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 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)

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)

432MHz AFS 2018

The final leg of the RSGB as usual is held in February starting at 0900, meaning waking up at 0500! This would be the first opportunity to try the newly built G4DDK Iceni 432MHz Transverter, along with the K3S, solidstate amplifier, HA8ET masthead Preamp and a 20 element yagi at 20'

As the first 2 hours overlapped a French Cumulative contest there was some interesting DX to be worked, which made up for the lack of ANY DL/ON/PA stations! There were a few snow flakes before and after the contest but during the contest freezing feet were the main hazard

The only equipment issue was the audio from the K3S. At times there would be buzz on the monitor audio on transmit. The monitor was set at level 2 and the buzz was equally as loud Eventually it was correlated with the fans running on the K3S.  The fan was running more often than previously as the Iceni was placed on top of the K3S, as shown above.

Searching the Elecraft website after the contest revealed there was a mod for this issue, although they were slightly confused as to whether the hum was on transmit or receive. The fix was a 470uF capacitor mounted on the KPAIO3. The modification was done and the issue resolved

The equipment seemed to have done the trick, again we won the open section:

Resulting in a winners certificate and , in conjunction with other Bristol contest group operators won the G0ODQ Trophy. Perhaps it was worth getting up at such an unearthly hour!

Feature upgrades for the 432MHz Solidstate Amplifier

Having upgraded the 2m solidstate amplifier I decided I should do the same upgrade to the 432MHz Amplifier.  The original writeup is here The Two same features were needed:-

1. There is no bypass position, so one cannot test the antenna SWR before letting the amplifier use it. This cost of the lack of this feature was learnt the hard way on my 70MHz amplifier
2. Currently the Elecraft K3 keys the DEMI transverter which keys the amplifier which sequences the antenna relays and preamplifier. There is no output for the Amplifier to properly sequence the DEMI Transverter along with the antenna relays and preamplifier, this needs to be added.

A bypass switch was added to the front panel along with a bypass LED. A phono socket was added to the back panel for the Transverter output. An hour was spent adjusting the PIC code and all feature additions were accommodated.