Need help diagnosing some issues with new scope - JDR Model 2000 - Page 1
Forum section: electronics / testgear

author: MatCat
author_rank: Frequent Contributor
timestamp: December 31, 2014, 05:12:14 am
content: Just picked this scope up today from a local ham for helping him out with his computer. He told me it had been sitting for 5 years and all he used it for before that was to repair Atari power supplies. In testing it seems mostly fine, time base seems to be spot on, though vertical is not exactly perfect, it seems more 'attenuated' on the left side of the screen then the right, and the vertical positioning seems to 'get stuck' plus gives issue with signal attenuation as shown in the video above.

author: tautech
author_rank: Super Contributor
timestamp: December 31, 2014, 06:41:07 am
content: To persue this fault you likely will need the Service manual.JDR may be a rebrand from the OEM, and others might recognise it.First port of call is always the PSU, all voltages must be to spec and ripple within limits.Vertical position relies on voltage offset of the CRT plates, just where this offset is applied will be shown in the Schematics.There definately seems to be an imbalance as Pos adjustments should drive the waveform undistorted off the top of the screen.Grab the Tek troubleshooting pdf from the first thread in the Repair board, it should offer you more understanding.

author: MatCat
author_rank: Frequent Contributor
timestamp: December 31, 2014, 07:02:54 am
content: Quote from: tautech on December 31, 2014, 06:41:07 amI found the manual for the Tenma 72-720 which certainly looks pretty identical to me, It has the full calibration instructions in it, though I am not that properly prepared to do a cal. I am going to take it apart tomorrow and give the pots a good cleaning, check out all the caps, take some pics.

author: tautech
author_rank: Super Contributor
timestamp: December 31, 2014, 07:13:30 am
content: Can you link the manual please.Often a Cal is not essential after a repair, just a careful check to prove it's not miles out.Visual check of caps is good, but the PSU performance should indicate if all is well.If it is OK, and they look OK, leave them be.

author: MatCat
author_rank: Frequent Contributor
timestamp: December 31, 2014, 07:47:31 am
content: Manual is here:http://bama.edebris.com/download/tenma/72-720/72-720.pdfAnd schematic is here:http://elektrotanya.com/hung-chang_os-620_sch.pdf/download.htmlI actually found a thread here on the forum where a few people had found these things rebranded under a gazillion different brand names, apparently super cheap but easily repairable.

author: free_electron
author_rank: Super Contributor
timestamp: December 31, 2014, 10:12:29 am
content: Looks like the vertical amplifier runs out of steam.These machines use electrostatic defelection. ( no magnets). The beam fly through two plates.In rest the beam hits center screen.To send the beam down they apply positive voltage on the bottom one and negative on top. Do the top plate repels the beam and the bottom attracts.To send it to top they apply negative bottom and positive top. So it looks like one half of the drive amplifier doesnt do its work.Look in the vertical defelection section of the schematic. There should be a bridge of 4 transistors driving the tune. Thats where the problem is. The supplies may have shifted as well.If top is 80 volt and negative is -120 the. That could cause such problem too. Check that the power supplies are nice and symmetric. If that is ok most likely one half of the drive bridge is wonky. Could be something simple as a burned resistor.EditJust took a peek at the schematic. Last page top right section is the vertical drive. Bottom right is the alt/chop control with the and gates. Follow the and gate outputs to the diodes. Anything left of those diodes is ok as both channels have the same problem. . Anything right of the diodes is suspect. Begin by checking the -120 volt and the +20 volt. Most likely one of em is not good.. Monitor the voltage as you shift traces up/down. And see if they drift.

author: MatCat
author_rank: Frequent Contributor
timestamp: December 31, 2014, 06:48:24 pm
content: Quote from: free_electron on December 31, 2014, 10:12:29 amGot her opened up on the bench now, the 120v and 20v rails to the vertical section look fine, 119.5v and 21.13v respectively, I do not see any change in voltage with vertical positioning. Upon further experimentation today I found that as long as the vertical position has it's ground reference 1 graticule higher then center or below it will not distort the waveform, even if it goes to the top of the screen.