Tektronix THS720 Backlight modification with dropin replacement LED Controller - Page 1
Forum section: electronics / testgear

author: zyxel13
author_rank: Contributor
timestamp: May 07, 2022, 11:09:23 pm
content: Hello,Few Weeks ago I finally had the chance to acquire a THS720A with the Probes, Leads and 2 Batteries that are still holding charge quite good, for a reasonable Price but the Backlight CCFL is dead ! The Scope and the Accessories (the ones I received with) are in Great Condition, FW 1.08, made the conversion to 720P.I reverse Engineered the existing CCFL just to see what is done and made a 1:1 Drop-in Replacement Converter for the LED Strip Backlight. I used a 12V 3mm thin LED Strip cut down to Length, this way the White CCFL Plastic Holder can be used again.The Led Strip I used is the :https://www.aliexpress.com/item/4000912596869.htmlanother option is this one :https://www.aliexpress.com/item/1005003193200776.htmldidn’t used it because the first one is very goodThe LED Strip can be Driven up to 15V with nothing getting really warm, at 12V Output the LED Controller draws ca. 120mA with 5V Input and ca. 40mA with 12V Input.Attached Pictures of the Conversion and the LED Controller.The Reverse Engineered CCFL in the last Attachment.Here a short Video i've made on the matter.Cheers,Konstantin

author: zyxel13
author_rank: Contributor
timestamp: December 02, 2022, 06:07:49 pm
content: Gerbers, Schematics and BOM List

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 24, 2023, 09:28:37 am
content: Nice work!I'm thinking of building one of these for my scope, one question though.. What is the purpose of the four voltage selections?

author: factory
author_rank: Super Contributor
timestamp: March 24, 2023, 07:46:06 pm
content: Seems to give choice of voltages to run the LED tape at, not sure running 12V LEDs at 15V would be good idea, sure to shorten the lifespan of the crap that is available these days.David

author: zyxel13
author_rank: Contributor
timestamp: April 30, 2023, 04:06:50 pm
content: Hello TERRA, sorry for the very late Reply, I just saw your Message.The Voltage Selection is just an Option that allow you to Increase the Supply Voltage of the LED Tape up to 15V in case the Display Transparency is not good anymore. Of course this comes with drawbacks :1) Increased Current draw (even not as high as the CCFL was drawing)2) Lifespan of the Tape could Decrease, even that there are 3 LED's in Series with a 560Ohm Resitor in Series for 12V (each smd 2025 Led is 3-3.5V, each chip draws ca. 15ma according to the Datasheet) by using a 560Ohm resistor they are driven almost half the rated current.Btw, in case you are interested I uploaded my design of an replacement External Charger for the THS7xx Series Battery Pack:https://www.eevblog.com/forum/testgear/tektronix-ths710-ths720-ths730-external-battery-charger-build/msg4840787/#msg4840787Cheers,Konstantin

author: TERRA Operative
author_rank: Super Contributor
timestamp: April 30, 2023, 05:57:40 pm
content: Great thanks!I have just laid out my own version of the LED driver board heavily inspired by yours just for the fun of it.Do you have the datasheet for the LED's available by chance? I'm interested to take a look...I have also used my i1 Display Pro screen calibrator to measure the LCD brightness with the LED strip at various voltages to figure out the voltage needed to replicate the original 70 cd/m2brightness and see how the voltage/current/brightness correlate all the way up to 12V.Once I have resistor values calculated, I'll post the info up here for others to figure out what voltage they want to run their LED's at.I also found that the original CCFL in my unit was only putting out a bit under 35 cd/m2, almost exactly half of the rated output.

author: zyxel13
author_rank: Contributor
timestamp: April 30, 2023, 07:33:56 pm
content: Hi Terra,attached the PDF I Have on the LED's and the Schematic of my LED ControllerCheers,Konstantin

author: alanbc
author_rank: Newbie
timestamp: June 24, 2025, 04:29:05 pm
content: Short version:Did you put your project up on PCBWAY?Long version:I hauled out my THS720 yesterday, for the first time in a VERY long time .. 5 years or more. I've been using a nice new colour scope, but it's 110 VAC only. And I have a nice little portable colour scope, that's not nearly fast enough for the project I'm currently working on.(I wasted a whole day of troubleshooting/programming before I figured out that the device WAS outputting pulses, but my portable scope just wasn't seeing them.)So out came the Tek, and the first thing I noticed was that the backlight is VERY dim. It DOES come on, no question, but it's really dim. It's darn near unusable. So I started to Google replacement options for the Tek, but even the comparable ones from China are way more money that I can spend. And after using a Tek, it's hard to use anything else.So I Googled 'THS720 and Backlight', I came across this post. Cool!Then I found another post where itappearsas ifsomeonehas put your project up on PCBWAY. Yay!While I can make my own PCBs, for something like this, I'd rather have them commercially done. And while I can solder surface mount stuff, I'd rather not, and PCBWAY has an option to do the soldering for me.So, was it you that put the project up ? If so, do you think PCBWAY is capable of soldering these up in a good manner?Thanks for any thoughts.

author: zyxel13
author_rank: Contributor
timestamp: June 24, 2025, 07:18:31 pm
content: Hi Alan,I Have Uploaded the Design on PCBWAY, this is the Link:https://www.pcbway.com/project/shareproject/TEKTRONIX_THS710_THS720_THS730_LED_Backlight_drop_in_replacement_Controller_180430eb.htmlPCBWAY is making a real good job and quite a lot of people ordered it as a PCBA there.And here my Video on Youtube:There is another one on PCBWAY, but this is just a lightly modified one from somebody else based on my Original Design.I also uploaded on PCBWAY a External Battery Charger Design I made Including the 3D Printed Casinghttps://www.eevblog.com/forum/testgear/tektronix-ths710-ths720-ths730-external-battery-charger-build/msg4840787/#msg4840787Cheers,Konstantin

author: alanbc
author_rank: Newbie
timestamp: July 16, 2025, 10:20:37 pm
content: Just a quick note to say that I got the boards back from PCBWay, installed things and it works GREAT!I guess that the stock tube had slowly but surely gotten dimmer, so slowly I hadn't noticed it, until one day, I went "this can't be right!".Well, now it IS right! Thanks again.Alan

author: alanbc
author_rank: Newbie
timestamp: July 19, 2025, 02:14:44 am
content: Hmm, maybe spoke a bit too soon.The LEDs do not turn off when power is off. They DIM, but do not turn off.I didn't notice at first because there was no battery in the unit, the battery long failed. But a new (from China) replacement just showed up and while charging it for the first time, saw this.Any ideas?