Tektronix 465B - Won't turn on... - Page 1
Forum section: electronics / repair

author: DkinG
author_rank: Contributor
timestamp: December 05, 2014, 08:14:29 pm
content: Hello!!... My name is David, and as you can see i'm a Chilean, so i'll do my best effort to be clear explaining the problem.One year ago, i bought a Tektronix 465B, it was working very good until someday won't turn on... so i started to see what happen.The first thing i saw was the fuses... the primary fuse has blown, so i thought that the issue could be something from the power suply.i started to measure all semiconductors, they are ok.in another forum, users told me to change the capacitors, because they are the first problems, when i did it, it still blowing the fuse.So i think you can give me a hint on whats happening here....If someone wants to write in spanish, i suggest to write in both languages so other people can understand and stay in tune.Thank you, I hope you can understand my english.Greatings!!

author: tautech
author_rank: Super Contributor
timestamp: December 05, 2014, 08:48:43 pm
content: Welcome to the forum.Get the Tek scope trobleshooting pdf from the first post in the Repair board.

author: JoeO
author_rank: Frequent Contributor
timestamp: December 05, 2014, 08:53:22 pm
content: http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdfHere is the link

author: PaulAm
author_rank: Frequent Contributor
timestamp: December 06, 2014, 02:21:06 am
content: The filter caps fail open, for the most part, so that was not a likely problem.The bridge rectifiers can short though, so you should check those.If those are OK, one of the power supplies may have shorted to ground. Check the resistances from the low voltage test points on the main board to ground (with the scope off, of course) to see if you can isolate which supply has the problem. Once you find out which is the cause, you can start tracking down the problem. The problem might be in the power supply itself, or further down the line. There are a few bypass caps around, one of those may have shorted. I think there are a few tantalums as well and those are a possilbility. You may have to end up taking things apart a bit in order to isolate the problem to a board.It's relatively easy to track down a shorted part if you have a milliohm meter once you have the general area located.It shouldn't be too hard to track down. I'll take a completely dead unit over one with an intermittent fault any day.

author: DkinG
author_rank: Contributor
timestamp: December 06, 2014, 02:53:08 am
content: Hello and Thanks for the answers...I've been reading the Troubleshooting manual From Tektronix and i think its interesting.I made all things that have been explained in the book as posible, i don't have much equipment to do the test, so it's a little hard for me to make troubleshooting.I think one of this days i'll check the resistances from the low voltage test points. (The scope is always Off due to the blown fuse of the primary transformer).So i'll be keeping you updated about this.Thank you all.. I apreciate your answers.David.

author: tautech
author_rank: Super Contributor
timestamp: December 06, 2014, 03:54:28 am
content: Quote from: DkinG on December 06, 2014, 02:53:08 amDavidDo you have the Service manual?If you make a dim bulb tester and use say a 60W bulb you will stop blowing fuses.Google dim bulb tester.If you disconnect the transformer secondaries and power on, what happens?

author: DkinG
author_rank: Contributor
timestamp: December 06, 2014, 04:45:17 am
content: Quote from: tautech on December 06, 2014, 03:54:28 amtautech:Hello!... Yes, i have the service manual and a lot of documentation of this scope.I forgot to say that i've been doing the dim bulb (with a 50 watt bulb) tester and i noticed that the light is fully Brilliant (without disconnecting the secondary of the transformer)...Also i forgot to try it with the secondary disconnected... Maybe i could try that soon...Thank you tautech for your answer, i would apreciate that you could tell me what to do in the different cases when i try the dim bulb.Greatings!

author: tautech
author_rank: Super Contributor
timestamp: December 06, 2014, 05:31:14 am
content: DavidWhat you are doing with the dim buld tester is perfect.Shining bright demonstrates there is excessive current being drawn.I suggest you use the tester on various other appliances to get a "feel" for what to expect.SMPS will show a different brightness on power on, usually a good healthy flash as the Caps charge then dim.A transformer type supply will show both the magnetising current and charging of the caps then dim.Transformer alone will be less of course.Always look at the DUT power label for the normal power consumption and select a suitable bulb.Like don't use a 200 W bulb for a 30 W DUT.

author: DkinG
author_rank: Contributor
timestamp: December 06, 2014, 05:55:40 am
content: Quote from: tautech on December 06, 2014, 05:31:14 amI don't understand what do you mean with "There is excessive current drawn"... You mean that the voltage isn't dropping on the primary of the transformer?... in that case what should i do?...Greatings and thanks for your help.

author: tautech
author_rank: Super Contributor
timestamp: December 06, 2014, 06:28:48 am
content: Quote from: DkinG on December 06, 2014, 05:55:40 amQuoteIf the bulb shines brightly the DUT would draw excessive current without the bulb in series.Thats why the fuses kept blowing.Use the tester on other appliances and see.

author: DkinG
author_rank: Contributor
timestamp: December 06, 2014, 08:48:58 am
content: Ok... ill try to do that... Thanks for your answer...I'll stay in tune.Greatings!!PD: Sorry if i don't write as well as you do... I don't usually write in english.David Mix

author: Marc M.
author_rank: Regular Contributor
timestamp: December 07, 2014, 01:07:53 pm
content: Chances are you have 1 or more bad tantalum capacitors downstream of the power supplies. 1st step is to check voltage on all the rails to find out which one(s) are bad. IIRC, the 465's were pretty modular making it relatively easy to isolate them. I usually just pull individual boards and do a simple resistance measurement between the affected rails and ground. Any boards that show low resistance are then further scrutinized with more resistance measurements of individual tantalum caps. Any caps that show low resistance are then pulled, tested, and replaced if needed. I've repaired a lot of Tek scopes from the 70's and 80's that suffered from failed tantalum caps - it's a very common issue. One good thing is that Tek designed the power supplies to shut down in case of trouble (the infamous won't turn on but it 'ticks' problem) and I haven't run across a p/s that Chernobyled due to shorted rails.

author: DkinG
author_rank: Contributor
timestamp: December 08, 2014, 01:24:31 am
content: So i'll check the tantallum caps...then i'll tell you how is it going on... First i have to make space here... i don't have any bench to work with...Greatings and thanks for your Answer...Edit: Now at last, I've bought a good Fluke 87 Series III so i can make a good check of most things you posted. Thanks and I hope i can find the problem...Cheers!