Help to Solve Oscilloscope problem

Discussion in 'The Projects Forum' started by webpower, May 10, 2015.

  1. webpower

    Thread Starter New Member

    Jul 13, 2012
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    I purchased an old Hamheg 203-6 a year ago and its been fine. Yesterday I switched it on and got the distinct smell of magic smoke. Front panel light appeared but nothing from the scope screen. The only thing I noticed is the three pin plug on wall socket was not in properly when I first switched on unit so I think when I pushed that in to seat it better, that is when the smoke came. But I am not sure on that.

    Anyway upon opening box. I noticed two things some melting of one of the two bridge rectifiers which come off some taps on transformer was slightly melted on one side (Although its so small it may have always been there) but no other component on any of the boards I could see was melted.

    Anyway I removed the bridge and it was still viable. So then I checked the continuity of the three wires from the transformer one is supposed to be the earth wire (brown wire) with other two (red wires) both sides of the AC for the bridge. But they all seem to be shorted as they all showing continuity. But maybe they show that when not runing? I don't know.

    Could it be that the transformer has burnt these wires together? It looks all fine on surface etc and no obvious signs of damage.

    But my guess is these three wires should not be showing as shorted. I have attached the schematic and the wires onto the bridge are shown at the component number D201 at bottom of document. Wires colors given in diagram.

    Can anyone suggest what I should try from here. Not keen on pulling the transformer out etc. But I am thinking I may have to replace it. Is there anyone here with experience of these scopes?

    I do not have the money at the moment to replace the scope. So I have no choice but to try and fix it. Any suggestions of where I should go from here are very welcome. I am being ultra careful as there are voltages in this unit that go up to 900V. Scary as its easy to make silly mistakes. Is it sensible to purchase some safety gloves and if so which type should I buy?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    disconnect the output of the transformer terminals 1,2,3 red wires, then put your meter on AC and power up the transformer and measure the voltage from it,should be 15V between red and brown, 30 V between the reds, if its zero then its blown, if there's voltage check the other outputs also.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    O'scope is not easy to fix it, do you have enough more than general EE knowledge?
     
  4. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2
    I have only ever dabbled in it as a hobby. But I do understand basic circuit theory.

    I also do understand the basic safety precautions. (I do not have an isolation transformer hence the idea of gloves) I have never worked on high voltage appliances before.

    At moment I am writing down all the circuit diagrams and brushing up on how all the blocks function. Since if I am going to attempt this fix I want to learn the circuit inside and out. As I want to treat this as a big learning experience at same time. That way if I fail to fix it all that time will not have been wasted. But I do want to be safe of course.
     
  5. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2
    Thanks Dave. Nice one. I checked it when it was connected and there was no voltage and they all were shorted somehow.
    But no I have not checked any voltages on transformer with output leads unconnected.
    I'll do that now.

    Many thanks for helping me. I will post back what happens.
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,357
    Personal safety is always a concern at any voltage and more so at higher voltages. You don't need any special gloves, just extra caution.
    One rule of thumb when taking any measurements with high voltage equipment is you keep one hand in your pocket. This is not a joke and the concept is taken seriously.

    We will attempt to resurrect your scope if you follow instructions and procedures very carefully. It would also help for you to have a digital camera handy so that you can post photos of your situation.

    1) Transformer windings have low resistance. Indication of continuity does not indicate shorted windings.

    2) Did the main fuse blow? F001 fuse.

    3) If bridge rectifier D201 was destroyed it may have resulted from a short downstream. C2001 2200μ/25V would be the first suspect.

    Instructions on how to proceed will follow in due course

    What Dave has suggested will confirm that the transformer 15V-CT-15V winding is still good.
     
    Last edited: May 10, 2015
  7. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2



    Thanks this is great, this is now going to be fun instead of scary and frustrating. Thank you for offering to help me like this.

    Okay this is the order and what I have done so far.

    1. I first checked that I had 240 from mains into transformer. Which I do.
    2. I then pulled out the fuse and that is fine so put it back.
    3. Because of the smell of smoke I inspected the whole board. The only thing I could see was a slight blemish on one side of the Bridge rectifier D201
    4. I pulled out D201 to look at that. But I am not sure how I test that out. Presmulably a simple battery in a series ciri=uit could test all the diodes in it are still working. But is it worth doing that check at moment?
    5. Upon Dave's instruction I unsoldered the outputs (15V-CT-15V) Voltages are ok on those. I think I have made a big mistake. As I tested all voltages before without the power button!
    6. So maybe I should solder the bridge back in! Then use the check strip and write down all voltages again this time with power button on.
     
  8. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2
    Okay I have now done following:

    1. Before I soldered the bridge back in I powered it on with that the bridge and got spot on tube showing.
    2. I soldered bridge back in and now only power light showing as before.
    3. I also checked the voltages on the checkstrip all show correct voltages as per the schematic document.
    4. One thing I have just noticed, is if I turn the time base knob then I see a phospher screen light up at a stationary single point on the tube. as I turn the knob. But otherwise there is nothing. So tube is working but the horizontal beam is in one fixed location.

    5. Also I noticed that I can move the flickering spot of light (whilst turning time base knob to keep it flickering) up in the Y and in the X. I have the probe connected to the test square wave lug on the scope. So maybe the Y voltage changes are not getting picked up from the probe??
     
  9. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2
    6. Weirdly the voltages are now showing correctly as 17volts and -18volts on the DC side of bridge.

    Not sure what to check from here.
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    if the bridge is in circuit, check the output from the regulator , and the +12V, -12V rails, put meter com on ground and use positive probe to check voltages.
     
  11. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2
    Also the schematic components do not seem to match the components on the board. For example just beyond the bridge it shows two capacitors 1000uf rated at 25Volts.
    But schematic does not show them? It shows two 2200uf at 25volt rated? It seems to be like that everywhere. Different components from document.
     
  12. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2

    Hi Dave. The regulator which is different to what is shown in diagram is bolted to frame near bridge. But that is showing +12V I can not physically get to the other pin with my meter probe.
    But on the checkstrip I am seeing +12 and -12 volts showing for those rails. (Those things marked in diagram as TS within circles I think go to that checkstrip which is shown at top of document.)
     
  13. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,969
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    i would check all the supply test voltages marked on the circuit diagram,which probably is a different model or design date,then see what isnt working on the scope.
     
  14. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2
    I must have probed and made a short as fuse has just blown. Any idea what size fuse I should use here. Manual says 315mA but so many things differ. Does that sound about right?
     
  15. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    if its a glass fuse it will be printed on the metal tabs.
     
  16. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2
    Hi changed fuse only had a 200mA so used that. Ok voltages on checkstrip all seem correct. But there is a difference between what is printed on the board than what I am measuring.

    the + and -12 volts is same. But where on the board is says 150Volts on checkstrip I am actually reading 147volts. But other than that all voltages are as they should be on checkstrip?

    So I suppose I need to check the other boards now. The CRT board only has one voltage 1900v on checkstrip. But it does look like the CRt is working because I saw a strong single electrons glow point in middle when the bridge was disconnected. And it now still flickers on and off if you turn the time base knob. There are no other checkstrips.
     
  17. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2
    What is best way to do this. Should I work from the D201 bridge outwards checking all voltages as I move out where and when I can. Basically go through whole circuit until I see an obvious voltage that is in error?
    or is there a better way to do this. Its interesting but difficult as I have zero experience of tracing through boards. I think I could possibly do more damage. But I don't have any choice. I must try.
     
  18. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2
    Well I have no idea what is going on as its now suddenly come back to life. But all I did was unsolder the bridge and solder it back in??
    I just checked the display screen again with scope probe on Square wave test lug and its showing a nice clean square wave as it used to do. Now wonder all the voltages were correct!

    But there is a slight horizontal back and forth movement now. But its not so bad you can't live with it.

    How could that happen! Its bizarre! I feel like I have wasted every ones time.
    Dave, thanks for your help anyway. I expect it will go weird on me again. Then its going to be hard as its then an intermittent fault. I do hope not.

    It still does not explain where the smell of smoke came from. And it was that very distinctive smell of something having burnt out. So something is burnt in there? The one thing I did notice is the regulator gets very hot.
     
    Dodgydave likes this.
  19. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    744
    must be taking excessive current than usual,to make it hotter, unless there is another component blown, like capacitors or resistors.

    post pictures of the areas affected.
     
    Last edited: May 10, 2015
  20. webpower

    Thread Starter New Member

    Jul 13, 2012
    16
    2
    I have put it all back together now as it has now been working okay for about an hour. I think there may well be something wrong. But there is no signs of burning or anything. Everything looks clean and no burnt parts except the tiny blemish on the bridge component. The horizontal trace does wobble a little side to side but I am not brave enough to go back in now and risk breaking something which is now working again.

    What I would like to do though is buy a broken Scope and fix it up. So I can then come back to this one later and have another go having gained some more experience fixing the other scope.

    Is there anywhere I can go to get old broken scopes other than ebay?

    Many thanks again Dave. I think it could have been a loose connection as the wires from the transformer were really worn. I put more solder on when I rejoined them and since then its all working as it was before. But that is just a total guess. But without your suggestions, I would still have a broken scope. As it is, I now have something I can use again. I am very grateful to have people like yourself willing to help like this.
     
    Dodgydave likes this.
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