Sketchy Oscilloscope Knob - Page 1
Forum section: electronics / general

author: tayexdrums
author_rank: Contributor
timestamp: December 17, 2010, 06:23:08 am
content: I have an old Hitachi V-423 40MHz scope that was given to me for free. Now I'm definitely not complaining here because of the fact that it was given to me for free. However, there is a small problem with the scope. Sometimes, when I use the calibration output on the scope there will be a distortion in it. I have to wiggle the volts per division knob in order for the trace to get back to the shape of a calibrated square wave. I have taken a video of it which I think will provide more information.Please let me know if anyone has any advice or information on how to fix it=]

author: PeterG
author_rank: Frequent Contributor
timestamp: December 17, 2010, 09:10:29 am
content: The cause is simple, the cure is hard. It is most likely caused by bad/dirty contacts on the rotary switch. The hard part can be getting to the switch to clean the contacts.regards

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 17, 2010, 02:33:35 pm
content: If the contact issues are caused by long term non-use, operating the switch a few dozen times may help clean it - always worth a try before diving in.

author: tayexdrums
author_rank: Contributor
timestamp: December 17, 2010, 02:51:18 pm
content: That's what I thought the problem was. I didn't think it was something that was very technical. I took the case off yesterday and looked to see if I could easily get to the switches, but they were definitely down in there. I would have to take some time and take somethings apart=\I will first try turning the switches a lot to see if that helps though.

author: Simon
author_rank: Unknown
timestamp: December 17, 2010, 04:44:46 pm
content: you could try WD-40, it does not conduct and is well known for cleaning switches and the like

author: ModemHead
author_rank: Frequent Contributor
timestamp: December 17, 2010, 05:35:09 pm
content: I'd be more inclined to use something like Caig DeoxIT or other spray contact cleaner than WD-40. In any case, don't forget to exercise and clean the pot for the vernier control as well as the switch. It's probably mounted on the back of the switch in question.

author: Time
author_rank: Frequent Contributor
timestamp: December 17, 2010, 05:42:39 pm
content: I got this same scope for free from the same guy, I will see if mine does this when I get some time.

author: tayexdrums
author_rank: Contributor
timestamp: December 17, 2010, 08:06:20 pm
content: I've already tried spraying the knob from the front panel with some contact cleaner that I picked up at radio shack. I guess the next step is to spray it from the inside of the scope.Let me know if your scope does the same thing!

author: Fraser
author_rank: Super Contributor
timestamp: December 17, 2010, 11:39:20 pm
content: This is a common problem in wafer switch technology, potentiometers are a little different in terms of the cause but more of that later.I have repaired many elderly pieces of equipment with the symptoms you describe and would advise the following.1. Switch contacts can suffer oxidisation of the silver plating causing intermittent or high resistance operation. The only way to cure this is to remove the oxide layer either by multiple operations of the switch wafer or, more reliably, by chemically cleaning the switch. The crucial detail in cleaning the switch is access and the use of the correct cleaner. Please do not use oil based cleaners such as WD40, they are not great for this application. I would recommend an aerosol contact cleaner available from electronics stores. Choose one that is 'no residue' and does not contain a lubricant as that is not really needed on wafer switches and contaminates the contact substrate. You can use a lubricated switch cleaner if you really wish to but I personally found that it makes a real mess of the wafers and attracts dust. The cleaner should be sprayed onto the contacts, left for a few seconds, and then the switch operated several times. Repeat around 10 times to ensure a good cleaning action. Leave the unit unpowered during the process and wait at least 15 minutes after the last cleaning session before applying power. If this action does not remedy the situation you are dealing with either severely contaminated contacts or contacts that have lost their plating due to wear. In either case the best solution is replacement of the switch assembly. If this is not possible, the switch wafers should be disassembled and all contacts manually cleaned with IPA and paper. Paper is slightly abrasive and when soaked in IPA will rub oxide off the contacts. If the plating is still OK this should clean it, damaged plating cannot be repaired economically but may clean up enough to work for a while at least. I recommend that you do not use cotton 'buds' to clean wafer contacts as they tend to shred and leave strands of cotton all over the contacts.2. Carbon potentiometers that are commonly used for variable ‘trim’ controls on equipment can suffer dirt ingress, oxidation of the wiper arm contact and deterioration of the carbon track. To remove dirt and oxidation build up, I recommend any cleaner that is designed for cleaning carbon potentiometers or a ‘no residue’ non-lubricated contact cleaner. IPA will also work well. It is possible to dismantle a potentiometer and clean the carbon track manually with a suitable cleaner and lint free cloth or paper. If cleaning does not solve the problem, the potentiometer should usually be replaced with a new part as carbon track damage is likely and not easily remedied. I have, in desperate cases, dismantled a potentiometer and re-aligned the wiper arm so that the contact traverses a different area of the carbon track . This must be done with care as poor alignment will rip the track up in no time at all.3. Never apply switch cleaner to switch or potentiometer shafts, it washes out the lubricant and achieves nothing in terms of cleaning the contact surfaces. Switch cleaner should be applied carefull directly to the contact surface or carbon track. Some potentiometers have a hole in the cover that permits spraying of the cleaner onto the carbon track. The alternative is to remove the cover to gain access to the track and wiper contact.All this is just my 2 cents worth and 30 odd years of experience refurbishing elderly kit. Others may know of alternative options.

author: Zyvek
author_rank: Regular Contributor
timestamp: December 18, 2010, 01:57:20 am
content: +1. Contact cleaner is your friend, this problem is quite common on old Tek scopes as well, but usually an easy fix. Some switches can be disassembled to the point of seeing the contacts but I doubt you'll have to do that.

author: tayexdrums
author_rank: Contributor
timestamp: December 18, 2010, 05:59:52 am
content: I think I've fixed it by using some contact cleaner and turning the switch many times. However, it does still do it intermittently on certain ranges.Thanks=]

author: FreeThinker
author_rank: Frequent Contributor
timestamp: December 18, 2010, 01:31:16 pm
content: Quote from: Aurora on December 17, 2010, 11:39:20 pmJust been reading this post while the wife was busy polishing, looking over my shoulder as she passed she commented-"IPA? can you still get that?"Me (puzzled) "Yes. Why?"Mary "Dad used to like a bottle of that, but I've not seen any for years"It was then the penny dropped.She thought it was India Pale Ale.I didn't put her wise, hope she doesn't try to clean any of my gear with it.Still chuckling now

author: Fraser
author_rank: Super Contributor
timestamp: December 18, 2010, 05:58:12 pm
content: Your wife is actually not far off the truth with the availability of IPA in the UK. I used to buy it in 500mL bottles from the local Chemists at quite a reasonable price. I tried to buy some from several chemists recently and all stated that under the UK Governments new controls over such chemicals, they could no longer supply IPA over the counter. IPA is apparently considered a controlled substance as it can be used in IED chemistry. I believe IPA is still freely available via electronic component suppliers though.Do not be tempted to use Chemists 'rubbing alcohol'.... it's IPA mixed with water ! Even that may have been controlled by now ?

author: qno
author_rank: Frequent Contributor
timestamp: December 18, 2010, 06:12:18 pm
content: Contact cleaner etches, sometimes you do more harm than good. I have seen audio equipment potmeters where the slider was almost dissolved by extensive use of contact cleaner. If you do not remove the stuff thoroughly you end up with green goowy stuff what used to be copper.The manufacture advises to use a cleaner substance after using the contact cleaner. Use a q-tip to put the stuff on.Silveroxide conducts as good as the silver itself. Don't be tempted to clean the blackend silver contacts.

author: FreeThinker
author_rank: Frequent Contributor
timestamp: December 18, 2010, 06:16:26 pm
content: Would Appear that Newcastle IPA is just as hard to find! Not that beer is brewed in Newcastle any more, that closed about 10 years ago....made in Tadcaster now I think.Whatever next.

author: Time
author_rank: Frequent Contributor
timestamp: January 05, 2011, 05:09:58 pm
content: Quote from: Time on December 17, 2010, 05:42:39 pmMine performs normally with none of the problems shown in your video.