UPDATE

Introduction

Over 2 years ago I acquired one of these amplifiers from K5VH, but it has taken until June 2003 to get to do anything with it. They are still being offered on eBay (May 2007). The following are some of my experiences trying to deploy it in readiness for the August 2003 contest.

Equipment Description

Toshiba were commissioned to design and build the amplifiers for a telecommunications project. The amplifiers are available in 2 versions. The UM2683A produces 40W and the UM2683B produces 20W when driven by 1mW. The only physical difference between the two units is that the 20W version has a right angled output connector, the 40W version has a swept right angled output connector.  They need a supply of 12.6V at up to 18A. 

Connections

Input and output are via SMA connectors. Power is connected to the unit through a DB-15 male socket mounted on the chassis. The matching connector is RS-276-1502. Pins 1,2,12,13 must all be connected to +12.6V . Pins 3,7,10,11 are all connected to the chassis. Pin 9 is grounded to enable the amplifier.

Power Supply

It was decided to dedicate a Power supply to the amplifier and Transverter. Others have suggested running the amplifier off 13.8V by putting two beefy diodes in series with the positive supply lead to reach 12.6V. However the logistics of mounting and heatsinking the diodes seemed too complex.  Surfing eBay one day I came across KE6F who was offering a 13.8V 20A PSU for $20 plus shipping. It comprised three series connected 5V 20A switched mode units with the voltage on each unit turned down to 4.6V. They also had the bonus of working off 120 and 240V line voltage. Five units were ordered (2 spares) and arrived. One had severe damage, accrued before shipping, but Bob quickly shipped a replacement.

An attempt was made to set the output voltage of a PSUs  to 4.2V using the voltage set potentiometer. Unfortunately it  will not set the voltage below 4.5V. So a 47K resistor was soldered in parallel with the 10k resistor next to the output voltage set potentiometer in each PSU enabling the pot to set 4.2V. Three modified units were mounted in a recycled 12x7x3" aluminum chassis. The line inputs were connected in parallel and protected by a switch and fuse. The outputs were all connected in series with thick insulated wire to reduce the voltage drop. The -ve wire of the series was connected to ground.

The power connector for the Amplifier was an RS 274-236 six conductor female connector. An additional 3 pin female XLR connector was fitted to provide 12.6V to the 3456MHz transverter

Amplifier Housing

Due to the heat generated by the amplifier (typically 150W) the amplifier HAS to be mounted on a heatsink. I used a 10 x 7" heatsink with 1" fins that was obtained new from Down East Microwave as part number HS107. For continuous operation (e.g. a beacon) fans will additionally be needed.

I was unable to obtain a suitable box to protect the amplifier and mount the connectors. A visit to the local hardware store produced some 1.25x0.125" aluminum strip and some 0.75x0.25" aluminum angle from which a 1.25" high surround was constructed.  A short length of semirigid cable with a chassis mounting 4 hole SMA female socket on one end and an SMA plug was connected to the amplifier input. For the output a 90 degree male to female SMA adaptor followed by a 4 hole chassis mounting SMA female to N  type adaptor is used.

 For the DC connections, an RS 274-226 male six conductor connector was mounted on the chassis. Pins 3&4 are connected to +12.6V Pins 1&6 are ground and are connected to the chassis. Two pins are paralled to cope with the current needs of the amplifier. A cable using two red 12swg flexible wires and two black flexible 12swg wires was  made to interconnect the power supply and Amplifier chassis. A 3.5mm socket was mounted on the chassis for the PTT connection. A green LED for 12.6V indication and a red LED for TX was mounted on the chassis.

The power cable is 2' long and consists of two 12gauge red flexible wires and two 12 gauge black flexible wires with an RS 274-226 connector on one end and an RS 274-236 on the other end.  

Tuneup

R217 and R210 were turned fully clockwise to set the input attenuator to minimum attenuation. They only need subsequent adjustment from this position if trying to drive the amplifier with more than 0dBm. The Amp chassis was then hooked up to the PSU chassis through a 20A ammeter and power switched on. A 1mW source should be connected to the input (but not yet switched on) and a 50W dummy load connected to the output through a power meter. The Green LED should light. Initially very little current should be indicated. The PTT socket is then earthed. The Red LED should light and the current should increase to around 15A. Drive should now be applied and output power should be indicated. R138 and R150 were then adjusted to maximum output. In my case the output was 50W; a value seen by others. Off load the voltage measured on pin 1 of the amplifier is 12.6V, on load (18A) the voltage is 12.55V

Power Output indicator

The 15 pin high density connector (as used for VGA, not the one that looks like a 9 pin serial connector!) provides lots of alarm voltages but the one signal it does not provide is an indication of RF output. When I took the cover off the amplifier devices I noted that there was a rectifier circuit connected to the input to the isolator which produce a "forward power voltage" . There was also a power detector connected to the terminated port that absorbs any power reflected from the antenna load. This provides a "reverse power voltage". These two voltages are fed to the alarm circuitry. The most accessible point for these voltages with the device cover attached is on the 6 pin connector at the output end of the amplifier board. The Third pin from the left is the forward voltage, the third pin from the right is the reverse voltage. To get this voltage to the outside world one of the pins on the 15 pin connector would be used. Pin 5 seemed to be doing nothing special so this was chosen. The white wire from pin 5 to the pcb was released from its housing with a small screwdriver. The wire was extended and pushed into the back of the housing at the 3rd pin from the left as shown in the following picture leaving the existing wire connected to pin 3 in place, hence the existing protection circuitry continues unmolested. This pin produces around 6V when the amplifier is producing 50W.

Update: DF6NA has pointed out that pin 5 is the VSWR Alarm pin. The pin is at "0" when the return loss is <6dB and "1" when the return loss is >12dB. If you need to use this protection use pin 6 to get the signal to the outside world.

As an analog meter was considered too fragile an LED bargraph meter was employed as shown in the earlier pictures above.

The circuit shown in Fig 1 was devised. R1 sets the current through each LED in the display and can be set for personal preferences.

A small PCB was designed as shown in Fig 2; the component overlay is shown in Fig 3.   VR1, C2, C3 are mounted on the track side of the board, both can be SMT types if available. IC1, BAR1 and R1 are mounted on the component side of the board.

Fig 2: PCB Layout                                                        Fig 3: Overlay

Table 1. Components for display

 Down East microwave have made the meter available as part RFP, which will also have provision to be driven by crystal detectors for other power measuring applications. The authors prototype board can be seen mounted through a hole filed just big enough to take the bar display in the top right hand corner of the following photo:-

Conclusions

Hopefully this article will encourage people to actually use these amplifiers. A lot have been sold but activity does not seem to show many of them are in use.

Not to be reproduced without prior permission of the author

Coming soon: How to mount the amp at the feedpoint of your EME dish

This page last updated 7-May-07