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Another HP 1741A Oscilloscope!

Discussion in 'Troubleshooting and Repair' started by atsu, Dec 13, 2013.

  1. atsu

    atsu

    8
    0
    Dec 13, 2013
    edit: I have created another thread. The first 4 posts in this thread were originally here: https://www.electronicspoint.com/hp-1741a-oscilloscope-t264568p18.html
    (*steve*)

    Dear Kris Blue Nz, I have been reading all about the lvps of 1741a so brilliantly analysed by both of you.
    Now I request you help me with information to solve my problem in a 1741a oscil I got recently.
    Unfortunately the lvps is smashed up and the transformer is the only part still ok.The HV unit is ok. My question is will the other section ,two channel amplifiers,time base system, etc. be worked by supplying the necessary voltages from an external source. Once I am sure they work I think I can search for an lvps. I do not know the voltages to be supplied
    as I have no clear schematic of the boards. The one I got downloaded is not clear. I saw some of the postings of lvps and hv supply schematics which are excellent. I thought I could request you to send me the particulars of the voltages and where I should feed them i the
    other boards and see if they function. I have a Tek465 and wavetek function generator.
    Y:confused:ou are experts in analysing the 1741 and surely can help me.
    Best wishes for a Happy Christmas. Thank you. ATSU 13th Dec 2013
     
    Last edited by a moderator: Dec 14, 2013
  2. tryppyr

    tryppyr

    228
    1
    Oct 22, 2013
    Hi ATSU, and welcome to the forums. As I think you know, I have posted the best schematic I have for LVPS, and would be happy to assist in any way I can. I'm attaching some close up images of the LVPS connectors that go to other boards, so you can consider whether your approach will work.

    As you can see from the images, LVPS is a pretty involved board, for obvious reasons, making connections that are important to ALL the other boards in the scope. Of all the cards to lose, I'd say LVPS is the hardest to replace, because it does more than just provide power... it provides a sort of central connection point for many other things. Take a look at the images.

    The first shows the rail going down to A12.

    The second shows the pins that attach A6 to the ancillary cards.

    The third shows the rail going down to A17.

    Let me know if you need more/better images.
     

    Attached Files:

  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,213
    2,695
    Jan 21, 2010
    I recommend you start your own thread as otherwise it will get *VERY* confusing.
     
  4. atsu

    atsu

    8
    0
    Dec 13, 2013
    Dear tryppyr, thank you for the prompt reply. I thought if I supply the marked voltages on
    the output plug of lvps supplying the other system, I could feed a sine wave and see if I can
    get an output displayed in my oscilloscope.The Hvps can be ignored for this test.If the
    system is working I can think of the lvps.I shall let you know about my tests. I will rig up
    a psu with all the voltages and try. This lvps was designed when single chip voltage regulators were not avalable.Thank you once again and I will be in touch with you for help. Atsu
     
  5. atsu

    atsu

    8
    0
    Dec 13, 2013
    another 1741a

    Dear tryppyr,
    I fed +15 and -15 volts to the termainals as per details you posted me as pictures. All the leds light up in the front panel and the Ics give the proper voltages in the
    input boards. I will have to test it further by feeding a signal at the inputs and pick the output
    at the xx yy plate outputs,with the probes and see if I can get a trace on my oscilloscope.
    I will post you the full details.

    After studying the shematic of the lvps in your other thread I find the board is only a by pass point for the chop blank and the Bf beamfinder.These actually function on the crt. All the crt potentials come from the hvps. I think these are applied to these electrodes in the crt at the
    hvps point. Even the Z mod is independent of the lvps board.
    Actually I retrieved the lvps from the junk along with the damaged 741A Hp oscilloscope I got
    .I went through a voltage analysis. I find the following readings. +15,-15, +48 all show only millivots output in my extech dvm autoranging. The 156 vots is 200+ and 120 is 166 volts.I feel we have to replace almost all the transistors and the three IC s that are opamps.I have not checked them yet.Also the voltage regulator transistors. Dont you think this will cost and we dont get exact equivalents.
    If we do substitution the control parameters in the circuits will not be those that the old HPveteran engineers designed. I do not know if this will function reliably.
    I have suggestion which I request you to examine and let me know if it can work.
    All that we need are the various voltages that the lvps is designed to supply. voltage control
    current control is necessary but I feel the high sophistication of the original board can be
    diluted. All that is needed is multi secondary coils in the transformer and fit, the two terminal
    regulaters to obtain the different voltages.. We can check if the whole system works. Theoretically I feel it would work as we are supplying all the voltages with a lesser degree of regulation.
    I want your advise in this. I can rig up the necessary unit to try.Thank you.
     
  6. tryppyr

    tryppyr

    228
    1
    Oct 22, 2013
    Honestly, I am not the one from whom you should seek advice. I am not well trained in electronics, nor have I yet had any success with this project. I think your idea has merit, but I'd fall back to the real experts on the board to comment and advise.

    - Greg
     
  7. atsu

    atsu

    8
    0
    Dec 13, 2013
    another hp1741a

    Dear tryppy,
    Thank you for your kind reply and I am astonished at the honest admission by
    you and asking me to seek advice from other experienced members. I have joined the forum recently therefore I have to request your assistense in this matter. I request you to put me on to the appropriate members.Or you can send me their names. I will still be contacting you .
    tiru anachi
     
  8. tryppyr

    tryppyr

    228
    1
    Oct 22, 2013
    Like you, I am a relatively recent addition to the forum membership, so I know very few of the other members. To the extent possible I will assist in any way I can. Just know that my expertise is very limited. KrisBlueNZ is the one who provided the most assistance to me so far.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,213
    2,695
    Jan 21, 2010
    I'm sure there's several people looking at this thread. However, we're not going to post "Sorry, I can't help".

    You have already said that you have looked at the schematic for the board you don't have and your analysis is that it is mostly interconnects. Your conclusion is that you should be able to make a replacement.

    If your analysis is correct, then your conclusion is reasonable.

    Perhaps you can cut the schematic of this board and post it as an image. I'm sure that will make it easier for others to see if they agree with you.

    In any case, if you have the schematic and tryppyr's excellent list of HP part numbers and their equivalents, you may be able to tackle it even if it contains quite a bit more than that. I'd be cautious about parts that are high voltage, current, or frequency, but there may not be any of those...
     
  10. tryppyr

    tryppyr

    228
    1
    Oct 22, 2013
    I think one of the other concerns raised was that using modern replacement parts might deviate from the design standards originally intended by the engineers. While I can't speak to the validity of that concern, I will confess I had a similar concern throughout the time I've been working on my scope. I found this to be particularly true as I look at the very tight tolerances of some of the passive components... resistors with 0.5% tolerance, capacitors with tiny tolerances. And then there are transistors and ICs that don't appear to have directly defined JEDEC replacements (like the CA3094AT). Even for someone like me who is willing to spend a fair amount of money to restore the old beast, it can be hard to believe that at the end of the restoration what you have will actually match the original performance.

    That said, I have been adding a lot more information to the spreadsheet, so if you are interested, here is the latest version of the spreadsheet and the latest version of the PDF.
     
    Last edited: Dec 21, 2013
    atsu likes this.
  11. elebish

    elebish

    177
    12
    Aug 16, 2013
    Is the defective lvps of your scope a 60hz transformer driven or a switching type supply. Most Tek scopes within the last 30 years will be using a switching type to save weight and cost. They are also more efficient but are more difficult to repair. Some of the resistors used in switching type supplies are non inductive, so be careful when replacing these. Substituting voltages to the scope circuits should be fine no matter what type of supply being used but be careful when substituting voltages in the high voltage section since good regulation is necessary. Ed.
     
  12. tryppyr

    tryppyr

    228
    1
    Oct 22, 2013
    It is transformer driven.
     
  13. atsu

    atsu

    8
    0
    Dec 13, 2013
    Dear elebish,
    Thank you for your answer. I have made a prototype power supply using the same HP power transformer. I have substituted the 5volt and 15 volt regulartors for the -15 +15 5 volt. THe other supplies like 146 volts 115 volts etc I have used Zener and sireis pass transistors. I have to get a pcb made or make one
    myself after I connect this to the main system. I am very careful in dealing with the HV supply.As it involves flood guns etc for varible persistence storage.
    If this feture is lost the scope is as good as any other.
    Meanwhile I muted the op amp feedback in the hvps that limits the transistor oscillating. In fact I removed
    the connection from the HV ferrite transformer. I fed unregulated 15 volts and it started the filament and the other HV voltages. The Tube is working and I get a sharp
    focus. I have left it at that. After I completely build the HVPS and get the scope commissioned I shall post all the pictures in the forum for our members benefit.
    Any way your suggestion of a switching unit is very good. All that I need is a Ferrite transformer core and winding wire, All the voltages can be obtained
    from the different windings.It will be simpler and lighter. Voltage can be regulated by feedback to the flip flop Ic. Thank you and please pass on any further suggestions.
    Atsu
     
  14. atsu

    atsu

    8
    0
    Dec 13, 2013
     
  15. atsu

    atsu

    8
    0
    Dec 13, 2013
    Thank you tryppyr. . I still feel all the effort is not wasted as in the process we learn new things.Restoration without damaging the original configuration takes some time. Thank you
    once again.atsu
     
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