A little help/guidance on fixing an old oscilloscope

Discussion in 'General Electronics Chat' started by Tealc, Mar 18, 2014.

  1. Tealc

    Thread Starter Member

    Jun 30, 2011
    140
    10
    My oscilloscope, an old Gould Advance OS3000 has developed a fault that manifests as a loss of trace/dead screen.

    I was working on a basic circuit and on turning a dial I lost the trace. The scope is triggering but nothing will bring the trace back, on either channel, in auto, normal or free run.

    At the moment the screen is super dim. There is no change to the brightness when I switch on and off. I think the glow I am getting is just the reactance to ambient light before I switch the light off.

    There is a glow at the rear of the CRT.

    I've injected -12v to the rear Z port and I get no dot. Said something about this in a tek document I found.

    I've probed the +/-15v and the +150v on some of the PCBs and they are close to nominal.

    Before I tear this thing down and look for blown stuff is there any particular area I should be looking at. Bear in mind I know very little about how these scopes work internally, although I do know to treat the high voltages with respect.

    Many thanks for your help.
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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  3. MrChips

    Moderator

    Oct 2, 2009
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    I would begin by checking the low voltage supplies, as you have done.
    Next check the high voltage supplies. You will need a HV probe in order to do this or you can make one.
     
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  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
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    Scott Wang's service manual seems well documented.

    Yes, yes and yes.
    Note the HV is negative on this scope.

    Has the connector on the back of the tube become loose? Check this before applying power.

    You can prove the tube by disconnecting the X and Y inputs to the tube itself. They usually just pull off somewhere.

    A working tube should give you a near centre spot, where you can adjust the focus and astigmatism controls as in p19 section 4 of the manual.
    I have further tested tubes in the past by applying an external dc bias voltage in place of the X Y drives and measuring the spot deflection. 0 to about 50 volts is needed. This checks the linearity of the tube.
     
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  5. Tealc

    Thread Starter Member

    Jun 30, 2011
    140
    10
    Thanks to all advice so far and the link to the manual. This is really helpful.

    That's definitely worth looking at. I moved the scope from a shelf to a bench on the day it stopped working so it may have worked loose and then when I fiddled with one of the knobs it could have dropped off. It was an instant loss of trace after all.

    There are 4 or 5 little ball and socket connectors around the circumference of the tube around 1/4 of the way up the tube from the rear and they are secure. These connectors are fed by little wires from the PCB mounted next to the tube. I'm not sure what they are for as haven't decoded the manual yet. The very rear of the tube is obscured so will need to take a few things off to get access.


    One other thing which I've noticed since moving house a few months back is that the first trigger on a square wave is longer that following ones. I doubt if it's related.

    A bit like this..

    [​IMG]

    I doubt I'd buy a HV probe just for this scope as the value in the scope isn't really high enough and I don't work in HV, at least not right now. I'd probably look at making one if it came to it and make sure it and myself are massively insulated from all that high potential. I'm nervous just thinking about it :D


    I like the warm green glow from these old scopes and you just can't beat the smell so will spend a good bit of time on this. :) Plus maybe I will learn a thing or two on the way.
     
  6. Tealc

    Thread Starter Member

    Jun 30, 2011
    140
    10
    Here are some photos of what I'm looking at.

    The fuse FS832 was blown as expected. I replaced with another 500mA which also went quickly.

    [​IMG]

    The PSU section looks fine to me. There is a transistor at a bit of an odd angle but it seems to be clean and making a contact.
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    The bright up board
    [​IMG]
     
  7. Tealc

    Thread Starter Member

    Jun 30, 2011
    140
    10
    The CRT connector, at least I assume it is. I've pushed and twisted but it's not going anywhere.

    [​IMG]

    The other side of the case
    [​IMG]

    And finally part of the EHT.
    [​IMG]

    The 'snot' seems to be all over the voltage multiplier section (at least that's what I think it is after reading the manual) and I'm wondering if it is allowing shorting or something.
     
    Last edited: Mar 19, 2014
  8. crutschow

    Expert

    Mar 14, 2008
    13,016
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    The "snot" is some type of dielectric conformal coat to block moisture and prevent leakage and arcing of the high voltage.
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    So you have a blown fuse FS832 on the +20V line that feeds the EHT section.
    That narrows the fault down to the ETH board.

    See if you can locate TR813 and check if this is blown.
     
  10. Tealc

    Thread Starter Member

    Jun 30, 2011
    140
    10
    I found TR813 mounted on the EHT box, it's a MJE520 NPN medium power transistor.

    I have found the datasheet and have determined that the junctions are working ok Base to Collector (1 diode drop) and Base to Emitter (1 diode drop) but I am getting 1.1v (2.2kOhm on resistance setting) on the diode setting from Collector to Emitter and it makes no difference which way the probes are presented. I get no reading Collector to Base or Emitter to Base.

    This might be because I haven't isolated the transistor from the PCB perhaps?

    Here's the rest of the EHT section anyway.

    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited: Mar 19, 2014
  11. Zerotolerance

    Member

    Sep 18, 2011
    56
    10
    That Green 4.7uF 63V cap looks like the bottom lead is possibly loose/blown out (Can't really tell). Unless that's just my eyes fooling me. I would also check those diodes near the fuse.

    For accurate testing of components it is always suggested to test them "out of circuit".

    On old oscilloscopes typically the problem with them are dried out caps. I would replace all caps bad or not then go from there. Especially due to the age of the scope.
     
    Last edited: Mar 19, 2014
  12. Tealc

    Thread Starter Member

    Jun 30, 2011
    140
    10
    Thanks ZeroTolerance.

    I will see if I can get those caps replaced.

    I was looking at D833, the 1N4148 as it has a little blob of solder on the leg and what could be a burn mark on the PSU board. This could have been there since initial assembly or could have been a previous fix but it is close to the 20v rail and the fuse and might be damaged. Will pop that off and take a look.

    I should have donor caps for every electrolytic in there apart from the big 4000uF 40V smoothing caps, which are well buried and rather large, although I could replace with a much smaller unit.

    The diode.
    [​IMG]

    And the caps.
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    They 'look' fine but without an ESR meter I can't be sure.

    I guess it's ok to replace with radial leaded caps and be creative with the leads as I have very few axial.

    I'm actually surprised how clean it is inside. You'd expect a near 40 year old device to be filthy and I did expect the caps to look a lot worse. The caps in my 3 year old router looked much worse than this.
     
    Last edited: Mar 19, 2014
  13. Tealc

    Thread Starter Member

    Jun 30, 2011
    140
    10
    Isolated TR813 and it seems to be fine.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    Last Advance Gould scope I had suffered from weak trace, I boosted the EHT by replacing the selenium sticks in the multipliers with series pairs of CTV TO220 style efficiency diodes - it had an adverse effect on the deflection (and therefore calibration) so I donated it to a local motorcycle mechanic for checking out ignition pulsers.
     
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  15. ian field

    Distinguished Member

    Oct 27, 2012
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    The mark on the PCB is a splodge of flux - I'd hazard a guess that someone has snipped the diode's lead to test it and/or isolate a part of circuit.

    The solder blob is simply where they've joined it back again.
     
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  16. Tealc

    Thread Starter Member

    Jun 30, 2011
    140
    10
    Well it certainly didn't have a weak trace until a few days ago :(

    I decided to test D833 as I had my soldering iron out. As I touched it one of the legs fell off immediately.

    [​IMG]

    So I soldered a new one on. It was probably as you say Ian, a snip and rejoin. Maybe this fault I've got now was in the scope recently in it's life and it was patched up somehow before selling to me.

    Checked my stock of salvaged and new caps and I've really not got much suitable so it looks like I will have to buy in replacements so might as well get the correct type if I can.
     
  17. ian field

    Distinguished Member

    Oct 27, 2012
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    Intermittent and/or weak trace could be tracking from the wiper of one of the pots in the HV divider to chassis, another possible is a high or intermittent open in a resistor thereabouts.

    Usually there's a chain of resistors, including pots from part of the EHT circuit and chassis.

    Don't just go poking in there with the DMM - there's usually a few HV capacitors about!
     
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  18. Tealc

    Thread Starter Member

    Jun 30, 2011
    140
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    I'm steering well clear of everything on the secondary side of the transformer in the EHT box. I'm nervous enough on the primary side and I know the voltage there should be low :)

    For a while I thought I had a dead short to ground from the EHT 20v supply but it seems to be showing 2.2kOhm now. Must have had the board shorting to the case, doh.


    There is actually a chain of 4x 3M3 on the bright up PCB between Grid Supply Vg Hi and Vg Lo. I don't know what potential that runs at. The bright up PCB is covered in acrylic so I assume there is some higher voltage than +150 there, as +150v is uncovered on the output driver PCB.
     
    Last edited: Mar 19, 2014
  19. MrChips

    Moderator

    Oct 2, 2009
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    I don't have the circuit in front of me now but you want to keep focused on that short from the fuse to ground. My bet is there is a capacitor shorting.

    I have to go find the schematic.

    Edit: Start with C814. Unsolder one leg and see if the short goes away.
     
    Last edited: Mar 19, 2014
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  20. MrChips

    Moderator

    Oct 2, 2009
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    So there is no longer a confirmed short from the fuse to ground. So you still have to figure out why the fuse is blowing.

    Unsolder the wire going to the collector or emitter of TR813. What are the colours of the wires, i.e. green, yellow and red goes to what part of the transistor?

    Put in a good fuse and see if it still blows when you turn on the power.
     
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