Stuck 0.050" hex grub screw on HP1740 scope timebase knob

[tggzzz]

I'd like to clean and/or replace the legend for the timebase of my HP1740 'scope. To do that I have to release a 1/20" grub screw in the knob of the timebase selector.

The screw is, not too surprisingly, stuck to the extent that the hex key bends significantly if I try to undo the screw. I'd be grateful for any suggestions as to how to release the screw.

So far I've doused it in WD40; next obvious option is light machine oil or penetrating oil. What are my other options?

Thanks

[DmitryL]

check here:
https://www.eevblog.com/forum/reviews/hp-scopes-knobs-removing/

[jlmoon]

A really hot & small easy out ?  You need to transfer a little heat to the set screw keeping it off the plastic surrounding it
If you can find a high quality (so won't round off on you) 1/4" socket to right sized allen set screw, I would recommend that first.  Use heat as a last resort!

[tggzzz]

A really hot & small easy out ?

I'm sorry, I don't understand that. Is it a commercial product?

You need to transfer a little heat to the set screw keeping it off the plastic surrounding it

I would have thought that would expand the screw into the colder surround, so making it more difficult to remove. What am I missing?

Thanks

[tggzzz]

check here:
https://www.eevblog.com/forum/reviews/hp-scopes-knobs-removing/

Thanks.

I'll investigate and cogitate before trying some of those ideas.

[robrenz]

If you have a battery powered drill or bit driver with a clutch setting this may help. Get a high quality .050" bit that can go in the drill or driver and set the torque setting to a value suitable for that size screw. In reverse at a slow speed let the clutch over run continuously. The mild torsional impacts of the clutch detents engaging and disengaging acts as a mild impact driver. Obviously make sure you are fully engaged in the socket of the set screw and maintain force on it to stay fully engaged.  FWIW

[jlmoon]

A really hot & small easy out ?

I'm sorry, I don't understand that. Is it a commercial product?

You need to transfer a little heat to the set screw keeping it off the plastic surrounding it

I would have thought that would expand the screw into the colder surround, so making it more difficult to remove. What am I missing?

Thanks

Yes, heat will create molecular expansion to some degree, but it can also help break the seizure.  Actually heat works best when removing screws that have been installed with loctite thread locker.  I really suggest a high grade properly fitting driver (short as possible to reduce flex of bit) and hope for the best.  I have a Hp 3580 spectrum analyzer I had trouble removing those knobs as you're having and resorted to that high quality driver.  Impact drive is another option as Robrenz stated.

[tautech]

check here:
https://www.eevblog.com/forum/reviews/hp-scopes-knobs-removing/

Thanks.
I'll investigate and cogitate before trying some of those ideas.

Maybe find a source of a good second hand knob in case it all turns to custard and you damage it and have to carefully break it off.
Alternatively
Have a look at the method that shaft is retained inside the scope and remove the retaining rings and withdraw the whole shaft with knobs attached.

[tggzzz]

check here:
https://www.eevblog.com/forum/reviews/hp-scopes-knobs-removing/

Thanks.
I'll investigate and cogitate before trying some of those ideas.

Maybe find a source of a good second hand knob in case it all turns to custard and you damage it and have to carefully break it off.
Alternatively
Have a look at the method that shaft is retained inside the scope and remove the retaining rings and withdraw the whole shaft with knobs attached.

Having previously repaired the timebase board (abraded track, electrolytic), I know that getting the knob+shaft out is trivial (albeit a little quirky!). Disassembling the knobs to clean/replace the timebase legend turns out to be less trivial.

Let's hope I don't need to find a replacement.

[mathsquid]

If you have a battery powered drill or bit driver with a clutch setting this may help. Get a high quality .050" bit that can go in the drill or driver and set the torque setting to a value suitable for that size screw. In reverse at a slow speed let the clutch over run continuously. The mild torsional impacts of the clutch detents engaging and disengaging acts as a mild impact driver. Obviously make sure you are fully engaged in the socket of the set screw and maintain force on it to stay fully engaged.  FWIW

This will work better if you use a left hand drill bit--if you can find one small enough.

[tautech]

@ tggzzz
Having done a bit of light engineering in days gone by, I like the proposal from robrenz.
But I now exactly how thin a 50 thou hex key is....f**ing thin!
If I had a long series key, I might consider cutting some off to fit in the drill.

There was another good suggestion, from David Hess, re grinding a 1/16 key down, which is also a good idea as it has a lot less flex.
https://www.eevblog.com/forum/testgear/hp-1741a-oscilloscope-restoration-project/msg506759/#msg506759

Personally I would attempt to "shock" the screw with a punch and small hammer.
The punch either needs to fit inside the hex or over the hex, I think I would go for over as hex screws are usually quite hard steel.
Should you attempt this, get another pair of hands, support most of the weight of the scope on the knob(cradled in/on something soft to minimize risk of breakage) and give it a good tap.
Ensure the punch you use is as big as you can fit down the screw hole.

If you have any engineer or mechanic mates use them as these types will have the "feel" required.

[tautech]

Let's hope I don't need to find a replacement.

If you do put an ad in the Buy/Sell/Wanted board.
and/or email these guys: http://www.sphere.bc.ca/test/hp-parts/hpparts7.html

Others might also suggest a source?

[RobertoLG]

do you have freon or something similar? spray over the screw and try again, it may work, and won't cause any damage on the knob

[tggzzz]

Thanks for all the suggestions. The problem has now been circumvented...

The original 0.050" grub screws are still as immovable as ever, despite gentle thumping/heating, WD40, gentle force. I didn't want to apply too much of any of those since the knobs were functional and I was afraid I would melt/break adjacent plastic parts of the knobs.

In the end the other two grub 1/16" screws on the delayed timebase knob were persuaded to turn, which allowed me to disassemble the knob and insert a replacement legend. Sure the font and colour aren't right, but at least I can read them!

[tautech]

Thanks for all the suggestions. The problem has now been circumvented...

The original 0.050" grub screws are still as immovable as ever, despite gentle thumping/heating, WD40, gentle force. I didn't want to apply too much of any of those since the knobs were functional and I was afraid I would melt/break adjacent plastic parts of the knobs.

In the end the other two grub 1/16" screws on the delayed timebase knob were persuaded to turn, which allowed me to disassemble the knob and insert a replacement legend. Sure the font and colour aren't right, but at least I can read them!

Good alternative solution. 

[apelly]

Um... PICS PLEASE!