70cm 250W Amplifier from secondhand DVB-T PA Part One

When I last posted in June I mentioned I had bought a secondhand DVB-T PA from a German eBay seller well I've finally got around to putting it all in a box and getting it up and running.

The PA module is simple to modify and in fact I found during an internet search a website by a Dutch amateur PA0EHG who had already modified one of these amplifiers and I followed his instructions which worked perfectly.

The PA is coupled with a LPF again from eBay which looks like it's a based on a design by Jim W6PQL the specifications where 2nd order harmonics -35dB and 3rd order -55dB with an insertion loss less than 0.15dB.

When measured with the VNA I found the 3rd order harmonic weren't rejected as much as specified so I expect it might need another coaxial stub filter to drop the 3rd order harmonic a little more.

70cm eBay LPF filter response.

I put the LPF in a tin box bought from Alan G3NYK though I had to buy a bigger box than was required and trim it down to size using tin snips.

To monitor the amplifier I was going to roll my own controller board but again Jim W6PQL has what he calls the ultimate amplifier controller board which at $50 for a kit I thought was a bit of bargain and a lot less hassle than making my own even if half the features of the board aren't being used.

I stuck all this in a rather large Hammond box so its ended up not being a small amplifier but you go with what you have laying around the shack.

I painted the case in acrylic paint and though the result was good looking I don't hold out much hope for it's long term durability.

Front Panel

Reat Panel

Top View

Internals

The amp requires a quite hefty PSU 31V 17A but more on that in later post.