Vacuum Relay Contact failure ???

Discussion in 'Amateur Radio Amplifiers' started by N4IJ, Jan 8, 2017.

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  1. N4IJ

    N4IJ Ham Member QRZ Page

    Recently put one of the W7RY QSK boards in a TL922A and used RJ1 relays for both input and output switching in the amplifier. I think I bought the relays at different times. Once I replaced a bad (Chinese) transistor off of ebay that was used in the board, it worked like a champ.

    For the past couple of weeks, I have found that over a period of maybe 5 minutes or so, the SWR from the input side of the amp to the transceiver would creep up, and then finally be really high.The xcvr shut down and no RF out from the amplifier. After a few minutes of rest, it would work again. Occasionally after this event I would notice the 3-500's glowing cherry red. I would have to shut the ampl down to stop what appeared to be runaway oscillation. I think I have the output relay identified as the culprit. Must not be making contact. What I am having a problem understanding is why it takes a few minutes of transmitting for this to happen. RF causing heating on the contacts on the relay ? I have the amp upside down on the bench with the bottom removed, otherwise i would never have noticed the tubes glowing red. I have used freeze mist and spraying the relay will allow a short fix. So far the tubes seem to have not been damaged. Most of the time they do not glow red and stay cool.

    Could someone try and explain why heat (RF) would cause this? Once again, if I fire the amp up right now, it will put out full power for a while. I know, just replace the relay/s, but I would like to know why the relay failed if possible.

    73 Doug N4IJ
     
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    I don't suspect ether relay to be the problem, I am thinking its something on the board that is causing the relay and the bias control on the board to come on....
     
    KA9JLM likes this.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    I would check R7. It may have increased in value. It would be good to check D1 and C3 too.

    If those are good RL-2-1 contacts could be a culprit.

    Good Luck.
     
    Last edited: Jan 8, 2017
  4. WR2E

    WR2E Ham Member QRZ Page

    Doug, what diagnostic evidence led to that tentative conclusion?
     
  5. W1QJ

    W1QJ Ham Member QRZ Page

    That board also controls the bias. Timing is of the essence with that board and if the timing is off there could be hot switching going on and the cut off/op bias timing could be causing the plates to get hot if it is hanging up.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Except 3-500Zs can operate fine with "zero bias." In fact, they were originally called "zero-bias power grid" triodes. We usually add a bit of operating bias when B+ exceeds 2000V, and add a little bit more at 3kV, and a little more than that at 4kV, mostly as a matter of efficiency. But in a TL-922, even with no bias at all they shouldn't glow bright red.

    Sounds to me like "something" is not activating the output relay. I imagine bad contacts in a vacuum relay can happen, as anything can happen, but unless it lost its vacuum I've never seen this happen.
     
    N2EY likes this.
  7. N4IJ

    N4IJ Ham Member QRZ Page

    As for the board being the issue, after this happened, I applied freeze mist to the board, areas at a time and no change (still failed).I then froze the whole board and it looked like a block of ice - no change.
    Last night I decided to freeze the input and output relays, and the output relay responded with the amp recovering and working for maybe 15-30 seconds - then failed again. Last night after it failed,
    I took a wooden ruler and carefully tapped the in and out relays, and the output relay would work again. Just in case, I disconnected the amp and re-soldered the RF conections. That didn't fix it.
    Definitely heat related, and a guess is RF heating on the contact. No proof yet. The W7RY board is set up to prevent hot switching of the relays. I didn't buy these RJ1 relays at the same time so maybe I bought one
    that had lost its vacuum ??? and then failed ??? I am just trying to understand (if I can) what happened so I don't repeat it. I have both relays jammed into the area on an "L" shaped bracket and cringe at
    the thought of taking it out to replace the relay, but it will probably come to that. Doug N4IJ
     
    AF6LJ likes this.
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    You used freeze mist around HV ?

    Maybe you do have a vacuum leak.
     
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    That is no proof that the board is Not the problem...
    Have you checked the voltage across the relay?
    What about the keying line are you sure it is at ground when the amplifier is in transmit mode?
    NEVER apply freeze mist to a vacuum relay, that is the best way to damage the seals and end up with a Non-Vacuum Relay.
    That is a real possibility; especially after freezing them.
    Good luck.
     
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    You only spray a hot tube once. :eek:
     
    AF6LJ likes this.

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