Introduction
Since it’s launch, the Teletec DXP-V220 power amplifier has maintained a good reputation for transmit performance. Unfortunately the manufacturer’s incorporated preamplifier, employing a J310 JFET, whilst being most aptly suited to the amplifiers commercial roots, is not "state of the art" by amateur standards. In fact using it in my 220MHz system increases the overall noise figure! An external preamplifier could be fitted but this would involve extra external relays and wiring. The extra gain would also make the system an intermod liability. It was therefore decided to try and replace the existing preamplifier and employ the existing internal relays.
Remember before commencing work on the changes that the warranty, if current, will be invalidated by breaking the seal and opening the box
Having successfully used the Down East Microwave Inc. VHFLNA in other projects, the design seemed ideal for incorporation. Looking at the circuit of the Teletec Amplifier there is a novel circuit to protect the back end of the preamplifier from having 25W of drive applied to it. It has been assumed that this circuitry is sufficient protection for the GaAsFET so it has been retained in its entirety. The existing low pass filter on the input to the preamplifier and the output bandpass filter have also been retained to maintain selectivity.
Method
1. The preamp board and components, less connectors and box were ordered as part number VHFLNAPK from Down East Microwave Inc.
2. Remove the covers of the amplifier by taking out all 12 cross head screws in the base of the unit
3. Carefully remove C48, L16, C2, L17, C65, R43, Q1, C46, L15, and C47 from the amplifier. See Fig 1.
4. Cut the preamp board to fit the gap left by removing the components as shown in Fig 2. Ensure clearance of all non-grounded tracks on the PCB. The mounting screw holding down the PA board is used through the hole in the preamp board (intended to mount a feedthrough capacitor) to mechanically hold the preamp PCB in place
5. Using a very hot soldering iron and tweezers completely remove the tracks connected to both ends of the position vacated by L15. Cover the plated hole nearest the centre of the board with insulating tape. Failure to do this step will result in the destruction of the amplifiers 78L06 regulator.
6. With the preamp PCB screwed down in its final position on the PCB scrape the solder mask (the "green varnish") around the edge of the PCB using a sharp knife to expose the bare copper. Fig 3. shows how the unit should appear at this stage.
7. Using 3/8" wide thin copper foil around the edge of the preamp PCB attach it to the PA PCB. A photograph of the installed unit is shown in Fig 3.
8. Solder the components on the Preamp PCB as per paragraph 2 of the DEMI Assembly Instructions. Connect the input capacitor C1 between the plated through hole (formerly connected to C48) on the PA PCB and the input to the Preamp PCB
9. Connect a short length of insulated wire between the output of the preamp PCB and the junction of C45/C58 on the PA PCB. Fig 4. shows the completed work.
10. Run an insulated wire from the junction of C9/U1 on the preamp board to the on/off switch on the PA front panel ensuring connection is made to the pole that goes to E7 on the PA PCB (against the large 22ohm 5W resistor)
11. Apply power to the PA. Ensure that 5V appears at the output of U1 and 3V at the junction of R3/R4. Ensure that when power is turned off using the on/off front panel switch these voltages disappear
12. Drill ¼"holes in the PA lid aligning with the positions of C1 and C2
13. In the absence of access to a noise figure meter, align the preamp for best signal to noise on a weak signal
14. Replace the PA cover using all the screws. C1 and C2 may need slight readjustment on a weak signal due to the proximity of the lid.
15. Put insulating tape over the trimmer access holes to reseal the
Box
Conclusion
All the amplifiers so far aligned have produced a noise figure around
1dB which is reasonable considering the losses in the filter
It is also possible to use the above procedure to upgrade the preamp
in the Teletec DXP-V175 power amplifier, the only difference being that
L1 on the preamp needs an extra turn.
