HP 8601a knob removal - Page 1
Forum section: electronics / repair

author: neil
author_rank: Contributor
timestamp: August 19, 2019, 12:59:34 am
content: I have an HP 8601A with the RF attenuator (and its concentric vernier adjustment) frozen in place. I'd like to pull the entire module to work on it, but don't have a driver that fits the knob setscrews. I assume they're allen-heads, but am not even certain of that.Any ideas as to the size and where to source an appropriate tool? Thanks in advance.

author: Tony_G
author_rank: Super Contributor
timestamp: August 19, 2019, 02:43:50 am
content: They use Allen keys, I want to say 5/32 but I can't really remember what I used on my 415A.TonyG

author: neil
author_rank: Contributor
timestamp: August 19, 2019, 05:56:03 pm
content: Thanks. After some other googling, I think maybe it's .050". Anyway, I've ordered a set of allen keys with small sizes, should be here tomorrow, so hopefully one of them will fit

author: SilverSolder
author_rank: Super Contributor
timestamp: August 19, 2019, 08:07:01 pm
content: Be careful using force on those small Allens - it is only too easy to come a gutser with those, as Dave might say.Applying a drop or two of "Weasel P*ss" (50/50 mix of ATF and Acetone) in advance of the procedure can work wonders.

author: Stray Electron
author_rank: Super Contributor
timestamp: August 19, 2019, 10:13:40 pm
content: I wouldn't use acetone on a plastic knob or on anything with a plastic front panel!

author: SilverSolder
author_rank: Super Contributor
timestamp: August 19, 2019, 10:38:13 pm
content: Quote from: Stray Electron on August 19, 2019, 10:13:40 pmI am talking about accurately applying one or two drops to the threads of the set screw, and only the set screw.Spilling stuff where it's not supposed to go is never a good idea, goes without saying!

author: neil
author_rank: Contributor
timestamp: August 19, 2019, 11:06:50 pm
content: ACK and thanks to all. Small allen wrench assortment should arrive tomorrow. Will update the outcome here.

author: Tony_G
author_rank: Super Contributor
timestamp: August 20, 2019, 04:39:53 am
content: Good luck and you should find that HP used the same size in a lot of other pieces of the same era. Buying a small size set was a good investment, much better than trying to find a flat head screwdriver that jams inTonyG

author: neil
author_rank: Contributor
timestamp: August 20, 2019, 05:13:00 pm
content: So the allen wrench assortment just arrived. And the winner is the 1/16" size, which was a very solid fit and enabled me to remove the knob easily. FWIW, none of the included metric sizes fit - the 2.0 mm was too large and the 1.5mm didn't fit securely (1/16" = 1.5875mm).Hopefully this information will be of use to some other forum user at some point.

author: factory
author_rank: Super Contributor
timestamp: August 20, 2019, 07:40:59 pm
content: Vintage HP test gear uses three different sizes of imperial grub screws;#4-40, uses .050" Allen key#6-32, uses 1/16" Allen key#8-32, uses 5/64" Allen keyAs the centres of HP knobs are usually brass & the screws use a coarse thread I've never had a problem removing them. However Tek knobs with steel centres are a PITA if rusted, but usually come out eventually.If you have a lot of vintage test gear invest in decent Allen keys, cheap ones bend or snap far too easily.Quote from: SilverSolder on August 19, 2019, 08:07:01 pmNever needed to use anything like that on HP test gear, if the grub screws are too rusted to come out using just the Allen keys then the rest of the unit is probably knackered too.David

author: SilverSolder
author_rank: Super Contributor
timestamp: August 21, 2019, 11:19:16 am
content: When rust is dripping off the front of the unit, you know you have a challenge on your hands, LOL! :-)The acetone/ATF mix works also if only one of the two parts is ferrous. See this video where the chemistry and the reason why this works so well is explained:I have an HP239A oscillator restoration project going on here. The set screws were in there for keeps... (iron in brass) - compounded by damage to the set screw hexagon from previous owners.Two drops and they were out!Iron gets oxidized even when it is sitting in brass... even if it isn't thick enough to look like rust. The combination of acetone and the wicking ATF is truly magic.