Tektronix TDS220 backlight not working - Page 1
Forum section: electronics / testgear

author: j03
author_rank: Newbie
timestamp: March 24, 2019, 09:14:17 pm
content: Hello,Just joined - appreciate all the shared knowledge and expertise here.Have recently acquired a TDS220 that does indeed work, but has no back light.After reading several posts/threads here, I've taken it apart and am checking out the power supply. (partno: SW422 Rev-C)The CCFL tube itself doesn't look to be in great shape. It's quite grey/black for about 1/2 inch on both ends. I'm thinking about replacing this tube.Back to the PSU, I also noticed that the C24 capacitor is pretty brown on the one end. There is a painted black dot on it, but you can see in the attached pic, it's suspiciously quite brown on the one side. (ie: opposite the C24 label). From my reading, I believe that this is part of the backlight power supply.I thought that the Q2 transistor (C3205) might also burnt, but it turns out that it only had some of that jelly stuck to it which make it look like the front face exploded, as you can see in the pic as well.I've looked around for a schematic, but haven't found anything yet. Does anyone have one for this board?But really looking for some help to get my backlight going again.Any suggestions, where to start?Thanks,J03

author: TurboTom
author_rank: Super Contributor
timestamp: March 24, 2019, 10:27:20 pm
content: Check this orange 0.68µF resonance capacitor close to the CCFL step-up transformer. Failed on mine as well long time ago before I donated the scope to a friend...

author: Miti
author_rank: Super Contributor
timestamp: March 24, 2019, 11:30:58 pm
content: I wouldn't even bother with the CCFL. For my TDS210 I bought an LED strip and driver from Banggood, I've cut it to the correct length and I replaced the CCFL with LEDs. It is easy to install and the brightness is amazing. Something like this:https://www.banggood.com/220MM-LED-Dimmable-Backlight-Rigid-Strip-Kit-Light-Update-CCFL-LCD-Screen-For-Monitor-p-1116862.html?rmmds=myorder&cur_warehouse=CNThis particular driver didn't work properly, the schematic is wrong so I had to modify it a bit but the LED strip is exactly what I needed, the right width and height.

author: j03
author_rank: Newbie
timestamp: March 24, 2019, 11:36:20 pm
content: Thanks TurboTom, I have removed that cap today, and it looks bad.I'm going to see about finding a replacement. I'll post back with some results.

author: j03
author_rank: Newbie
timestamp: March 24, 2019, 11:39:21 pm
content: Thanks Miti.I've also been reading about this option. Seems like a good one, and I will consider this if debugging the PSU becomes too painful.Could you please tell me how you powered the strip? (It's 12V DC right?)

author: Miti
author_rank: Super Contributor
timestamp: March 25, 2019, 12:09:23 am
content: Yes, the same 12V that powers the CCFL inverter is used to power the LEDs constant current driver that comes with the strip. I added a trimmer potentiometer on mine to adjust the current.Make sure you disable the inverter though, I think is L4 that you need to remove.

author: j03
author_rank: Newbie
timestamp: March 31, 2019, 09:31:22 pm
content: Replace the blown cap with a new one 0.68uF and now working again!If this blows again... i'll look at the LED strip solution.thanks guys!

author: joelwx39
author_rank: Newbie
timestamp: June 22, 2020, 07:59:37 pm
content: J03Can you tell me more about the specific type of capacitor you used to fix your scope? That cap on my scope has blown out and I need to replace it. I see that it is 0.68 uF, but what type and voltage should I use?Thanks.

author: tautech
author_rank: Super Contributor
timestamp: June 22, 2020, 08:38:55 pm
content: Quote from: joelwx39 on June 22, 2020, 07:59:37 pmWelcome to the forum.It's just a resin dipped MLCC, a 50V rated one should be adequate.I had the same issue with a TDS1002 where it had drifted to pF values, way too fast for the poor little bipolars to keep oscillating.

author: TurboTom
author_rank: Super Contributor
timestamp: June 22, 2020, 09:51:07 pm
content: No, it's not an MLCC, it's a 63V polyester film capacitor (clearly marked MKT). For a replacement, I'ld try to get an MKP (Polypropylene) capacitor of the same or higher voltage rating. A replacement like that should last forever.

author: joelwx39
author_rank: Newbie
timestamp: June 26, 2020, 07:34:26 pm
content: Thanks for the info. I got some caps and did the repair. Now the backlight comes on. A pretty simple fix.

author: tautech
author_rank: Super Contributor
timestamp: June 26, 2020, 08:28:23 pm
content: Quote from: joelwx39 on June 26, 2020, 07:34:26 pmSure is !Did you measure the dud cap ?