Repair of a Tektronix 485 - Page 1
Forum section: electronics / repair

author: Ranayna
author_rank: Super Contributor
timestamp: February 14, 2018, 02:25:03 pm
content: Hi everyone,A couple of weeks ago, I started browsing Ebay for old Testgear. I immediately fell in love with some of the old Tektronix Equipment. Just for the looks of some of the older stuff I needed to have some of them.Long story short, I got myself an old Tek 485, a broken one though. It has not arrived yet, so I can just provide the information I have from the seller: No beam, Scope was still working when it was put in storage. From the pictures it looks in very good shape, so I assume dry storage conditions.I already have acquired and printed the Service Manual.The first thing I will do when I get the scope, is tearing it down, I will not try to power it up at first. Things I will look for are leaking caps, corrosion in general, loose connections and broken components.What are some general tips you experienced folks can give me? For example, how can I make sure that the tube is still ok? What should I definitely not do as someone who is not experienced with high voltage stuff?Tools I have available:- Rigol 1054Z- BM235 EEVBlog meter- Uni-T 61D- Crappy function generator- One of those 20$ component testers- Soldering station- Desoldering stationThis will very much be a learning experience for me. And even if I do not get the scope to work, in my opinion it would also just be a nice decoration pieceGreetings,Ranayna

author: tggzzz
author_rank: Super Contributor
timestamp: February 14, 2018, 03:02:18 pm
content: Quote from: Ranayna on February 14, 2018, 02:25:03 pmNot much. The usual strategy is to assume the CRT is good, and make sure that all signals going to it are correct. Once they all are, then you guess the tube is faulty.Your strategy will depend on what signs of life you do and don't see and hear when you turn it on. Follow the faultfinding guides in the manual (available on the web). With luck it will be a problem with the low voltage supply, classically decoupling tants which are short circuit, or electrolytics that have dried up. Be aware there is a "hidden" 13V PSU rail with several 15V tants on it, doh! See R967, and findallthe tants connected to it.If you are less lucky there will be an HT or inverter problem; they are a pig to debug, partly because some components are buried right in the middle of the scope.If you get it working, it is a delightful scope, and the one normally resident on my bench.

author: MaxFrister
author_rank: Regular Contributor
timestamp: February 15, 2018, 03:51:03 pm
content: First check that the fuse is the correct one. People have a tendency to keep upping the fuse size once they see smoke. BTW, if it smells burnt do not turn it on.I usually start by assuming that the crt is okay but the power supplies are bad.You can do a tremendous amount of debugging before you even open the case. Tek has an application note called "Trouble shooting your oscilloscope: getting down to basics" that has lots of info.High quality scans of old tek scope manuals are available (online or Artek). There used to be a forum on yahoo dedicated to old tek gear but I think Yahoo is kaput.Once you have it in your hands and can describe the symptoms I'm sure people can help more.

author: perdrix
author_rank: Frequent Contributor
timestamp: February 15, 2018, 04:27:39 pm
content: tekscopes has moved to groups.io. Yahoo! isn't quite dead yet, even if if rigor appears to have set in ...Do get the manual (from e.g.: ArtekManuals) and also if you can beg/borrow another scope on loan as you often need another 'scope to help fix one.READ that manual, it is *full* of information on how the 'scope works.Dave

author: Ranayna
author_rank: Super Contributor
timestamp: February 16, 2018, 02:17:43 pm
content: So, the scope has arrived. I am not disappointed, it looks to be in very good optical shape. I will upload pictures over the weekend. I did not yet have much time to do anything with it though, except playing around with all those knobs, levers and buttons. Very therapeuticOne problem now occurred to me... while the 485 is not so big as I feared, it still is quite a large thing, especially compared to the 1054Z. I have to do a major bench overhaul, just to be able to properly service it.How ESD sensitive is the scope? The seller used broken styrofoam pieces as packing material. Small fragments of that stuff went everywhere, and i'm sure some got into the device as well.

author: PA4TIM
author_rank: Super Contributor
timestamp: February 16, 2018, 03:00:16 pm
content: Quote from: Ranayna on February 16, 2018, 02:17:43 pmNot, so don't worryFirst thing to do with old Teks is pull every transistor a few times in/out their sockets.As it was long not used (few years) then be careful with the PSU caps. Reform them. Check also Tanalums.Then check psu for the right voltage and ripple. You need a HV probe.Do not start aligning or adjusting things as long as you do not have the gear to do that, it is easy to cripple it but it can be a hell of a job to get it good after someone messed it up. You needs tuns of gear for that and the calibration part of the manual shall be somewhere between 50 and 100 pages and on every page there are many steps.

author: mr.fabe
author_rank: Regular Contributor
timestamp: February 16, 2018, 03:12:09 pm
content: Quote from: PA4TIM on February 16, 2018, 03:00:16 pmIs there a recommended way of checking the tantalum capacitors or do you check it the same way as the electrolytics?Sent from my XT1575 using Tapatalk

author: PA4TIM
author_rank: Super Contributor
timestamp: February 16, 2018, 03:43:50 pm
content: Tantalums last very long and have a very low dissipation factor (= related to ESR) but they do not survive overvoltage. That includes transients. That is why they fail a lot in the early smps's. Do not test them on leakage above the rated voltage.They short if they fail. If you are lucky you can follow the smoke and sound. If you have bad luck you need to find the shorted one by measuring resistance, current or temperature.I made something for that although I did not need it. It was just for fun.http://www.pa4tim.nl/?p=5339Most times they are easy to find if you have the schematics. If a rail is down measure the resistance, is it very low then look for the location of the caps on that rail. Desolder them and if it is short you found it. It is not necessary to replace them all if one it dead. But if on that tail there are more tantalums I would replace those.If you have good desoldering gear , you can measure them out of circuit. If the DF or ESR is substantially higher it will die sooner or later. If they measure good there is no worry. They most times do not fail from age. The bright coloured drop-shaped dry ones, there are also wet tantaliums in the aluminium colored housings. I have seen one failing in a NOS probe amplifier psu just autonome without a voltage on it. (http://schneiderelectronicsrepair.nl/?p=303)

author: Ranayna
author_rank: Super Contributor
timestamp: February 24, 2018, 12:14:50 pm
content: Ah, time, the eternal enemy of all hobby projectsI finally got around to rearraning my bench and taking a closer look at the 'scope. I also took a couple of pictures, but most turned out not so good.All in all, the device looks in very good shape, internally and externally. The only apparent damage that i found is that one of the back feet has been cracked. The lip pressing against the back panel is completly gone, and the hit was strong enough to even bend one of the screws. Also the retaining ring is dented directly beneath the foot, as if someone used exessive force when tightening the screws of the foot.On the inside, everything looks very clean. I see no signs of dust at all. Many transistor and ic legs are quite tarnished though, so reseating them might be called for.I also have some translucent deposits on some of the plastic headers. I assume it's the same stuff as in this thread:https://www.eevblog.com/forum/repair/tektronix-485-what's-this-encrustation/In my 'scope it is not as prominent, it's more translucent.The fuse seems to be the original proper one. It does not have traditional modern markings: One end has just 250 VOLT, the other BUSSAGC3, Google tells me that's a fast 3A Fuse, so the correct one.I have uploaded the pictures to imgur:https://imgur.com/a/O0Kb3Also thank you very much MaxFrister for pointing out the Troubleshoot your Oscilloscope document. That one is goldI think i will try powering it up and i will follow that guide at first.I will keep you all posted, this time hopefully with not such a big delay.Greetings,Ranayna

author: mr.fabe
author_rank: Regular Contributor
timestamp: February 24, 2018, 12:21:50 pm
content: Quote from: Ranayna on February 24, 2018, 12:14:50 pmLooks like it's in nice condition. Great score. For your first boot, try to bring it up slowly with a variac if it hasn't been powered on in a while...Sent from my XT1575 using Tapatalk

author: tggzzz
author_rank: Super Contributor
timestamp: February 24, 2018, 12:25:18 pm
content: Quote from: mr.fabe on February 24, 2018, 12:21:50 pmDon'tuse a variac.Tek 485s contain a switching power supply that will attempt to deliver the correct output voltages and thereby the correct output powers. If you reduce the input voltage, you will increase the input current - simple power conservation. Increased input current could damage the SMPS, and that's a pig to debug/repair.

author: mr.fabe
author_rank: Regular Contributor
timestamp: February 24, 2018, 12:29:29 pm
content: Quote from: tggzzz on February 24, 2018, 12:25:18 pmOh.. sorry about the bad info... I was told to do that with my old 485 to allow the caps to reform. I guess I was lucky not to cause other issues with mine. I appreciate the correction!Sent from my XT1575 using Tapatalk

author: tggzzz
author_rank: Super Contributor
timestamp: February 24, 2018, 12:43:27 pm
content: Quote from: mr.fabe on February 24, 2018, 12:29:29 pmYou're welcome.The first 485 that I resurrected had a cap that needed "reforming". Once reformed it stayed good(ish) for 12 hours, and then needed reforming again. I replaced all the "similar" caps. For that and other reasons, I'm not a fan of reforming caps.The symptom was that it would take 60s to spring into life, and was good thereafter. Only being able to attempt two transient debugging sessions per day was slow.(Sent from my keyboard and mouse without WackAWhine.)

author: Ray Gianelli
author_rank: Contributor
timestamp: February 24, 2018, 04:52:19 pm
content: These are classic scopes of that era. I'd start checking the power supplies to see if all the voltages are correct. IIRC the +50 volt regulated output provides a reference for some of the other supplies, so it that goes south you'll lose other voltages as well.I was seeing filter cap failure on these (and the 465/475 series) 25 years ago. They haven't gotten better with age.

author: Ranayna
author_rank: Super Contributor
timestamp: February 24, 2018, 08:48:43 pm
content: So, I powered up. No magic smoke escapedAfter a couple of seconds, I got a line. After a couple of more seconds though, it disappeared againAny tweaking and twiddling about did not get it back. There is one exeption though: When i set the scope to X-Y mode, I get a dot. As soon as I set it to anything else, I see that dot flittig to the left, and it's gone.The next step in the Troubleshooting Guide now tells me to input -10 volts to the Z-Axis Input at the rear of the scope. I see the BNC, but i am unsure about the polarity though. Can I just use my bench supply and put positive to the outer connector and negative to the inner?

author: tggzzz
author_rank: Super Contributor
timestamp: February 24, 2018, 10:36:29 pm
content: Quote from: Ranayna on February 24, 2018, 08:48:43 pmNO! CHECK THE POWER SUPPLIES for voltage and ripple.Note that there is a "hidden" +13V supply (diagram 17 R967). If you trace that through to the other diagrams (very manual), then you will see several 15V tants on that 13V line. I had one tant short short circuit, and destroy R965.

author: anachrocomputer
author_rank: Regular Contributor
timestamp: February 24, 2018, 11:33:13 pm
content: The behaviour of the spot when switching out of X/Y mode sounds very much like a power supply problem. Check all the voltages, including the "hidden " +13V one that tggzzz mentioned, and let us know. I suspect that a tantalum capacitor has gone short and taken out a supply rail.

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 25, 2018, 09:32:13 pm
content: Having repaired quite a number of the 4xx scopes, I will third (or fourth) the suggestion to check the power supplies to be in spec before doing ANYTHING else. Power supply problems can cause all manner of weird symptoms on these scopes and you'll be chasing ghosts unless you verify they are working correctly. They need to be in spec for voltage AND ripple.The trouble shooting procedures were not written for a 30 year old instrument and shorted tantalums and bad power supplies are a major failure mode. The good news is that these are pretty easy fixes.

author: Ranayna
author_rank: Super Contributor
timestamp: March 03, 2018, 01:03:10 pm
content: Quote from: tggzzz on February 24, 2018, 10:36:29 pmWow, you are spot on here. I first measured all documented testpoints on the power supply board, and they were more or less fine, if a bit low. I then looked for the elusive R967, which I have to admit it took me quite some time. And lo and behold, next to it, R965 is busted. The laquer is gone in the middle, it shows visible signs of charring. It is even more visible now than on the first pictures. So, the hunt for tantalums is on now.Would i have to expect more broken things except the tantalums and that resistor?

author: tggzzz
author_rank: Super Contributor
timestamp: March 03, 2018, 01:15:00 pm
content: Quote from: Ranayna on March 03, 2018, 01:03:10 pmI am omniscient; I just wish I could have persuaded my daughter that was the caseMake sure you find c861 c911 c965 c2101 (possibly c844) and ensure they are at least 25V (there may be others). The specific capacitance values aren't critical.