Vacuum Relay Contact failure ???

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

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  1. AI3V

    AI3V Ham Member QRZ Page

    No its either 300v OR 500ma IF power dissipation and junction temperature limits are otherwise met.

    AF6LJ likes this.
  2. WA7PRC

    WA7PRC Ham Member QRZ Page

    The schematic shows some series R, as well as the input relay in series. I didn't research any further to guesstimate the actual current. You could just measure it (or the voltage).
  3. N4ZAW

    N4ZAW Subscriber QRZ Page

    Measurement will be tricky to take, but I do plan to do that if and when I need to go back into the amp. And yes, it looks like there may be an issue with mine there.
  4. N4IJ

    N4IJ Ham Member QRZ Page

    Bryan (WA7PRC) and others - I saw the post about your W7RY board having the MPSa42 relay driver transistor fail a couple of times on you.
    My original post was about my TL-922 and vacuum relays which is working ok now. I recently put a W7RY board in an SB-220, got it working ok, and now with an hour of testing it looks like the MPSA42 driver is gone also like you had. The transistor does not shut off completely and leaves the relays keyed all the time - with correct base input. Maybe it is something to do with heat, as the series resistors in the keying line
    are nearby on the circuit board. Or maybe the base voltage when keyed? More to come.

    Doug N4IJ
    Last edited: Apr 28, 2017
    AF6LJ likes this.
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    Mine has never failed. I suggest you contact Jim W7RY. Since he is the designer, manufacturer, and seller, he would be the only one to deal with it.
    AF6LJ likes this.
  6. N4ZAW

    N4ZAW Subscriber QRZ Page

    That could be the case for sure.. And well -- while I hate seeing you, or anyone else have this issue, I'm glad to learn that I didn't simply buy two bad MPS42's in a row. BTW, I purchased more of those , and built another W7RY QSK board in the event it ever fails a third time.

    My amplificator is still hanging in there for now, but I'll be anxious to learn of your findings, even if I go "toes-up" before it does again. Are you running twin-vacuum relays, or reed/vacuum combo? I was also wondering (since you're in there as we 'speak'), if you could perform a voltage/current test so we can compare your voltage drop on the relay circuit under operation (something I failed to check) against that of the theory of operation outlined by the design.
    Last edited: Apr 28, 2017
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    Are you guys following ESD protocol??
  8. N4ZAW

    N4ZAW Subscriber QRZ Page

    Too late to edit (again), so my sorries for double-posting in-advance...
    But (and I realize this is a very rare long-shot) this issue could still be the MPS42, whwn placed in-real-world applications like this, is not meeting the spec shown on it's datasheet. That would share some very 'rare air' with other components, but would not be impossible (like the original "ECG123" when they went from metal case to plastic).
    We're ALL still here, on this side of the grass, (I pray). Besides, let it be known, that if I'm ever found to be at room-temp, with my hand in an amplifier, that this was one way I would actually prefer to 'go out'... Not to convey that I'm ready. ;)
    Last edited: Apr 28, 2017
  9. N4IJ

    N4IJ Ham Member QRZ Page

    I suppose ESD damage is a possibility for the MPS A42, although I have read that ESD on BJT is rare. If it failed due to ESD it would be
    a case of it deteriorating over time since it was working ok for a day or two. I use ESD protection but to be honest I usually only worry about it on
    MOSFET devices. The apparently failed MPS A42 has 0 volts on the base in RX mode, and .7 volts in TX mode. This is on the other side of a resistor from the transistor driving it. which
    has about 0 volts RX and a little less than 8volts volts TX (good). The MPSA42 collector that feeds the relay string varies from 26volts in RX and 24 volts TX mode.
    It should have about 115 volts RX (transistor cut off) to 0 volts in TX (transistor totally on. The plus (+) side of the relay string is a proper 115 volts. Not having gone any further yet,
    I expect that 24 to 26 volts on the collector means there is enough current through the string to activate the relays. The relays are activated all the time. Since I have not documented all the voltages on a properly operating board, I don't know what a good board would be putting on the base of the MPSA42. I am suspicious that on a good board it may be approaching the maximum base voltage of the
    MPSA42 which is 6 volts according to the specification. No proof of that though.

    I looked around for another tougher replacement for the MPSA42 and didn't find anything.

    Doug N4IJ
    N4ZAW and AF6LJ like this.
  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    ESD = Electrostatic Discharge (link). The potential is for damage to the device, not the technician.

    While the most potential for damage from ESD is to MOS devices, with a significant charge, a PN junction can be destroyed or even damaged (and fail prematurely). The thing about ESD damage is, you don't know WHEN the device will fail.
    AF6LJ likes this.

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