TDS524a O'scope HVPS what is this part?  (E25) - Page 1
Forum section: electronics / repair

author: TerrySt
author_rank: Regular Contributor
timestamp: April 29, 2023, 03:36:53 pm
content: I'm working on a TDS524a CRT based O'scope with an intermittent problem with the high voltage power supply. It works for a few minutes, but then the high voltage drops and the screen starts flickering. After ruling out the shutter driver board, I found that this component in the CRT high voltage supply responds to being hit with a spray of component cooler. It lives under the flyback transformer. I don't know what it is. It is marked E25 on the silk screen. I don't have a schematic for the 524a, but I found one for the 544a that is similar. It shows E25 and the symbol for it, but I'm not sure what it is. I am guessing it is some sort of voltage limiter, but I can't figure out how it could become thermally intermittent or what failure mode it might have that would make the high voltage drop. It is in the HV sense feedback so if it started leaking I would expect the HV to increase, not decrease. It had some markings on it, but they were obscured by dirt and as soon as I tried to wipe the dirt away, the markings left with it.It reads open on Ohms or Diode test and about 0.08uF when cold and 0 uF when warmed slightly, although the 0.08uF when cooled could just be caused by the freeze spay residue (and the dirt). I have tried to ensure that I was really only cooling this part, and not something near it, but of course that is nearly impossible since it lives under the flyback transformer and is a b**** to get to.Does anyone know what this is, or better yet , where to find a replacement? I know I could ditch the CRT and install a VGA LCD panel, but the CRT looks so good when it is working.Thanks,Terry

author: TerrySt
author_rank: Regular Contributor
timestamp: April 29, 2023, 03:37:41 pm
content: The silk screen:

author: TerrySt
author_rank: Regular Contributor
timestamp: April 29, 2023, 05:00:50 pm
content: The somewhat scary test setup. The HVPS is buried in the scope and the CRT has to be removed to get to it. This is the only way I could figure out to troubleshoot it.

author: fzabkar
author_rank: Super Contributor
timestamp: April 29, 2023, 05:08:36 pm
content: Gas arrestor or gas discharge tube.https://www.circuitstoday.com/lightningsurge-protector-circuit-using-gas-discharge-tube-gdt

author: TerrySt
author_rank: Regular Contributor
timestamp: April 29, 2023, 05:17:01 pm
content: Thanks for that fzabkar. That helps a lot. I assumed it was some sort of voltage limiter, but I hadn't come across one before.Terry

author: fzabkar
author_rank: Super Contributor
timestamp: April 29, 2023, 07:01:16 pm
content: Would it be worth cleaning the "stain" on the PCB in that area?

author: jwet
author_rank: Frequent Contributor
timestamp: April 29, 2023, 07:26:49 pm
content: It would be worth cleaning up that residue- HV doesn't need any more encouragement to arc over- that looks a little messy. I guess that its just dirt that the freeze spray washed off. Be a little careful with this circuits and ones like it. That gas discharge tube is designed to limit very high voltages by arcing over- they are used in lightning protection sometime. Be aware that it might be a safety system to make sure the the tube voltage doesn't get too high and create a danger of X-Rays. Oscilloscope anode potentials can be 15kV nominal and if its goes out of control could go over 20 kV, this is XRay territory- dangerous, ionizing radiation. Does the brightness fluctuate before it starts to flake out. It may be that this GDT is arcing and protecting you.

author: TerrySt
author_rank: Regular Contributor
timestamp: April 29, 2023, 07:46:45 pm
content: Thanks for the suggestions. Yes, I cleaned up the pcb (the whole pcb, carefully). The stain was from the black dust that accumulates around high voltage stuff combined with the freeze spray.The failure didn't make sense for the gas discharge tube. So after I cleaned everything up, I checked the components in the area of that part. Like I mentioned, it is impossible to be sure that you are cooling a specific part with freeze spray, and only that part, especially since the parts were mostly hidden under the flyback transformer. Well, I found a resistor (R13) that was right next to the E25 part. It was marked 680 ohms but measured 780 ohms. You can see it in the picture next to E25 and on the schematic across the primary of T11. I couldn't see any signs that the value changed with heat or cold, but after you touch something that is intermittent, all bets are off.Anyway, I cleaned everything, changed the bad resistor, re-installed the gas discharge tube and put it back together. Everything is working good and has been for a few hours, so fingers crossed, I have a fix.I love the way the display looks when it is working. I have a VGA LCD panel on order that I will compare against the CRT. At least I'll have a backup for if (when) the CRT gives up for good.Thanks for the help. I was really curious about that gas discharge tube. I had seen PCBs with sharp pointed copper that is shaped like arrow heads that perform a similar function on some scopes, but never a component like E25.Terry

author: jwet
author_rank: Frequent Contributor
timestamp: April 29, 2023, 08:18:47 pm
content: I hadn't looked at the symbol on your first schematic- that circled symbol is for a cold cathode vaccuum tube which I guess is technically what that device really is. Its the same symbol used for little neon lamps like the NE-2, etc. NE-2 were sometimes used as overvoltage protection or zener like devices in the dark past. Good luck.