Systron-Donner Spectrum Analyzer
Type 809-1 (10MHz - 12.4GHz), page 4

The second IF (260MHz) amplifier is a little inconspicuous milled aluminium enclosure next to the coupler after the isolator on the output of the first LO Yig. Could this be the faulty item I was searching for? I could test it easily by connecting it up to my Network Analyzer and verify if it was working, but I would first have to make up a pair of SMA to SMB leads. I had a better idea. I had a brand-new Watkins-Johnson gain-block which I connected up to my Network Analyzer and verified the gain to be around 13dB. The supply voltage was the same (+15V), so wiring it into the RF-deck was not going to be a problem. However the W-J amp used SMA connectors so I had to be creative in

connecting up the output! BINGO! It worked … Or more precisely, the Spectrum Analyzer now worked. I had a nice ‘fat’ trace on the display for +10dBm at 400MHz with 30dB of input attenuation. The photograph on the left shows the ‘belt and braces’ lash-up, but it worked. I could have left it like it was and lived with it, but I suspected that the failed amplifier might be an Avantek MMIC and I was sure I had one tucked away somewhere. I put it on my VNWA (Vector Network Analyzer) and confirmed that it was now an attenuator (about 30dB). It was an easy matter getting the PCB out of the enclosure. I was right, it was indeed an Avantek GPD-462. See the above photograph. Note the corrosion around the base of the can. I found my ‘spare’, a GPD-405 which is essentially a beefed up 462, capable of delivering higher power. I cleaned up the tiny PCB and fitted the GPD-405 only to discover that the can was slightly ‘taller’ … so it was necessary to drill a hole right through the enclosure to allow it to fit. See the photograph below.