Project of the Month

A 100 Watt UHF Power Amplifier.

This project is for the advanced constructor who is proficient with a soldering iron, and comfortable with the principles of radio and electronics. We will build a 100 Watt power amplifier using cheap, readily available junk radio's. Current draw of this amplifier is expected to be in the order of 28 Amps, so make sure your power supply is up to it.

The project

The Tait T-196 is a crystal-controlled 25 Watt UHF commercial radio manufactured circa 1980's. Hundreds of thousands were built, and can be found in junk boxes of commercial radio suppliers and repairers everywhere. I bought 20 of these radio's through eBay for $10 plus $34.50 postage, and another 14 of these radio's through eBay for $20.50, pickup from Cleveland. For this project I pulled 4 from my collection.

The transmitter final output block, known in the radio industry as the 'PA' (Power Amplifier), can be easily removed from the radio and operate as a stand-alone PA. When removed from the radio, the PA operates from 460 Mhz to 512 Mhz, with 25 Watts out for 1 Watt in. We will butcher four radio's, and connect four of these PA's in parallel to get 100 Watts out for 5 Watts in, theoretically. And for less than $100 !!!!

The schematic diagram of the Tait T-196 PA can be seen here. It's very poor quality, but that's how I got it.

You will need:

4 x Tait T-196 UHF commercial radio. Cheap on eBay.

A 1 Watt UHF radio, for testing, and a patch lead from that radio to this project.

A 5 Watt UHF radio for final testing, and using.

A Power / SWR Meter, and a patch lead from that meter to this project.

4 or 5 x 2-point-2 picofarad ceramic capacitor.

444mm of 40mm x 40mm x 3mm Angle Aluminium.

2 x BNC or SO239 panel mounting socket. I recommend BNC's, because they are quick-connect/disconnect.

A 30 Amp power supply with a current meter.

1,890mm of RG-174 thin coaxial cable.

Step 1

Remove the PA. Do this for all four radio's. Keep all the screws.

Undo the screws in the grey lid and slide it off. The RX circuit board is hinged. Undo the two screws opposite the hinge and swing the RX board open. Remove the screws from each end of the PA, releasing it from the frame. Cut the wires, including the thin coax cables. You can now discard the radio, unless you want the bonus transistor.

Bonus

As bonus, the TO-5 transistor, Q411, in the top left-hand corner of the TX board is an MRF-630 or equivalent. This transistor is the most commonly blown transistor in Uniden CB's and Philips PRM-80's. The transistor is no longer manufactured, and the service centres that still have some want thirty dollars or more for them.

If you have a soldering iron that can get hot enough to remove this transistor, then definitely grab it.

De-solder it's leads, then heat the surrounding PCB with a blow torch while pushing the transistor out from the topside of the PCB.

Step 2

Make the two side rails.

Side rails are really optional, but they provide support to the PA's, provide a common ground, and give somewhere to mount the input and output connectors. I have drilled the connector hole in my side rails 9.5mm for BNC connectors. If you use a different connector, drill the connector hole the appropriate size. Mount the BNC connectors. Screw two solder lugs on each of the two holes near the BNC connectors.

Step 3

Rework the PA's.

Remove the shield. There should be nothing holding it on. Cut the brown, red, blue and white wires off and discard them. Cut the orange wire short and solder it to the positive input, that is, the same solder pad as the big red wire. The orange wire is the mode selector. When connected to plus 12 volts, the PA is in transmit mode. Without this connection there will be no power out from the PA.

From the RG-174, cut eight pieces 210mm long. These will be the half-wave links.

Cut two pieces 105mm long. These will be the quarter-wave links.

If you use any other type of coax, these lengths may not be correct.

Strip the outer insulation on each end of each link back 5mm, no more. Fan out the braid and fold it around until it is a single bunch of strands poking out sideways. Tin (soak) the braid with solder. Watch the inner insulation. As you heat up the braid, the inner insulation will change colour from a cloudy translucent to clear. This means it is molten. At this point, do not push or pull the inner conductor, or it will pull through the insulation and short to the outer conductor (braid). Do not grip the coax with pliers, either. The same thing will happen. If the inner insulation starts to bubble or expand, remove heat immediately. Wait for the coax to cool before trying again. Once the braid is tinned, wait for the coax link to cool. Strip the inner insulation back 1mm, and tin the inner conductor. If you do not wait for the coax to cool, you will pull the inner insulation off from where it is molten, and the the link will be rooted. You can't cut it shorter, so you will have to throw it away and cut another one.

Un-solder the existing coax on the input, L501. Discard the coax.

 Replace it with a half-wave link. Solder the coax inner to the input pad at L501. Heat up the solder pad on the ground track, and add fresh solder. When it is nice and molten, press the braid of the coax link into the pool of solder. Press the braid down with the tip of the soldering iron. Once the braid is in the pool, remove the heat and start blowing on it. We don't want to melt that inner insulation and cause a short at this point.

Do this to all four PA's.

Un-solder the coax from the RX feed out, C522. Discard the coax.

There is a brass collet near the PA output, L527. In the centre of this is a white plastic sleeve with a wire that goes down to the Tait connector. Unsolder this wire from the pad at L527, and cut it off at the white sleeve. Fit a half-wave link. Solder the coax inner to the output pad at L527. Heat up the solder pad on the ground track where the brass collet is soldered, and add some fresh solder. When it is nice and molten, press the braid of the coax link into the pool of solder. Press the braid down with the tip of the soldering iron as before.

Do this to all four PA's.

Measure inner conductor to outer conductor of the coax links with a multimeter. There should not be any shorts.

Connect power leads to the Tait connector. Refer to the picture below for the connections. Confirm with a multimeter. Solder bridge all the pins to the left of the coaxial socket and solder the negative lead to them. Solder bridge all the pins to the right of the coaxial socket and solder the positive lead to them. Be careful not to bridge the positive to the coaxial connector, because this is ground, and will short out the positive.

Do this to all four PA's.

Step 4

Tune the four PA's.

The PA's must be tuned for best performance before they are combined. If they are not matching, the lower performing PA's will 'suck' the power from the others. Put the shield on and screw it down on the long sides.

Solder a BNC connector to the input of one of the PA's, and solder a BNC connector to the output. Connect the PA to your power supply. Connect the Power Meter to the output, and your 1 Watt radio to the input. Transmit 1 Watt (and 1 Watt only, Priscilly) into the PA while watching the power meter and the current meter. If you do not get any power output at all, note the current meter and stop transmitting immediately. Check your coax connections for shorts or opens, the inner insulation of the RG-174 melts easy and could cause a short circuit. If the PA was not drawing any significant current at all at the time, there is probably a short circuit or open circuit in the input coax. If the PA was drawing more than 7 amps, there is probably a short in the output coax. If the PA was drawing less than 7 Amps, there is probably an open circuit in the output coax. Also check that the orange wire is connected to plus 12 volts.

There is only one tuning adjustment, fairly obviously the large trimcap poking through the hole in the back of the PA shield. Transmit into the PA. Adjust the trimcap until you get maximum power out. You should be able to get 25 Watts easy, but if you can't, just make sure that they are all nearly the same output power. You can always drive them harder to get full power. These PA's will do 30 Watts each, but don't drive them any harder than that.

Unsolder the BNC connectors from the PA. Solder the connectors to the next PA for tuning and tune it as above. Do all 4 PA's.

Step 5

Link them together.

The schematic diagram below shows what we're going to do.

Solder the two quarter-wave coax links to the output connector. I looped my two around and to a couple of solder tag posts on the side rail. Screw down two solder tags to the side rail for each coax link. Solder the braids of the coax's to the solder tags. This will make it easier to join them to the four half-wave links.

Solder two of the half-wave coax links on two of the PA's together, and solder them to one of the quarter-wave links. This is where those solder tag posts come in handy. Then do the same for the other two PA's and the other quarter-wave link.

Tuck the cables under the PA's and screw them to the side rails. Leave the screws loose until they are all started.

Join all the positive power wires together, and all the negative power wires together, and screw them into a terminal block along with the main incoming power lead.

Add four 20mm stick-on rubber feet on the side rails and there you have it!

Antenna Switching

This amplifier is not like the commercial units. It's a one way deal. You can only transmit through it. The antenna is not bypassed for receive. To do that properly, you would need a co-axial relay, and they're not cheap or easy to find. The problem with standard relays is that they leak too much RF at UHF frequencies, causing all sorts of problems. The easy way around this is to have a separate radio and antenna for receiving.

The circuit below is a relay driver circuit that switches the antenna over when it senses TX, but using a standard relay affects the SWR, loses power, and some relays latch on.

Final Notes

I was not able to get the theoretical '100 Watts out for 5 Watts in' that I had hoped for. I suspect that my four-capacitor-power-divider on the input is less than perfect. Still, the prototype unit here got a respectable 50 Watts out for 5 Watts in, and 90 Watts out for 10 Watts in.

And in conclusion, let's sing a little nursery rhyme. Altogether now -

It is illegal to use this amplifier on CB frequencies.

So there.