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 http://pa0ehg.com/70cmpa.htm 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)
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| 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.
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| The completed amplifier (with lid off module) |
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