Siel "RFB300" 3mtr linear amplifier to 2mtr conversion.

From a fellow amateur I got a damaged 3mtr linear amplifier, the 2 BLV25 FETs are burned away because of a mismatched output, the controller didn`t do it`s job to shut down the amp and so the FET`s are gone. It also got a lowpass filter and a directional coupler build into it to measure in/output power, and swr.


It is a shame to salvage the amp for parts, there is a nice power supply in it, so why not use all this and put a new circuit board in it to work on 144Mhz. The lowpass filter has to be renewed, it is about 20Mhz to low, as it was used for in the 100Mhz range, as I measured on my spectrum analyser. I can build a new one,..perhaps better with 9 or 11 poles or a banpass filter instead.

A good way to get the amp operational again is to build anew circuit in it with a BLF248 FET, one is enough for 300Watts, two of these can`t be handled by the power supply it is only rated at 750VA. A nice circuit board can be ordered from , see the board for just one FET. The controller board can be purchased by and look for the "Upgraded Amplifier Controller:, you can buy it as a kit or fully build and tested.




Here at the left you see the RFB300 actual amplifier,..the FET`s are soldered out,..circuit board is damaged where to much heat was.
Left you see the insides and what is what, actual amplifier, filter/coupler, controller, power supply. To use it for 2mtrs I need put in an extra 2 coaxial switches operated by my tranceiver, newly build lowpass or bandpass filter, and a few other mods.

At the left the setup as I planned it.
From Ron, (VK4DD, NL-Call PA4K) I bought the circuit board for the amp and ordered the parts at , then I started to solder the parts onto the board, at the right the progress at sept.13, 2009.
Questions about the board and building/adjusting it is very easy with RON,..I can Email him and ask questions in my own language, as Ron is from the Netherlands from birth


Here the PCB almost finished inside the RFB300 cabinet, mounted onto the 6mm thick copper heat spreader wich will be bolted onto the heatsink.
A nice replacement for the old 3mtr PCB. Even the original power transistors for the power supply can be re-used with a new PCB to regulate these transistors.

The VK4DD PCB is bolted onto the copper spreader by using small 0,8mm washers, so the top of the pcb is around 2,85mm from the top of the copper heat spreader. This way the BLF248 can be soldered and then fastened onto the copper without stressing the solderings and the FET. So no future cracked joints. (??crack,...joint???)



To adjust C6 no variable C, but ATC-B capacitors. I measured different 82pF with 3,3pf and 3,6pF ones and I got the the following combinations I can use to tweak the output of the amplifier.


Further i`m building a SWR coupler, here at the left the final result. I re-drawn it from the pages of amateur station PANHC , he has some nice designs with layout on his site te reproduce. And when I`m at it making these why not a few more for future use. This coupler measures the SWR for the W6PQL controller board and a meter to see SWR and F/R power.

The size of the microstrip in the middle is made optimum with a microstrip calculator and has a width of 2,8mm on FR4 board of 1,6mm thickness.

The same way I`m also want to make a 6-9 pole lowpass filter to put between the antenna and the amplifier to supress harmonics. According to Ron (VK6DD) this is not neccesary with this amplifier.


The "directional coupler" as seen above was re-designed after it was build and after some measurements. The coupling now is not 50Ω but 100Ω impedance with a 100Ω resistor, and a BAT85 detector diode. After I soldered it together and put on some BNC connectors I made a test on my spectrum analyser for what the directivity would be at 0dB input.
Reflected power measured was approx. -38dB at 50Ω input/output impedance. I also measured the directivity with the FT817 connected to it with 5Watts output power. The directivity was the very same as measured before.
I choose for a 100Ω coupling so the coupling has lesser influence on the 50Ω stripline.

Left the result of the coupler with lowpass filter build into a metal sace with N-connectors onto it. The box is 7x8cm in size and made of 0,7mm thick tinned metal plate. On top you see two feed-through capacitors for the measurement outputs. The output of the coupler goes with a short piece of 50Ω coax to the input of the 9 pole lowpass filter wich is adjusted for around 145Mhz. The -3dBm is at 147Mhz and rejection of the 1st harmonic at 288Mhz is -22dBm. Loss at 144.300 is -0.2dBm.
The green stuff on the lowpass filter is green polyurethane lacker, Connecters are N-type.



I dismanteled the case from above again and made a new one for the coupler alone, the lowpass filter was to bad in rejection when in the case,..out of it the rejection at 288Mhz was around -60dBm. So the case was much to small and will not be used in this amplifier.
The above gives the power measured from the coupler with increments of around 5Watts with my TS790 on a 50Ω dummyload good towards 4Ghz.



The rear cover with the connectors is also foreseen with some information.
With Autocad I made a design on 1:1 scale with text and some lines for around the different connectors of what they are. I printed this with a laserprinter in mirror on a whith A4 paper. I cleaned the rear cover. Then I put the paper on the right place and fixed it with some cellotape. Then I warmed the rear cover up with a small electrical stove. With a cloth drenched in some laquer thinner I pushed hard on the paper till the tonor was coming through, take not making it wet. this way the tonor is pressed onto the cover and also burned onto it by the stove. As a finish I sprayed some clear coat over the complete cover.

Detailed picture here.


detailed picture here

Sunday, May 9th I put the last hands on the wiring and connectors. Power supply connected and a J-38 key to get it on transmit. Lets see what happened after I switched everything on, the sequencer switches the coax relais/bias/main power and the 230Volt blower. So everything is working fine. The 28Volt from the power supply is set at 27.8V. Last thing to do is solder the BLF248 onto the circuit board. Adjusting output coupling, adjusting the bias current and that should be it.

Here at the right a modified schematic diagram of the "BIAS" circuit. Here are D1 & D2 normal 1N4148 diodes. These are here to surpres the negative spikes from the power supply, these can kill the BLF248
Zener ZD1 is a 5.1V zener diode, a backup insurance if the 7805 fails, so there won`t be more then about 5V to the bias. The extra 520Ω resistor is there to maximize the bias current. Special thanks for this mod goes to Ron, VK4DD

20th of June everything connected to adjust the amplifier, antenna analyser connected to the input of the amp so I can adjust ine the input C for minimum SWR. With the FT817 connected I get now a 22W output with W input. I adjusted some more and got a stunning 100W out with 2W input. Efficiency is a bit low, around 45%. This has to go up to around 75% or so.

Wednesday June 23th, I adjusted C6 some more and I got an output of 300W with 15W input in FM. Power supply input power is 380Watts (27.8Vx13.6A). Efficiency now 79%. I go for lesser efficiency, around 75% so I get a bit more power out,..IN FM (CW) 400W out.

July 11th; Adjusting C6 some more by changing the C combination and move it a bit. This gave me a power increase of a easy 400Watts in FM, and a efficiency of  75% now. Not bad at all. I also put my TS790 under some tests connected to the spectrum analyser to see if there are some bad harmonics from it.

The TS790 gives in FM with 15W out on 144.300Mhz the following harmonics data: on 288.600Mhz, -8dBm (0.16mW) and on 432.900Mhz, -23dBm (0.005mW)
At full power of 40Watts out in FM same frequency this gives: 288.600Mhz, +7.2dBm (5.5mW), and on 432.900Mhz, -19dBm (0.012mW). On full power I do not care because I can`t use it to drive the amplifier.

With 400W out in FM on 288.600Mhz, +6dBm (6.5mW). 432.900Mhz, +11dBm (12.5mW). So the amplifier magnifies the 3th harmonic more. I have to get this down within a limit of  ≥60dB, this because this is stated here in the Netherlands by our law. So I`ll see if I make a bandpass on the output of the amplifer . See the site of  PANHC for some nice details.

July 16th;  I adjusted the controller/sequencer to shut down the amplifier when there is a high SWR on the output. I adjusted it to a SWR of 1:2. I did this not by connecting a 25 or 100Ω dummyload. But instead I used a hand held small 3 element beam, I adjusted the gamma match of this antenna to 1:2 and connected it to the output of the amplifier with an extra SWR meter in between. Next I transmitted on full 400Watts in FM, and while transmitting I adjusted the small resistor till the amp shut down. Repeated this a few times to make sure it is adjusted well. So the high SWR power down is also OK now.

Further I checked everything to see of all was done and put on the covers,..well I finished the thing. Filters for harmonics are screwed on the output N connector so I don`t have a need to open it up again

Well,..up to the next project,..400W 70cm amplifier.