TDS 540A Oscilloscope PSU Repair - Page 1
Forum section: electronics / testgear

author: reidkersey
author_rank: Newbie
timestamp: August 22, 2017, 08:07:19 pm
content: I just bought a non-working Tektronics TDS540A (My very first O-Scope!).When I got it, it would not power on. Once the switch was turned on it would make a high pitched squeal, not disimliar to a bad capacitor, and it would squeal in ~10 second increments. When the soft power button was pressed, nothing would happen.I took the PSU out, and found a problem. A nice little burn spot. However I am having trouble diagnosing it beyond this point without any schemeatics. Does anyone know what would cause the burn spot, and where I could obtain some schematics?Thanks!

author: steven4601
author_rank: Contributor
timestamp: August 22, 2017, 08:32:03 pm
content: The circuit apears to be part of the primary side . Two zener diodes clamping the primary mosfet. Could you figure out if this is part of a flyback circuit?The extreme dark color could be due to age & being hot for a long time, but also due to a very recent failure. This being your first scope, You already have a DMM to perform basic trouble shooting?

author: hendorog
author_rank: Super Contributor
timestamp: August 22, 2017, 08:50:35 pm
content: That long trace heading upwards in the bottom image also looks discoloured compared to the rest of the board.I'm no expert at this, but I'd trace that and see where it goes and test the components at both ends of it.

author: reidkersey
author_rank: Newbie
timestamp: August 22, 2017, 09:11:21 pm
content: The long trace is discolored. It goes to a transformer, and a 200K resistor (reading 168K). The diodes seem to be functioning fine as far as I can tell.I do have a DMM, prety much a full basic lab with the exception of an O-Scope. If needed I can borrow one from work for a day or two.I dont know how to tell if it is part of a flyback circuit. It kind of looks like some of the flyback diagrams online, just with 2 diodes.

author: steven4601
author_rank: Contributor
timestamp: August 22, 2017, 09:21:08 pm
content: There are many things you can try. But id recommend this for a first start.- figure out which part of the supply this powers, crt / analog / processor- you already checked those darkned/burned zener / clamp diodes? They probably are rated at 100 volts each.- check the diodes on the output / secondary side of the flyback transformer.- inject known correct voltage into the rails, see if a rail draws excessive amount of current / shorted / difficult to get to a known valid voltage. (don't do this if the voltage is nowhere listed on the board) This also should not be attempted for the HV part of a crt.

author: reidkersey
author_rank: Newbie
timestamp: August 22, 2017, 10:25:10 pm
content: I am replacing all components in the darkened area. They all look okay, and test the most basic of tests, but since they are so cheap, why not.I cant inject known voltage, because none is specified.I dont see where the output side is. I lose the traces under a large inductor, and next to a transformer.If anyone knows where to find the schematic, I know they are out there somewhere because other people have found them, that would be a great help!

author: JFJ
author_rank: Regular Contributor
timestamp: August 22, 2017, 10:37:14 pm
content: Quote from: reidkersey on August 22, 2017, 10:25:10 pmTheTektronix TDS520B Component Service Manualhas power supply schematics (starting from page 5-138) that may be sufficiently close to your power supply, to be useful.

author: tautech
author_rank: Super Contributor
timestamp: August 22, 2017, 10:50:02 pm
content: Do lots of reading and study before floundering around in equipment like this.Bits in this might be of help:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 22, 2017, 11:38:55 pm
content: Hi,There are two versions of the bias supply in the TDS5xx and TDS7xx.Here they are:Q9 is a BU508A. VR4 1.5KE220CA and VR5 1.5KE250CA The 'C' indicates bi-directional.I would be tempted to change as many parts as possible. It is a pain to the PSU in and out of the scope.This is the later version:The first thing to do is determine which version you. I would then pull all the semiconductors and test out of circuit.Regards,Jay_Diddy_B

author: reidkersey
author_rank: Newbie
timestamp: August 25, 2017, 06:21:43 pm
content: Jay_Diddy_B Thank you so much for your help! Is there any way that I can get the whole version of the top schematic? I need to do a bit more trouble shooting.I replaced Q9, VR4, VR5, R25 & R29, and the squealing stopped, but the scope still does not power on. I measured C12 at 160Vdc, and got various wrong readings off of T3.THANKS!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 25, 2017, 11:07:15 pm
content: Hi,The image is from a manual set that was scanned by Artek. You can buy the manual set for the TDS520 from this link:http://artekmanuals.com/manuals/tektronix-manuals/Search for TDSThe TDS520 will be very close to your TDS540A.There is very little component level information on these scopes and this is one of the best resources.Regards,Jay_Diddy_B

author: chronos42
author_rank: Regular Contributor
timestamp: August 26, 2017, 01:10:32 pm
content: Hi,If you have the early version of this PSU then check C17 for high ESR. Replace it with a low ESR cap and very high quality part.