Building a Hendricks 41dB Step RF Attenuator

Two or three years ago I had a browser tab sitting open on the Hendricks 41dB Step RF Attenuator from Pacific Antenna/QRP Kits in California.  For some reason I never ordered one and when I finally remembered that I was going to get one, they were out of stock.

I dropped them an email back in March this year and James told me that they were hoping to either redesign or restock and then in September he emailed me to tell me that they were restocking.  As soon as the kit appeared back in the shop, I ordered one straight away and it arrived just a few days later.

There’s not really much in the attenuator but here’s everything that came in the kit followed by a picture of the circuit board and all the components ready to go.

Hendricks 41dB Step RF Attenuator kit

Hendricks 41dB Step RF Attenuator kit

Hendricks 41dB Step RF Attenuator kit - All the components

Hendricks 41dB Step RF Attenuator kit – All the components

I was struck by the quality of the circuit board.  Very solid, good through hole plating and very decent sized pads on each side.

It didn’t take me long to fire up the soldering iron and get to work.

The first two resistors soldered into the Pacific Antennas QRP Kits attenuator

The first two resistors soldered into the Pacific Antennas QRP Kits attenuator

Back of the first two resistors fitted to the attenuator

Back of the first two resistors fitted to the attenuator

Because the solder pads are so large on each side, rather than rely on solder to flow from one side to the other, I made a point of soldering on both sides.

All the 'vertical' resistors in position

All the ‘vertical’ resistors in position

I fitted all these resistors and then moved on to the ones below the switches.  I worked from left to right and when I’d fitted R3, I realised that I had no more left.  There was no R16 in my pile of components.

Panic.

I looked around, I checked the bench, I checked the packaging, I checked the floor but there was no sign of the missing resistor.  I checked my component rack but I didn’t have any 270R 2W 5% resistors in there.

I quickly realised I was going to have to find a spare resistor from somewhere.  I can’t remember exactly how it happened but I got into a conversation on a local repeater where I happened to mention that I was a resistor short and a good friend popped up a few minutes later to tell me that they would drop one over to me later that afternoon.  I asked where they’d got it from and they told me it was a local shop.  I was planning to head over to town later anyway so I said “thank you very much, I’ll pick it up myself”.

When I collected the resistor, I looked at it with disappointment.  It was the correct size and value but the style was very different to the resistors already on the board.  Even though the board was going to be mounted in a box, I wasn’t happy with how it was going to look.

The non standard 270R resistor in the attenuator

The non standard 270R resistor in the attenuator

You can see that it’s totally and utterly different.  You can also see that it’s not soldered in, I just couldn’t bring myself to solder it.

eBay called and I found these.

They looked perfect, correct value, correct power rating and correct tolerance.  They came as a pack of five and I ordered them, paying the extra 20p for next day delivery.

About an hour after ordering these, I rolled my chair back and it stalled on something.  I looked on the floor and there was the missing resistor.  If resistors could smile, I’d swear it was smirking at me.

Within thirty seconds it was soldered in place.

All the resistors fitted to the 41dB step RF attenuator

All the resistors fitted to the 41dB step RF attenuator

The next thing to fit was the large slide switches.  These are big and chunky and feel very positive when chunked backwards and forwards.  I fitted them all with no problems at all except somehow (and I genuinely have no idea how) this happened.

The broken switch

The broken switch

I tried to find a replacement switch locally of the same style with no joy so I emailed James to ask about getting a spare.  James was very helpful and just a few days later a care pack arrived from California.

Two spare switches

Two spare switches

Because I’d done such a good job of soldering everything in, getting the broken switch out was a nightmare.  It took me a good half an hour of working with both a solder sucker and some desolder braid.  It goes to show the quality of the board that I was able to spend so much time on it with a hot soldering iron and not damage it at all.  I’ve never used desolder braid before but that and the solder sucker eventually did the job.

The circuit board showing where the switch was removed (click for a full size version)

The circuit board showing where the switch was removed (click for a full size version)

With the broken switch removed, I fitted one of the replacements which left me all ready for the next stage.

Hendricks 41dB Step RF Attenuator PCB all built and ready to go

Hendricks 41dB Step RF Attenuator PCB all built and ready to go

The next stage was testing so I hooked up my DVM and checked the resistances were all correct with the associated switches.  All looked good.

After this, it was time to prepare the box.  The supplied box has a nice brushed finish and  decided I was happy to leave it like that rather than paint it.  There are some nice decals provided for labelling and the instructions recommend spraying the box with “Krylon clear” first.  This is a USA brand of lacquer spray and not available in the UK so I sourced this alternative from Wilko.

Wilko clear lacquer spray

Wilko clear lacquer spray

After a couple of minutes shaking and spraying, the box was left to dry for 24 hours.

The box, freshly sprayed and drying before applying the decals

The box, freshly sprayed and drying before applying the decals

This looks fine but once the lacquer dried, it didn’t really look very attractive.  I mentioned this to a friend who very kindly offered to hydro dip the box and lacquer it for me.

Once it was all done, I assembled everything into the box for testing.

The finished attenuator

The finished attenuator

It’s quite a dark finish but I think it looks good.  I used my analyser to test the actual attenuation and on 40m, it’s within half a dB or so of specifications and so this will be a useful addition to the shack as it’ll allow me to match the output of my two WSPR transmitters for proper aerial comparisons.

Considering the complexity of some things I’ve built over the years, this should have been a simple build.  It just goes to show that the gremlins can creep into anything and everything seemed to go wrong with this one. I’m just glad it’s finally completed and working as intended.

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