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Vacuum Relay Contact failure ???

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

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  1. KA8NCR

    KA8NCR Ham Member QRZ Page


    Is the rubber mallet for crystal calibration or frequency stability for Swan radios?
     
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  2. N4ZAW

    N4ZAW Subscriber QRZ Page

    True, the 220's do have that feature, and it partially, (though, inadequately IMHO) addresses that issue underneath the chassis. In the SB220, even the tube pins still can overheat, melting the solder in them. One way to tell if a 3-500 tube from an SB220 amp has been subjected to overheating, is to inspect the pins, is if they are concave, as opposed to convex. This clearly indicates that the solder has melted after production, and the tube's internal vacuum sucked the solder in. They can literally lose their vacuum if it is extreme enough.
    I've kicked-around the idea of mounting a 160F thermal switch (as found in coffeemakers and hair driers) to one of the innermost grid pins with a bit of JB Weld. I'd wire it in series with the keying circuit. The circuit would open at 160F, thus, disabling amp TX until it cools down. RFI may be an issue I need to overcome, but cheap and Cool, huh?
    I may still do that, if when mine quits again.. Though I don't overheat amps. ;)

    I think Pete was also referring to airflow around the W7RY's PCB components in addition to the ones in the relay string.
    This board has power resistors actually mounted on it which get very hot. Not bad news on it's own, but if that component generates some of it's own heat, we have to safely dissipate it. The ways to partially-address this is to elevate and separate them, if you cannot heat-sink them, and there is very little real estate for that in this application. Maybe a baby aux fan -- Ok, I'm getting silly. Where's my coffee cup?
     
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  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    Yes... :D
     
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  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    All you need do with an SB-220 is leave the spacers on the front feet.
    That lowers back pressure and the air the fan moves under the chassis has somewhere to go.
     
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  5. N4ZAW

    N4ZAW Subscriber QRZ Page

    I bought the 220 I have now in working condition, with concave solder pins. I sold the tubes on Fee Pay, and bought a new matched pair. Yeah, they were still good -- but it's like buying a used 'Vette that belonged to a 19 year old drag-racing drug dealer... No doubt, it was "rode hard and put away wet". ;)
    In this case, an AM'er with political and alcohol issues.
     
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  6. N4IJ

    N4IJ Ham Member QRZ Page

    Don't use common flux like you find at hardware stores (lowes) - I have been using Oakey No 95 flux. It has metal particles in it to aid in solder flow.
    I used it on solder wick to suck solder out of PCB holes. If you don't do a really good job of cleaning it off after using it - you may get what I had -
    an arc to ground. In my case it carbonized the pin and I had to cut the trace to remove the megohm short. I actually had one major carbon arc and two monors
    ones. All fixed now. Explains all of my problems, and confusion. It did apparently cause the vacuum relay to heat up too much and fail.

    Now I have to think if there was any other project where I used the flux. The actual can of flux is now in the trash.

    Doug N4IJ
     
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  7. N4ZAW

    N4ZAW Subscriber QRZ Page

    I was referring to the solder that the tube was made with in the pins, but that's a good tip on the flux, and will heed your warning. I use Kester #186 liquid. I use it for everything in the shop. On solder wick, SMD's and IRDA flow, and it can be thickened by evaporation to make a nice component pin prep paste.
    I'm not sure what Taylor an Svietlana use for tube assy, but it goes molten over the 180F (Ithinkitis) of the tube spec. So if you see it concave on the pins, that tube has gotten so hot, that the solder was drawn into the pin by the tube's vacuum -- a sign it has been abused like a fighting chicken.
     

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