Modifying the Spectrian 2.4GHz PA

Transcription

1 Modifying the Spectrian 2.4GHz PA Two types of modification: 1. Basic required for the PA to function 2. Enhancements additional monitoring and functionality The unit normally mounts vertically in a rack. However, the mods will assume that it will rest flat on a table or similar. In these notes, the Top of the unit is the side closest to the power switch. The Bottom is closest to the handle. The Top cover only has a handful of screws holding it on. The Bottom has several dozen. Note that the PA contains a mixture of Philips Head and Torx screws. You will need size T10 (?) and T15 Torx drivers. 1. Basic Modifications 1.1 Removal of Unnecessary Items Remove both covers and front panel. On the Bottom side (RF side), remove the input/mixer board. It s the one in the front corner with only a few components on it. Keep the hardline leads for later. On the top side, remove all of the boards and associated ribbon cables. (The large board is the 48 volt to 24 / 12 volt DC-DC converter. The other board is the supervisory / controller). If you want to add the current metering enhancement, leave the two gold power resistors (0.01 ohm) that are bolted to the chassis. Remove the power switch / circuit breaker. Remove the multi-pin connector on the rear panel. Either desolder or cut the hardline coax right at the connector. The coax will be re-used on the input later. 1.2 Power Supply Mount the three switching supplies to the top panel, centred. Viewed from the front of the PA, the AC connections should be to the left and DC outputs to the right. The supply with both 5V and 12V modules should be at the front. The power supplies have threaded mounting holes. Mounting screws are M4, and must be very short (5mm max thread length). Use washers to limit their penetration if needed. Wire the 5 volt supplies in series using heavy gauge wire capable of carrying the 60A peak current. I only used one of the 12V supplies, leaving the other 12V output unconnected.

2 To feed the wires into the PA, I cut two slots in the lid and fed the wires through grommets (see photo). It means that, to take the lid off, I first have to disconnect the DC wires from the power supplies.

3 Wire all the AC inputs in parallel. You can then run a power lead straight from that, or add a power switch as per the Enhancements below. 1.3 Cooling The PA requires a LOT of cooling, as it dissipates nearly 1 kw at full power. You need high flow muffin fans or, ideally, a blower with a manifold to couple it to the cooling intake. I used 4 x 85mm 24V muffin fans (one-off lot from ebay). Equally, you could use 12V or 6V, but I d recommend DC fans so that you can do the Cooling Enhancement (slow them down during Rx periods). I used two strips of 12mm x 12mm aluminium angle to hold them in place. To hold the strip in place, there are four bolts, one hole in from each end of each length of angle, going right through the fans and back panel to hold them in place. Each fan is bolted to the alu angle. Wire the fans in series/parallel and connect to 24V or 12V supply as appropriate (or Cooling Controller (Enhancement)). Feed the wires through the hole left by the rear panel multi-pin connector. 1.4 RF Connections The PA has over 43dB of gain, so it s strongly advised to use hardline coax for both input and output. I used UT-141 for output and UT-085 for the input. The input uses the coax recovered from the old input board. I used a solder SMA panel mount socket on the end, recovered from some short leads I had. The output uses a length of UT-141 coax with an SMA solder connector on one end and a screw termination, panel mount N connector on the other. Both connectors are mounted on a bracket made from 25mm x 25mm alu angle mounted to the cooling fan bracket. The hardline is carefully bent to fit through the

4 hole left by the rear panel multi-pin connector. I had to carefully grind away some of the end fan body so that the leads would fit past it. 1.5 Enable The Enable input puts the PA into transmit state when it is grounded. The input is through an RCA connector on the same bracket as the RF connections. Wire the RCA connector to the point where the thin orange wire goes in the photo below. Note that it connects to the upper pin (furthest from the PCB). This is the Enable for the Driver stage. Now the Driver Stage Enabled signal (+12V) needs to be wired through to the Output Stage Enable input. This is done by the two purple wires in the photo. The lower purple wire connects to pin 4 of the open connector (the second pin from the left, top). Note that I ve used an unconnected wire in the cable between the boards to pass the signal across. You could equally just wire pin 4 across to the pin of the power transistor directly. Note also that the white wire from pin 4 is used for the Enhanced Cooling mod.

5 Enhancements (To be Completed) 2.1 Metering Add an LCD voltmeter to the front on a bracket. Include a toggle switch to select between Current and Temperature Current Sense voltage drop across the paralleled 0.01 ohm resistors Temperature Tap off one of the output stage temperature sense outputs preferably from the middle PA board as it s likely to be the hottest. 2.2 Fan Control (High speed when Tx or High Temp) Put a resistor in series with the fans to slow (and quieten) them. Use a comparator on the temperature sense line to short the resistor when temp rises beyond a threshold (50 deg?). Could also switch to high speed when Tx.

6 2.3 Front Panel Status LEDs Chop off the front corner of the control PCB containing the LED s (see lower picture on page 2). Include the mounting holes. Wire the LED s back (with appropriate dropper R s etc.) to suitable points: DC ON the 28V supply ENABLE 12V PA activation voltage FAULT another comparator on the temperature line set to say 80 deg?? 2.4 Power Sensing The output combiner includes FWD and REV power monitoring. additional circuitry, power metering could be added. With some There is also a 30dB sense output that comes out the front panel.