Help Me With My Oscilloscope. (Dial Up 56K Warning)

Discussion in 'The Projects Forum' started by Mckayman, Jun 22, 2009.

  1. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Well right now I am trying to restore an old HP H77-120B Oscilloscope. I can't seem to find any information at all on this thing. The scope was working just fine until decided to take all the tubes out and clean them and then sand all the rust of the edges of the scope. When I went to put the tubes back in I realized that I had no idea which spot was the correct one for the tubes. ( Stupid me for not labeling the original spots correctly.) I finally had some help from a similar scope and put the tubes in spots that seemed to work best. After many trial and errors I was able to get the scope to come back on. But now the only problem is that I can't adjust the horizontal or vertical nor to the inputs pick up and signals. I tried new tubes and I still got the same results. Does any one know about these ancient things and can help me?

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  2. hgmjr

    Moderator

    Jan 28, 2005
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    For around $20 US you might want to try purchasing a manual from this site. I don't have any experience with the site but you may want to check into it.

    hgmjr
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You're very lucky; there is a set of manuals available for download at no cost here:
    http://bama.edebris.com/manuals/hp/120b/

    In case you didn't know, those old tubes after they've been used awhile are extremely fragile, particularly when they're not in their sockets. It's really easy to break the heaters or cause the envelope to crack/leak while removing/handling/inserting them. So, handle those tubes like newborn baby birds. For the moment, unplug the 'scope and leave the tubes where they are.

    Also, the 'scope was calibrated with the tubes in their original positions, and no two tubes are precisely alike, even if they have the same part numbers - much like transistors. Even if you insert a 12AU7 tube into a socket where a 12AU7 is supposed to go, it may very well not work properly unless the biasing is re-adjusted.

    Randomly plugging tubes into various sockets may have damaged/destroyed several of your tubes. I haven't made an exhaustive study of the materials available; that's up to you.

    What you need to do is first identify which sockets are supposed to contain what part number tubes. White surgical tape and a Sharpie will help. Remember that isopropyl alcohol will remove Sharpie marker.

    Then determine as best you can which tubes are installed in incorrect sockets, most preferably without disturbing them. Remember, the more that you handle them, the shorter their lives will be - and good tubes are hard to get nowadays (read:expensive).

    It's best to wash your hands well first, and wear white cotton gloves while handling the tubes. Finger oils left on the tubes' surfaces will accelerate their inevitable demise. A careful cleaning with some isopropyl alcohol on a clean rag will remove the contamination.

    Try your best to minimize "tube swapping". Make notes of where all of the same-type tubes are used, and where they started out. You may very well wind up having to swap tubes around several times to get things working properly, but make lots of notes about where the tubes were, and where you moved them to so that you don't lose track.

    Good luck!
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Gee, that looks very familiar,

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    I've had the same problem with manuals, so I thank you for the post.

    I must say, yours looks much prettier. I picked mine up from a ham flea market around 20 years ago for $10.
     
  5. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Thanks i'll see what I can work up tomorrow. I do however have access to an entire stock hold of tubes never been used from the 50s/60s. I'll see what I can pull up.
     
  6. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Ah, Yours is a 120B! The manual for yours is on the link provided in the thread. Yours is different from mine in the way that you have a build in sweep generator. The one that I have does not. (just x and y inputs) The tube placements in ours are similar on the power supply ends of the machines, but our tubes differ from the signal and data ends.
     
  7. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Well I went ahead and replaced the tubes with new ones and then I put in a new resistor were one was cracked, but this did not fix the problem. It still turns on but I can't adjust the horizontal or vertical nor to the inputs register anything.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Start by checking out the low voltage power supply; in particular the -150v supply.

    But first, get familiar with the low voltage power supply.
    Start at paragraph 4-34 on page 4-5. After you've read that entire portion, skip to paragraph 5-9 on page 5-2. Be certain to read & heed the CAUTION in paragraph 5-13.

    Then skip to paragraph 5-27 on page 5-6. Note the part that reads "If any voltage is not within tolerance it is probable that ... the replacement of a tube or part has caused the misadjustment."

    Then check out tables 5-4 and 5-3 on page 5-9. You're going to have to adjust all of those voltages in accordance with the instructions provided in the manual.

    As I've already said, each tube is going to act somewhat differently. There probably wasn't anything wrong with the tubes before you removed them and started mixing them around. Substituting brand-new tubes won't fix the problem either, as they'll respond differently than the old tubes did.

    You have to take this one step at a time, read the service manual, and follow it step-by-step. If you've been operating the scope with the low voltages out of specifications, things aren't going to work well (if at all) and you may cause additional harm.
     
  9. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Alright thanks, I'll see what kind of reading I can get.
     
  10. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
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    Hey Bill,

    I'm curious, do you have an amateur radio license?
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    If I ever get around to renewing it I do. KB5BNW.
     
  12. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Okay well I pretty much when to town the the multimeter. What I have found is that the four tubes, which are for the (Horizontal and Vertical Input amplifiers and just amplifiers) which are located at the front of the machine where the controls are, are not coming on. I am reading high amounts of voltage around this area but then I will find little dead zones around the tubes where no voltage is present. The tubes are new and never been used. The horizontal and vertical output amplifiers are working though and I assume this is why I still get a dot for the trace in the center of the screen.



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  13. Wendy

    Moderator

    Mar 24, 2008
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    Are the filiments all lighting up? A dumb question, but I have to ask.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Not a dumb question at all, Bill.

    12AU7 tubes (aka "valves" in the UK & down under) have the "ends" of the heater filaments on pins 4 and 5, with a center tap on pin 9.

    This board appears to be different from the schematics in the 120b.pdf file. I see no resistors with a reference designator in the R400 range, and the wires' color scheme is different.

    However, looking on page 5-18 (relative page 41) the vertical and horizontal deflection amplifier tubes V1, V2, and V101 share a common filament supply from T302 pin 14; one side of a 6.3v secondary - but more importantly, from the 19v supply. If the 19v supply were significantly higher or lower than it should be, it would either rapidly burn out the filaments (if too high) or not be high enough to cause the cathodes to emit electrons.

    V1 and V2 have the filaments wired in parallel using pins 4 & 5; dropping 12.6v. V101 is using the filaments' center tap (pin 9) and pins 4,5 wired to ground, dropping 6.3v, for a total of 19v.

    If there were a problem with the 19v supply, or any of the filaments in those tubes or the connecting wiring, there would be no horizontal or vertical deflection.
     
  15. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Yes, Bill the tubes are not lighting up. =\

    I went and checked the various pins that you mentioned in regards to V1 and V2. I did see where pins 4 and 5 are wired in parallel connecting V1 and V2 ( The voltages running across these areas were very low not close to 12.6 at all. The voltage varied from 0.8 to about 1.8 from pins 4 and 5 to the ground. And then for tubes 401 and 402 (they seem to have identical configurations to V1 and V2) On theirs pins 4 and 5 the voltages varied form 1.8 to 3.3 from pins to ground. As for pin 9 on V1 and V2 the voltage was .8 from pin to ground. And as for pin 9 on V401 and V402 the voltages was 2.6 from pin to ground.
     
  16. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    When pins 4 and 5 are tied togerther, then the tubes work on 6.3 Volts with repect to pin 9.
    The 6.3 Volts will be AC.

    See this page for the connections and more data.
    http://www.nj7p.org/Tube4.php?tube=12AU7

    Greetings,
    Bertus
     
  17. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Ahhh..A/C eh? - Well I just measured the voltages in A/C and all were zero, for all 4 tubes.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    According to the 120b.pdf, the tubes in the horizontal and vertical amplifiers' input stage should be powered from the 19v supply. I believe that's to decrease the possibility of noise interfering with the low-level input signals.

    However, this particular 'scope isn't wired as shown in the 120b.pdf. So, you're going to have to figure out whether those tubes' heaters are being powered from a 6.3vac secondary winding from T302, or in series from the 19v supply.

    That part really shouldn't be terribly difficult to figure out.
     
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