Just drill a triangular hole in the top of the grub screw and use one of these:
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=351508;image)
Drill a triangular hole, how do I do that? ![Face Palm :palm:](https://www.eevblog.com/forum/Smileys/default/facepalm.gif)
There's a chance that a triangle driver might work in something larger but for your 50 thou grubscrew.....forget it.
I've done a lot of hex driver work and there are two things you need ensure, first that your key is in good condition, edges sharp and not rounded and the end square too.....grind as needed.
The hex hole need be
clean to full depth to ensure full engagement.
Neither matter much for
most fasteners but when you get a tight one it really does.
With something as small as 50 thou there's not many options when things go wrong, only three IMO, heat, percussion (to loosen the bond), and drill the bugger out but that generally results in damage to the thread.
I've been racking my brain (what little I've got) to remember if you need to remove the timebase knobs as IIRC the retaining collars on the shafts near the two timebase PCB's actually hold the shafts in....I should whip the covers off the 1740A I have here but it's a job I don't want to get into just right now.
Have another look....and IIRC the two timebase shafts need be 'timed' so that they can be slid out the front without fouling on the way.....or maybe it was the selectors on the PCB's.
![Tweak :-/O](https://www.eevblog.com/forum/Smileys/default/smiliey_cal.gif)
(main and delayed)