Sunday 8 August 2021

Plisch 432MHz amplifier module

 When I visited Frederichshavn in 2018 I purchased some small Plisch dual BLF861 device 28V 432 MHz amplifier modules  which had been used in TV trasnsmitters. I also got a larger two stage module with a single device driving  the same dual BLF861 devices.

When I returned I tried using the dual stage device but it saturated at 100W, even after doing the published modifications to the output stages at and help from G4ERO. So the project was put to one side

I recently rediscoverd the modules and decided to try again, firstly with the dual device modules., With the same modifications I again got 100W output with 4W of drive. with each dual device set to 1A standing current.  I then reread the modifications and realised a PAIR of 5.6pf ATC capacitors were added across output capacitor, not a single one like I had been doing. Adding the capacitors and retweaking the input variable capacitor resulted in 190W output with 5W of drive, which is generally considered to be normal

A summary of my mods :-

Remove C62 and C63 (3pf)

Remove C7 and C27 (5.1pf)

Add TWO 5.6pf ATC capacitors across C9 AND C29

Add 22pf trimmer across C2 and C22

Add a 2N7000 FET at to turn the bias OFF on receive by earthing C19 V7 base junction (needs 5V on gate on receive)

The module after modification

Next task was to put it in a box. I did find the chassis from the defunct 6m 300W amp with coax relays and PIC controller so decided to repurpose it. 

The completed amplifier (with lid off module)