Tektronix TDS 640A Power up stall - Page 1
Forum section: electronics / repair

author: pete99
author_rank: Contributor
timestamp: December 07, 2018, 08:44:28 pm
content: Hello,I recently purchased a used Tektronix 640A 'scope. It passed the power up diagnostic tests and worked fine for a couple of weeks. Now when I push the ON button, all the lamps light up, the CRT flashes briefly, the lamps stay on forever, the CRT is blank, and nothing else happens. I sometimes have to push the ON button a couple times to get something to happen.Any suggestions?Thanks in advance,Steve

author: nctnico
author_rank: Super Contributor
timestamp: December 07, 2018, 10:44:28 pm
content: Did you replace the capacitors yet?

author: pete99
author_rank: Contributor
timestamp: December 07, 2018, 11:09:39 pm
content: I inspected the boards and didn't see any obvious signs of leakage around the capacitors. So no, haven't replaced the caps yet.Thanks,Steve

author: chrisl
author_rank: Regular Contributor
timestamp: December 08, 2018, 06:16:51 am
content: Sounds like it is the time to replace those caps before they leak and make things worse.I wish I changed the caps when it stated acting funky...I waited too long on mine and it is a nightmare to reverse the damage done to the board....

author: pete99
author_rank: Contributor
timestamp: December 12, 2018, 07:44:56 am
content: I usually shop at DigiKey, where can I find the part numbers for the 2 values?And are the same type of caps used as replacements or is there something better?Thanks y'all for the suggestions.Steve

author: Fiveten
author_rank: Newbie
timestamp: December 25, 2018, 01:40:54 am
content: Just search for caps with the same specs. You should be able to refine your for size. Higher voltage is ok.

author: madao
author_rank: Frequent Contributor
timestamp: December 28, 2018, 06:15:24 am
content: TDS640A is a bit later modell: possible have Tek no problems with capacitor. I have one Tek TDS520, which their CPU Board has orginal capacitor, but no fault. Why? Because, this CPU Board is a later reversion (1993, maybe a replacement for repair)A check: Soldering on capacitor, if it smells very bad (urine), this is a alarm signs.If yes, someone trace is etched trought. Find of this fault, which Tek doesn't make complete startup. It is very bad !One tipp: Doesn't power up it anymore and desoldering Dallas NVSRAM and save their data.I have a bad experice with halfdead cpu board: It can make NVSRAM Data corrupt.That you can read in dallas NVSRAM, what have Tek a problems. I'll writing into NVSRAM, even if it doesn't make a sign on CRT.TDS640 ACQ Board Schematic exists. But not one for CPU Board.http://w140.com/tekwiki/wiki/TDS640

author: Armadillo
author_rank: Super Contributor
timestamp: December 28, 2018, 08:04:46 am
content: Connect external monitor to the VGA to get more information of the startup error.And see inside one row of LEDs, the indication of the startup.That's [if I can remember correctly].

author: Dacke
author_rank: Regular Contributor
timestamp: December 30, 2018, 07:31:57 pm
content: My TDS 420A was doing similar until it stopped working altogether, I assume parts of it may be similar to the 640A. They're right about recapping. Virtually every surface mount electrolytic on the acquisition board had very high ESR and some were leaking. Same was true for the power supply, there were maybe a dozen electrolytics there as well and three were leaking and started to eat into the board. I just went ahead and broke down the entire scope and replaced them all, as well as the two lithium batteries. It's a bit of work, but the scope has been fine since.