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Thursday, 8 September 2011


Back in January 2010 I built and blogged a PTO I threw together using either a Starbucks or a MacDonalds straw as the former and a small brass plug and M5 Nylon bolt that generated a fair bit of interest. It appears Rev George Dobbs from GQRP club referenced it in an October 2011 Practical Wireless article. Anyway after a brief email exchange with George I am going to see if I can create a 'kit' of sorts for the GQRP convention 2011 at Rishworth of all the mechanical parts and the electronics. I'll update the blog with progress. If anyone has ideas then glad to hear them.

The ideal is 100kHz swing preferably not needing more than 3 full revolutions of a tuning knob.
If I have time will be usable for both a direct conversion setup so will be 80m or 40m or will be usable in a superhet type vfo+IF option. (11.0592MHz maybe??)

Slow motion would be nice but I know this is easier (read cheaper) to obtain by using fine threads OR A BIG TUNING CONTROL.
I found using a blass slug not an iron one was easier so that way you work from a maximum inductance and it drops as you insert the brass slug (made the tuning the 'right way' on the tuning knob too!).

Will be either colpitts or vacker type vfo as stable, I am sort of familiar with them and part counts are low.
Active device will be a FET J310, 2N3819 or similar depends what I have a few of.
Caps will be black spot NPO ceramics.
I'll try and see if I can include a voltage stability at 6V or 8V but will wait and see.

There are two main options
1.The PTO assembly sit 'proud' of the main board (pretty much as my original bodge did, and as per George design) or
2. An on board design a bit like the MMR-40 from KD1JV.
Go to and you'll find docs for it or there was an ARRL article (I might update these with real links)

The assembly needs to turn a rotation into a horizontal motion. There are probably as many ways to do this as there are resistors in a E24 series but in short you move either the coil over the slug or vice versa, the later is easier. George has already whimiscally shown one option using a used up old lipstick for this.

Anyway more later off to work at the salt mines....


Ricardo - CT2GQV said...


I made some mechanical tests you can see here:

I like this design (not mine):

And this idea also nice:

73 de Ricardo - CT2GQV

gj7rwt said...

Voltage regulation would be a good idea. I would go for 6v. Would be a good idea to house the regulator outside the vfo case as they can generate alot of heat which will make the vfo less stable. Goodluck with producing your kit.

gj7rwt said...

I would use a 6v regulator but leave it outside the pto as they can produce quite alot of heat which will make your pto less stable. Good luck in producing your kit