Tektronix TDS410 Power Supply - Page 1
Forum section: electronics / repair

author: dan3460
author_rank: Frequent Contributor
timestamp: December 17, 2016, 01:32:02 pm
content: I got this scope a few days ago, after following the troubleshooting guide it pointed out to replacing the power supply. So of course I'm trying to fix it.I can hear a faint click and I get a burst at the base of the transistor that I think provides the switching, I had zoomed in a portion of the burst.Should I shotgun all the lytics? What can I test to get to the problem? Most power supplies that I have worked on a had a schematic to follow and understand the circuit.

author: Arjan Emm
author_rank: Regular Contributor
timestamp: December 17, 2016, 03:25:00 pm
content: This video might be of some help. Mr Carlson repairs a very similar psu. It looks slightly different from yours, might be for the Tds210 or so. I never watched the whole episode. good luck!

author: dan3460
author_rank: Frequent Contributor
timestamp: December 17, 2016, 03:50:01 pm
content: Thanks, I did saw that episode but once he put the supply back together it pretty much worked, so I don't know if the symptoms were about the same on that supply. He did replace all but the two big caps and the two opams on the vertical board.

author: dan3460
author_rank: Frequent Contributor
timestamp: December 17, 2016, 04:14:17 pm
content: I was looking at the board and I saw a discoloration on the A1 board, could something blew up here?

author: Daxxin
author_rank: Regular Contributor
timestamp: December 17, 2016, 05:46:08 pm
content: there is an LeCroy scope with same PSU brand and very similar you can download schematics , repaired it in the past replacing all the caps and there also 1 diode 1n4148 it was shortened

author: dan3460
author_rank: Frequent Contributor
timestamp: December 19, 2016, 12:42:20 pm
content: I downloaded the above schematics and indeed look very similar. I don't know if I want to go ahead with the expense and the time to replace all the caps. I replace two that people replace on similar units but not such luck.I put this thing on my bench yesterday stare at it for 10 minutes and put it back on top of the scope.

author: RadioJoe
author_rank: Newbie
timestamp: January 17, 2017, 09:48:09 pm
content: Hi, I'm a new member. I'm currently working on a couple of Tektronix scopes, TDS420A & TDS460.If you are still interested, I can offer some help with your power supply.

author: dan3460
author_rank: Frequent Contributor
timestamp: January 18, 2017, 12:45:07 pm
content: Sure I'm interested. Let me know what you would like me to do.

author: RadioJoe
author_rank: Newbie
timestamp: January 18, 2017, 06:33:39 pm
content: Hope I can help.First, about my scopes. I have a TDS420Athat powers up, and a TDS460 that wouldn't turn on. The power supply in the 420A looks just like yours, while the 460 has one that looks like the one pictured in Mr. Carlson's lab. Both look different, but are much the same electrically. My hopes were to compare the two supplies.I think your first step should be to test the Green Module. It contains the oscillator that makes everything work.Before we start, I want to mention the shutdown circuitry. Each of the power supplies is being monitored for voltage and current. All are or'ed together along with the temperature and can shutdown the supply when a problem arises. It does this by blocking the oscillator output. Any one of them, can also stop the supply from powering up. When I tried to run my 420A supply on the bench, it either wouldn't come on, or it would pulse intermittently. This is the good supply! I decided that the supply out of circuit, unloaded, could cause a shutdown.I devised a way to disable the shutdown circuitry, so I could troubleshoot the supplies. By jumpering pins 1 & 2 of U1, the optical coupler, the emitter will never turn on and block the shutdown signal.I gathered all the info I could find including schematics of the LeCroy power supplies. I even found some info as to what may be contained in the Green module. I'm sure it has errors, but it looks to be logically correct. I'm attaching a working drawing that I came up with.All measurements on this side (primary side) of the power transformer are in respect to C2 minus. This also connects to pins 1 & 19 of the green module. I tacked a wire on C2(-) as a reference point to make things easier. I also used an LED with limiting resistor to constantly monitor 5V at the edge connector.I'm guessing that you have already checked the B+ voltage at C1 and found it to be @ 330 Volts. If so, you should have voltage available at pins 13 (5V) & 17 (15V) of the green module.Pin 14 of the module is the oscillator test point. It's free running and you should see a ramping signal from 0-10volts. If not, the module may be bad.Pin 10 is a test point for the shutdown signal. In our case it should be high.Pin 18 is the oscillator output that controls the pulses on T2 & T1. This is controlled by the shutdown signal and will be off when a fault occurs. Since we disabled the circuitry its output should be a pulse train that goes from 10V to 40V.I'm also attaching some of my measurements with the power supply working.

author: dan3460
author_rank: Frequent Contributor
timestamp: January 19, 2017, 12:40:24 pm
content: Thanks, I have two project on my bench that I have to finish. As soon i'm done with that I will get into the power supply.

author: RadioJoe
author_rank: Newbie
timestamp: January 19, 2017, 03:52:56 pm
content: Your scope traces look encouraging. Looks like the supply ramps up but goes into shutdown.

author: slbender
author_rank: Regular Contributor
timestamp: July 25, 2018, 03:11:55 am
content: Quote from: RadioJoe on January 17, 2017, 09:48:09 pmHi RadioJoe,Its been some months, I hope you are still around. I just got a TDS-420a yesterday off eBay. I spent some time watching Carlson's videos on the PS and the 420 scope and found something interesting. Mine also has the 4th board for OPTIONS Centronics and RS232 and it appears to have "other stuff" like another power supply or something, anyway it was installed badly by whoever opened it last - shorting one side of that board to a metal brace running down the front to back of the chassis. Seems the CRT isn't lighting up and I suspect that the 6.3 volt supply for the CRT Filament is blown. Set starts but no display on the CRT. Just turns on the fan and the ten front panel LEDs - and then it hangs. The CPU BOARD has a 7 Segment led and it stops at .8 ...Any thoughts or suggestions - it does smell like "bad caps" but I can not see any goop or corrosion or failures.Thanks,Steven

author: james_s
author_rank: Super Contributor
timestamp: July 25, 2018, 07:38:34 pm
content: I have three of these now, two from scopes I have and another defective one that someone kindly gave me. One of mine has a bad hybrid, the other I'm not sure yet what's wrong, the output voltages are significantly higher than they're supposed to be. I started to lay out a replacement for that hybrid based on the LeCroy schematic I found but after looking at xrays of my defective hybrid I noticed a few things didn't look quite right. I can dig out the KiCad files I was working on if anyone would like to continue this effort. I intend to finish it myself one of these days but I have not had time.

author: slbender
author_rank: Regular Contributor
timestamp: July 26, 2018, 06:51:43 pm
content: Hi Radiojoe, and Dan3460 -My TDS-420a also has the identical Power One PS as in Dan3460's picture, with the 6pin opto in one corner and two big input caps on the other, and two vertical heat sinks between the sets of coils... In my set, with the board no longer shorting to ground, still nothing shows up on the CRT, the ten front panel leds light and the set hangs, CPU board shows ".8" ...When first taking the cover off, upon inspection, I noticed that one of the plugin boards, the one closest to the PS (centronics/serial card) was installed with the back of the card shorting to a metal brace running front to rear in the chassis - due to faulty insertion of the plastic retaining clips for those plugin boards.At this time, I have yet to pull out the PS. Can someone post the ratings (uF and voltage) of those five output caps, so I can look and see if I have any replacements for them at the moment? Thanks,Its now almost ten days later, if you have this supply and it has never been removed... There is a "hidden" torx #10 screw besides the four clips. Need to remove the fan as this screw Is behind the transformer and is not easy to get to. I struggled for an hour and even broke a screwdriver trying to lift the board out while this central retaining screw was still in place. Once the screw is out, its simple, four clips and it just lifts up, then go towards the rear, turn outward towards you and its out. It seems there are four voltages 5.1 at 15 amps, +15 at 3.5 amps , 6.4 at 5.5 amps, and -15 at .75 amps.Steven

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 30, 2018, 09:27:56 pm
content: Quote from: james_s on July 25, 2018, 07:38:34 pmWas the schematic looking wrong or the X-ray photos of the defective module?

author: james_s
author_rank: Super Contributor
timestamp: July 30, 2018, 09:41:14 pm
content: I was having trouble correlating the two. It may be that nothing is wrong, it's difficult to tell precisely what is what in the xray.

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 30, 2018, 11:34:48 pm
content: The 8 pin IC is the TL431.Dark rectangles are all ceramic caps, the 2 larger dark blobs with a thin strip through the middle are tantalum caps.3 dark squares with 3 lighter rectangles are obviously SOT23 packages.Then all the resistors are thick film stuff, they show very very faintly on your photo, look in places where tracks appear to dead-end. To make things easier some run under IC's.

author: james_s
author_rank: Super Contributor
timestamp: July 31, 2018, 12:26:48 am
content: Yeah I had figured that out more or less, I forget where I ran into some confusion. Take the xray and try to identify each one of the components and see if everything matches the posted schematic.

author: seronday
author_rank: Regular Contributor
timestamp: July 31, 2018, 12:09:54 pm
content: The attached photo of the inside of the green module may help.It is from another thread on TDS4xx power supply problems.https://www.eevblog.com/forum/repair/tds-460-power-supply-problems/msg759498/#msg759498Regards.

author: james_s
author_rank: Super Contributor
timestamp: July 31, 2018, 03:47:19 pm
content: Ah! Very helpful, thanks!I wonder how they de-potted it so neatly? I made a feeble attempt at digging into one of my dead ones and found the coating to be hard as rock.

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 31, 2018, 06:52:43 pm
content: Quote from: james_s on July 31, 2018, 03:47:19 pmWith much patienceThe biggest problem is to not alter the thick film resistors by digging into them.The only point in anyone going through all this again would be to read some of the markings off the SOT23 devices, and make better (lower) resistance measurements than the ones stated on that schematic.

author: james_s
author_rank: Super Contributor
timestamp: July 31, 2018, 07:17:08 pm
content: Well now that I've seen this I don't need to go through it, although I had considered trying to repair one of the ones I have. Do you have a schematic that correlates to the reference designators on that photo? I'll get back to work on my replacement PCB. If anyone else has already done this let me know, no sense in duplicating something that is already available. My plan was to build one to test the layout and then release the board layout open source once I know it works.