Flickering backlight Tektronix TDS220 - what could it be? - Page 1
Forum section: electronics / repair

author: nneilboss
author_rank: Newbie
timestamp: August 21, 2020, 04:06:41 pm
content: Hello - lapsed Electrical Engineer now messing about with vintage electronics after 25 years. I bought a Tek TDS220 digital scope for an amp build project. I thought it was a bargain - display a bit clunky but easy to use and small footprint. But after a week or so the screen started flickering. Instead of sending it back to the seller, I thought I would be clever by replacing the CCFL backlight. Did that , very fiddly, but got it done. The damn thing is still flickering like before - it still lights up and it is not related to vibration, so probably not a bad contact. What could it be? The inverter? Any thoughts? I know it supposedly has 1470 Vp-p supplying the CCFL , but I cannot measure it because my oscilloscope is the damn problem.Any thoughts? Is it worth trying to replace the power module? Should I just bin the thing and buy a cheap generic scope on Amazon?

author: Testtech
author_rank: Regular Contributor
timestamp: August 21, 2020, 04:25:40 pm
content: It might be the inverter, I replace the CCFL backlights with an LED strip and driver. Much brighter and adjustable intensity. I obtain the LED and inverter kits from eBay. The LED strip is trimmable for length in 3-LED increments.

author: nneilboss
author_rank: Newbie
timestamp: August 21, 2020, 05:18:10 pm
content: Thanks - that sounds interesting. Was this for the same family of scope? Could you share the eBay listing, or at least the appropriate search term?

author: nneilboss
author_rank: Newbie
timestamp: August 21, 2020, 05:19:53 pm
content: This could be more cost effective than buying the PSU - I have seen those listed between $50 and $250, which was more than I paid for the scope

author: tautech
author_rank: Super Contributor
timestamp: August 22, 2020, 09:37:39 am
content: Some of these Teks used a free running push pull bipolar inverter for the display backlight, the frequency of which is controlled by a simple RC.The cap drifts with age until the frequency gets that high it surpasses the Ft of the bipolars.Check the cap value and replace as necessary.I don't now remember the inverter frequency however it might be listed in the service manual and you can easy check it's there abouts (20-40 KHz range) by clipping a probes reference lead to the probe tip to form a RF probe and just hold it close and check the readout frequency.

author: nneilboss
author_rank: Newbie
timestamp: August 22, 2020, 07:56:45 pm
content: Yes, would love to do those measurements, but unfortunately the dead scope is my only instrument other than a multimeter. Very odd. It seems to need about 5 minutes after power up and then the backlight flicks on and off. I have noticed that it seems to respond and flick back on when I whack the red resonance cap lightly. There are no obvious loose connections.I have ordered a replacement capacitor and will try that anyway.By the way, for the LED replacement option I am wondering where I will get the required 10-30 V DC. The available voltages are +- 5VC and -24V. How would I power the LED driver? Have I missed something?

author: Testtech
author_rank: Regular Contributor
timestamp: August 22, 2020, 08:35:58 pm
content: Quote from: Testtech on August 21, 2020, 04:25:40 pmSomething like eBay item 201798321089, is what I buy. There are many other listings with different length or width LED strip, etc.I remove the CCFL and inverter, cut an LED strip to length, mount the LED supply and connect it to power. You also usually have to connect the LED power supply DIM pin to some Vsource via a current limiting resistor or divider to set the desired brightness, as the LED strip may washout the display.I usually have to glue the LED strip into the old ccfl reflector to keep the light aligned with the diffuser.

author: nneilboss
author_rank: Newbie
timestamp: August 22, 2020, 10:32:16 pm
content: Thank you, I will try ordering one. Where did you pull the DC voltage from to power the LED ?