TDS 540C - Partial failure - Horizontal Scale - Page 1
Forum section: electronics / repair

author: Tantratron
author_rank: Frequent Contributor
timestamp: December 21, 2019, 08:28:21 pm
content: Hello,One knob of my TDS 540C front panel does not always respond correctly.The Oscope works fine except that on some selection of the horizontal scale, when turning the knob it will suddenly change the value by a factor of 10 or 100. In other words, the coded knob seems very unstable and not fully decoded on some values which end up doing huge skip.However the way to avoid this trouble for the moment, when I press Horizontal menu then select Time scale push-button below the screen then I can choose the correct time division scale if turning the main bigger selector knob on top-right of front panel.No idea what is the technology of these knobs and push-button from these TDS 500 - 700 serie A B C and D. Do you know if the issue is bad contact of the horizontal scale Knob or should I rather change the all front panel assembly ?Thank you, Albert

author: james_s
author_rank: Super Contributor
timestamp: December 21, 2019, 08:39:12 pm
content: They're mechanical rotary encoders, notorious for getting flaky, especially on the TDS3000 scopes. It's tricky to get it in there but a bit of contact cleaner/lube usually improves things tremendously.

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2019, 09:01:07 pm
content: Did you replace the capacitors on the front panel as well? If things take a turn for the worse the electrolyte can eat away the pins of the encoders.

author: wn1fju
author_rank: Frequent Contributor
timestamp: December 21, 2019, 09:48:21 pm
content: As nctnico said, remove and carefully inspect the front panel board.On my TDS520, the Shift button was not working correctly. I pulled the front panel off, pulled off the membrane switch pad and cleaned the contacts. Itdidn't occur to me at the time to actually look at the bottom side of the board where all the components are. Had I looked I would have several puddlesof wet electrolyte. Needless to say, my button "fix" didn't last more than a day. It probably only worked for a short time because all the manipulationsmoved the puddles around.Assuming any damage is not severe, the board is easily re-capped (there is plenty of clearance for normal radial caps). Simply twist off the old SMT capsgently with a needlenose, clean, clean, clean and clean again, then tack on the leads of a normal cap (or use SMTs if you must).If it still doesn't work, then you can start fiddling with the encoder. And, by the way, if you do replace the entire front panel assembly, that needs to bechecked for bad caps also.And just to spoil the rest of your day - when you open it up, you should inspect ALL of the boards.

author: madao
author_rank: Frequent Contributor
timestamp: December 22, 2019, 07:03:35 am
content: I have similar problem, trigger-level encoder is away.I have buying "ECW1J-B24-AC0024L" and file their axle down and remove their spring for detenting. (I have glued him after disassemble)Now runs fine :-)ECW1J-J24-HE0024 or ECW1J-J24-CE0024L is correct, but where can buy i this type?https://uk.farnell.com/bourns/ecw1j-b24-ac0024l/digitalpotentiometer-winkelenco/dp/1200080regardsmatt

author: Tantratron
author_rank: Frequent Contributor
timestamp: December 22, 2019, 07:27:06 am
content: Quote from: madao on December 22, 2019, 07:03:35 amHello Matt,Attached quick picture of my A12 front panel un-removed from the TDS540C Oscope.It seems the rotary encoder have 4 pins soldered to the PCB whereas the Farnell link mentioned has only 3 pins.Do you mean we cannot purchase anymore this rotary encoder being phase out ?By the way, did you just need to desolder the pins from the PCB to repair or change one rotary encoder ?Thanks, Albert

author: madao
author_rank: Frequent Contributor
timestamp: December 22, 2019, 07:41:36 am
content: Oh, it is depents of serial numbers. Your is older than my.It is double potiometer wiht 90° angle of two wiper and not encoder !I have a bone-yard-TDS540A. I can picked one good for you. If using of contact cleaner doesn't help.regardsmatt

author: Tantratron
author_rank: Frequent Contributor
timestamp: December 22, 2019, 02:48:59 pm
content: I've removed the A12 front panel board, please find attached pictures where I'm aware the need to remove the main ribbon connector.What do you think visually, do you forecast any external issue on the PCB, should I remove the other side of PCB or can we consider globally no electrolytic cap to be culprit of malfunction of the horizontal scale selector ?By the way, on this TDS540C model, this seems to be an encoder... when I dial to the right or the left, it will always click. Same for the Vertical scale controller so I'm not sure this would be a double-potentiometer but I could be wrong.Do you think it would be enough to clean the rotary knob, if yes what best method and product ?

author: madao
author_rank: Frequent Contributor
timestamp: December 22, 2019, 03:33:33 pm
content: Eletrolytics on newer TDS oscilloscope is not cause for failure. Reason: Problem with bad SMD eletrolytics (leaking, corrosion) is solved since 1994/5. B, C and D-Serie is too new for this Problem. Your Photo show, it is good. Solder joint of eletrolytics is glittering. A good sign.Yes, potiometer with click exists.Contact cleaner would help here. I recommens t6 from teslanol.

author: Tantratron
author_rank: Frequent Contributor
timestamp: December 22, 2019, 04:19:46 pm
content: Quote from: madao on December 22, 2019, 03:33:33 pmWould you have a picture showing exactly where in the culprit potentiometer I'd apply the teslanol ?I mean easy, efficient lubing of the potentiometer not requiring to de-assemble anything from A12 board ?Thank you very much, Albert

author: Tantratron
author_rank: Frequent Contributor
timestamp: April 26, 2020, 06:45:55 am
content: In order to complete the full repair of this oscilloscope (other threads discussed EEPROM failure), attached a picture of the front Panel where I've noted the failed potentiometer then isolated two pins to excite with GND and +5Vdc external.To save me time and cost, what easy method do you suggest to verify if the potentiometer is failed or some other IC, PCB lines are failed. In other words, if I probe the 2 outputs of the failed potentiometer, what simple oscilloscope method to conclude then order either a single potentiometer or just order a complete working PCB from eBay ?

author: madao
author_rank: Frequent Contributor
timestamp: April 26, 2020, 11:03:45 am
content: https://www.qservice.tv/vpasp/shopexd.asp?id=8900&bc=noBut it is not new, used.This potiometer is never produced for market, only for Tektronix.

author: Tantratron
author_rank: Frequent Contributor
timestamp: August 29, 2020, 03:02:33 pm
content: Hello,I've finally found a potentiometer to replace the failed one but I need some advice, help on how to properly complete the desoldering. In the attached picture of the A12 front panel, you will see that I did remove the potentiometer, 3 holes among the 4 are empty using my pump but I cannot get the 4th one (bottom which is GND).What happened initially, I was trying to de-solder the 4 pins from the PCB but the last one would never move or come. So after I did cut each pin from the pot then extracted what was left in the PCB with my pliers and solder, it worked OK on the 3 holes. Only problem I discover, there seems to be a GROUND plane inside the PCB which dissipates lot of heat so each time I try to extract or pump the hole, the solder is not hot enough to clean the 4th hole by sucking.Thank you, AlbertAny advice is more than welcome

author: Jwalling
author_rank: Unknown
timestamp: August 29, 2020, 04:53:02 pm
content: I'd just bend or cut the pin a bit and tack solder it.