Friday 18 July 2014

432MHz Solidstate Amplifier Completed

Over 3 years ago I bought a W6PQL 432MHz amplifier module  with the matching LPF/Directional coupler. Before I moved out of the McKinney house I quickly mountedthe modules on a heatsink inside an aliminium chassis to protect it in storage. Over the last few weeks I have finished the metal bashing thanks to WA5YWC and built a PIC controller and LCD display.  The biggest issue was testing it as I no longer had a 432MHz transmitter in the USA. W5LUA kindly offered to let me use the facilities in his shack which included drive source, Power meter and dummy load.
The first thing we learnt was that the RF got everywhere inside the chassis. Initially there was no screening around the PIC board which caused measurement issues. After a trip to the local hobby store for some .015" ali sheet and the donation of many 1nF bolt in feedthroughs by W5LUA that issue went away. It was necessary to put a screening can over the directional coupler (W6PQL only suggests it for his 1kW amplifier) as the calibration of the bargraph meter changed dramatically when the cover was put on the box
When completed the amplifier gave 420W for 6W drive when running 17A at 48.0V. Looking at the W6PQL figures I could hit the amplifier harder to get the full 500W but this way I have a safety margin. All harmonics were over 65dB down on the fundamental power.

Monday 7 July 2014

Fujitsu 251-9735-010 6GHz amplifier fixed

I was given a Fujitsu 251-9735-010 6cm 5W amplifier by WA5VJB about 12 years ago for my Transverter but never got round to trying it out, this weekend was an opportunity. I borrowed a 5760MHz California Microwave brick which gives 16dBm at 5760.000MHz from W5LUA to save needing to retune my DB6NT 5616MHz LO. This needed a -19V supply, provided from my 57A 28V PA supply (yes overkill!) According to Kent's notes the PA needed 4A at 12V and 200mA at -12V for the bias for which two other additional PSUS were found (a variable one and the 13.8V rig supply). Applying the -ve bias the power supply immediately current limited, but the resistance to ground from the bias supply lead was 12kohms. Time to take the lid off. LOTS of screws later the lid was off. The -ve bias supply feeds a 7905 regulator. As a quick test (to avoid more screws) the regulator was cut out and the -5V applied to the regulator output pin. With the 12V supplied 5W was seen. The control board was removed and a new regulator fitted. It still produced 5W. 
The opportunity was taken to measure some voltages and currents. Full output was produced until the positive supply dropped below 12.00V. Full output was produced until the negative bias dropped below -6V, The measured currents were 12V at 3.9A and -12V at 50mA. It was noted that the detected output was a negative voltage. Not good for Microchip PICs! The detector diode was reversed and resistors adjusted so that the detected output was +10V at full output.