Trying to get a RCA VTVM WV-98A working.

Discussion in 'The Projects Forum' started by Mckayman, Sep 30, 2009.

  1. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Alright, right now before I try to work on any another equipment I feel that it is more important to get my VTVM working. The VTVM is an RCA WV-98A. The battery that was in it leaked and corrosion occured on some of the compontents. What I have done so far is taken a toothbrush with a little bit of warm water with dishsoap in it and burshed off most of the corrosion. The next thing I did was use de-ox-id contact cleaner and applied it to all of the switches and vitial components. After that I replaced all of the capactiors in it, except for the electrolyitc cap (it seemed to be in good condidtion) and a cap that was located on the function switch. This cap was rated 6uf to 30uf, i'm not sure what kind of cap this is. It looks like a paper cap. After that I applied power to the VTVM and tried taking some test measurements.
    - Problems that occurred were when the VTVM was set in the lower ranges (1.5V and 15V) the needle moved all the way to the right indicating voltages but I wasn't applying voltages at the time. The Zero control did not help to move the needle over either. Nor did adjusting some of the resistors in the VTVM. :confused: I am kind of lost now on what I should do to fix this.

     
  2. beenthere

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    Apr 20, 2004
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  3. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Thanks, i've been reading through the manual but I just am confused on what is going on here. I read that if the meter is giving you problems like the one I described it could mean that the bridge is out of balance from a bad 12AU7. I've tried different 12AU7s but it did not seem to help.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    If some other component isn't hopelessly deteriorated, that might be the case. Have you verified all the resistors and soldered connections? Some terminal strips got a ground connection to the chassis (speaking of older equipment in general) with a riveted joint - those can become very resistive.

    It's the usual fun of verifying every voltage point and every capacitor, as well.
     
  5. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    UPDATE: Well good news! I finally got it working! I checked the resistors and they all seemed to be okay. But I think that some of the componets were just too deteriorated. What I did was I went and got another VTVM just like it and then used parts form both and built a working one. :p When I put everything together I got good results. I tested the high voltage probe with it using a 0-3kv power supply and it proved to be very accurate.


    Reason why I decide to go a ahead and used the parts from the other one was that the first one I had just had way too much corrosion over everything. And I noticed that a lot of the boards in the first one were cracked and a ton glue was squirted all over them from long ago. The second one had non of these problems.
     
    Last edited: Oct 6, 2009
  6. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Take back everything I said about it working. It just quit on me. Now it is unresponsive, only reads 1v on the 1.5v on (AC and +DC) and goes into the negative on (DC+). Sigh, what a bummer.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Great - you changed out all the suspicious components, and now they're all suspect again.
     
  8. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    I've gone through and tested most of the components. Everything seems to be in check. Caps, Resistors, Tube voltages. However there is one capacitor called C2 (5-30 uf.) The manual says that it is a trimmer cap. It is located in the circuitry that is associated with the first stages of the input of a voltage source. The cap looks like a cylinder. When I take it's readings of uf's I get readings that jump from 4 uf to 22 uf. This is the only cap that I haven't replaced because I don't really know what it is or what to replace it with. Maybe this could be the problem since the meter worked perfectly for 3 hours then just quite responding on me?
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    Try jumping in some modern cap of about 10uF and see if it settles down.
     
  10. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    I'm sorry I was mistaken. The "cap" that I was refering to was not C2. C2 is a trimmer cap of 5uuf to 30 uuf. It is only involved with measuring AC voltage sources. So it is not responsible for the meters problems. The thing that I was referring to earlier is actually a 7Meg ohm resistor. (Stupid mistake on my part.)

    What happened when the meter stopped working was, I was measuring 380v +dc. All of a sudden the meter started to slowly drop from 380v. It continued to slowly drop until reached zero. I mean it was crawling very slowly. At first it was very confusing to observe. Now it is just unresponsive to any voltage input. However the zero function can still be properly adjusted. It can be adjusted form 0% to 60% on any DC range, which according to the manual means that the meter is functioning properly for DC voltages.
     
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