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Lab Volt Model 790A will not power up

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

  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,160
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    Jan 21, 2010
    Yes, electrolytic capacitors require you place then in the circuit the correct way around.

    Before you remove existing electrolytic capacitors make very sure you record which way around the old ones were. If the is no polarity marking, the capacitors may not be electrolytic.

    It is also important that you get replacement capacitors with the same or higher voltage rating and approximately the same capacitance. Going higher in capacitance is ok, but don't take this to far. The same or next highest available value is safe. You can go as high as you like with voltage rating, but it comes with increasing size and cost.
     
  2. Willard

    Willard

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    Dec 3, 2018
    Okay, Thanks Steve! I will do that.
     
  3. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Sir Willard . . . . .

    And so I said to myself . . . . . . . MYSELF, now doesn't the front panel of that unit seem all so familiar . . . like a B&K ?
    I then see by the physical scaling of the mu metal shield around that units CRT, that it is smaller than expected, with me
    initially thinking of it being a 5 inch CRT , but it now is looking like a 3 inch.
    I look into B&K and find two of their 3 inch units of that vintage, as a B&K 1403 and 1403A .
    With one unit having a bit snazzier set of front panel knobs.
    Now your Lab Volt unit seems to agree with the B&K on the power transformer voltages, the use of the same pair of
    Hoz drive transistors and they just LOVE the use of plenty of numbered terminal stakes.
    I am cutting up and prepping up a select schematic of the unit so that you can then reference its power path, where the
    burnt parts are..
    in the interim, confirm if that one 500VAC ORANGE " hot line" comes to a terminal stake and then connects to the cathode of that HV
    rectifier diode . . . . . that we were wondering about its condition.
    Confirm that there is ALSO being ANOTHER like diode, that goes over to the side . . . to circuitry.
    The 500VAC ORANGE wire might connect to stake #37 and if so designated, the associated diode might be D5.
    The low potential ORANGE wire might go to stake #38 and the overlooked diode might be D6 and its cathode connects to
    chassis ground.
    Thereby we have a voltage doubler circuit that ends up with - 1000VDC upwards, to feed the CRT electrodes.
    And if my schematic agrees with your unit, the GRAY 0.1 ufds handle half the voltage and the BLUE cap handles the FULL negative supply and should be rated at 1.6KV, while the GRAYS were 1000V rated.
    If the BLUE cap shorted or done a constant arcing it would put the total voltage across a series resistor to burn . . . . if that worst smoked resistor is labeled as R51.
    The OTHER heated resistor . . . if it is R20 and a 12K 2 watt unit . . . is being a collector load resistor of Q4 vertical plate deflector.


    Now, isn't this being your rebadged unit ?
    Excepting, your sync switch being located on the BACK of your unit, and one minor control knob, possibly being an edge lit graticule, light level control ?

    [​IMG]


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


    Life is sexually transmitted !

    .
     
    Last edited: Dec 18, 2018
    Richard9025 likes this.
  4. Willard

    Willard

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    Dec 3, 2018
    God, I love your writing and humor. I kind of need that right now. I'll get on your questions as soon as I can. My mother unexpectedly died Thursday morning and as you can imagine all hell is breaking loose right now. The Funeral is Friday so my life will slow down after that. I'll get back to you.

    Thanks much
    Will
     
  5. 73's de Edd

    73's de Edd

    2,576
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    Aug 21, 2015
    Condolences . . . . . . . until that pain in your heart subsides . . . . for the most adored woman in your life . . . and for certain, the most IMPORTANT one .
     
    Richard9025 likes this.
  6. Willard

    Willard

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    Dec 3, 2018
    On to the new normal. One of the orange wires from the transformer connects to Stake 37 and from there to the rounded end of D5 which is one of the HV diodes. It goes through the diode and directly to our severely burned resistor which is R51. That same orange lead also connects to the square end of D6 which is also a HV diode. The other Orange wire connects to stake 38 and proceeds directly to the bottom ends of the .1 microfarad 1000 WV caps which are C16 and C17. It is close to what you intimated but as I said stake 38 goes to the caps. The other orange lead goes to both diodes, but they are opposite each other if that makes sense. They enter the diode at the yellow dot on one end and on the other diode at the opposite end of the yellow dot. They are opposite each other.
    The slightly discolored resistor is in fact R20 and it is a 12K resistor.
    If you need more please ask. As always your help is invaluable!
    Thanks.
     
  7. Willard

    Willard

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    Dec 3, 2018
    Sorry I overlooked one confirmation. The yellow dot on the HV D6 diode does go to chassis ground. I am assuming the yellow dot is the cathode side of the HV diode. The cathode or yellow dot on the D5 HV diode connects to the orange wire. Then the anode of the D5 diode connects directly to the badly burnt R51 resistor. I am assuming this is the voltage doubler that you are refering to.
     
    Richard9025 likes this.
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It looks to me that the dead resistor is between a high voltage rectifier and the 0.1uF 1000V capacitor.

    There are a couple of reasons why those three components could be involved in the resistor getting hot enough to die:
    1. Failed diode(s) - This will cause AC to alternately charge and discharge the capacitor leading to higher currents and higher dissipation in the resistor.
    2. Leaky capacitor - This will cause additional current to flow, possibly leading to the higher dissipation in the resistor which caused it to fail.
    The failure of either the capacitor or the diode could damage the other. I would replace both of them. It looks like there are 2 high voltage diodes (there is almost certainly a voltage doubler).

    The secondary of the transformer is rated at only 2mA, so a suitable rectifier is not hard to find (something like this one I bought recently would be fine -- with a rated current of 250mA, overcurrent from the transformer is unlikely).

    The 1000V 0.1uF capacitor(s) could be replaced with these.

    Hmmmm I think you're in the US, so this particular seller may be tricky. What's your preference? Digikey (diode and cap) and/or Mouser (similar diode and similar cap)?

    And obviously replace that resistor too. Is its value known?
     
    Richard9025 likes this.
  9. Richard9025

    Richard9025

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    May 24, 2016
    Schematic & parts list for the B&K 1403 (a unit that's very similar to your scope).

    [​IMG]
    [​IMG]


    Look at the schematic carefully.
    Check if that good fuse has the value that's in the schematic&parts list ( 0.5Amps size 3AG <6,35x32mm>)

    Leave the electrolytic (polarised) capacitors alone, they're good.
    The 1kV oil capacitors don't have a very good reputation...

    All the secondary voltages coming from the transformer go into diodes D2, D3, D4... D7.
    Check all of them with the diode function of your multimeter.
    For D1 you should get a low reading, between 0,200-0,400 because it is a germanium diode.
    For D2&3, a reading of approx 1,4 is good, they're some special suppression diodes. They work as rectifiers for the 180V secondaries.
    Imagine D4 as a transistor that has 3 legs, but this isn't a transistor, it is a double diode. On the schematic, there are two diodes head to head. The center leg (the base) are the cathodes (negative) terminals of the diodes. The other 2 legs are the anodes (positive)
    To check D4, put your multimeter in diode mode, your negative black probe to the center leg, and with the positive red probe you check one of the other two legs, then the other one. You should get.. 0.7V? for each diode. As long as it isn't shorted (0,000 reading) or open (OL) then it's good.
    D5 and D6 are some high voltage diodes. They might read OL because they have a higher voltage drop that your meter can't read.

    For burnt R51:
    leaky C18 (high chance) alongside C16 & C17.
    shorted D6 (low chance)

    For heated good R20:
    Hmm, I can't see much, I'm too tired. One side of R20 connects to the B+ rectified voltage...

    Leakage is a very tricky thing. If you pull out a capacitor and measure its capacitance, it might read good, but in circuit, it may leak DC voltage.

    Don't shotgun-replace all the electrolytics, you may cause more harm than good. Do a step by step repair, following the defects. Also when replacing a lot of capacitors, replace 2-3 at a time, power the unit, see if something changed, make a log of it, continue...

    Also, if you got absolutely nothing, even no hum, when you powered the unit, set your multimeter in continuity mode and check:
    One prong of the plug to one side of the fuse, then the other side of the fuse to the power switch, then one side of the power switch to the other while the power switch is pressed (to check if the power switch is good) then from there to the transformer.
    For the other prong, check continuity between it and the voltage range switch, then from what you chose (117 for example) to the transformer.
    In Short: Make sure that the line voltage is getting into the transformer.

    Also watch out for dangerous voltages, even if its powered off, dangerous voltages can still be present in the circuit if the unit has been powered on recently.
    By working on such old and dangerous equipment, you acknowledge all the dangers in what you're doing. Always take safety measures and its good to have someone by your side when working on such equipment.

    Read Edd's replies, he explains how the diodes and capacitors work in R51's vicinity, making a voltage doubler circuit that drives the CRT. Now that the schematic is at hand, everything becomes clearer.
     
  10. Willard

    Willard

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    Dec 3, 2018

    In Edds post he pegged the resistor at 1K based on the surviving colors on the resistor posted on one of my pictures I uploaded.
    Many thanks for your help and especially part numbers. I was going to go for Digikey. I have an account with them, but they don't have both parts so Mouser it is.
     
  11. Willard

    Willard

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    Dec 3, 2018
    I have to head to work for now, but I will get on this tonight. Thank you so much for the schematic. I'll study it and see what I can learn. One question though. On checking the diodes do they need to be removed from the board to check or not. Also in regards to voltages. I am aware of the shock hazard or worse that capacitors can hold. I know they need to be drained or shorted before you handle them. This unit was last powered up over 4 weeks ago. Many thanks.
     
  12. Richard9025

    Richard9025

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    May 24, 2016
    The diodes don't need to be pulled out.
     
  13. Willard

    Willard

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    Dec 3, 2018
    I checked the D5 and D6 HV diodes and they both test OL on my Fluke multimeter. To make sure I did this right I also checked the smaller diodes which are D2 and D3. They read about .583 and .586 volts one way and OL the other way. They should be good, correct?
    D4 checks out as .562 and .563 with the black lead on the cathode. OL when you reverse it. I can't find D7 yet. My eyes are getting tired. I'll check the schematic and find it there and then check the board in the morning.
    The fuse has a .5 A on it but that is all. No 3AG. I can tell you that the filament is so small I can barely see it.
    As always many thanks. We will persevere.
    Will
     
  14. Richard9025

    Richard9025

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    May 24, 2016
    D1 maybe.

    D2,3,4 are good. Also D5 and 6 may be good even though they aren't showing any reading, they drop too much voltage and it's normal.

    The fuse is also good. 3AG means the dimensions of the fuse. Automotive Glass size 3.
    You can also order replacements for the 2 resistors (be sure to keep track of their wattages) and the three 100nf capacitors (note that two are rated at 1kv and one is rated at 1.6kv). Order capacitors with the same or higher voltages.
     
  15. Willard

    Willard

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    Dec 3, 2018
    I studied the schematic last night and found D1. It checks out with a reading of .365 one way and OL the other. That should be good also. I also found D8 and D9. I don't recognize these diodes. They don't look like what I would expect. Diode D9 reads .634 when it is hooked up correctly. Black lead on cathode. The other way it reads 1.925. The D8 Diodes reads .591 when hooked up correctly. Black lead to cathode. The other way it reads 2.162. Are these readings consistent with a zener Diode?
    I don't seem to be able to find D7. Also the schematic doesn't have a D9. On my board though the cathode of D9 is connected to the Anode of D8. Also the anode of D8 connects to chassis ground. I am wondering if the numbering is just wrong on the board because it appears to follow the schematic. I'll post a picture with annotations so you can see what I mean.
    One last question. On the schematic it references a .3 A fuse in the lower right. Is my .5 A fuse too big? It doesn't correspond to the schematic.
    Edd. I am looking at the schematic that was posted on the second page following this one. For the R51 resistor it has a value of 100K at a 1/2 watt. Would that be correct or should I go with the 1K. There is a big difference. Also how do you tell wattage sizes? I am assuming it has to do with overall size but I don't know what is watt. Slight pun intended.
     
  16. Willard

    Willard

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    Dec 3, 2018
    zener diodes.jpg D8 and D9 are zeners correct?
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I doubt they are zener diodes. The reverse voltage reading may be due to something else in the circuit.

    I would lift one end and check them again.
     
  18. Willard

    Willard

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    Dec 3, 2018
    Would you, by chance, have a suggestion for the blue 1600WV .1 MFD capacitor? I looked up the parts you suggested on Digikey and there are way too many options for me to choose from. I'm simply not well versed enough to know which one is would work.
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I'll have to do that tomorrow. It's just me and my phone at the moment and these sites are a pain on a mobile platform.

    I would be looking for something with a 2kV rating because I like to have plenty of margin.
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,160
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    It looks like I have you a reference to a Digi-Key part where I was meaning to link to the mouser capacitor.

    So, let me reach you to fish! (Some of this may vary on a desktop vs mobile view)

    1. Go to the mouser site
    2. Search for "polypropylene capacitor"
    3. Click on the link for "capacitors" (showing 32,000 hits)
    4. Click on the link for "film capacitors" (with 26,000 hits)
    5. Select "in stock", "axial" termination style, "polypropylene" dielectric, "0.1uF" capacitance, and every DC voltage rating from 1kV to 2.5kV, then scroll to the bottom of the page and hit "apply filters"
    6. Hit "show products"
    7. Here is where it gets harder on a mobile platform. Ideally I would sort by price and pick the cheapest options. Given the prices, I might pick the 1600V and the 2000V caps as replacements for the 1000V and 1600V caps.
    https://au.mouser.com/ProductDetail...GAEpiMZZMv1cc3ydrPrFwTA9MNepSrRK5%2b5qMyUU8w=
    https://au.mouser.com/ProductDetail...=sGAEpiMZZMv1cc3ydrPrF1fw9wOI4QJUpue8b8hmZOc=

    Sorry for the ugly links.

    Check that these are physically compatible, i.e. that they have a diameter less than or equal to the ones being replaced. I would also check the datasheets to make sure that the DC voltage is a working voltage rather than a peak voltage.
     
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