Tektronix 2440 PSU repair - Page 1
Forum section: electronics / repair

author: LazyJack
author_rank: Frequent Contributor
timestamp: March 03, 2016, 09:34:21 am
content: Hi,I was recently given a Tek 2440, which looks absolutely dead. It still has calibration stickers intact from 5 years ago, so it was not opened since then. My first check revealed that there is no power at all, none of the power rails have anything on it. I have already removed the PSU and confirmed that the primary DC voltage is there, but id does not show any sign of starting. There is no obvious damage, like exploded parts or such.I'm planning to do measurements based on the service manual, but first I'm asking if there is any know part that usually fails in these PSUs. So where to look first. Any help appreciated, thanks.Related question. I do not have an isolation transformer, do you think testing the PSU from an UPS unplugged and running form battery is OK?

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 03, 2016, 11:01:37 am
content: If it is a SMPS isn't oscillating.

author: LazyJack
author_rank: Frequent Contributor
timestamp: March 03, 2016, 11:52:41 am
content: Quote from: SAUL BRITTO on March 03, 2016, 11:01:37 amYes, its a switching psu. built around an sg3525an pwm ic. Once I have time, I'll get my scope an see what it is doing.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 03, 2016, 12:02:33 pm
content: Quote from: LazyJack on March 03, 2016, 11:52:41 amSeems an easy fix, to me.Maybe an open resistor.

author: LazyJack
author_rank: Frequent Contributor
timestamp: March 03, 2016, 07:39:36 pm
content: OK, got a bit further in debugging. Seems like the control supply, which should initially power up the pwm ic is not supplying the required 11 volts only about 3.5. However,it seems that it is overloaded, as all components are good and desoldering it output it works.It powers a couple components in the PSU, so I'll have to check one by one. I'm thinking of replacing the SG3525 in the blind, just to make sure.

author: tautech
author_rank: Super Contributor
timestamp: March 03, 2016, 07:53:41 pm
content: Quote from: LazyJack on March 03, 2016, 07:39:36 pmDon't assume the control supply is OK.Check the dropper resistor/s from HV DC, any electrolytics and flyback diodes.Plus usual other suspects of course.

author: LazyJack
author_rank: Frequent Contributor
timestamp: March 04, 2016, 08:40:07 pm
content: Thanks, looks like you have given a good lead.looks like the capacitor (180uF) that is charged by the resistor from the HV DC and also from a transformer once the psu starts up is bad. I found it by giving supply to the control psu from an external source, and that would make the pwm start up. Measured the cap, my meter says it has an ESR of over 1 k, so it would never get charged once the pwm start.Anyway, I patched in a similar value replacement, and the PSU now seem to start up. Unfortunately I have to wait till Monday to buy a replacement that actually phisically fits in its place, so i can reinstall the PSU and see what happens. Probably it's also a good idea to replace all the other caps.

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2016, 05:31:10 am
content: Quote from: LazyJack on March 04, 2016, 08:40:07 pmYou're on to it.

author: oldway
author_rank: Super Contributor
timestamp: March 05, 2016, 08:47:53 am
content: Warning: you need an external fan to cool the scope: AD-converter and CCD hybrids may suffer irreparable damage within a few minutes if operated out of the metal enclosure without extra cooling.

author: LazyJack
author_rank: Frequent Contributor
timestamp: March 05, 2016, 12:28:22 pm
content: Thanks, yes, I've read that in the service manual.But I'm one happy guy now!!!Managed to get replacement caps on Saturday, replaced them and now I have a fully working 2440, it passes all self tests and seems to be in a generally working shape. Yipeee! (So now I guess it's time to get a replacement NVRAM and transfer the content, before this one dies on me and loses the factory cals.Although I have to figure out why I'm now left with one extra screw fter putting everything back together. Doh!

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 07, 2016, 02:51:54 am
content: Wow,good!

author: chihaxinh
author_rank: Regular Contributor
timestamp: March 15, 2016, 01:32:44 pm
content: I have one 2440 , but have error gain and ccd error in start up, some time pass and some time fall event running in 30 min (warm up ).I thing psu have problem ( cap over 30 yeards old ). I want to replace all cap , but i have one questionHow to i checking vol rip ? can i remove psu low volt and psu regulator board , and mearsurement with 2 board only ( no load )?

author: LazyJack
author_rank: Frequent Contributor
timestamp: March 15, 2016, 02:03:09 pm
content: Quote from: chihaxinh on March 15, 2016, 01:32:44 pmThe voltages are accessible on the PSU panel near the fron of the scope. You should be able to access the PSU by folding out the large upper panels (unplug the rear BNC flex cable from the panel). You may need to take off the metal cover off the PSU, but I think the test points are accessible even with that on. All voltages are writen on the PCB. Also, I think most of the voltage test points are also accessible on the side panel.It's is fairly simple to remove the PSU, but takes some wigling around some cables to move it out of the scope. You need to remove screws from the rear, fron the side and from the top to get the PSU loose, and unplug some cables from it.But before you do that. You can get CCD and gain errors if the calibration is lost. Do you stil get these if you do a self cal? Is the NVRAM ok? Do you get a 6000 error?Also, if you have not yet done, do read the service manual, it really has a lot of useful info on how to take things aparts, and what to check.

author: chihaxinh
author_rank: Regular Contributor
timestamp: March 15, 2016, 02:13:35 pm
content: Okie , many thank , pls wait me 10 or 15min for take picture about error !

author: chihaxinh
author_rank: Regular Contributor
timestamp: March 15, 2016, 02:42:21 pm
content: Here its some picture about errorhttps://photos.google.com/share/AF1QipOd83UeUdLWGs1CukrX1NeLO-95kJRKViVHdgLPYton93ml_Pg05AA8g4wcjSv03w?key=ZmdkSno3S3ppeHEyZDhZSVctdFFLSkcxN3dRVG1BGoogle photo not show direct link for insert in forum

author: LazyJack
author_rank: Frequent Contributor
timestamp: March 15, 2016, 03:28:10 pm
content: Quote from: chihaxinh on March 15, 2016, 02:42:21 pmOK, so you are getting these errors during Self Cal. This may be hardware problem then. Definitely worth checking the supplies for ripple. The supply value is ok, it is checked by the selftest.Read the service manual (for example here:http://w140.com/tekwiki/wiki/2440) it has a detailed description on what may be bad based on the faults. Based on the various Gain fails, I suspect the DAC subsystem.

author: chihaxinh
author_rank: Regular Contributor
timestamp: March 15, 2016, 03:43:13 pm
content: Hi , sefttest some time pass and some time fail . Some time only 7000 CCD , if i have only 7000 CCD error i have 2 trace display with gain error !

author: LazyJack
author_rank: Frequent Contributor
timestamp: March 15, 2016, 06:23:24 pm
content: Quote from: chihaxinh on March 15, 2016, 03:43:13 pmI suggest, check the obvious. Reseat connectors, plugs. Check the PSU voltages and ripple.Run a self cal and if you can, an ext cal too for attentuator gain and trigger gain (easy, you need 0.2V, 2V and 20V voltages for that).Otherwise go trough the steps in the service manual for these diag fails.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 16, 2016, 11:01:13 am
content: Quote from: LazyJack on March 15, 2016, 02:03:09 pmCaps guy, Don't forget.

author: chihaxinh
author_rank: Regular Contributor
timestamp: March 16, 2016, 02:03:07 pm
content: Hi AllTo day i take cover and checking voltage and rip! All ok, noise rip about 20mV p-pand when i force cooling by other fan ! oh like magic ! all tesst running pass ! I thing environment temp have in case ! how to calibration match real temp ? i no ussing osc long time( 2 years)Many thank all to support !

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 16, 2016, 05:48:52 pm
content: Quote from: chihaxinh on March 16, 2016, 02:03:07 pmAre your cooler running at the voltage and normal velocity?

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 16, 2016, 06:08:05 pm
content: Look if it can be a dust case or something very hot, in lack of your heat sink.