Fixing Sticky buttons on HP 8111A Function Generator - Page 1
Forum section: electronics / repair

author: tino_so
author_rank: Contributor
timestamp: May 05, 2017, 06:23:02 pm
content: My apologies in advance if this has been discussed already, but I searched using all the keywords I thought that would apply to this issue and it seems no one has posted questions related to this specific type of repair yet.The problem I am having with this unit is that some of the buttons are getting stuck in pressed position, specially those that are mechanically connected, for example, when selecting the waveform, sometimes the Pulse function button will stay engaged when one presses a different function, such as he Sine wave button.Another similar example is the Amplitude button. This one is part of another group, that once you press say, the Frequency button, it should disengage itself to allow one to select the frequency.I had to clean the boards on this unit really good, as it had a film of something, not greasy really, but noticeable, yet easy to wipe off, that had a very strong smell to so harsh chemical. I wonder what kind of industry this kit spent most of its working days at.Aside from the buttons being a bit sticky, and looking to be a bit out of cal, specially in the higher end of the frequency range, it has held up fairly well so far.I have attached some photos of the buttons I am referring to. The brass plate that joins them, which is actually composed of at least two separate thin layers, where one rides over the other and I am guessing this is where some kind of lubrication might be required.Perhaps some more lubrication in the springs would also help? Using a graphite based lubricant perhaps?. You know where this is going, I am at a loss when it comes to bringing some "spring" back to these buttons and I would really appreciate your suggestions.The schematic of these switches is shown in the on page 311 and on 313 of the manual I found at the Keysight support website. I attached also a crop of those pages.Thanks in advance,Tino

author: VK5RC
author_rank: Unknown
timestamp: May 05, 2017, 08:00:28 pm
content: Hi tino-so,I am away from a decent screen size, so I can't easily find the reference on EEVblog forum.I recall a post of a fix of a bank of ganged push button switches, I think in a Fluke.The postee I recall cleaned everything then lubricated the 'outer' mechanism and even took apart the switch for cleaning and light 'lubrication' with a contact cleaner.Regards Rob VK5RCPS I recall the discussion commented on the difference between the true switch mechanism lubricant (ie that found in switch cleaner) and a more typical grease for the outer mechanism.

author: tino_so
author_rank: Contributor
timestamp: May 06, 2017, 02:18:50 am
content: Thanks for the tip Rob, I used the suggested keywords and found this thread about a Keithley Microvolt DMM that Dave is repairing. I noticed towards the end of the video he uses a contact cleaner that also lubricates:https://youtu.be/kdCfAR06GeI(45min 19s)I checked the specs of the contact cleaner I used, and sure enough, it does not seem to be good for lubrication:http://www.mgchemicals.com/products/cleaning-products-for-electronics/cleaners/contact-cleaners/electrosolve-contact-cleaner-409bI have this other contact cleaner I have used in the car, I think it might do the trick, I just have to be extra careful when applying it:http://www.canadiantire.ca/en/pdp/motomaster-electrical-contact-cleaner-150-g-0381740p.html#srpI will look a bit more and post any other info later.Cheers,Tino

author: xrunner
author_rank: Super Contributor
timestamp: May 06, 2017, 02:31:40 am
content: Quote from: VK5RC on May 05, 2017, 08:00:28 pmIt may have been me.Those banked switches have a crimped metal bar assembly at the front. What can happen (and I've ran across it) is that the metal crimps overpower the plastic they are crimping against and start to push in the plastic over decades of applied force. If you bend out the crimps a little it can free the switch up and they will work again!

author: tino_so
author_rank: Contributor
timestamp: May 06, 2017, 02:41:13 am
content: Thanks XRunner, I will disassemble it tomorrow and have a closer look

author: Smith
author_rank: Frequent Contributor
timestamp: May 06, 2017, 07:07:59 am
content: I had the same problem with 2 HP 8116A function generators. The simple push buttons where sticky, and therefore made quite some noise. There was some sort of sticky film on the plastic parts, just like you said. I had just removed the front panel PCB from the front panel and cleaned all switches with quite some IPA. Let them soak a few minuted (maybe use a antistatic brush or something to help), in the meanwhile press them a couple of times, and rinse off with IPA till it's clean. I did NOT disassemble the switches themselves! You can immediately tell the difference between the clean switches, and ones that are still dirty. Did mine about 2-3 years ago and they are still working like new.I don't have the longer switches, but i guess the same procedure will work, as it's plastic from the same era. Although I mostly use an ultrasonic cleaner for these types of switches. You can disassemble these switches too, but be really careful when taking them apart.I would not recommend a graphite based lubricant. Maybe a small amount of teflon spray or something non conductive, NON sticky. You don't want to start collecting dust in your switches right away. I have cleaned similar switches on other devices, and I never used any lubricant except for some very heavily deteriorated rotary switches. My guess is that cleaning your switches will be enough to make them move more freely.

author: pelule
author_rank: Frequent Contributor
timestamp: May 06, 2017, 12:45:41 pm
content: That pushbuttons are used at several HP tools (3455A, 3456A, 8116A).The "sticky" may have different reasons:- dirt (typical example dryed orange juice) - clean with IPA- internal metal spring - reform- broken plastic (worst case)There was a thread here describing that in detail, but I can't remeber for which HP tool. There was also a YouTube video on./pelule

author: tino_so
author_rank: Contributor
timestamp: May 13, 2017, 01:39:29 am
content: Thank you all for your input. I used a bit more of the MG contact cleaner, having the boards on a different orientation (as in the photos below) to let gravity do its work, while pressing the buttons. I also used a paint brush (green one on one of the pictures) to apply the other contact cleaner, but only along the brass plates and to the rusty springs.The buttons are working much better nowThanks,Tino

author: coromonadalix
author_rank: Super Contributor
timestamp: May 13, 2017, 11:02:15 am
content: The best thing i've used is a brake cleaner, like kleen flo, no too strongNo gunk products, they smell for hours, dried all the switches, and use Prolab pl100 lube or m55 lube, with an syringe needle at the endDo not use gigaloo or wd40 ... gigaloo attack plastics, and dries out ... pl100 is my best one ...When i had access to ultrasonic cleaner, i could plunge the board in it, it was doing an incredible job on old rotatives tv tunners, switches, pots ...

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 17, 2019, 09:00:18 am
content: Quote from: tino_so on May 05, 2017, 06:23:02 pmhi, just want to ask you if you amplitude vernier works fine. What pk-pk wave you get on you oscilloscope when you set the amplitude vernier to 1.6v, 8v and 16v. I am having some issue with mine at the moment.