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AL-80A band switch repair question

Discussion in 'Amateur Radio Amplifiers' started by N8VIL, Apr 10, 2019.

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  1. N8VIL

    N8VIL Ham Member QRZ Page

    I have an AL-80A amplifier that has burnt band switch contacts for 80 and 40 meter only on one side of the wafer and the back side has redundant contacts for 80 and 40 meter which allows the amplifier to be operational and have used it since this way. The burnt contacts were there when I acquired the amplifier about about 5 years ago. I took it apart to do my yearly cleaning and decided to repair the switch. I happened to have a defective MFJ antenna switch which had similar switch wafers and the contacts were the same as the band switch wafers. The Ameritron price for a new band switch was $103.00 for the complete switch so I decided to repair it as it is in great condition other than the burnt contacts. My question is what hardware should I use to secure the replacement contacts to the porcelain wafer? I was thinking tiny brass screw and nut to secure and solder the nut in place to prevent loosening.
    Any idea what would have caused the contacts to arc to cause the damage? The redundant contacts on the back side of the wafer are undamaged.

    switch.jpg
    DSCN2133.JPG
    Burnt contacts

    DSCN2134.JPG
    Donor switch
     
  2. K6BRN

    K6BRN XML Subscriber QRZ Page

    This is a very common failure mode in tube amps that use a wafer switch for tank circuit band selection. What causes it? Well...

    1. Driving too much power into too high an SWR
    2. Improper amplifier tuning
    3. Switching the band selector while transmitting

    Just takes one "Ack! Awsh*t!" moment and the wafer is toast. Pain in the tush to repair, too. SB-200 and many others have the same issue. Henry, in their infinite wisdom, chose to use a cam and finger contact approach on the 2Ks. Almost indestructible. Until age causes the plastic cams, gears or levers to crack and fail.

    Brian - K6BRN
     
    N8VIL likes this.
  3. N8VIL

    N8VIL Ham Member QRZ Page

    I just found it odd that it only happened to the contacts on one side of the wafer.
     
  4. W1QJ

    W1QJ Ham Member QRZ Page

    I use #3 brass hardware. After the nut is secured tightly I Dremel off the balance of the screw beyond the nut head with a cut off wheel and dab solder on it so if does not loosen. Works perfectly and have been doing it for years. Most of the time a band switch will arc when too much drive is applied when the amp is too lightly loaded. There could be other reasons but when everything is right and it arcs it is because the amp is too lightly loaded. Sometimes it is a good idea especially with 3CX800 and 8877 amps to advance the load a little after more drive is applied in the loading process. Pre set markings help to avoid the whole loading process from scratch after band changes. Why they arc only on one side is probably based on which side of the switch has the least surface contact to the wiper. That is only a guess.
     
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  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    That is kinda weird, but maybe one side of the switch is better at conducting RF than the other. The lesser of the two probably heats up under stress and goes blammo first.

    edit: Lou beat me to it. :p
    Maybe the lesser of the two sides makes the other side heat up... gotta flip a coin on that one!
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The "light loading" is a problem in a lot of tube amps, and one thing a lot of users do is start out at low power to tune up the amp, then increase power and re-tune as they increase.

    This can lead to the "light loading" problem!:)

    I'd always recommend with "over-loading" an amp by using much less loading capacitance (that's a "higher number" on the panel dial on an AL-80A/B or most other amps) and then dialing it "down" to more loading capacitance to find the sweet spot for efficiency -- rather than the other way around (which would be, beginning with "light" loading, or a lower number on the dial, and then increasing it as drive increases). This way is easier on everything including the bandswitch.
     
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  7. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I witnessed this issue in the shop of one of our good Zed friends. He started with a “new to him” amp with the load control all the way down, keyed the amp, and before I could say something, arced the control to its grave.

    That took about 1/2 a second.

    The “drown your sorrows” margarita afterwords was pretty tasty...
     
    Last edited: Apr 10, 2019
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  8. N8CBX

    N8CBX Ham Member QRZ Page

    Setting a triode amp load control: There's a point where the grid current will climb at a higher rate (grid accelerates faster upward while setting the load when keying the amp) and you don't want to load it there, but increase the loading more. It can be seen saying "hello" while setting the load control.
    Jan N8CBX
     
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    ^^^ indeed ^^^
    Another corollary of this happens when someone want to run the system at medium power. They'll 'load' the amp up at 20 watts drive or so and the overshoot spikes from the exciter (little short bips of high power before the power limiting kicks in) can arc the cap.

    The moral of this story is that it's better to have much too much than a little too little as far as Loading goes. Start with the control to the right of where it should be, and don't try to peak the LOADing until you are running at high power.

    Sorry N8VIL! One off topic de-merit point for me... but interesting pictures that you took. Thanks for posting those.
     
  10. N8VIL

    N8VIL Ham Member QRZ Page

    Don't be sorry. Always a great discussion and learn a lot. That is one thing I have always done is have the loading control set heavy when tuning for the unknown. Off to find #3 machine screws and nuts.
     

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