Main time/div knob stuck on HP 1742A oscilloscope: what should I do? - Page 1
Forum section: electronics / repair

author: acacia
author_rank: Newbie
timestamp: December 23, 2017, 08:17:56 am
content: Hello! I've searched the forums here and the internet in general, but I haven't found a way to fix this.The main time/div knob is stuck on 0.1 ms/div on my HP 1742A analog oscilloscope and I can't move it. The delayed time/div knob (the ring around the main one) works fine. The main knob flexes a little, but never clicks into another position. I haven't put too much torque into it for fear of breaking the plastic.The insides look likeVideo 803with the 1740A, but with an extra plastic cog wheel and its board for the 1742A. None of the plastic gears seem stuck to the boards, and none of the spring-loaded pawls seem to have a problem. It's easy enough to manually move the springs. If you turn the delayed knob a tiny bit, its metal axle moves a tiny bit. But if you turn the main knob a tiny bit, its axle doesn't move at all.It's very clean inside, except for a little bit of dust. Nothing looks obviously jammed and it doesn't look like there should be anything blocking movement. What do you think I should look for or do at this point in order to try and fix it?I was able to findthe manualonline, but I didn't see this problem in there. I'm familiar enough with digital o-scopes, but this is my first time with an analog one. I'd be happy to provide any additional information as well.

author: tautech
author_rank: Super Contributor
timestamp: December 23, 2017, 08:31:58 am
content: Welcome to the forum.The timebase shaft may have been incorrectly installed by a previous owner.If it's similar to the 1740, remove the two collars on the shaft and slide it out the front. You may have to rotate it some to get the rear half of the shaft to engage in the flats of the front timebase selector switch as you draw it forward.There should be instruction of how the switches must be set for reassembly in the manual.Some time studying the front dials and the steps on the rotary switches can also allow you to nut it out so that the shaft and switches are all in the correct alignment before reassembly.

author: acacia
author_rank: Newbie
timestamp: December 23, 2017, 08:57:19 am
content: Thank you for the reply!I did notice that there was a configuration where the flats of the main shaft and the delayed shaft align. I can set the delayed dial in that position, study the settings of the front panel (and probably take a photo, just in case) and then loosen the two hex nuts on the collars inside. At that point, I guess the concentric time/div knobs should be able to come out?I did find the section of the manual that lists the front panel control settings to be used for adjustment, so I'll make sure those are all set as they should be. I saw an exploded diagram in the manual that shows the parts (with part numbers) as well, so I'll be looking at that too.

author: tautech
author_rank: Super Contributor
timestamp: December 23, 2017, 09:06:58 am
content: Quote from: acacia on December 23, 2017, 08:57:19 amYep, you're onto it. Just work with the retaining collars and draw the whole lot out the front.Start trying to get the knobs off will only turn to a disaster. At least a couple of members this year have wrecked the timebase knobs on their HP's.Work slowly, methodically and take note of what you see. Nutting it out isn't to hard especially if you have some mechanical aptitude.Put some pics up if you get bluffed, there's a good # of these HP owners on the forum that can guide you through.I've got a non-working 1740 that if I have to I can take a look at how it's exactly set up. I had another years back that gave me the small understanding of them that I have.

author: acacia
author_rank: Newbie
timestamp: December 24, 2017, 07:35:38 am
content: Thank you so much for getting me in the right direction.Sorry for the late update, but I've been trying to unscrew the screws on the retaining collars, and they're in very tight. Twisting the hex driver harder isn't a problem, but the PCBs and metal shafts are bending more than what makes me comfortable. I'm wondering if there might be rust/corrosion.Should I use a drop of WD-40 or machine oil to try and loosen up the screws? I know oil and plastic don't always play well together, and the plastic gears are right there.Attachment is a photo of where I'm at now.

author: tautech
author_rank: Super Contributor
timestamp: December 24, 2017, 08:01:51 am
content: Quote from: acacia on December 24, 2017, 07:35:38 amSort of a corrosion as it's a dissimilar metals reaction between the grub screw (steel) and the alloy collar.Can you get the key in from the side and use the longer fulcrum of it's shank, if only just to crack it (break it free) and then use the key like it's in the pic to undo it some once it's been cracked ?Adding some support for the shaft can allow you to apply more force.QuoteI'd use a more dedicated penetrating oil but if WD-40 is all you have then use it and leave if for a day or two. Apply with a toothpick to attempt to restrain where it might end up.It might seem that the collars have never been off as this tightness is typical of steel in alloy that hasn't been shifted for decades. Take a step back and carefully examine what you have staring you in the face and try to see the mechanical functionality of it....or not. Good light and a magnifying glass and check for something that's seized or frozen up.When these tight grub screws break free you'll think you've broken something by the crack they can sometimes make.When you're reached the limit of force you're comfortable applying support the underside of the collar with some steel of some mass and shock the grubscrew towards it with a drift/punch and a small hammer blow. This can drive the screw slightly into the shaft and in doing so losen its tightness or just break the thread to thread bond.Good luck.

author: acacia
author_rank: Newbie
timestamp: December 24, 2017, 08:52:39 am
content: QuoteI can, but the way the screws are aligned I only get about 1 cm between the end of the key and the PCB. At distances like that, the key just bends.QuoteI've been poking around with a flashlight to check what's supposed to move and what's not, comparing with the working knob and the EEVBlog video. I can steal the magnifying glass from my soldering station too. I think it has to be something behind one of the plastic pieces, since that end of the shaft is the most resistant to turning, and I can't see any visible obstructions after looking at it with my phone camera from a few different angles. My guess is something stuck between the plastic and the PCB, or to the axle itself. There is zero rotational play on the back end of the shaft, whereas there's a bit on the front end.I don't mind getting some actual penetrating oil, since it's something I'd still use after this. It's night here, but hardware stores will be open in the morning. Whatever I would use, it would be a tiny amount.

author: tautech
author_rank: Super Contributor
timestamp: December 24, 2017, 08:57:16 am
content: Quote from: acacia on December 24, 2017, 08:52:39 amUnderstood. Try some thin metal tubing on it to take some of the flex out of it.

author: tautech
author_rank: Super Contributor
timestamp: December 24, 2017, 09:11:08 am
content: Another thing that comes to mind....if you have another hex key in the same size....they're not always made the same.

author: acacia
author_rank: Newbie
timestamp: December 24, 2017, 10:01:32 am
content: QuoteI do have two 3/32" hex keys, but they're both very thin and springy. All my nice hex screwdriver bits are metric.If I try to crack it with just force, I'll want to brace the shafts so they don't bend too much, but that shouldn't be a problem.

author: tautech
author_rank: Super Contributor
timestamp: December 24, 2017, 10:38:38 am
content: Quote from: acacia on December 24, 2017, 10:01:32 amGood quality hex keys won't break or bend permanently. Flex heaps yes.A lot depends on how much mechanical experience you have and knowing just how much pressure you can apply before rounding the hex or splitting the grubscrew. Heat is another thing you can try but take care of the plastic parts nearby. Heat cycling a couple of times and let cool and try again. Sometimes heat cycling can permit penetrants to better get in around the threads.There's quite a # of tricks you can use but a lot depends on accessibility and robustness of the parts and in this case you don't have a lot of either. If you can grab another set of hands, try the impact trick. For a drift/punch use a piece of brass that won't hurt the grubscrew. Your helper need hold something with a little weight under the collar while you give the drift/grubscrew a belt. Not so much a hit as a hard slap. You don't want to bend or bugger anything.There's not many tricks left up this old sleeve so work with what I've suggested unless someone comes up with something better.

author: tautech
author_rank: Super Contributor
timestamp: December 24, 2017, 10:50:09 am
content: If you have a 1/4" drive ratchet and 1/4" socket, screwdriver tips will fit it. It won't flex at all and if you can get a 3/32" tip you'll be able to give it some hurt.

author: tggzzz
author_rank: Super Contributor
timestamp: December 24, 2017, 07:46:15 pm
content: On my 1740, it isveryeasy to get the two shafts in the wrong position, thus restricting the angle of rotation.As for the collet/grubscrew, that kind of thing can be a pain. Sometimes thermal shocks can help, but they shouldn't be the first resort.Heat the collarorgrub screw with a soldering iron, or with a hot air gun of some sort. In the latter case, use kapton/polyimide tape to deflect air away from other components. Obviously too much heat could damage neighbouring components.Alternatively, use freezer spray to cool the components.

author: acacia
author_rank: Newbie
timestamp: December 25, 2017, 06:18:06 pm
content: After letting it sit for several hours with some penetrating oil, and using pliers to turn the hex key, I was finally able to unscrew the collars. It's a Christmas miracle!...but they're still stuck. The shaft slides in and out of the plastic gear for the delayed knob, but it's still stuck on the ones for the main knob. So it slides in and out of the front board fine, but the back two just bend. I have a short video of what's going on, in case I didn't explain it well:Seems like there's nothing left to unscrew, but there's still something keeping the shaft in there. I thought at this point I would be able to pull it out. What do you think might be keeping it in?

author: tautech
author_rank: Super Contributor
timestamp: December 25, 2017, 06:35:37 pm
content: Quote from: acacia on December 25, 2017, 06:18:06 pmTypically you'll align both the flats on the two shaftsandthe timebase wafers to draw the shaft.For yours, in some way you'll need to 'walk' the shaft out by rotating the shaft as you go.

author: Torsten
author_rank: Newbie
timestamp: December 27, 2017, 08:13:05 pm
content: Anyone know what hex key is needed to unscrew this shaft ?I tried 1/16, 5/64, two 3/32 and metric 2 and 2.5 but they all are either too small or too big... only T10 torx kinda fitsSeems like only grinding one 3/32 or playing with thermal expansion might help, but with my skills, hehe...QuoteDoes that also ends up with not working delay ? Seems like previous owner did that mistake and now delay knob can be only rotated as long its setslowerthan main time knob

author: tautech
author_rank: Super Contributor
timestamp: December 27, 2017, 08:28:10 pm
content: Quote from: Torsten on December 27, 2017, 08:13:05 pmWelcome to the forum.acacia did state he was using 3/32 but check for burrs or damage to the grubscrew hole.Quote from: Torsten on December 27, 2017, 08:13:05 pmThat's correct and as it should be.Delayed timebase is only available at slower settings than the main timebase.

author: Torsten
author_rank: Newbie
timestamp: December 29, 2017, 04:59:52 pm
content: Sanding the 3/32 bit + cleaning hole with wire brush on rotatory tool + plenty of force solved the problem.Quote from: tautech on December 27, 2017, 08:28:10 pmIm confused now, isn't the delay supposed to be faster as it highlights part of main sweep ?Meaning, if the 'Delyed time/div' knob is set slower than 'Main time/div' knob, it will just pointlessly highlight the whole sweep all the time ?

author: tautech
author_rank: Super Contributor
timestamp: December 29, 2017, 07:38:25 pm
content: Quote from: Torsten on December 29, 2017, 04:59:52 pmYep. you're right. Poor choice of words on my part. Sorry.

author: acacia
author_rank: Newbie
timestamp: January 01, 2018, 10:50:14 am
content: Thank you for helping me understand what's going on inside here, I really appreciate it.I sprayed it a bit with some contact cleaner, which loosened it up enough so that I could pull out the shaft (yay!) so that's going well. Out of the two main time/div plastic gears, the front one can rotate but is still sticky/gummy, while the back one still won't move. It looks and feels like there's some sticky gunk and crud still in there. When I'm removing the time/div shaft, it feels like I'm pulling it out of a jar of honey.Should I be looking for debris jamming it, spraying it for the stickiness, or something else? The "open" parts of the main time/div plastic gears seem to be aligned with each other already.(Happy new year! And sorry about the hiatus, I was visiting family out of town and couldn't work on the scope.)

author: tautech
author_rank: Super Contributor
timestamp: January 01, 2018, 11:09:35 am
content: Quote from: acacia on January 01, 2018, 10:50:14 amIIRC removal of the shaft was effortless and smooth but the time/div PCB cards can tip some and bind on the shaft as it's slid out. I seem to remember I needed to hold both PCB cards and my tongue at the right angle for it to slip out effortlessly.In your pics the shaft looks clean but if it feels sticky give a wipe down with IPA.

author: acacia
author_rank: Newbie
timestamp: January 01, 2018, 10:44:04 pm
content: Success! After a few rounds of cleaning and trying to move the plastic gears without the metal shaft, the main time/div knob is working again. Got it aligned and put back together, and it seems to be working as it should.Thank you for all your help!