No trace on Tek 475A

Discussion in 'General Electronics Chat' started by spinnaker, Nov 6, 2010.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I bought a Tek 475A off of Ebay about a year ago. I haven't had much need to use it but it worked fine the few times I have used it. Pulled it out this morning to work on a little DIY generator (from a floppy disk drive) I was working on. Probably should have waited to buy.


    It worked fine this morning. I accidentally left it on and ran out for some errands. When I returned there was no trace.

    Everything I try I cannot seem to get a trace. It powers on. The intensity is at max. I tried adjusting the Horiz and Vert position. The beam finder does not show the beam. I do notice a quick display (more of a dot) in the center of the screen as I turn on the scope.

    I tried following through the manual and seem to have covered everything but I am not very confident in the operation.

    Any other ideas of what to check?

    I don't have another scope to fix it and my analog skill level is not exactly top notch. Do I now own a boat anchor?
     
    Last edited: Nov 6, 2010
  2. tom66

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    May 9, 2009
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    I'd do some simple checks first.

    Perhaps try to open the scope up and look for burnt components.

    For example, here is the case of a Tek 2445A oscilloscope which has no focus on the display:

    http://www.cromwell-intl.com/radio/tek2445a.html

    In this case, it was a single resistor in the focus amp that had failed. Replacing it with two 680k restored the scope to working order.
     
  3. spinnaker

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    I'm thinking it is heat related. I am trying to keep my hands off of it dor a few hours to cool.


    I should at least be able to see a beam when I hit beam find correct?

    I have not used the scope much and the last time I used one to any great extent was about 30 years ago so I am sure I am rusty. I was worried that it just might be operator error.
     
  4. tom66

    Senior Member

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    Possibly the EHT supply has failed. That's usually a potted module and difficult to repair, but you might be able to get one on the 'net.

    Beam find will not work if the scope is faulty.

    When my scope went faulty, a can of compressed air and 5 minutes fixed it. Try that as well.
     
  5. jpanhalt

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    I had an e-bay TEK 2235 (AN488) scope fail because the resistor to ground on the input was open/intermittent. That resulted in a very simple fix and a very nice 100MHZ scope. Don't despair.

    It could be serious, but look for simple things first.

    John

    Edit: I should add that I bought 3 of them on e-bay. One worked from the getgo with a few bad switch contacts, one (as mentioned ) was fixed and is the best of the lot, and one was chopped up for parts, including a good CRT. I'm happy with the result, but you have to be prepared for garbage.
     
    Last edited: Nov 6, 2010
  6. spinnaker

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    I sort of realized that. I guess my question is other than intensity being too low could operator error cause the beam find not to work?

    I just fired it up again after letting it sit a few hours so the problem is probably not heat related. This really sucks. :mad:
     
  7. bertus

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  8. spinnaker

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    Thanks. As I said in my post I already looked at the manuals. I popped the cover and had a look around. I did not see anything blown out.

    Of course that was just what I can see.

    I can't imagine trying to fix it. Those things are packed with components.
    Just taking it apart and getting it back together would be a challenge

    I will probably just end up selling it and hope I at least get a few bucks for it. And learn a sad lesson. ")
    .
     
  9. gootee

    Senior Member

    Apr 24, 2007
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    Don't give up too easily. Sometimes you get lucky. Did you try both channels? If you have a DMM there is a lot you can do. I'd first check all of the power supply voltages and make sure that they're within the tolerances given in the service manual. If those are all OK, then look at the troubleshooting section and see what looks relevant. Study the schematics a little bit and find the sections where the vertical deflection inputs are near the CRT, and work backward from there. Maybe the beam is just deflected way off the face of the CRT. You should also be able to assume that the vertical circuits for the two separate channels are all OK and concentrate on what's between them and the CRT deflection inputs. The same situation could be true for the horizontal deflection circuits. But you could probably similarly assume that all of the complicated timing stuff is OK and that it's somewhere relatively close to the CRT end of things. Maybe find the last low-voltage parts of the deflection-signal chains and measure the DC voltages, there, and see if they vary according to the position knobs as they should. That type of testing should give you some confidence and narrow things down a lot. If you're lucky, it will be a problem in a low-voltage section. Looking for the effects of the position knobs on the DC voltages can be an easy way to find certain problems.

    As mentioned, the "focus chain" resistors are a prime failure mode in a lot of scopes. They are a series chain of high-value resistors in the focus section. Measure their resistances (with power disconnected). You might get lucky.

    Remember to be SAFE. Keep one hand in your pocket, at least. And watch out for the high-voltage sections, even just so you don't blow up your meter.

    You should also ask for help in the TekScopes group, at yahoogroups.com, and in the sci.electronics.repair group at http://groups.google.com. Some of those guys already know what the top ten most-likely problems are, and might lead you to a very easy fix. They will certainly give you the least-painful way forward, every step of the way.

    At least give it a try or two.

    Cheers,

    Tom
     
  10. spinnaker

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    Thanks for the support. You and the others have inspired me to take another look.

    One think I noticed that the focus control binds a bit. Is this normal. I pop the lid on the HV section (with power off) and tested the resistance in circuit it appears to operate OK.

    I might try that group you provided after maybe taking a few voltage measurements.
     
  11. PRS

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    Aug 24, 2008
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    You won't see the beam finder if the caps in the high voltage section blew.
     
  12. PRS

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    Checking the power supply voltages is the first thing to do. Nothing works without the proper voltage.

    This may be a good troubleshooting scheme. But it starts from output to input as if trying to take a short cut. I usually go the other way around. I go from input to output. I therein try to find the point where things are screwing up. This means I trace the input signal to the output step by step.

    As before, I go input to output. And where that fails, therein lies the problem.

    This sounds like good advice, maybe.

    Personally, I would trace from input to output using another scope. The point where signals are not what they are supposed to be represents the faulty area and then you test all the components of that area.
     
  13. spinnaker

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    Thanks. How do I identify the caps? Where are they located? Near the focus control? Under the high volt shield? How can I tell if they are blown? Will it be obvious?
     
  14. spinnaker

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    Problem is this is my only scope. All I have is a DMM.

    i am having trouble finding out where I can measure the various voltages. I found the table in the manual of all of the various voltages and there ripple but can't see where it tells where to find the voltages in the scope,
     
  15. spinnaker

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    Found them! They are printed right on the circuit board! Looks like I do not have -8V. It measures + 1 V.

    My 50V is high it reads 61.6. Manual says tolerance of .5%

    5V measures 5.6V. Manual says tolerance is 1.5 % so that is OK.

    15V measures 18V, Manual says so that is high.

    I have not found a TP for -15V.
     
    Last edited: Nov 9, 2010
  16. retched

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    Your on your way.

    Are any caps leaking electrolyte or bulging?

    It sounds like bad caps may be your problem.
     
  17. gootee

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    Apr 24, 2007
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    Yes, a short cut was exactly what I was aiming to try to give him, if the power supply voltages had all been good.

    Whether you choose to go from input to output or the other way around, there's not much difference. You can go from input to output looking for where things start to go wrong or you can go from output to input looking for where things start to go right. But if you suspect that the problem lies closer to the output, then you might save a lot of time by starting at the output and working your way back upstream.

    But, in fact, neither of those ways would be the most efficient. If you have no idea where the problem might be in a long chain of circuitry, it would be best to start in the middle, so you'd immediately know which half you needed to go through. Once you know that, you'd go to the middle of that half, and have it narrowed down to a fourth, and so on. It's called a "binary search" scheme (which I first learned about when studying number-sorting and lookup algorithms in a class on Fortran programming, back in 1975 - arrgh!).

    Better yet, of course, is when you can deduce where in the circuit the problem is likely to be, based on the symptoms and the schematic, and go straight to the problem.

    Cheers,

    Tom
     
    Last edited: Nov 10, 2010
  18. gootee

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    If you can't borrow another scope or an ESR (Equivalent Series Resistance) meter, and the rectifier diodes and other easily-checked semiconductors in the -8V section test OK, and nothing else looks amiss, you should consider replacing all of the electrolytic caps in the -8V section (but keep in mind that I don't have the service manual in front of me and can't remember how the -8V is derived).

    If you can't get ahold of an ESR meter but you can borrow another scope, and you have a square or pulse generator or can throw together a simple 555 circuit, you could try checking the electrolytics, first (instead of just replacing all of them), with the method at http://www.fullnet.com/~tomg/esrscope.htm .

    Cheers,

    Tom

    P.S. The above is if you're lucky. If you're not, then there's something wrong with something unrelated to the power supply, in some section that uses the -8V rail. If there's a zero-Ohms resistor in the -8V rail, after the power supply but before anything else, it's probably there so you can lift one end of it and see if the -8V PSU output goes back to normal, to help you determine whether the problem is upstream in the power supply or downstream in the circuitry that runs off of that power supply rail.
     
    Last edited: Nov 10, 2010
  19. spinnaker

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    I do not see any bulging. Those caps are so buried but I see what appears to be a tiny amount of goop under two of them. Some left over flux maybe?


    It would be a shame investing in caps if that is not the problem. I can imagine they would be expensive.

    I am having a hard time determining what is what on that manual. I do not see anywhere where it outlines the power supply section for example. It could be in there but it is so huge.


    Why are the 50V and 15V so high?
     
  20. marshallf3

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    Jul 26, 2010
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    Tek had a habit of putting a drop of glue to bond the caps to the PC boards, it's not uncommon to see this in a lot of equipment that contains HV as it helps to keep static charges from building up on the outer casings of components.

    The odd thing here is that more than one voltage is involved, see if they're derived from the same initial supply source or if there's a circuit board somewhere that uses mainly the ones that are affected.
     
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