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Anyone near Rotherham who could test my Scope?

Discussion in 'General Electronics Chat' started by bigal_scorpio, Aug 10, 2008.

  1. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    What have you got the TIME/CM switch set too.?

    If its fast, the square wave from the probe test could look like a straight line.!!!
    Switch to 50mS/sec. try again.

    What do you get when you touch the probe with your finger [ 5mV/CM setting]
     
  2. bigal_scorpio

    bigal_scorpio Active Member

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    A) finger produces a wave, not a smooth wave, not quite sine but close!

    The adjustment you mentioned? Would it be the CAL pots Rv151 & Rv152?

    I'm having trouble finding the right ones! Damn them for not silk screening it!

    Al
     
  3. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Al,

    The green ringed POSITION pots on this dwg.

    I hoping we havnt cooked any of the power resistors on the Y1 plate drivers,, we will see.
    The rough finger sine wave sounds promising, what about the probe test.?

    Ignore the old voltage readings on this dwg, I have used an old dwg.

    EDIT:
    DONT adjust any of the trim caps by mistake...

    EDIT:
    Dont touch the CAL pots.
     

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    Last edited: Sep 2, 2008
  4. dave

    Dave New Member

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  5. bigal_scorpio

    bigal_scorpio Active Member

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    Baffled!

    Hi Eric,

    I have been studyingb the drawing for almost 2 hours now and cannot find RV102 or R117 that feeds it. Can you point me to it?

    Also I notice on the schematic you marked that there is RV102 with adjacent RV101 fed from M151 and below there is an identical RV102 and RV01 but this time fed by M251, are there 2 pots or one, and is it RV101 and RV102 I need to adjust or does the schematic confusingly show a sinle RV as 2 parts?

    Truly baffled............Al
     
  6. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    It appears that RV101/102 are duplicated as are R116/117 and C117.???:confused:

    Do you see the 150R on the Gate of each FET, can you trace that bit of track back to the pots.

    I dont think you sent me any photo's of the RV101/102 board layout.
    Do a shot of the underside of the pcb near the 150R's
     
  7. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Al,

    I believe it must be these black furry pots.

    Make a small pencil mark on each pot, before you try to move the trace.
    It appears that there is only 1 pot/trace.

    The other components nearby check out.
     

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  8. bigal_scorpio

    bigal_scorpio Active Member

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    Pictures

    Hi Eric,

    As requested some pics of the 592s in place and the undersides of the same.

    Hope you can make more sense of it than I can ;)

    Al
     

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  9. bigal_scorpio

    bigal_scorpio Active Member

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    Further

    Hi Eric,

    Just to mention that the set of 3 resistors at the top part of each 592 are Brown Green Brown - Gold.

    They just don't look quite right with the flash but they are the 150r's.

    Al
     
  10. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Al,
    Look at my 14:40hrs posted image, thats the ones Im sure, just adjust them slowly, watch the traces as you do it.

    Dont touch the ones in your pics they look like CAL pots.
     
  11. bigal_scorpio

    bigal_scorpio Active Member

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    Result!

    Hi Eric,

    You were as usual spot on! I now have both traces centred with the knobs centred.

    Now, what can I use to test the scopes setup?

    Battery, mains through a transformer, or is there something I can knock together simply?

    Al :D
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Hi Al, at last.!:)

    I would apply some dc voltages from a variable psu if you have one else one or two small batteries. Use a DVM to check and compare what the scope measures. Do that for both traces.

    Touch the probe tip with your finger, as the mains is 50Hz, ie 20mSec that will give a rough freq check.

    Then you could knock up a 555 astable with a freq pot and try a few square waves etc.

    Lets know what you find on the dc input voltage check.
     
    Last edited: Sep 2, 2008
  13. bigal_scorpio

    bigal_scorpio Active Member

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    Update

    Hi Eric,

    Think I have another problem now!

    A probe shows 1v for 1v52, 2v1 for 3v2 and 5v5 for 7v8 so seems like adjustment needed.

    B probe shows nothing for 1v5 unless I switch to 50mv/cm and then it only shows a minute deflection so I think something is wrong.

    I have tried swapping probe leads to no avail.

    Both traces centre perfectly by adjusting the vertical and seem to move about the same distance per degree of rotation as each other.

    If I switch to XY instead of dual then I just get one stationary spot.

    Horizontal movement seems fine and gives full scale with no problem.

    All measurements were done on AC and at the same frequency.

    Any Ideas mate. Even though I am much happier with a working "A" channel than the "B" it would be nice to have both, but if it comes down to it I will manage with just the "A" :)

    Regards...........Al
     
  14. bigal_scorpio

    bigal_scorpio Active Member

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    forgot

    Hi Eric,

    Forgot to say that touchiong with my finger gives a wave of about 1cm when set on 20ms.

    Al
     
  15. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    That sounds OK, 20mSec/cm.

    If you have some zener diodes or voltage refs, you could make up some test voltages.
    Check your Wien osc on the ESR.!!!!!
     
  16. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Hi Al,
    If you are sure about the Y A being off, slowly adjust the CAL pot near the NE529
    Change the suspect FET.
     
  17. bigal_scorpio

    bigal_scorpio Active Member

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    Update

    Hi Eric,

    I got the "B" channel working, it has a bad connection in the BNC socket, if I hold it to the side it reads as "A" does! :)

    just going to see if I can fix it, damn strange though when "B" was good and "A" bad then "all change at Euston", I seem to attract weird problems ;)

    Will keep you informed and hopefully take some pics of the display so you can see if it looks ok.

    Al
     
  18. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Al,
    I wrote this reply before I saw your latest missive, I'll post it anyway, it may help.:)

    Make up a simple 1KHz pulse gen, using a 555 or 4093 schmitt.

    Connect the +5V square wave output from the pulse gen to the suspect Y channel via the front connector.

    So you now have a square wave to measure internally on the suspect Y channel, using the scope probe connected to the input of the working Y channel.

    Using the circuit diagram you can track the square wave thru the circuit.

    Be aware due to the design of the Y channel GAIN selection you may see the 5V signal being attenuated
    before it gets to the FET and NE529 inputs.
    After the NE529 the signal should be increased in amplitude [bigger voltage swing]

    From what Nigel observed when he looked it over, its most likely the FET source follower thats defective.
    Measure on either end of the 150R connected to the FET Gate and then Pin#1 of the NE529.

    How did the re-Cal go on working Y channel, did you have enough Gain control.?

    EDIT:
    I have one or two chassis mount bnc sockets if you need a new one.
     
    Last edited: Sep 3, 2008
  19. bigal_scorpio

    bigal_scorpio Active Member

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    Mending

    Hi Eric,

    Just to let you know I havn't done any calibrating yet, thought I'd get the iffy part swapped out first.

    I'm hoping to get a BNC today, got to take my mums dog to the vets today, so I may be able to call at Maplins (though its more like the Hi-De-Hi version) than an electronics store, never have anything in stock nowadays! May have to swap the "Ground" BNC to the "B" channel? Not quite sure why I need a ground one anyway? Does it get much use usually?

    Another thing I noticed while trying to source a BNC was that they seem to come in two different impedences 50r and 75r, I always thought that connecters were just passive and the cables and or comonents gave the impedence - strange! I hope it's not too important as most of the BNCs I have seen available don't even specify which they are.

    Anyway I will let you know if I'm successful.

    BTW I think the BNCs must have been fitted before the unit was riveted together, its a bugger to get at the nut and boy is it tight!

    Regards...............Al
     
  20. bigal_scorpio

    bigal_scorpio Active Member

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    Phew!

    Hi Eric, Nigel and all,

    At last I have managed to get the BNC off and swapped it with the EXT one which looks a bit better! It was a true struggle for room to get a spanner or anything in. I would have liked to fit all three new BNCs while I was at it but sadly as usual Maplins didn't have any in store! Big surprise ;)

    Anyway I have at least got two working channels now and am just about to set about calibrating it all.

    I will mark any presets before moving them and be very steady to adjust.

    I will let you know what happens in the next post.

    Regards........Al
     
  21. bigal_scorpio

    bigal_scorpio Active Member

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    Cal

    Hi Eric, Nigel and all,

    I just calibrated the scope traces and seem to have got them to an acceptable degree of correctness, well they are showing the same reading as the MM is!

    Still having trouble with the BNC connectors none of which are perfect, but I'm beginning to wonder if then leads I bought have slightly smaller pins than perhaps the old ones did, they seem to have a bit of play in them when plugged in, metric plugs and imperial sockets? Is there such a thing in BNCs?

    Anyway just going to knock up square wave generator as Eric advised and play with the scope a bit more!

    Al
     

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