Tenma 72 3060 oscilloscope repair - Page 1
Forum section: electronics / repair

author: tyguy2
author_rank: Unknown
timestamp: April 22, 2017, 02:46:12 am
content: Hello all!I recently purchased this oscilloscope online, and, of course, its got something wrong with it (despite the seller's insistence that its fully functional). I've noticed that it only shows half the wave form on screen, so I'm guessing it has something to do with the deflector coils in the horizontal direction. I've found manuals for the scope as well (It's a re badged scope ):(User manual)http://www.jameco.com/Jameco/Products/ProdDS/2145246%20UsersManual.pdf(Schematics and board layout)https://www.electronica-pt.com/esquema/func,startdown/id,21183/I've tested the amplifier transistors, and the caps look fine. All the voltages I've measured on the board are within spec as well (I haven't measured anything too high voltage because my multimeter is cheap). Any ideas?Here's a link to a video of the fault:https://www.youtube.com/watch?v=PWL868Z81qo&feature=youtu.be

author: vk6zgo
author_rank: Super Contributor
timestamp: April 22, 2017, 04:50:43 am
content: First of all, it doesn't have deflection coils (Google Electrostatic Deflection).Yes, it looks like your horizontal deflection is offset to one side.A couple of people on this forum have had similar problems with other types of 'scope,some of which may be "badge-engineered" versions of yours.Hang in there, give us a bit more information, & somebody will probably be able to help.

author: tyguy2
author_rank: Unknown
timestamp: April 22, 2017, 05:23:08 am
content: Quote from: vk6zgo on April 22, 2017, 04:50:43 amAlright, my bad.So I think I've narrowed down the search area for the fault. If the traces are being deflected to far to the right (left if looked at from the oscilloscope screen), then that means that the right deflection isn't working. Looking at the schematics, and referencing the repair manual, it looks like the circuitry connected to X+ (which is the right deflection connector) is going to be the place to look.Am I right, or did I mess something up? I tested the high voltages on the board and everything is within 5% tolerance.

author: george.b
author_rank: Frequent Contributor
timestamp: April 22, 2017, 05:45:09 am
content: I'd check the transistors on the X+ section, going backwards from the ones closest to the output.Actually, I'd also check that resistor next to the h-position pot, as well as the pot itself, just to be sure.

author: tyguy2
author_rank: Unknown
timestamp: April 22, 2017, 06:17:33 am
content: Ah Ha! I got it working, sort of. I adjusted the Mag Cent. potentiometer (I had to twist it all the way left for it to work, I don't know if that's good or bad), and the trace moved back to the middle of the screen. I say sort of because it takes some finagling to adjust the trace to the middle of the screen. But hey, if it works, it works.

author: tyguy2
author_rank: Unknown
timestamp: April 22, 2017, 06:19:29 am
content: It's still very buggy though, especially if I put a signal through it. I'm still going to have to fix this fully

author: george.b
author_rank: Frequent Contributor
timestamp: April 22, 2017, 06:34:43 am
content: I take it that you have tested that +12V-R rail there, yes? Are the resistors around that mag cent. pot within their nominal values?

author: tyguy2
author_rank: Unknown
timestamp: April 22, 2017, 08:25:01 pm
content: Update:I'v found that if I lift the scope up with one hand on the left side of the casing, everything works. Looks like its just a broken connection somewhere inside the thing.