Tektronix THS720 Backlight modification with dropin replacement LED Controller

[zyxel13]

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.html another option is this one : https://www.aliexpress.com/item/1005003193200776.html didn’t used it because the first one is very good 
The 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

[zyxel13]

Gerbers, Schematics and BOM List

[TERRA Operative]

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?

[factory]

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

[zyxel13]

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/#msg4840787 


Cheers,
Konstantin

[TERRA Operative]

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/m² brightness 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. 

[zyxel13]

Hi Terra,

attached the PDF I Have on the LED's and the Schematic of my LED Controller

Cheers,
Konstantin