Tektronix MSO4054 - rotary encoders failure [SOLVED] - Page 1
Forum section: electronics / repair

author: dbator
author_rank: Contributor
timestamp: April 14, 2025, 05:17:46 pm
content: Hello,As I promised under my other thread, I'm back with another issue which I encountered and which in my opinion deserves to be shared here. And that's about failing rotary "encoders" (symptom: skipping values) mounted on front panel boards of MSO4000 series of scopes (probably it does apply to all DPO/MDO4000, B and non-B series too).12/24 (detended/non-detended rotary encoders).First, these are not ordinary encoders. They work more like switches. There are to versions of them (at least in my case). 12 and 24 positions per cycle - detended and non-detended respectively. Sadly, they are customized, obsolete (not available anymore) and not desired to be dismantled without damaging them (not desired for second assembly). The encoders were manufactured by NOBLE Teikoku Tsushin Kogyo Co.,Ltd. especially for Tektronix. Relaunch of manufacturing is possible for MOQ = 5000 pcs. - No thanksEncoders with 36 positions per cycle (non-detended) are still available on ebay and Aliexpress. But they would be to precise I think, so inconvenient.The bad news is, the encoders are not easy repairable/refurbishable, since the enclosure (probably made of duralumin) has short clips holding a metal plate which is keeping the PCB stay inside the enclosure and the whole encoder stays intact. Any attempts of squeezing the clips to release the plate (and the rest) causes usually they break easily. It would require gluing them after eventual repair.Below are decoded connections (open/close to common pin) of both of the mentioned encoders.12-position encoders have one common pin (for confusion it's pin #3, not #1). Pins #1 and/or #2 are shorted to pin #3 in a sequence of 4 modes (combinations) and all that 3 times per cycle (3 times 4 makes 12 positions per cycle).So, starting from position zero and moving the shaft in CW direction:1. Pins #2 and are #3 short together.2. No pins are short.3. Pin #1 and are #3 short together.4. Pins #1, #2 and are #3 short together.24-position encoders work the same way, but with a difference.Pins #1 and/or #2 are shorted to pin #3 in a sequence of 4 modes (combinations), but and all that 6 times per cycle (6 times 4 makes 24 positions per cycle). Coding is the same, but starting combination is different:1. Pin #1 and are #3 short together (like point #3 of 12-position encoder).2. Pins #1, #2 and are #3 short together (like point #4 of 12-position encoder).3. Pins #2 and are #3 short together (like point #1 of 12-position encoder).4. No pins are short (like point #2 of 12-position encoder).From the first look at the board, these encoders are grouped in sections where pin #3 of usually every of 4 encoders (they are connected to each other) is connected to the same (only one) I/O port of the front panel MCU.Pins #1 and #2 are connected through diodes (every pin through one of two double diodes) to +3V3 rail (cathodes to encoders, anode to +3V3), but they are connected alternately (quasi muxed) with another group of 4 encoders (diode A is connected to 1st pin of channel #1 Volts/div encoder and diode B is connected to 1st pin of Vertical channel #1 position encoder. I', pretty sure, hardware designer wanted to save ports on front panel MCU and in order to reduce amount of the to be required, they decided to measure voltage level. The way they saved a lot of ports. But still, I don't understand, how the MCU recognizes the signal differences (which encoder is changing its position), when two groups of 8 encoders (plus monitoring of one state change of Zoom/panning encoders, which are connected through diodes too) are connected together. For instance pins from channel #1 Vertical Position and Voltage Scale are connected through one double diode. I drawn partially a schematic of the encoders section, but it does not look not nice and it's not complete, so not available for publishing it right now.Now, there are few options to solve the issue with rotary encoders.1) Refurbish the encoders - quick, cheap, but not long lasting. Explained later.2) Silver plating service of encoders PCBs (since judging on the color, the original pads on these PCB are silver plated) for all PCBs at once (single price). Surface of PCB tracks will not be even as originally eventually, almost no work on my side, it's quick (about 10 days + plus delivery time to the plating service company and back), but very expensive - around 140 USD for all PCBs (around 10.5 USD for 1 pc.). The more PCBs per process the less the cost for one pc.3) Measure dimensions of both PCBs, design them using a CAD software and have them professionally printed with silver plating, solder new pins, clean wipers and put some petroleum jelly on new PCB, put together and glue the encoders - more work (time spend), reasonable price, and if properly measured dimensions of the PCB and the quality of the PCB will be very good, it will be long lasting. Explained later.4) Purchase an ordinary rotary switch which will fit into front panel enclosure, front panel PCBA and between the PCBA and metal enclosure of the scope, configure the switch electrically so it works as the original one and solder wires to the PCBA - quick, low cost, not aesthetic, loud, and stiff working, mid-lasting since such rotary switches have usually around 10000 cycles.5) Design additional hardware (logic) or design hardware and write software (controlled by an additional MCU), purchase ordinary 12 and 24-pulse rotary encoders connected to the mentioned hardware which will convert or simulate functionality of the original encoders - much work (time spend), not expensive if one does good at programming, long lasting since Alps rotary encoders can even have 30000 cycles guaranteed.As you see, there are plenty of options and since using a drill solution I read somewhere on this forum is not an option for me I decided to design my own PCB for these encoders. Creating two PCBs took me about two days total - quite fast as for an amateur designer. There is only one error related to hole to be solved and arc pads shapes (drawn manually) must be correct because they are not smooth enough and the project could be sent for manufacturing. Since all that took already too much time, projects require to be corrected and I was not sure gluing of the encoders will be successful anyway, I decided to suspend finishing of the PCB projects and just refurbish the encoders first in order to test new epoxy glue it was recommended to me and see how long the encoders will work without issues.I purchased a sand paper #3000 (finest I could get, polished the PCB pads, cleaned the PCBs using isopropyl alcohol, cleaned the wipers using a contact spray, then on dry PCB i put a bit of petroleum jelly. After that, areas which will be glued (metal plate and the enclosure) I grinded using a sand paper #60 and a fine screwdriver (initially I was using sand paper #320, but it turned out to be be too fine), cleaned the mentioned area using isopropyl alcohol, assembled the encoders and using two clothespins each on one corner (diagonally) to hold encoders intact I glued other two corners of the encoders. After two days I repeated the process on remaining corners, but without clothespins since the glue was holding the encoders intact enough in my opinion.Tests revealed, the Autoweld (from CX-80) glue I was using is holding the parts enough good, but stronger prying can remove the glue. Especially from the duralumin enclosure it goes off easy. Not something what makes me satisfied enough. But, if it will not go off self, then it will be a good solution until next refurbishment I think. As backup I considered use of an adhesive tape and stick the encoder to the PCB and/or eventually place ESD foams between every encoder and scope enclosure as support, so if the glue adhesive will lose at least the metal parts will not be moving lose inside the scope. It was not enough reliable method as other idea to use carefully chosen springs to be glued to the encoders prying a bit against encoder vertically or bending two thin plates around enclosure of the encoders and drilling two holes and screwing in two screws plus two nuts. Since I have no springs, I have no idea where to get from such a thin plate and it was was too much of effort and the scope was too long disassembled and I couldn't wait to use it for repairs, so I finally did not use anything as backup.Before assembly of the encoders, after assembly and after soldering I did check them electrically and they are working fine. I also covered the small slits with an ESD tape to protect inner side of the encoder from dust.I will see how long the encoders will work without skipping and whether the encoders will fall apart or not. I hope they won't. I think best verification will be if I will count oscilloscope operating hours and time period from the date of gluing process respectively.Hint!For disassembly and assembly of the scope, when putting its front downside I recommend you to put it on something with 13 holes made for shafts of the encoders so weight of the scope (about 5 kg) does not push them to the front panel board. Especially after they are glued.Since attachments limitation is up to 10 files per post, I will continue in next post.I attached:1) Pictures of the rotary encoders, incl. pictures of their PCBs.2) Basic datasheet of the encoders received from the manufacturer proving my theory of how they work.I asked the manufacturer for the PCB layout files, but they stated they do not manufacture the PCBs recommending me to ask at Tektronix. probably they misunderstood me because Tektronix does not have nothing on file for the PCBs. Yes, I asked them the question3) Screenshot from Diptrace presenting one of the unfinished projects of the PCBAs.4) Picture of gluing process.5) Assembled refurbished encoders.

author: dbator
author_rank: Contributor
timestamp: April 14, 2025, 05:33:19 pm
content: Now the part with Zoom/panning encoder which turned out to fail electrical test (bad contact) - I was not aware left side panning at lower speed (1st stage) was not working properly because there was no opportunity to use it before disassembling of the scope. On one of the pictures you will notice grease contamination preventing electrical contact.Panning encoder has 5 pins.First two pins are for left panning, next two pins are for right panning and 5th pin is COM1 (common pin #1).Panning encoder is switching pins A or B or A and B together to COM1 pin depending on how far left or right the panning wheel is pulled. The wheel has thin bulges on its bottom which are pushing down the A and/or B contacts to common pin. Spring provides force back pulling the ring back to its central position.Zoom encoder has 3 pins. First two pins are A and B signals and 3rd pin is COM2 (common pin #2). It works technically like ordinary encoders you can find in many devices nowadays. The difference is that contact pins are static and moving part is the wheel with bulges on its bottom side pushing down the contacts and shorting them to COM2. Bulges are arranged so the A and B pins are short to COM2 in order AB or BA.I removed the main shield by bending the side clamps of the metal shield. Then I cut the melted (for protection) plastic of the body to have access to the second shield covering the wheel and contacts. Luckily the shield has its own clamps which you have to release from the bottom side of encoder body - there are three holes where you have to put in a tin precise screw driver and carefully pull to release the clamps of the shield.While I had full access to the contacts, I removed the rest of grease of petroleum jelly or whatever it was, cleaned the contacts with isopropyl alcohol and used a contact spray on them. I also put a bit of petroleum jelly on contacts and some silicone grease on moving parts to increase lifetime of the encoder of which as you see reproduction is rather not possible at home. Then I tested functionality of the encoder electrically an it worked fine. After assembly of the scope it turned out however, that when the panning ring was pulled to the left side it stucked there. Not enough good checked before assembly. Probably putting grease (or petroleum jelly, I don't remember) on the spring which rubs the PCB pad was not a good idea. Or the grease was too thick. Luckily using the panning ring fast repeatedly changed that and it does not stuck anymore on any of the sides. Also zoom knob would work with more fine click if I did not use the grease or use a lighter one.I hope that's all I wanted to share here.I attached pictures of Zoom/panning encoder.