(Solved. with pics) Got a new scope but it has some issues Tenma 72-6820... - Page 1
Forum section: electronics / repair

author: OilsFan
author_rank: Regular Contributor
timestamp: July 19, 2014, 07:18:38 am
content: Hi all, newbie here with a few questions. I got a 50 buck chuck scope off ebay as Dave suggested. It's a Tenma 72-6820 100MHz scope and after a bit of fumbling around I did get the test Cal to show properly on the CRT so I think it basically works but it seems a bit off of the grid lines though so I think it needs some adjustment.The bigger problem though is this scope has those infernal rotary encoder pots that just spin around till you hear a beep.They are used for nearly every control Ver/Hor Position, Volts/Div, Time/Div, Trigger level etc. Of course they don't work very well and turning the knobs can make the adjustment go either way or just not move at all. Weird thing is all of them seem to have a problem so they are either trashed, need to be cleaned or there is another component causing it. Can you give me some ideas what to check when I crack it open? Also I haven't found a service manual yet if someone knows where one is.Here is a pic of the Cal trace I got...

author: tautech
author_rank: Super Contributor
timestamp: July 19, 2014, 07:34:56 am
content: WelcomeThe Cal waveform is NOT a precision waveform.Your display of it is as one might expect.Sometimes there is provision to adjust both frequency and amplitude, but for it's intended use it is not necessary.Encoder problem is concerning, might be worth removing one to check it's functionality.

author: mij59
author_rank: Frequent Contributor
timestamp: July 19, 2014, 07:57:21 am
content: Hi,The switches inside the rotary encoders are probably at bit crusty, it may help to turn the rotary encoder a lot.If not post a photo of the encoder(s).

author: G7PSK
author_rank: Super Contributor
timestamp: July 19, 2014, 08:20:23 am
content: Try driving it for a bit longer before you start to take it apart, you may find that when one switch is in a certain position another has no effect and that is how it is meant to be, if possible find the manual as well.

author: elliott
author_rank: Regular Contributor
timestamp: July 19, 2014, 04:48:02 pm
content: I don't know if it will help you any, but I'm pretty sure that is also a GW Instek GOS-6103.

author: Stonent
author_rank: Super Contributor
timestamp: July 19, 2014, 04:53:51 pm
content: From what I've seen the Tenma scopes are all rebrands. I have a Tenma scope that's a rebadge of the well known Hung Chang 620 that was the bargain scope that many dealers (Dick Smith, JDR etc) rebadged as their own. Amusingly enough some of the revisions of my scope, the brand name is held on with screws probably so they can remove it and change it when it arrives at the store.

author: Stonent
author_rank: Super Contributor
timestamp: July 19, 2014, 04:55:15 pm
content: Manual for your scope:http://www.mcmelectronics.com/content/ProductData/Manuals/72-6820.pdfSpec Sheet:http://www.farnell.com/datasheets/12983.pdfSorry no schematic I've found yet. The manual for my Tenma scope has a full schematic.

author: OilsFan
author_rank: Regular Contributor
timestamp: July 19, 2014, 11:10:48 pm
content: Thanks for all the help so far. The rotary controls are definitely not functioning as they should. I should be able to just turn the Volts/Div knobs at any time but mine just jump around or don't change increments when I turn them. Is it possible to clean rotary encoder switches?

author: elliott
author_rank: Regular Contributor
timestamp: July 20, 2014, 12:34:02 am
content: You can clean them, I've had success with 90% alcohol in an ultrasonic bath. You should lubricate them afterwards with an electronics safe silicone lubricant.Since you have to take them out anyway you might want to look into just replacing them.

author: OilsFan
author_rank: Regular Contributor
timestamp: July 20, 2014, 02:24:39 pm
content: Last night I decided to pop the unit open and see what I could find. I noticed that one or two of the encoders had been changed before. There was some bad soldering in there that gave it away. I decided to pull one of those off to see what condition it was in. I found it dripping wet inside with some kind of lube or cleaner. I know these things are supposed to have a bit of lube in them but there is no way anything is going to achieve proper contact if it is absolutely drenched with the stuff.I pulled the encoder apart and cleaned it with Deoxit, dried it off and reassembled it. I soldered it back on the board for a quick test and what do you know, it worked! I removed and cleaned the rest of them, it took about an hour and now the scope controls seem to work fine. They hiccup a tiny bit but for the most part it's 100% better! $50 and a couple hours work and I have my first 100MHz scope!I took pics of the whole process and could post them if anybody is interested.

author: GreyWoolfe
author_rank: Unknown
timestamp: July 21, 2014, 12:22:03 pm
content: Congratulations on your repair. Of course we want to see pics!

author: Hydrawerk
author_rank: Super Contributor
timestamp: July 21, 2014, 01:04:52 pm
content: It is always easier to change a rotary encoder than to change a big mechanical multicontact rotary switch as on old analog scopes.

author: urbis
author_rank: Unknown
timestamp: July 21, 2014, 03:45:21 pm
content: I'd be interested in seeing the pics!

author: OilsFan
author_rank: Regular Contributor
timestamp: July 23, 2014, 08:01:03 am
content: OK since people wanted to see them here are some shots of how I 'fixed' the rotary encoder pots on my oscilloscope. Hopefully it's not too many pics or too large.1. After removing the top cover this is the control board on the front of the machine.2. I removed this little daughter board. It came right out.3. I then removed four phillips head screws with a small screw driver.4. That free'd up the board from the chassis. Now I had to remove two flat flex cables to get the board out.5. There was one at the bottom. I used a small flat head to push the connector free.6. And another one at the side of the board.7. Here is the top and bottom of the board.8. The idea is to take apart the rotary encoders for cleaning. They consist of a shaft, a contact plate, a shell and a mounting plate. The mounting plate is soldered on to the board (two big solder pools) for stability but you do not have to remove it. You just need to desolder the three pins that lead to the contact plate (at one end and between the two big solder pools).9. I used an iron and solder sucker to desolder the pins.10. Then I turned the board over and used a small flat head to very carefully bend over the four straps that hold the encoder to the mounting plate. They bend easily so be careful.11. Then the encoder (except the mounting plate) should pull right out.12. Here is the encoder from the bottom. I sprayed it with Deoxit at this point.13. Then I carefully removed the contact plate from the shell.14. The shaft will also come out at this point. My pots were loaded with a greasy oil that I cleaned off of everything with paper towels and Q-tips.15. I then inspected the contacts to make sure they were sticking out at this angle and lined up closely enough. They need to be at this angle to ensure contact with the metalized surface on the bottom of the shaft. Be very careful.16. After reassembling the encoder and placing it back into it's mounting plate, I carefully bent the four straps back over to hold it together. Be careful here as well but make sure you get the straps bent back so that they hold everything firm.17. Then solder the three pins back on and you are done.

author: mij59
author_rank: Frequent Contributor
timestamp: July 23, 2014, 08:58:13 am
content: Very nice pic's and explanation.Great fix, almost as good as new!Thanks.

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2014, 09:32:56 am
content: VERY NICE.Would you consider renaming this thread to make your fine fix easy to find for Tenma owners?

author: dentaku
author_rank: Frequent Contributor
timestamp: July 23, 2014, 09:30:28 pm
content: What great photos.This go me thinking, how would you clean the rotary switches in old scopes like my Hameg HM-605.I use DeOxit Fader F5 is pots but I wouldn't dare use it in those old switches.

author: OilsFan
author_rank: Regular Contributor
timestamp: July 24, 2014, 02:29:05 am
content: Quote from: dentaku on July 23, 2014, 09:30:28 pmI'm not an expert or anything but I think it would be OK to use the Deoxit. I wouldn't let it sit in there forever though. Spray it on, work the switches good then blow out the excess with air. That's what I do. It might be hard to get at those kind of switches though.

author: Smokey
author_rank: Super Contributor
timestamp: July 24, 2014, 05:17:08 am
content: Quote from: OilsFan on July 24, 2014, 02:29:05 amDoesn't that get DeOxit all over the place? I remember it being really slick and oily feeling. I can't imagine blasting the stuff with compressed air.

author: Hydrawerk
author_rank: Super Contributor
timestamp: July 25, 2014, 09:49:22 pm
content: I like that buttons. Are they better than a membrane keyboard? I think yes.

author: elliott
author_rank: Regular Contributor
timestamp: July 26, 2014, 05:25:05 am
content: Quote from: Hydrawerk on July 25, 2014, 09:49:22 pmI've had issues with my Hitachi's buttons, they are little dome switches inside with an LED.I'd take cleaning a membrane over these things any day. My channel 4 button is still not right, I have to hit a specific corner to get it to work.