I had been looking on the web for 1296 MHz Power amplifier designs and had stumbled across G4BAO web site which has a design for a 45W PA which was featured in the RSGB magazine Rad Com. Unfortunately the kit of parts is no longer available for this design but also on John's web site is an article about converting old Andrews 900 MHz GSM amplifiers for use on the 23cm band. Recently I purchase one of these Andrews units off Ebay, it was the dual MRF9060 type. When it arrived I was surprised how small the unit was. John's article is very useful with details of how to remove the transistors from the heat spreader. My wife would have my guts for garters if I was caught using the ceramic hob to heat the unit so I used an old non steam iron set to linen and held upside down in the jaws of a vice in the workshop. Let the iron warm up first for five minutes as some irons have a lot of temperature overshoot. Place the amplifer on the iron using some suitable wide jaw pliers.The PCB is very heat conductive so in less than a minute all the components can be lifted of the board with long nosed pliers with no trouble at all. A note of caution here, the iron must be secure, it is easy to get burnt if the amp falls of the iron. Wear a pair of those cheap cloth/leather gardening gloves,better to be safe that burnt!
Having got this far a second Andrews MRF9060 amplifier appeared on Ebay so I bid and won this one with the intention of combining the two together. When it arrived it was a MRF9045 type, DOH.I could have returned it, it was an honest mistake by the seller who fortunately had a few more MRF9060 ones so I bought all four for a really big amp! Well I bought the MRF9045 as it is much easier to convert no need to heat the whole PCB just change the matching sections. I followed John's mods and within the space of a couple of hours I had about 20W out for 1W drive. Over the course of a couple of evenings I modified the output matching to obtain 38 W output for 1W drive at 2.6A current. Saturaded power output about 50W.John's original input matching seems to be spot on, better than 10dB return loss without any trimming. My output matching line is some what longer with less fixed capacitance. At only 2.6A current this would make an ideal mast head PA.
Returning to the MRF9060 amp it's time to solder a single MRF9060 device back onto the heat spreader after removing the copper ridge between the the original milled cutouts in the heat spreader.Place the unit on the preheated iron and lightly push down on the transistor. If you have removed all of the original solder then first pre-tin the area the transister will bond to. Using an iron you cannot slide the amplifier of the heat as you can using a ceramic hob but just switch of the iron. The transistor will be hot for longer but this did not seem to be an issue with my amps. Inital results were very diappointing 1W in, gave 5A current with only 3W output,input return loss only 3db ! Adding a 4p7 capacitor to the end of the output matching section increased the power output to 50W and with a few other mods 2.5 W in will give 60W output for 4.6A. Saturated power output? well I have seen 80W but my chip capacitors keep catching fire. At this power level you can easily get a nasty RF burn and remember your eyes are even more sensitive to RF damage.
I am still experimenting with the amplifier design as I need four units with similar performance. My next mod will be to drill two new holes to bolt the unit to the heat sink, this should allow the input and output sections to be made longer. My thanks go to John for the original article and his help, I dont think I would have bought the units without knowing that someone else had already done most of the hard work.