Tektronix 466 with magic smoke escape issue - Page 1
Forum section: electronics / projects

author: nakchak
author_rank: Contributor
timestamp: April 22, 2013, 01:21:35 pm
content: Hi,Hoping that someone out there more knowledgeable than I may be able to help me fix my trusty old tek 466. I bought it off ebay 10+ years ago for not very much at all (IIRC about £35) so if its un-salvageable i guess i have more than got my moneys worth and will probably sell for parts.A few months ago i was debugging a circuit, I had left the scope on for a couple of hours when suddenly the traces got very noisy, followed by some clicking sounds then a re-enactment of the start of the outer limits (do net adjust your set etc.) on the screen. Thinking it may have over heated or something like that i left it an hour or so to cool down and then turned it back on again. This was when the magic smoke escaped and the main fuse popped. Now i know i must have some issue on the time base or y amp side of things (probably some dead caps from what i have researched) but the most pressing issue is the PSU now. Having taken the lid off i started looking for where the smoke came from and one of the rectifiers has clearly blown (big crack in the package and smoke stains on the pcb), i can get a suitable replacement from farnell/rs easily enough, but i am hesitant to power it on again once fixed in case i cause further damage, AFAIK its one of the low voltage circuits +15v i think, but my concern is what could have caused the rectifier to explode.Ideally i would like to fix it my self, but incase im biting off more than i can chew, does any one know of a place in the UK that i could have it repaired at? (preferably in the south west)CheersNak.

author: PaulAm
author_rank: Frequent Contributor
timestamp: April 22, 2013, 02:00:00 pm
content: Common failure modes for these scopes are the bridge rectifiers and filter caps. If you have an esr meter, check the filter caps - they tend to fail open and result in huge amounts of ripple on the power rails; that makes everything act wierd.Otherwise, just check the resistance between the power test points and ground, you should not see 0 ohms anywhere, if you do you still have a short somewhere.If everything looks good, power it up and, if you don't have another scope, check the ripple on the power rails with a dvm on an ac range, maybe run though a blocking cap. It should be in the mv range, but the specs are in the service manual. Check ALL of the power supply voltages; they all need to be in spec.Tek scopes are pretty solid, it's fairly rare that a power supply failure will take out something else.If you don't have a service manual, you should be able to download one from the usual sites. There's a manual at the Tekwiki, but I don't know if it's just the operating manual or the service manual.

author: nakchak
author_rank: Contributor
timestamp: April 22, 2013, 05:27:43 pm
content: Thanks for the reply PaulAmI dont have an ESR meter (yet..., been on my to get list for a while), i assume the dirt cheap from hong kong boards are not worth bothering with, from what i have seen the "russian go" or one of the Bob Parker designs would be the way to go?I have both an electronic and original (gotta love the huge fold out schematics) service manual, and have calibrated the unit in the past so that should be fun to work through again.I will meter through the PSU and report back, hopefully i can bring it back to life.Luckily i bought another scope on ebay last year which should make life easier...nak.

author: qno
author_rank: Frequent Contributor
timestamp: April 22, 2013, 07:10:51 pm
content: the tantalum capactors are prone to fail to a short.

author: KJDS
author_rank: Super Contributor
timestamp: April 22, 2013, 07:26:17 pm
content: Quote from: qno on April 22, 2013, 07:10:51 pmThat's true, and after a while they can go from short to open, so if you just look for a short you could miss a broken cap. They do usually show signs of discoloration when this occurs.

author: nakchak
author_rank: Contributor
timestamp: April 22, 2013, 08:03:27 pm
content: Thanks for the pointers guys.Guess i will be looking for knackered tantalum's once i fix the psuNak.

author: JoeyP
author_rank: Frequent Contributor
timestamp: April 23, 2013, 03:59:54 am
content: Quote from: KJDS on April 22, 2013, 07:26:17 pmAnd there is often a low value series resistor at each cap, which tends to burn when the cap shorts. So if you find shorted caps, look for open resistors as well.