TDS 540 problem - Page 1
Forum section: electronics / testgear

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 17, 2013, 05:15:45 pm
content: Today I've got a TDS540 oscilloscope with options 13, 1M and I think 2F (I don't know exactly).The problem is it doesn't turn on.Despite I read a lot of comment regarding capacitor leaking in my unit none of the capicitora are leak. (A lot of fortune!!)I think it is a psu problem.When I plug in the power cord and turn on the instrument, nothing happens.Near the psu I can hear tick-to sound.Any one have a schematics for that power supply circuit?The model is: 22904700 rev F.Any body have some idea on what ca be wrong?Thanks for the attention.Best regards, Alberto Vaudagna.

author: kxenos
author_rank: Frequent Contributor
timestamp: May 17, 2013, 06:12:26 pm
content: If the PSU does audible sound like tik tik tik (or tak tak tak) then I think you will have to check and replace the elec. caps. Good luck

author: nctnico
author_rank: Super Contributor
timestamp: May 17, 2013, 06:31:05 pm
content: Are the fan and CRT unit connected? If those are not connected, the CPU won't start (it will go into overvoltage protection on the 24V bus).You can download the TDS520B component level service manual from Tektronix. There is a diagram in there but not all the PSU's are 100% the same.

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 17, 2013, 07:43:02 pm
content: Yes they are. So look at the capacitor?

author: wraper
author_rank: Unknown
timestamp: May 17, 2013, 07:52:43 pm
content: That sound tik tik tik could mean shorted diode in PSU secondary or some power rail shorted.

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 17, 2013, 08:02:17 pm
content: Ok, i'm going to chek those situation. Do you know if there some common component failure?

author: wraper
author_rank: Unknown
timestamp: May 17, 2013, 08:15:38 pm
content: I don't know about particular oscilloscope, but sounds that SMPS can't start because of overload. You can also check flyback transistor that drives HV transformer. If it is shorted then power rail also will be.

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 17, 2013, 08:21:29 pm
content: No it have to be in psu because it do that without any other board connected.

author: wraper
author_rank: Unknown
timestamp: May 17, 2013, 08:26:44 pm
content: SMPS without load will function incorrectly. It also can be with strange tik tik sounds. SMPS needs some load, otherwise it will skip pulses to not overshoot output voltage. Some PSUs wouldn't start at all or go off after some seconds because of protection.

author: nctnico
author_rank: Super Contributor
timestamp: May 17, 2013, 08:38:03 pm
content: If the PSU does tick-tick-tick it is mostly ok. In standby its in hick-up mode. There usually is a small button on the processor board (close to the flatcable connectors which connect the PSU to the acquisition and processor board). This is an on/off button as well so try to press that and see what happens.

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 17, 2013, 08:55:40 pm
content: Ok I will try.

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 17, 2013, 09:25:55 pm
content: Ok Also whne I push the on board switch nothing happen, There some voltage check point? in the board?

author: nctnico
author_rank: Super Contributor
timestamp: May 17, 2013, 09:38:06 pm
content: You better get the TDS520B component level service manual first:http://www.austexsoftware.com/documents/tektronix/service/070971003%20TDS520B%20Mod%20CM%20Digitizing%20Oscilloscope%20Component%20Service%20Manual.pdfYou can try and disconnect the power to the acquisition board. Still your scope should have leaky capacitors. Could you post some pictures from the boards (with readable IC numbers)?

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 17, 2013, 09:45:59 pm
content: Ok I will post some image of both cpu and acquisition board.

author: Watermelon
author_rank: Contributor
timestamp: May 17, 2013, 09:54:07 pm
content: The ticking sound varying from about 1 to 2 Hz is common on these scopes.

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 17, 2013, 10:35:34 pm
content: So here there is the media fire link the zip that containes all the images of my TDS540.(The image is too big to upload to the site!);link:http://www.mediafire.com/?b3y1xxgguouod1dThere some testpoint in the cpu or acuisition board to check the voltages?

author: nctnico
author_rank: Super Contributor
timestamp: May 17, 2013, 11:18:23 pm
content: It seems someone already had a stab at replacing the capacitors which is why the boards look so clean. However, when soldering these capacitors it is easy to make a solder bridge (=short circuit) to the can of the capacitor. I'd start with checking the capacitors on the processor board and try to power it while the acquisition board and front panel are disconnected from the PSU (and remove the bridge between the acquisition board and the processor board. While you are at it, also disconnect the cable to the CRT unit.

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 17, 2013, 11:31:54 pm
content: Ok I've jsut checke all the capicitor and they are good. The fan not spin also when I disconnect both front panel and acq board.At the end it is a psu failure? There some voltage check point in the boards?

author: nctnico
author_rank: Super Contributor
timestamp: May 18, 2013, 12:27:08 am
content: The voltage checkpoints should be in the service manual.

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 18, 2013, 12:32:37 am
content: Ok I will look to that. Thanks for the helps.

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 18, 2013, 01:05:08 am
content: I've just finish to check the voltage and if I take correctlly, all the voltage are zero!!Securly a psu failure.Any idea in what could be the problem? Bad capicitors? bad transistors?There are Any others common failure of this psu a part of the capicitors?Thanks for the helps.Best regards, Alberto Vaudagna.

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 19, 2013, 06:48:42 pm
content: So just find some component blow on psu. VR4 and VR5 seem overheated, Q9 is sorted and a 200ohm resistor read about 100ohm (low consider the resistor is 5%).I wish it could solve the problem.