220 MHz Micor conversion PA built by WD7F, KD7TXO and K7IOU

 

Ours was the 100 watt model PA.  You will need to remove the PA board. Don't forget to un-solder the power connections for the feed through capacitors before removal on the red and black wires where they feed through the board. I removed the PA board and then ground down, filed, sanded off the raised bosses for the power transistors on the heat sink. heatsink.jpg (214872 bytes)  We direct mount the power modules to the heat sink pa1.jpg (597464 bytes) The stand off's pictured were removed for our later design. I had added them to mount the capacitors to, but decided for the design in the later pictures.  John wd7f came up with this design for the power modules I chose. wd7fdualmodule.jpg (99290 bytes) I decided to use the RA30H2127M purchased from RF Parts as they require only 50 mW drive each, so we didn't need to build a pre-amplifier to drive them.  I noticed after the purchase of these modules the fine print says not for repeater use due to the duty cycle. So we shall see how well they perform and will keep you posted on the duribility.

 John wd7f came up with a TU-50 (Anzac) power divider he purchased off Ebay. The TU-50 uses SMA connectors. I built 90 degree brackets and mounted chassis mount SMA connectors to the heat sink and direct soldered the input pin 1 to the connector. Craig and John made the SMA cables out of RG-400 cut to 1/4 wave length of the frequency of 224.74 for the power divider.

Craig made the power combiner- bridge for the two modules with the suggestion of Stanly Bradley wa7wnm . Craig kd7txo met Stanley at the FAA academy in OK. Here is Stanley's design wa7wnmbridge.jpg (65290 bytes) 

 
 Next we installed the capacitors and wired the assembly. They were grounded using a ground lug as in the picture. You want them as close as possible to the pins. Also note the braided grounds at the base screw of each module. This braided strap connects to the A- on the power terminals. I also used a piece of heat shrink on it as it passed the left module. I used a piece of 3M body side molding tape to install the TU-50 power divider. The input (TU-50) will extend through the shield for the SMA connector from the exciter. I also made 90 degree brackets for the output of the modules (pin 4) and mounted BNC chassis connectors. I used 1 1/4" stand off's to support the BNC bracket with a star washer on the bottom and top for (bond). This raised the BNC connection up through the protective screen on the PA. This made the bridge hookup simple.    

Here is the layout of our PA. pabridge.jpg (69347 bytes)

We tested the gate voltage and the Motorola design has a negative voltage on this wire. John is working on the design and also SWR fold back protection .  We also tested the modules separately and had 31 -34 watts out of each. We also restricted the output of the exciter with a 180 ohm resistor at R436 for 110 mW output. 

We then tested our design with the Marconi, exciter @ 100mW, through both the exciter filter and the PA filter. This was the output. pabridge6.jpg (40394 bytes)

We spent many hours working on the PA with the dual modules. We hope this design will last many QSO's. An interesting point of the bridge design is the dummy load and the 270 degree cable is if a module fails the working module won't burn out. Picture of the bridge. pabridge5.jpg (70037 bytes) This will be wound up tighter and zip tied to the protective screen on the PA. Notice the dummy load in the center of the picture. 

Here is a picture of the finished PA and combiner with the power control wired up from the original Micor. pacombiner.jpg (76308 bytes) and more pictures of the repeater and happy campers 224.742.jpg (76560 bytes)  PA and combiner224.743.jpg (82944 bytes) 224.744.jpg (84056 bytes) 224.745.jpg (69202 bytes) Dave wrapping it up 224.746.jpg (74506 bytes) PA and combiner with shielding off 224.747.jpg (62986 bytes) 224.748.jpg (69131 bytes)

More pictures to follow. de k7iou

Page recreated by: David Stanford k7iou Copyright 8/07/2005 B.A.R.T. and k7iou