QRO? That'll b-u-r-n nicely Sir!

As some of you may know, I recently acquired an HF linear and needless to say I've been putting it through its paces recently! I have a doublet antenna and use an MFJ-974H balanced tuner.

The problem came when the other morning I tried to tune up on 80m, and at 100W there was a lot of fizzin' and poppin' coming from my ATU! Closer nasal inspection revealed an interesting aroma, ooh errr.

So, I opened it up and could immediately see where the problem was - the inductor rotary switch had definitely gone to the undertakers - here's a pic:

The central core had pretty much been eaten away by heat. I reached for the handbook thinking 'Crikey, that's a tiny switch for 300W'. It was then that I realised the spec was 300W p.e.p. not key down! I had been using 300W on RTTY the night before... Here's a close up of the switch:

So I thought 'This just won't do, need a new ATU'. Had a good look round and resigned myself to having to shell out a large sum for a Palstar. I decided to ring my good friend and mentor John, G0DVB. His response (the publishable bits) was something like 'Buy? Have you gone mad? Call yourself a radio ham?' You get the gist! His first suggestion was rip out the switch and use croc clips to connect the tappings which would get me going immediately, and then look at replacing the switch, or replacing the switch and coil with a roller coaster.

Indeed, I whipped out the switch, connected up a croc clip to each of the tappings in turn and was able to tune even better than before. Things were looking up! I ran like that for a bit and realised it was going to be quite an annoyance to go poking round inside the ATU each time I needed to change band.
 

So, had a poke around an old ATU Andy, M0GVQ gave me, and lo and behold it had a very heavy duty rotary switch in it. I made some quick measurements and noted that it would just fit in with a bit of drilling here and there. With some trepidation I pulled out all the guts:

Here's the panel the switch needs to be mounted on, marked out:

I did like the system of differentially tuning the capacitors, although I have to say the workmanship was dreadful. It certainly gave me some ideas for a future homebrew design.

So, I screwed on the  new switch, made up some new tappings for the ones that weren't long enough and soldered them to the tags on the switch (after checking I still had enough space to get the assembly back in). There were only 6 positions on the switch, but with the old switch I had noted that I only needed 5 to tune all bands anyway. Gently put the thing back together and here's a couple of pics of the result:

Moment of truth! Switched on and tuned up. Groan - wouldn't tune on 20m or above, but really sweet LF. Thinking cap on, realised that the croc clip I had checked the taps with was probably inductive - tested, indeed - .5uH!  Quick resoldering to more appropriate taps and voila! tuned up brilliantly. 300W key down produced absolutely no heat on the switch - hurrah!!

So, I ran with this for a couple of days until I thought I'd better put the covers back on. It was then I noticed that the  input 1:1 balun (the MFJ-974H is actually a tee-match, brought to balanced operation  by the balun) was VERY hot, and I'd only been using 100W on 80m. Quick perusal of the handbook said something like 'balun keeps cool, even at 100W'. So, deeply suspicious about its efficiency I delved into Sevick's book, 'Building and Using Baluns and Ununs'.  He made the very point that he doubted the ability of such a balun to not heat up with a moderate amount of input power. I should add that the balun is a piece of RG-303 with about 50 ferrite rings around it:

Reading on, for high power operation he recommended a 2.25 inch HF (red, to you and I) toroid with 9 turns of RG-303 on it for high-power applications (1.5kW!). I happened to have a 2", so I wound 8 turns, reasoning that the smaller diameter would be about right for 1 less turn. What a complete pig trying to wind double screened 303 round a toroid! Didn't have any single screen, and didn't fancy stripping off the outer sheath as that would complicate the issue. Anyway, with the help of some cable ties it went together, and I had already worked out I could fit it on the back panel next to the SO239 output connector. Found a bit of paxolin and cut it to size to form a retainer through which I could affix a bolt/nut to the back panel. Fixed the whole lot in before I cut the pigtails to length, so that the connections would be under no stress if cut exactly to the right size. 

Here's the finished result, oh, after I discovered I really should have put 9 windings on it as couldn't get a decent swr on 40m and had to build version 2! My MFJ analyser plugged in to the ATU showed that on best SWR the impedance on 40m was about 30 ohms, so the extra winding brought that up quite a bit.

 Now I can easily tune up, run key down 400W with no problem, AND I've saved a fortune (thanks for the kick up the *rse John!).

Well, that wasn't the end of the story. You may have noticed that the 239 connector is missing its connection. That was because when I first reassembled, keyed up 400W CW, there was some horrible arcing INSIDE the connector. Oh err mother!! I figured I'd never use it and just disconnected it. It was connected by a piece of silvered wire to one of the balanced outputs. THEN I noticed that after a short while the SWR was creeping up on key down and discovered the very same post the 239 was connected to was getting hot. All it was was a little nut on the back of the binding post was loose - tightened it up an no more heat. Guess this was causing the problem with the arcing on the 239 although haven't reconnected it as I won't be using it.

So, sat back and enjoyed using the radio for an evening and realised the front panel scale was going to be bugging me - it only went up to 300W. I forgot to mention that I was easily able to adjust the meter to read 600W full scale just using the adjustment pots on the pcb. So, pulled out the meter, scanned in the old scale and with a bit of work in Photoshop had it adjusted to show the new values. Here's a pic:

Well, that's about it! I will probably need to revisit the power meter circuitry to check the component values aren't overloaded with the new power levels, although I have to say it's working fine at the moment. Oh, probably need to get some better, probably ceramic, binding posts too!

 Endpiece

Replaced the binding posts today - here's a couple of pics: