Up one level
Back to Home
Two Meter Conversion
of VT-3 Transmitter
and VR-3 Receiver
By Gary Hendrickson W3DTN
The Daniels synthesized VR-3 receivers and VT-3 transmitters are designed to operate in either the 132 to 150 MHz (low split), or the 150 to 174 MHz (high split) frequency range, using practically identical synthesized oscillator modules. The only significant difference is the frequency range of these oscillators. For repeater operation in the two-meter amateur band, the receiver local oscillator (L.O.) module needs to operate in the 150 to 174 MHz range, and the transmitter module must operate in the 132 to 150 MHz range. The operating frequency of each of these synthesized oscillator modules is programmed by a set of 4 rotary BCD switches.
When I originally received them, these units were configured to operate as voice (analog) repeaters operating in the 150 to 160 MHz (high split) frequency range, so the receiver oscillators were operating in the 132 to 150 MHz range, and the transmit oscillators operated in the 150 to 174 MHz range.
Rather than trying to alter the tuning range of the synthesizers, i.e., modify the transmitter synthesizer to tune over the 132 to 150 MHz tuning range from its original tuning range of 150 to 174 MHz, it turns out that just swapping the receiver and transmitter synthesizer modules makes the modification project a lot simpler.
Therefore, modification essentially consists of swapping oscillator modules between the transmitter and receiver, along with a few adjustments and modifications. This way, the original transmitter oscillator, which operates in the 167 MHz frequency range, will now serve as the receiver's local oscillator, providing high-side L.O. injection, producing an I.F. of 21.4 MHz. Likewise, the original receiver oscillator, which operates in the 146 MHz frequency range and originally provided low-side receiver L.O. injection, will now serve as the transmitter oscillator, operating directly on the two-meter transmitting frequency.
I have to warn you, first, that this project would be almost impossible without having the Daniels Instruction Manual in front of you. As detailed later in this document, there are many jumpers that have to be properly set in order to configure the equipment in the desired operating manner. Unfortunately, not all of the jumper numbers are silk-screened on the printed circuit boards, and they are not laid out in a logical order, so you can't just guess where they are. You have to have the pictorials in the manual to find most of the jumpers. You can contact Daniels Electronics, Ltd, at 43 Erie Street, Vancouver, B.C., V8V 1P8 Canada, phone: 1-800-664-4066, e-mail www.codanradio.com/lmr/ or contact your local two-way radio service shop; they may have a manual. It turns out that these radios are very popular at locations where commercial A.C. power is not available, so battery power and solar cells are used. Internal jumpers can be set to minimize the current draw in stand-by, making them very desirable at remote locations. They are rugged, well designed, and small. A complete 35-Watt repeater takes up only 5-1/4 inches of rack space, plus power supply, duplexer, and for amateur radio use, a controller.
In my case, I was preparing the radio for use on Army MARS frequencies, where a controller is not needed. But there are many options for interfacing a controller, as detailed in the manual.
According to the manual, the synthesizers tune in either 5 kHz or 6.25 kHz increments. There is a formula for determining the settings of the rotary BCD frequency selection switches, and for the math-challenged, there is a multi-page table for setting the switches to any specific frequency within the full operating range.
Well, as it turns out, the synthesizers in unit I was working on changed frequencies in 2.5 kHz (0.0025 MHz) increments instead of the 5 kHz or 6.25 kHz tuning steps that are described in the manual. Therefore, both the formula and the table were useless! But I was able to do some "reverse engineering" to determine the correct formula to calculate the switch settings using 2.5 kHz steps.
So, before you start tearing things apart, first remove the cover from the transmitter/exciter, as described below, then plug it back into the mainframe and fire up the equipment on its original operating frequency, connect a frequency counter and dummy load so you can measure the transmit frequency, and change the Least Significant Digit (LSD) rotary switch just one step to see how much the frequency changes. If it changes by + or - 5 kHz or 6.25 kHz, you can use the formula or tables in the manual. If the frequency changes by + or - 2.5 kHz, use the following formula to determine the settings of the four rotary switches:
Fxmit - 128.6 Switch # = ============= 0.0025
where "Switch #" is the setting of the 4 rotary frequency selection switches, "Fxmit" is the transmitting frequency in MHz, and "0.0025" is the 2.5 kHz tuning steps, expressed in MHz (2.5 kHz = 0.0025 MHz). For example, 146.00 MHz would require switch settings of 6960. Note that the 4 rotary switches increment the synthesizer frequency in the following order:
MSB = 2.5 MHz increments 2nd = 250 kHz increments 3rd = 25 kHz increments LSB = 2.5 kHz increments
If the frequency changes by some other amount, substitute that amount in the denominator of the above formula.
A similar formula is used to set the receive frequency:
Frec – 128.6 Switch # = ============ 0.0025
where "Switch #" is the setting of the four rotary frequency selection switches, "Frec" is the receiving frequency in MHz, and "0.0025" is the 2.5 kHz tuning steps, expressed in MHz.
First, disassemble the VT-3 transmitter module by removing the four Philips screws on the front panel, and the four large Philips screws on the side of the module then slide the cover off. The oscillator module is attached to the main printed circuit board by a single Philips screw offset from the center of the module. Unplug the RF output connector, and carefully unplug the module from the motherboard. Be careful not to bend the connector pins on the bottom of the module. Remove the cover from the module to be able to access the oscillator tuning capacitor, by removing the eight small Philips screws around the edge of the cover.
Remove the VR-3 receiver cover by removing the four Philips screws on the front panel, and the four Philips screws on the side then slide the cover off. Remove the two Philips screws on the front panel that hold the front-end assembly, and one screw at the back end of the front-end assembly, and tilt the assembly out of the way. Similar to the transmitter, the oscillator module is attached to the main board with a single screw offset from the center of the module. Unplug the RF output connector, remove the mounting screw, and un-plug the oscillator module from the motherboard. Then remove the cover from the module.
VR-3 Receiver Modification:
The receiver mixer is followed by either a high-pass, or a low-pass, image rejection filter. The high-pass filter needs to be removed from the circuit, and the low-pass filter connected in its place. To access these filters, which are part of the front-end PC board, remove the tin cover from the receiver front-end module by straightening the small metal tabs. Then carefully remove the solder jumpers on Jumpers JU-1B and JU-3, and install solder jumpers on Jumpers JU-1A and JU-2. These jumpers are in the compartment closest to the front panel. Use a small tip soldering iron, and use solder wick to remove the original solder jumpers. Re-install the tin cover on the front-end module.
Now, the synthesizer needs to be tuned to the correct frequency range. Set the four rotary oscillator frequency tuning switches to a frequency near the middle of the two-meter band. Use the table in the manual, or the above formula, as appropriate, to determine the switch settings.
Temporarily mount the transmitter oscillator module on the receiver motherboard. Be careful with the connector pins on the bottom of the oscillator module. With the receiver on the bench, apply a +9.5 volt D.C. source to pin B-6 on the receiver's multi-pin connector. Connect a high-impedance DC voltmeter to Test Point TP-4 on the oscillator circuit board (it's just to the left of the mounting screw hole), and use an insulated tuning tool to adjust tuning capacitor C-24 for a voltmeter reading of around 4.5 volts.
To check the frequency range over which the synthesizer will remain in lock, rotate the frequency-set switches above and below the originally tuned frequency. As you tune away from the tuned frequency, the synthesizer will eventually drop out of lock, and the red LED will light up. It should have a lock-in range of at least +/- 3.0 MHz from the center frequency, covering the entire two-meter amateur band.
Once the synthesizer is tuned, you can align the five receiver front-end resonators by connecting a signal generator to the RF input connector, and tuning the resonators for maximum sensitivity. First, remove the screw caps from the tops of the resonators then tune the caps with a small screwdriver. It might be easier if you carefully plug the receiver back into the mainframe, and reach the tuning caps by using a skinny screwdriver thru holes in the ventilated cover. If the original operating frequency was in the 155 MHz range, you should only have to turn the tuning caps clockwise about 1/2 turn.
On the side of one of the front-end resonators is a label with a checkmark for the frequency range of this receiver. Be sure the correct frequency range is checked. You might also place a notice on the outside of the receiver and transmitter cases noting that they have been modified to tune the 132 to 150 MHz frequency range.
Don't install the cover yet, as several jumpers, as described below, need to be set in order for all of the modules to work together in the desired manner.
VT-3 Transmitter Modification:
Modification of the transmitter/exciter follows a similar procedure, but the receiver synthesizer, which is now being used on the transmitter, first requires one modification. As shown on the schematic diagram for the synthesizer, resistor R-22 is 215 ohms. It is a small surface-mount resistor right next to the edge of the PC board, about 1/2 inch from tuning capacitor C-24. In the receiver synthesizer, this is either a zero ohm resistor labeled "000", or a small jumper wire shorting across 215 ohm R-22. This shorts out the audio, which would otherwise frequency modulate the RF oscillator, so this needs to be fixed. If there is a zero ohm resistor, carefully remove it and replace it with a 220 ohm 1/4 or 1/8 Watt resistor. If it is just a jumper wire, carefully remove and discard it.
Now, mount the modified receiver synthesizer module on the transmitter/exciter PC board. With the transmitter on the bench, and the cover still off of the synthesizer, apply +9.5 volts D.C. to pin B-6 of the connector, and measure the synthesizer tuning voltage on test point TP-4 of the synthesizer PC board. Set the frequency select switches to the desired transmitting frequency, connect a dummy load to the RF Output connector, key the transmitter on with the front panel "Key Tx" toggle switch, then tune capacitor C-24 until the tuning voltage reads around 4.5 volts. The red LED should not be lit if the synthesizer is in lock. As with the receiver, the settings for the four BCD rotary transmit frequency selection switches use either the appropriate table in the manual, or the same formula as that used for setting the receive frequency:
Fxmit – 128.6 Switch # = ============= 0.0025
where "Switch #" is the setting of the four rotary frequency selection switches, "Fxmit" is the transmitting frequency in MHz, and "0.0025" is the 2.5 kHz tuning steps, expressed in MHz. Once modification and tuning is finished, screw the cover back onto the synthesizer module.
The power amplifier module in the transmitter/exciter needs a little bit of "tweaking" due to the lower operating frequency. Remove the two screws on the front panel, and the four screws on the PC board that hold the PA module to the PC board. Turn the module over and remove the bottom cover. Then, carefully solder a 27 pF dipped mica capacitor across C-31, which is in series with the output of the amplifier transistor, and add a 10 pF cap in parallel with C-37. Keep the capacitor leads as short as possible. Then reassemble the PA module and install it back onto the transmitter motherboard. Check the jumpers, as listed below, to be sure the correct jumpers have been installed or removed. Don't install the cover yet, as the deviation will have to be adjusted, once all of the jumpers are properly set.
These relatively simple changes complete the modifications needed to bring the Daniels transmitter and receiver from the 150 to 174 MHz range down to the 132 to 150 MHz range. Using the procedure in the manual, the transmitter output power can be adjusted anywhere from about two watts up to about 12 watts. If it is being used to drive a Daniels AMP-2 higher power RF power amplifier, use an RF power meter to adjust the output to no more than four watts. Also, follow the procedure in the manual to adjust the deviation, which will need to be fixed after changing resistor R-22 in the transmitter synthesizer, and after setting all of the jumpers in the three modules to their proper settings, as described below. The Daniels AMP-2 35-watt RF power amplifier is broad-banded, so it does not require any modification.
Jumpers (Lots of jumpers!):
The Audio Control, Transmitter and Receiver modules all have jumpers on the PC boards that need to be properly set so the units will function in the desired manner. Having the Daniels Instruction manual is a must for determining the locations of all of the jumpers - there is no rhyme or reason as to where they are located on the various P.C. boards. The following description is intended to configure the equipment as a voice (analog) repeater. If desired, jumpers can be re-configured so the equipment will function as a base station, with wire-line remote operation, for digital transmission, or other modes of operation as desired. Consult the Instruction manual as necessary. Note that the equipment has no provision for station identification. For amateur radio operation, an external controller will have to be interconnected with the Daniels equipment if automatic station identification, and any form of remote control, is desired. Refer to the Daniels Instruction manual to determine how to connect an external controller.
Audio Control Module:
Receiver A audio needs to be routed to Transmitter A. Install jumper plugs on JU3-A and JU4-B. Solder shorts on JU-1 and JU-16. Open JU29 and JU36. Install a jumper wire from P2-2 to P2-6. Screwdriver adjustment pot R5 labeled "TX-A" on the front panel sets the level of receive audio going into the transmitter. In conjunction with Input Level pot R31 and Compression pot R38 on the transmitter Audio Processor PC board, adjust the audio levels so that an input signal with 3 kHz of deviation produces 3.0 kHz deviation on the output signal.
If a TS-64 CTCSS Encoder/Decoder is plugged into CTCSS Module A socket, receiver discriminator audio needs to be routed into it. Short JU44-B. Open JU45. The switches on the TS-64 board set the desired CTCSS tone. Go to the Communications Specialists, Inc., Web site at www.com-spec.com for more information on the Model TS-64 "Miniature 64 Tone CTCSS Encoder-Decoder".
Switch SW-1 is used to enable/disable the receive CTCSS decoder. Short JU25 and JU28, open JU30. In the "Off" position, repeater access is by carrier only. "On" requires a CTCSS tone on the incoming signal.
CTCSS encode tone needs to be routed to the transmitter tone input. Short JU101-A, JU56, JU115, and JU47-A. Open JU46, JU54 and JU100.
Switch SW-2 is used to enable/disable the transmit CTCSS encoder. Install jumper wires from J3 to J54, J4 to P2-31. Open JU33, JU32, JU34, JU36. Short JU115.
In the PTT circuit, install jumper plugs on JU56 and JU63-B. Open JU62. Short JU116. Adjust pot R90 for the desired transmit "hang" or Drop-Out-Delay (DOD) time.
On the main PC board, solder jumper shorts on the following points: J19-Y, J20-Y, J22-Y, J23-Y, J25-X, J18-X, J16-X, J2-X, J4-Y. Be sure that the other side of all of these jumpers are open.
On the Audio Processor Board, solder jumpers on the following points: JU11-Y, JU36-Y, JU3-Y, JU22-X, JU2, JU6, JU43, JU33, JU16. Open JU24, JU25, JU26, JU7, JU21, JU8, JU31, JU32, JU34. Adjust pot R29 for the desired peak deviation, using the local mike as an audio source.
Most of the jumpers on the receiver should already be properly set. However, check all of the following:
Solder jumpers should be installed at the following points: JU8, JU9, JU11, JU13, JU15, JU17, JU18, JU22, JU27, JU35.
These jumpers should be open: JU10, JU12, JU14, JU16, JU20, JU21, JU23, JU24, JU26.
Note that when adjusting the Squelch pot R-88, the setting may be very close to the fully CW position. You may also want to play with the setting of the Squelch Hysteresis pot, R-93, to get the squelch operating the way you want it to. These are multi-turn pots, so they might require a fair amount of diddling.
After all of the modifications and tuning are completed, screw the covers back on, put a label on the front of each unit indicating its operating frequency, and place it into service. You should have a compact, efficient, reliable unit that, I hope, will give you many years of service.
The author can be contacted at: his-callsign [ at ] verizon [ dot ] net.
Back to the top of the page
Up one level
Back to Home
This page originally posted on Tuesday 07-May-2013.
Article text © Copyright © 2013 by Gary Hendrickson W3DTN.
Hand-coded HTML by Robert Meister WA1MIK.
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.