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The Tektronix again, 7603

Discussion in 'Electronic Design' started by [email protected], Oct 21, 2013.

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  1. Guest

    I posted this in SER and figured maybe someone here would like to take a shot at it :

    Sorry, I just like this scope. In my (excessive) spare time I have been poking at it again. I was getting a really weird display and I found it to be related to the readout circuit. I can live without that. I DO have a spare readout board though, so after getting the thing to actually work normally,I will look into that. For now it is mostly disconnected.

    Now, vertical is OK. Horizonal is compressed and will only deflect on external or line input, it will not trigger even on auto or P-P auto. That may be related to the problem, it may not. If you remember, originally it had notrace. This was before I goofed by accidentally switching two plugs on theZ axis board which dumped 130 volts on the 5 volt line.

    This would not be worth persuing had I not a parts donor. I have a 7613 which had some sort of a short. Looking at

    https://dl.dropboxusercontent.com/u/29948706/tekamp01.jpg

    You can see that I obviously wiped out U510 which is of course pure unobtainium. I changed it from the 7613. Originally I tried using the whole H amp board but it didn't work so I put the original back in with the good IC. I figure it's good because the donor had a short, this IC was not the short so there is no reason to believe it is bad.

    There was a mishap along the way and it wound up frying R558. After replacing R558 the sweep was even more compressed ! A look at the print might explain it. I don't have the unit in front of me at the moment but what is going on here is that CR530 and 533 are mostly reverse biased. Replacing R558 simply reverse biased them more.

    Since originally there was no sweep and at the time I did not switch it to line input I do not know if it was compressed at that time or not. That's why I do not know if the two problems are related or one is what I created. I hate it when that happens, my blind ass is getting more useless by the year.

    I have two horizontal plugins which display identical symptoms so I am ruling that out temporarily. I know they use the 5 volt line but only one of them was in the unit at the time of the 130 volt incident.

    In the manual (available at bama) the description of the operation of U510 is that when the XY shutdown goes high the input of the chip switches to the input from the readout board. The readout board is putting out a tri level waveform at about 60 Hz to the XY shutdown, but not quite at the line frequency. It is also putting out square waves to the vertical amp which was causing a dual trace when it was not desired. It is obviously whacked out but it seems to me it should work with those inputs (XY/RO) disconnected. Indeed it did straighten out the vertical sweep. In fact the horizontal is actually closer to working now but there is not enough gain and it is clipped (truncated) at the sides.

    I did highly suspect U510 at one point but I have pretty much dismissed that. I also considered that maybe there was a source not getting to the plugin. The output therefrom is terminated with 50 ohm resistors, so there is not much of a signal there. When I disconnect P501 I get quite a bit more amplitude but looking at the print for the 7B50 I see that it is pretty much acurrent source so that is to be expected. I do notice though, that when disconnected the signal is riding on a negative voltage.

    I have checked pretty much every semiconductor on the H amp board and they all seem OK. There is a slight DC imbalance, and I am not quite sure what to make of it. Since this thing is a differential amp a fault on one side can give misleading readings so it is hard to tell exactly where the fault lies.

    The centering and centering range controls have very little effect. I couldsee how R525 would have limited or no effect due to something wrong with U510, but not R535 which is after CR530 and 532.

    The voltages at the deflection plates are low, though not specified they are much lower than one would expect owing to the fact that they used a +130 volt source for them. The trace is also expectedly blurry.

    All the sources are there and on the money according to Hoyle. Where is allthis positive voltage that is obviously supposed to be there ?

    Any help is welcome except sending me to Yahoo. Sorry, Yahoo left a bad taste in my transistors a long time ago and I simply won't go there.

    Thanks in advance.
     
  2. Guest

  3. Frank Miles

    Frank Miles Guest

    Yuck.

    Have you gone through the bias conditions? Is the 7613 amplifier an identical
    part or might it be a little different? Are all the power supply voltages OK?

    IIRC the interface sensitivity is 50mV/div, biased close to 0V. You should be able to check that whether
    the signal going into U510 is reasonable (i.e. not grossly distorted) by checking
    the differential signal at that point. You do have another 'scope to check
    this, don't you? If the signal there is ok you can ignore readout and other
    complexities until the deflection amp is shown to be working.

    "Fuzzy" is bad - though if the bias is significantly off you could have some
    oscillation somewhere (higher frequency than the display, might need a spectrum
    analyzer or really fast scope to verify).

    It may be a good thing that you have all that spare time...
     
  4. Fred Abse

    Fred Abse Guest

    On Mon, 21 Oct 2013 13:24:32 -0700, jurb6006 wrote:

    Number one rule when troubleshooting electronic equipment:

    *Check the supply rails first*
     
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