service.
The overall view of the transmitter sections; the exciter and P.A. boards. OEM specification for the intermittent duty transmitter is +50dbm, however, for repeater use this needs to be down by 2-3 db. The ferrite beads keep most of the RF out of the transmitter's circuits for hostile sites.
For repeater use, beside dropping the rated power a few db, fan cooling is also necessary, especially for the mobile type of transmitter. Running the fan 24/7 wears it out. A better choice is have it come on only when needed. This is done by a thermo switch, normally open, then closes at 120F. For redundancy two of these switches are mounted, in parallel. Be sure to use a good layer of heat sink compound under each switch because the mounting surface is not perfectly flat.
Exciter board
The exciter board is the "heart" of the transmitter, therefore most of this document will discuss it. It holds the frequency determining element (channel element), the modulation section and all the multiplier and amplifier stages to a usable level. The type of modulator is very important for a flat system, therefore, the 4-pin exciter board is used for this project. If 3-pin exciter boards are the only available type the channel element can be modified for direct FM, discussed in another document.
The board also needs +12v, +9.6v and switched (keyed) +9.6v. The latter gets it from a regulator in the stock mobile package, therefore, for this project a regulator and keying switch will need to be added. The LM-7810 3-pin regulator is used. The output goes through a simple rectifier diode (6 tenths voltage drop) causing the output of 9.4 volts which is close enough to the stock "9.6" voltage. The 12 volt section runs the last output stage, while the "9.6" runs most of the other circuits on the board. The left image is the older, obsolete mounting location for such 9.6 regulator (the PTT switch was located elsewhere). More recent version (2009 and later) have the parts in line with the (not used) PL deck's pins, P401, as shown on the right, circled. In the event the components needed replacement is an easy task working the pins as solder mounts. To get started on this you first need to cut down the pin length (so the top of the parts still fit in the chassis) then tin them heavily.
This is first done by making the regulator have a very small "footprint". Shown here is the .22 uf cap and dropping diode, making this a "sub-assay" to be mounted on the pins. The cap is very important for the regulator's output to prevent oscillations on the DC line.
Actually, several were made up for several transmitters.....
Mount (solder) both the switch transistor and the regulator assembly on the pins. Front and back views.....
Left shows detail where to mount the PTT in-line resistor. Besides the 9.6 regulator modification, there's a slight heat issue problem with some boards. The second to the last RF stage runs a little too hot during long transmissions. To reduce the heat, but still provide enough RF drive to the last RF stage Q405's collector voltage is lowered a little by adding a 27 ohm (1watt) resistor in series with L413. (circle indicates the area.)
This equipment is old and subject to wear. The (floppy) RF output coax is no exception. During normal mobile use, plus modification handling can cause most or all of the wire shield to break off which would cause level and interference problems. The coax is just long enough to reach the filter it plugs into so cutting it is not an option. If this is the case with your board, remove the broken parts of the coax and board's eyelets. Take a new piece of shield from a spare piece of coax and solder-wrap around the damaged shield to extend it to a point it can be soldered back to the board as shown here. Left shows a repaired shield on the coax output. The "shorty" RCA plug's ground fingers get loose over time. It's a good idea to press all four in slightly for a tight fit on the filter module. Be careful not to slip and bend them in too far, when doing this. The right shows the completed exciter board modifications on the solder side, plus some alignment-test wires; power, ground, PTT and audio. Old design was to remove the board's pins which was a time consuming and unnecessary task. For this version they may be left and the I/O wires soldered to them.
Mechanical
The mechanical part of the project is working with the panel, drilling, mounting the standoffs and fastening with screws and bolts. Most of the bolts are 6-32, 8-32 and 10-32 machine with phillips head. Phillips head (and pan head) make it much easier to work with, especially in an angle, such as securing inside a cabinet. Even the terminal blocks, TB-1 and TB-2 have these types of screws to hold the wires and lugs:
Modifications
This procedure was developed in September 2009, based on research from 2000, using the TLD5132A exciter board. There are two versions; "A" from the 1990's and recently, September 2009 version "B" was developed, however, not proven in service. The next paragraph will describe the modifications for both versions.
Install the 10v regulator and PTT switch on the P401 pins per the schematic and board layout drawings. Install a 1K resistor between P401 pin 1 and P401 pin 2. This is the PTT line input. Remove JU405 and install a 2.2K resistor in its place. (2.7K is okay for a substitute.) Remove C457 and replace that spot with a wire jumper. Also, jumper this point to the eyelet feeding P902 pin 5. This now a ground run. Remove C410 and replace with a 100uf, 25 v electrolytic capacitor with positive lead going to P902 pin 7). This is a general filter for the regulator. Remove C402 and replace it with a jumper, now called JU402. This grounds the F1 select line for single frequency operation. Optionally (for general cleanup) you can also remove JU401, JU402, JU403, JU404, C464, C465, and C466. Remove C405; then remove C463 and install a new tantalum with the leads reversed; the positive lead going to CE1 pin 4 and the negative lead going to the F4 select line (where part of C405 was). This is now the "Tx AF input" (P902, pin 18). Both versions have Q406 powered full time by A+. Being class C no RF output will occur during this state. The differences between the versions are with the "switched 9.6v" circuits and what they power and what's running in standby state. The next two sections discuss the differences.
Version A ; Channel element running full time:
This version keeps the crystal and channel element running full time via the continuous 9.6v line. This improves stability. The other circuits after the CE are off during standby condition; that would include pulse amplified IC401, mic audio IC402, CE buffer Q401, pulse amplifier Q402, and the multiplier stages of Q403, Q404, and Q405, via the keyed 9.6v line.
For this version, cut the PCB run between IC401, pin 9 and CE4, pin 2 and C406 area. There's a straight run at IC401; right near it's pin 16 is a good place to make the cut. This isolates the continuous and keyed 9.6v lines. Jumper P902, pin 4 to pin 8, and pin 17. This feeds the voltage input of the PTT switch and other continuous 9.6v circuits. Jumper P902, pin 9 to pin 13. This feeds the keyed 9.6v circuits and components such as the ICs and multiplier stages. Jumper the eyelit near P401, pin 7 to the area of the cut, just before C406. There are two eyelets that work great for this; one next to P902, pin 7 and the other above C406. This completes continuous 9.6v to feed only the channel element continuously, during standby state.
Version B ; Channel element and ICs running full time:
This version keeps the crystal and channel element running full time via the continuous 9.6v line. This improves stability. Some of the other circuits after the CE are also running during standby condition via the continuous 9.6v line; that would include pulse amplified IC401, mic audio IC402, CE buffer Q401 and pulse amplifier Q402. The the multiplier stages of Q403, Q404, and Q405 are off during standby state, via the 9.6v keyed line.
For this version, cut the PCB run between P401 pin 9 and Q402 emitter. There's a straight run closer to the former to make the cut. This isolates the continuous and keyed 9.6v lines. Jumper P902 pin 13 to P902 pin 8 and pin 4. This completes the continuous 9.6v line. Jumper P401 pin 3 to pin 9. This completes the keyed 9.6v line.
Version C ; Channel element running full time; this version is similar to version A, except for two items:
SRG design will use this version.
board modifications-Layout For version A
board modifications-Layout For version B
board modifications-Layout For version C
Schematic modifications Page 1
Schematic modifications Page 2
Install the appropriate channel element for your frequency. The formula is carrier frequency divided by 12 for the channel element frequency. For this SRG alignment the 147.20 frequency was used. You might consider sending the entire channel element into crystallization so it can be compensated as well. Doing this also has the advantage of the crystal company being responsible for the proper formula; all you need to supply is the carrier frequency. Next, tune the exciter board as usual per the service manual. In the event its not available you can use this procedure:
The meter socket pins 1~7 are functional either with a stock test set or a simple meter. If using the latter typical voltages are as follows (when properly tuned):
Pin 1 is IDC-audio level.
Pin 2 is channel element output; no tuning here, just an indicator of typical + .865v.
Pin 3 is the pulse amplifier output of Q402; peak L401 and L402. Typical voltage is -1.877v.
Pin 4 is Q403 output, or Q404 input; peak L403, L404. Typical voltage is -1.174.
Pin 5 is Q404 output, or Q405 input; peak L405, L406. Typical voltage is -.529v.
Pin 6 is ground.
Pin 7 is ground.
Peak L407, L408 for power out using a power meter or spec analyzer. Minimum level is a +26 dbm to properly drive the P.A. section.
Typical level is +28.451 dbm (700 mW for math challenged folks).
Checks and partial parts listing:
#2 panel, white, bud radio source.
Several colored LEDs, one flashing, T 1 3/4 diffused type.
Terminal block: the I/O, series 140, 10 position, Mouser El # 538-38770-0110
Terminal block: the PA power, series 142, 2 position, Mouser El # 538-38211-0102
3 switches, SPDT, Mouser El # 10TCC320
8 Standoffs, female 8-32 threads each end, .812" in length (about 20.6 mm).
1 mic connector, 4-pin, Hosfelt El # 4PMCS.
1 fuse holder, 3AG type, All El # FHPM-31.
several machine screws and nuts.
This may be copied in complete form only for non-profit purposes, such as for the knowledge for the Amateur Radio Service, with AK2O credited as designer. For other arrangements please contact the author.
Copywrite: AK2O 2006~present viewing date
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