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Need help with Heathkit io-10 o-scope repair

Discussion in 'Troubleshooting and Repair' started by kdogzxc, Feb 20, 2014.

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  1. kdogzxc

    kdogzxc

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    Feb 20, 2014
    Alright. I have a Heathkit io-10 that I got from a garage sale, guy said it worked. Of course I got home and pluged it in and it did not. I just got a green dot in the center of the screen. Befor this I had never worked with vacuem tubes before, but I am willing to try. I found a manual and a schematic, took the bad boy apart and started to check connections, solder joints, and what not. Found one cold joint and a loose wire but over all from what i can tell the guy followed the directions to a t. Fixed these problems, powered it up to check the tubes. v10 the 6x4 tube didn't light...replaced it. the new one would start to heat up then spark on the inside a little and fail, figured it was a bad tube. I got another one, this one sparks on the inside a bit but doesnt fail like the last one. Though, now there is arcing across pins 4 and 5 on the v8 IV2 tube and this blows the 1/2amp fuse when it happens. Again i figure bad tube, so i replace it. Arcing still hapends. Could it be a bad transformer? attached the schematic, need the manual?
     

    Attached Files:

  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    You are starting with a dodgy project. The scope has very high and dangerous voltages.

    The problem with the 6X5 is probably due to leaky electrolytic capacitors which have 'gone off'. I would replace rather than reform.

    The sparking at the high voltage rectifier could be due to a leaky valve base. Is it paxolin or ceramic? Is it clean? If necessary the valve could be replaced by a silicon diode but it will need a PIV of 4000V or so.

    The transformer seems to be standing up to abuse rather well. Take out both rectifiers and see if the transformer is happy.
     
  3. kdogzxc

    kdogzxc

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    Feb 20, 2014
    Yeah, for real, I am very keenly aware of the high voltages. im a little afraid whenever i flip the thing on without the case.

    I have testeed the caps and they test good, high resistance with the proper capacitance. could they test good but still fail in operation? Ill check them again though.

    Between the two pins that are arcing there should only be a ac voltage of 0.625v which is why im thinking maybe bad tranformer. because there is no short in the tube and the unit has arced between these pins before when i left this tube out by mistake while testing (forgot to mention that before). I geuss its paxolin cause it is definatly not ceramic, the pins are clean if not singed a bit.

    So just pull out v8 and v10, turn it on and see what happens? what would be a good result; nothing happens?
     
  4. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Without the rectifiers, the transformer will only be supplying the heaters so it should run with minimal warming. It will be a big limp and have a long time constant so leave it for several minutes. and yes, nothing should happen.

    You coud put a 100 ohm resistor in the cathode of the 6X5 to measure the HT current. Use leads with crock clips at each end or meter leads with clips so that you do not have to be near the scope when turned on.
    I do see how you know the resistance of the capacitors without testing them at high voltage.
    Beware that the capacitors will store high voltage when the power is turned off.

    If you have sparking when the HV rectifier is absent, then the valve base is shot. Change this to a good ceramic base. Do not stress the transformer.

    I have not studied the heater connections but it looks as if they are in a parallel/series circuit, . removing one valve can overstress the other one in the same part of the circuit.
     
  5. kdogzxc

    kdogzxc

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    Feb 20, 2014
    First I will try getting a new base for this tube. I have definatly seen the thing arc without the tube in there, so I am thinking that the base is the issue. I will also check some of those caps on 6X4 and see whats up with them, and then get back to let you know the results. The parts will probbaly come from china so it will be like a month. Thanks for the great advise, I was lost.
     
  6. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    The valve base is noval or B9A.
    Use a ceramic or PTFE base.
     
  7. kdogzxc

    kdogzxc

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    Feb 20, 2014
    sorry for taking so long to post my results. I replaced the base and there is no more arcing but the unit still does not work. it starts to warm up all the tubes light but then the fuse blows after about 10 seconds. I'll be going over the manual and schematic tonight.
     
  8. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Does the fuse blow when the rectifiers are removed?

    If no, then the reservoir capacitors could be faulty.
    If yes, then it could be the transformer. To test the transformer properly, then all the secondaries should be disconnected.

    Rather than replace the fuse over and over again, you could use a lamp limiter, just a 60W bulb in series in the mains lead. It doesn't take long to make one and it is possible to apply some power long enough to take voltage measurements.
     
  9. kdogzxc

    kdogzxc

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    Feb 20, 2014
    I have tested the scope without the 6x4 rectifier plugged and it powers up fine. All the tubes light and the fuse survives. So I'm thinking its the capacitor bank. I'll probably have to hack something together if I can't find a replacement.
     
  10. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    There are four posibilities.
    1. The transformer is faulty. With the rectifier removed and the set unpowered, test the continuity of both sides of the HT winding. If one side is open circuit, this could saturate the transformer core giving excess input current.

    2. The 6X4 rectifier is faulty. A bodge can be used with silicon diodes but this is not recommended. In any case, the rectifier has had a very hard time.

    3.Faulty rectifier socket. You have had one faulty perhaps you have another.

    4. Faulty reservoir capacitor, this is most likely. There is a 40µF 400V capacitor running on 380V, this is very close. Also these capacitors 'go off' when not used. You could disconnect this capacitor and see if the fuse blows, the scope will not work correctly in this case.
    If you replace this capacitor, use a 500V capacitor. My valve book gives a maximum reservoir capacitor for the 6X4 of 10µF or 40µF depending on the maker. In any case, do not exceed 40µF.
    Something better than a 'hack' is wanted here.

    If you disclose your location,then someone may be able to help with sourcing.
     
  11. kdogzxc

    kdogzxc

    8
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    Feb 20, 2014
    And a year later I finally got around to working on this project. I replaced the the two multi section capacitors with similarly rated caps. The 3x 100mfd 150v was replaces with 3x 100mfd 250v and the 40/30/40/40mfd 150/250/350/400v was replaced with 47/33/47/47mfd 400/300/400/400v. Even after replacing these caps the fuse would still blow after the unit warmed up (5-10 sec).

    I pulled all the tubes in the power supply block (v8,v9,v10,v11) to try and track down where the problem was. The fuse would only blow when both v9 and v10 were plugged in. Then I started pulling tubes in the sweep block (v3 and v4), because that is where I saw problems initially the very first time I powered up the unit. With all the tubes plugged in except v3 and v4 the unit power on, did not blow the fuse, and the remaining tubes lit. I checked, and output voltages were spot on coming out of the power supply block of the circuit 148ish on the 150+; -74ish on the -75; and 328ish on the 330+. Adding v3 back into the mix (v4 worked out fine) caused my my 6x4 tube to fail and go dead, instead of just slowly warming up it warmed up then started flickering and flashing a bit then died. I was seeing this same thing happen when the fuse would blow also. So i figured the problem was somewhere in the area of the v3 tube.

    I pulled the neon bulb, which is on v3 tube's circuit (v3b between sweep position and the fine freq). This seems to have fixed it!

    Now with all the tubes in place they warm up fine, and the unit starts up and does not blow the fuse. After I went through the set up guide in the manual I have a pretty decent output. Im still not sure if it is fully functional because the manual I have does not go over the specifications of the unit, but it will read a signal and the adjustments seem close enough.

    Im still going to look into the sweep circuitry, because Im not entirely sure the unit is sweeping correctly. Ill post pictures and stuff later

    thanks for the help and advice!
     

    Attached Files:

  12. kdogzxc

    kdogzxc

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    Feb 20, 2014
    Here are some picks of the working unit
     

    Attached Files:

  13. kdogzxc

    kdogzxc

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    Feb 20, 2014
    Now my only question about the function of this scope deals with the sweep. As I increase the horizontal dial from x1 to x10 to x100 the sweep only gets smaller on the screen (see pics). I would think that increasing this would still allow the sweep to span the screen but increase the number of periods in the wave you could see. Does this make sence and I still have work to do or do i just not know enough about 50 yr old dc o-scopes?
     

    Attached Files:

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