Accessing the brightness potentiometer of a TDS684C to fix a dim CRT - Page 1
Forum section: electronics / testgear

author: tverbeure
author_rank: Regular Contributor
timestamp: May 08, 2025, 06:42:29 pm
content: I didn't realize there was a separate test equipment repair boards, please don't reply here. I'll repost it there.My TDS684C has a dim CRT. I know there's the option to ditch the CRT altogether and use an LCD, but before I do that I first want to make sure that it can't be fixed with the turn of a potentiometer, as described in the service manual. Has anyone had success with that?In the service manual of the TDS500/600/700, in the "Display Assembly Adjustment" section, they have the following instructions:Access the inside of the oscilloscope: See Removal and InstallationProcedures in Section 6 to remove the cabinet.Adjust the display rotation: ...Adjust the display brightness: Locate R403 (BRIGHTNESS).It is one of the adjustments on theleft side of the instrument located just ahead of the fan.It is thefourth adjustment from the fan.The way it is described, it seems like it's just a matter of removing the case and accessing the potentiometer right next to the fan. Of course, when you actually try that, there are no potentiometers to be seen. Some further digging revealed that these potentiometers are located on a display board that doesn't seem to be immediately accessible.In practice, does this mean that I have to take apart the whole scope just to access those potentiometers or is there a location where I can squeeze in a screwdriver and still access it without taking the scope apart? (Not that inserting a screwdriver inside a dark place with CRT voltages is very appealing either.)I've been looking for YouTube videos that show me how it's done but so far I've come up empty handed.

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 09, 2025, 07:22:28 am
content: When you remove the case, on the left side (viewed from the front of the unit) of the scope where the fan is, take a look next to the fan under the floppy drive.You'll see some holes there. Use a ceramic screwdriver to tweak the trimpots.There may be a bit of metal in the way of one or two trimpots, it's just a shield for the floppy drive, but it can be pulled out if you are feeling energetic, or maybe just maneuvered around. I have been known to snip a bit of the shield away to gain easier access....The 4 accessible trimpots in order from left (fan) to right (front of scope) is: Trace rotation, Vertical position, Brightness, Contrast.I have the Tektronix J16 Photometer and I have found that most CRT's are a little on the dim side these days, so just a slight tweak gets the displays looking nice again.Here is the schematics of the display module, so you can figure out what all the trimpots do:https://hakanh.com/dl/docs/hardtofind/TDS544A_A30-32(Disp).pdfAlso, double check the screen brightness setting in the menu. It is at 85% by default (and should be set to 85% when doing adjustment), so just check that yours isn't turned down somehow...Press DISPLAY -> Settings (on bottom display soft buttons) -> Display (pop-up)

author: tverbeure
author_rank: Regular Contributor
timestamp: May 11, 2025, 09:00:42 pm
content: Thanks! As mentioned in my Repair thread: the trim pot holes were completely covered with dust so I couldn't see them. I have successfully increased the brightness and the scope is like new now (but I will soon need to replace the DS1486...)