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Lab Volt Model 790A will not power up

Discussion in 'Troubleshooting and Repair' started by Willard, Dec 3, 2018.

  1. Willard

    Willard

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    Dec 3, 2018
    I recently was given a Lab Volt Model 790A oscilloscope that didn't work. I am relatively new to electronics so I am looking for some guidance. I opened the scope up and noticed a resistor that was burned up. It has the number R51 next to it. The color coding on the resistor is burned off so I can't tell what ohm it is. I am looking for a schematic for this scope to determine the resistor number. If I can find the resistor specification I can replace it, however, I am wondering why it burned out. Is there something on the board either before the resistor or after it that caused this failure? I tried to upload a picture of the board but I am getting a failure notice so no pictures for now. Any help would be appreciated. If you need more information please let me know and I will do my best to provide it.
    Many thanks.
    Will
     
  2. kellys_eye

    kellys_eye

    4,289
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    Jun 25, 2010
    Schematics for these types of equipment aren't easy to come by. However it is often possible to make very good guesses based on the actual component layout and associated circuitry.

    Post clear images of the board and its surroundings. You will need to resize them for the forum - say 800x600 pixels.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I can't find any details on that specific model, but let me make some observations:
    1. It may be rebadged. See if you can find evidence internally of the original model and manufacturer.
    2. If it has a CRT (which also seems likely) then beware that there are dangerous voltages inside. These voltages may persist long after the power is removed.
     
  4. Willard

    Willard

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    Dec 3, 2018
    I will resize the pictures tonight and get them posted. Also, thanks for the warning about hi voltages. I was aware of that but it always pays to be careful. It also has a bunch of capacitors that I am assuming will need to be changed but that will become more apparent when I get the pictures posted. Many thanks for your replys.
    Will
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    24,861
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    Jan 21, 2010
    In in the middle of working on an oscilloscope myself (actually 3 of them, but I'm too embarrassed to post about all the at once :)).

    I assume you just want to make it go again, and are not too concerned about keeping it looking original.

    I note that you've found a suspicious looking resistor, but can you explain more fully what you meant by "didn't work"? Was there no sign of life at all? Did you get a power on indication, but nothing else? Did you smell smoke? Was there any odour when you opened it up? (All the stuff we can't tell from the pictures you'll post soon).
     
  6. Willard

    Willard

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    Dec 3, 2018
    The first picture shows a partial board layout. The burnt out resistor is located below the crt and above the large right side gray capacitor. The second picture shows the burnt capacitor located in the center of the picture below the left gray capacitor.

    When I tried to power it up I get no response from the unit at all. No lights, no hum no smell. Nothing. There is also no smell of smoke. This unit has been sitting in a school in a storage cabinet for an unknown amount of time. When I first opened it up I detected no smell at all. Judging from the look of the board and the resistor and the lack of smell, I am guessing it has been in storage for a very long time. I'll get a couple of more pictures of the other side of the board on the other side of the crt. Please feel free to ask for more information.

    Thanks Partial board layout.jpg Burnt Resistor.jpg
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    24,861
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    Jan 21, 2010
    Before you do anything, set your voltmeter to its highest voltage range (1000VDC if you have it) and measure across those capacitors. Make sure the voltage is under 20V or so. If you've left the scope a couple of days since you last tried to power it up, it is *probably* (not certainly) safe by now.

    Check for continuity of the fuse, and also the resistance for that burnt out resistor. If the resistor hasn't fallen apart, you might get a reading that will come in handy later.

    I would be looking with extreme suspicion at those electrolytic capacitors. The 250V 47μF and anything similar would be on my list for replacement. You'll be looking at between 50c and $2 for one of them.

    I'm not sure what type of capacitors C16, C17, and C18? are. Are they all 0.1μF 1000V? It may be worth considering replacing them with film capacitors (polyester or polypropylene) at around $1 to $3 each. Although -- if these are currently film capacitors, they may be safe to leave in place. I'm not an expert in identifying old capacitors :)

    A photograph of the rear side of this board will help us in determining what that resistor is connected to, and possibly why it failed. Try to get us photos perpendicular to the board on both sides. The more of the writing on the board that we can read the better. A couple of extra shots to show it all would be appreciated (or mark up a photo).
     
  8. Willard

    Willard

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    Dec 3, 2018
    The first picture is from the front obviously. Just for clarification. The second picture is from the left side of the chassis. The third is from the right side of the Chassis.
    I checked the fuse for continuity right from the start and it is good. I noticed the resistor after that.
    I was wondering about the capacitors. I was going to ask if they should be replaced. From my reading on the web it sounds like capacitors tend to leak over time and I believe this has been around a while.
    After looking at my posts I will take some more close ups. I'll also work on removing the board so I can get an idea of what the back side looks like. I am assuming I should keep all the wires connected if at all possible. This board has no plugs to remove. All the wires are wrapped on studs protruding from the board.
    I'll continue tomorrow. My insomnia is waning for now. Thank you for the help! Front of Oscilliscope.jpg Left Side of Circuit Board.jpg Right Side of Circuit Board.jpg i
     
  9. kellys_eye

    kellys_eye

    4,289
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    Jun 25, 2010
    'fraid you're going to have to do more work on dismantling in order to resolve this issue.

    The board needs to be completely out of the case and we need close-up pictures of both the component side and the track side of the board. You should be able to do this without disconnecting any of the wires although it could get a bit 'tangly'.....

    On the upside the resistor may be a current in-rush limiting device and be burned because of the collapsed capacitors in the circuit. Ideally you would test them for ESR (equivalent series resistance) to determine their quality but quite often it is just simpler, cheaper and less trouble to replace them regardless.

    If you go down this route just cut the leads as they enter the body of the capacitor and use these free ends to solder the new parts in place.
     
  10. Willard

    Willard

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    Dec 3, 2018
    Sorry for the delay. I'm having trouble getting my phone to upload pictures. Also real world work is interfering with my available time. I'll post pictures as soon as I have everything sorted out.
     
  11. Willard

    Willard

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    Dec 3, 2018
    The first picture is of the back of the board. It turns out there was an inspection panel that I could take off. I know, I'm not very observant. I missed that completely as I was focused on the front of the board. The last 3 pictures are of the front of the board. I took them in a series so I could get them a little bigger in the picture. You should be able to stitch them together. They are the top, middle, and bottom of the board. I will have a couple of more pictures in the near future of the board back lite. You can see some of the traces and where they go. This should be somewhat helpful. board back.jpg board 2.jpg board 1a.jpg board 1.jpg
     
  12. 73's de Edd

    73's de Edd

    2,442
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    Aug 21, 2015
    Sir Willard . . . . .

    I can see that is being one clunker of a minimalistic old silly scope .
    It is using recurrent horizontal sweep . . . . and not TRIGGERED sweep . . . .therefore capable of only a slippery-slidery type of locking onto a waveform.
    It is so dated such that it also uses Banana connectors for inputs, providing no input shielding, as would be provided by even a Cro-magnon SO-239 or our current day preferred BNC connectors .
    It's using Japonie transistors and caps, metric hardware, along with the phenolic circuit board.


    Your most informative photo has been the post #11's second photo down,where I see the electronics for the driving of a set of CRT deflection plates ( probably its vert ckt) with its outputting from stake pins 20-21 into a length of clear twin line. That is all being on the left half of the board.
    This deflection area should be using 100's of volts , in order to be able to even create electrostatic deflection of the CRT deflection plates.
    ( Those 2SC995's spec out at 300VDC. )

    On the right half, is the best photo of THE burnt resistor, yet I additionally saw an overheated 18? or 1.8 K? resistor, in another area .
    I have put your two board photos side by side and I am seeing that they do not relate to being the SAME board. It seems like I can not find the foil path layout of the FUSE and the fried resistor that would be located at the 9:00 position from it.

    The board foil side being shown is one with another set of deflection plate amp / output transistors on it and several trim pots on the board, so this must be the hoz sweep section and also having some power supply portion.

    So we really need to see the foil side paths of board picture #2 . . .albeit, probably being a BITCH to get out into photo positioning.

    DIFFERENT BOARD PAIR . . . .
    [​IMG]

    73's de Edd
    . . . . . . . . . .


    I don't SUFFER from insanity, I absolutely enjoy every minute of it.

     
  13. Willard

    Willard

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    Dec 3, 2018
    Yea, I know it is an old scope. I seem to draw old broken things to me. I will work on getting the board out. I was working on drawing a schematic of the caps and how they relate to the burned transistor but I haven't finished that yet.
    I usually try to resist disassembling things more than I have to. Less chance for me to make it worse. You are also correct with the other overheated resistor. I was planning on replacing that also. It still tests okay but it is obviously stressed.
    Thanks for the reply and I'll get back when I have the board out.
    Thanks!
     
  14. Willard

    Willard

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    Dec 3, 2018
    So I took the plunge and started desoldering the wires that absolutely had to be done. This was done with a lot of pictures to help me put it back together again. The first Picture is obviously the back. The next picture is the front. I have added the last two pictures to hopefully add clarification and to make tracing tracts a little easier. Please let me know if you need more. Thanks!
    Post edit. If you look at the right side of the burned resistor you can follow the trace down to a black torpedo looking object. I am guessing this is an old diode? Anyway the trace attaches to an orange wire. This orange wire goes to the transformer. I hope this helps.




    Final board back.jpg

    Final boardfront.jpg

    Final backlight 1.jpg

    Final backlight 2.jpg
     
    Last edited: Dec 9, 2018 at 3:00 PM
  15. Willard

    Willard

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    0
    Dec 3, 2018
    In the previous post I highlighted the burned and stressed resistors in red. In this post I blew up the picture of the burned resistor on my phone and it looks like the first two bands are brown and black. Then we hit the badly burned area. I also included a close up of the stressed resistor. I hope this helps. burned resistor.jpg stressed resistor.jpg
     
  16. 73's de Edd

    73's de Edd

    2,442
    819
    Aug 21, 2015
    Sir Willard . . . . .

    Wheeeee ! . . . . I now see . . .that you got the board out for some nude photos . . .very well done . . . and additional KUDOS on the backlighting of the
    poil faths.

    I placed and 1x scaled the two views side by side and concentrated on only the points of interest. I now was able to find that fuse "landmark" easily this time and I used fine . . . . .minimally view encroaching . . .YELLOW lines. Establishing boundaries.
    Using the corner board mounting screw hole, one screw 180 deg from it and the holes thru the board established 3 corners and the 4th just fell in as the automatic right bottom apex.

    MIEINE EVALUATION . . . . .

    What I seem to be " foil path reading " here, is a negative HV supply area that is associated with the different levels of voltage division needed for supplying the focus and sub voltage levels needed by the sections / elements of the display CRT.
    Three 0.1 ufd at 1KV HV caps are being used for HV filtering, along with intermediate voltage divider resistors.

    On velly hi dollah scopes, that use much higher voltages and accelerator electrodes, they usually build them up as a dedicated flyback type of supply
    On minimalistic scopes, they usually depend upon a sole separate/isolated high voltage AC winding on the power transformer that is up in the 5 hundreds PLUS of volts, just a bit less than that 1KV. Or they could use half that voltage and voltage double up to get that level. I don't see that being done here..
    I do see that on the 4th photo down, of your new cluster, that I am able to see writing on the side of the power transformer that is giving voltage ratings, related to its windings .
    Pass that info on to us and I suspect that one of them will be the single higher level that I mentioned earlier. Or there may be another transformer that was not in a photo..

    Proceeding, prematurely, in a highly educated manner and by taking in what I am seeing, I believe that if you will take the second photo down of the 4 shown and note that there are two ORANGE wires that come into the PCB, that connect to the two right ORANGE CIRCLES on my right foil side display.
    That would seem to be the HV AC input.
    I am taking the top wire as going up and connecting to the cathode of a high voltage rectifier diode, (as WHITE A on the left photo) and it is outputting negative voltage on its anode up to WHITE B, which foil loops on down and connects in B PRIME to a first section of filtering with the "GRAY LINE" filter.
    Therewith, you should successfully have your first and highest level of Negative high voltage, with no problems.
    And then . . . .AND THEN . . . comes the series insertion of the dreaded and cremated R31 resistor . . . a carbon film type . . . no less.

    Concentrating on the left photo now . . . .
    It has its left end connecting into a quadrant of 4, left sloping pads. The top two I am wondering about, as they are staked and on day one, probably looked as good as the vertically arranged WHITE and PALE YELLOW wire wraps just to their slight upper left.
    Now it looks to me . . . yet for you to confirm . . .that the top stake is open unless the GREEN wire takes a hard twist to the right to then connects and wraps on it.
    BUT it looks like to me that both wires are solder connected to the lower stake of the top two..
    Then we look at the second pad up of the four and it is going upwards from WHITE X to a high value of resistor that than goes upward into a cluster of 1 meg and 100K + values of resistors . . .they would NOT load down that supply line, to fry that R31.
    ALSO that cluster of resistors are then connecting back down thru ONE high value resistor to the center GREY high voltage capacitor, so that line also could not load down the main supply.
    Now that finally leaves the bottom pad of the 4 that is receiving the voltage output of R31 at WHITE C that then passes down the foil path
    to WHITE C PRIME line that directly connects the BLUE 0.1 ufd filter caps negative connection..
    Now if that capacitor is shorted / badly leaking/ constantly arcing over . . . . that condition could fry that R31 series supply resistor.
    OR a miswired connection, of that previously mentioned PALE YELLOW and GREEN wires, if one is not belonging there, if someone else with a poor memory has worked on the unit.
    Now let's look at the other lower ORANGE CIRCLE connection of the right photo. It obviously being the LOW potential side of the transformer ? supply .
    It immediately routes to the left for making two + connections "grounds" of the two GRAY filter caps. BUT do note that the bottom connection of the
    BLUE capacitor is not involved.
    Look at it and t seems to go to a possible repaired service loop that is wound around it and goes up at a 45 degree angle to another pad with a wire inside.
    Also look at the nearby YELLOW corner apex where there seems to be a Phillips screw inside of a Hex alum-i-ninny-yum- yum spacer.
    Am I just seeing a shadow there or is it blackened out , such that an arcover to that area has occurred and burned a carbonification of the phenolic board ? Considering that standoffs screw received a metal shield that is eventually grounded to chassis.
    Last associated consideration . . . since that BLUE cap did not share the paired GRAY caps grounding, there may be a voltage divider at the bottom of that cap in the BLUE supply so that a lower voltage negative supply can be acquired.
    IF one of those resistors happened to be the scorched R20 12K ?, all of the massive overloading of the R31 could have slightly overloaded R20.

    Looking at your final photos two resistors, I just commented on the R20.
    Now for your MAIN R31 ? 34? and its photo, I see its definite untouched BROWN first band and likewise a BLACK second band, I now think that in knowing this type of scope circuitry and the expected voltage and currents involved.a 100 ohm would be too low of a resistance to fry that resistor as it is.
    And THAT a 1K value would be just right, to burn up the same , and a 10K value would only discolor, just about a much as the R20 did.( If R20 is not the last leg of that BLUE supply and you find one of its leads connected to chassis ground. )

    YOUR HOMEWORK . . . . .
    ALL of the above answers requested . Also check the number ? . . . if possible . . . of the H V diode and if any, or particularly, if the BLUE capacitor is shorted.

    Now . . .Vat chew tink ? . . . .

    ILLUSTRATIONS ILLUMINATA . . .

    [​IMG]





    73's de Edd
    . . . . . . . . . .





    TOP BUNK:
    Where you should NEVER put a child wearing Superman jammies.
     
    Last edited: Dec 10, 2018 at 12:30 PM
  17. Willard

    Willard

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    Dec 3, 2018
    WOW. I'm on it! I am going to have to print your post out and refer to it while I check your updated photos. Give me a bit. It's Monday morning here and I'm shipping corn to the river. Not as exciting as working on a scope, but it pays the bills.
     
  18. Willard

    Willard

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    Dec 3, 2018
    Okay, I'm getting started on your requests. I removed the transformer and flipped it on its' side so you can see the voltage readings of the windings and trace the orange wires back to the board. As you can see I do have one of the leads off the transformer in the 500 volt range. Unless I am totally blind this is the only transformer in the scope. orange transformer wires.jpg
    The next picture shows the Green and White wires going to the second stake. They are both soldered together onto the stake at location 27 as seen in this closeup photo. They do not connect with the first stake. It only has the white wire on it. I would like to add that in my opinion this board has never been repaired. All the soldering looks to be original in my opinion. There doesn't seem to be any evidence that this board has ever been taken out. green whitewire.jpg
    In the next picture I have included a somewhat out of focus picture of the stand off screw. My eyes tell me that there has been no arcing in this area. It looks pristine. standoff post.jpg
    In the next photo I highlighted in red the lower orange wire from the transformer. I am not following your question about a possible repaired service loop. As far as I can tell this looks like it is all original. Unless this isn't what you are talking about. What say you?
    Final backlight 2 Red.jpg
    I did my best to find numbers on the HV black diodes but to no avail. I thought about asking digikey for a part number for a black diode with a yellow dot on it but I'm preeeetty sure that isn't going to work.
    I also checked the blue capacitor with surrounding pads and found no shorts with components near by. I also have no continuity through the capacitor as checked with my volt meter.
    The following two pictures show a few more capacitors. I am assuming that I should replace these also just because of age but that they aren't related to my power issues. Correct?

    pot capacitors.jpg capacitors at back.jpg
     
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