Bringing an Ameritron AL-811HXCE back to life - part 3

Have some time off this week and so got some more done in the shack and so, As per blog part two, ed work on the 811, first thing to do was count the windings.

This was not as easy as it first sounds.

I didn't want to remove the windings immediately, but count them on the ceramic former, belt and braces I thought.

In the end I resorted to using a head mounted magnifier (you've got one of those haven't you ? - if not get one, I use it for surface mount construction and repair but it's really useful for lots of other things !) to count them. 

After some experimentation - I developed this technique - using a pair of fine tweezers that I have for surface mount (I also tried a cocktail stick - didn't work as well) I dragged it across the coil (very lightly) to give me a little tactile feel (click) for every turn whilst I watched very carefully, and I do mean carefully.

I did this in a quiet room, with no-one else in the house as it really needed concentration and no distractions - but that might just be me ! 

I did this three times to get an average 

I then removed (unwound) the former to see if I was right (a further double check). 

I then checked what the SWG of the wire was (removing the enamel) and measuring the wire itself - you did know that SWG is measured without insulation didn't you ? I didn't until I read about it !! Thus turned out to be 18SWG - and luckily I have some in "stock" and so  after a little consideration, and a careful marking of the former where windings started and finished - and more importantly if you read Tom's article, where the gaps are and then started winding..

The result is shown below :-

Apologies for the worktop :) at least it shows it gets some use !!!

And this is the MiniVNA plot for TL which shows where the various resonances are :-

As you can see the peaks are out of the amateur bands (main peak at about 12.4Mhz), there does seem to be one quite close to 17M (one of my favourites), when I ran the same check centred on 17M there was no resonance / problem shown, so should be OK I think. It would be good to check an original one, it is interesting to note that Tom W8JI in e his RF plate choke article talks about resonance on his original choke that went into the Ameritron was at 12.5 Mhz, and so mine seems to be close which is good news!

Also I not that there is a hand written 12.9 on the original coil - possibly this is its primary resonance ? anyone know is this assumption is true ?

I've also measured its inductance as 222 uH which seems to be quite close to one very similar on Tom's site which is 245uH - so again pretty close methinks.

Also the spare in the RF Parts catalog shows its inductance as 235 uH which again is not to far off.

Not too shabby for an approximation, reverse engineering a burnt and broken coil :)

Onwards to the parasitic board reconstruction .......