Tek 2431L (2430A) Power Supply Problems  - Page 1
Forum section: electronics / repair

author: TobyLatino
author_rank: Newbie
timestamp: August 28, 2023, 09:58:21 pm
content: I'm attempting to repair a Tek 2431L scope (almost identical internally to 2430A). It is displaying symptoms that have led me to follow the dead start procedure in this service manualhttps://data2.manualslib.com/pdf7/159/15831/1583003-tektronix/2430.pdf?87a103b2d0d33d02ac48fe9682d9795eI have no or very little voltage coming out of the voltage regulators (410). This is my first post here so please excuse any rookie formatting mistakes. I don't know the best way to put images in here, so I'll reference the service manual pages.I have tracked down the primary error to be in the creation of the +10 and -5v reference voltages found on page 410 grid J2. I have a perfect 10.00v coming out of U180 but it drops to 2v after the RC filter. The inputs to U900 are both in that low 2v range. I was thinking that this was indicative of a bad op amp, but I replace the chip (LM308) with a TL070CP and the issue was better (less of a drop) but still incorrect and some component started heating up horribly (Q279, Q479, Q779, Q879) and the overcurrent shutdown kicked in, so the replacement was either not adequate or there is a bigger issue downstream.I have an ESR meter on order, but other than that, I was wondering if anyone could point me in a direction to look? I'm lost.Thank you so much in advance, and if there's any other info I can give, please let me know.EDIT: Attached Images

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 28, 2023, 11:12:45 pm
content: If you took a screenshot of the schematic section and a picture of that section of the board it would make it easier for us to debug it.To add a picture just add it as an attachment (it’s under the text box)

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 29, 2023, 12:19:14 am
content: Most of the time its the electrolytic capacitors in the power supply and HV section.All of them except the 2 large ones and replace the safety caps, 4 of them.If it still has a problem fireing up, check the MOSFETs even if you checked thembefore these are the most common problems in all of the 2400 series scopes.Jeff

author: TobyLatino
author_rank: Newbie
timestamp: August 29, 2023, 02:49:11 am
content: Thanks, images attached

author: TobyLatino
author_rank: Newbie
timestamp: August 29, 2023, 02:50:22 am
content: Thank you, I attached some images of the schematics, but I will definitely be recapping. I wanted to wait on the ESR meter so I could try to track down what exactly happened. Experience much appreciated

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 29, 2023, 03:30:06 am
content: First check if the 10V ref rail is shorted to ground with a multimeter.I suspect the 1uF capacitor (C900). Sometimes capacitors get leaky. If the 1uF capacitor is leaking significant amounts of current, it could drag the output of the RC filter down (especially since R is quite large). I've seen cases where capacitors test good with a multimeter, but leak when voltage is applied to them.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 30, 2023, 11:23:57 am
content: The 2431L was a cheaper version of the 2432A being a 300MHz not a 150MHz scope.The selling price was less than the 2430A.Jeff