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Refurbishing a Kay attenuator

Discussion in 'Homebrew and Kit Projects' started by WB0RIO, Jun 13, 2018.

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  1. WB0RIO

    WB0RIO Ham Member QRZ Page

    I recently purchased a used Kay attenuator found on Craigs List for $15, the date code was 1966.
    Here's what it looked like when I got it:


    The rubber switch covers had lost their flexibility and some of the switches would not stay in the up
    position. I removed all of the switch nuts and carefully clipped off the rubber covers with a pair of wire
    cutters. Some of the switches were intermittent so I sprayed the inside of the box with de-oxit spray, worked
    all of the switches and let it dry for a couple of days to evaporate the solvents.
    I cleaned the outside of the box and switches using rubbing alcohol and tissues.
    Everything seems to be working nicely now.

    I'm pretty impressed with the quality of this device, the switches are custom and the interior is
    all silver plated. There are even cast metal separators in the center of each resistor pi network
    for increased isolation.

    One unusual thing I noticed was a number of small capacitors (?) across the grounded resistors,
    I've never seen that in a step attenuator. I'm guessing they are some kind of compensation devices
    for UHF. Ideas?
    VK4HAT, W1BR and AI3V like this.
  2. VK2TIL

    VK2TIL Ham Member QRZ Page

    A very nice instrument from what I call the electr0-mechanical era when mechanical construction was as important as electronics.

    Kay were attenuator specialists in their day and would have known all the "tricks"; the capacitors are probably, as you surmised, for compensation.

    The frequency limit might be about 100 - 200 MHz; just a guess, it would be interesting to know how it performs.

    I have a nice Kay rotary attenuator bought from the US in the good early days of ebay when "junk" could be found at low prices and postage was affordable; it goes to about 120 MHz.

    You were fortunate to get it at the price; Kay attenuators are very rare in VK so I had to make a copy;

  3. AD5HR

    AD5HR Ham Member QRZ Page

    I have the model 30-0 432d.
    It is labeled as 50 Ohms, and Pmax= 1 Watt.
    Also says 0db insertion loss. It has no switch covers, and straight BNC connectors.
    Labeled KAY on lower left, and Pine Brook, N.J. on the lower right.
    I got it in a box of patchcords.(think it was $5.00 with 20 or so cords)
    Milliwatting anybody?
  4. W9GB

    W9GB Ham Member QRZ Page

    Last edited: Jun 13, 2018
    WB0RIO likes this.
  5. N3DT

    N3DT Ham Member QRZ Page

    I have an HP one that has the long shaft that switches each section at it's location, but someone put too much power in it and burned up some of the resistors, plus the micro-switches were intermittent. The shaft had cams on it to turn on and off the microswitches. Anyhow I replaced the resistors with 1% from Mouser, and cleaned the switches as best as I could, but the thing never did work right. Too bad, it was well made too. 0-12 dB range.
  6. W1BR

    W1BR Ham Member QRZ Page

    I can give you a suggestion on how to get the Microswitch contacts working. Resolder the connections. That apparently cleans off the internal residue that causes poor conductivity on the contacts. It worked for two of my vintage HP attenuators.

    KD2ACO likes this.
  7. W1BR

    W1BR Ham Member QRZ Page

    Those capacitors compensate for the stray inductances for the wire leads and switches. You will often see the same thing done on RF transformers to compensate for inductive reactance.

  8. KD2ACO

    KD2ACO Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    Pete's suggestion for burning the film off of a switch contact is a nice one. There's another method that works with switches that don't have to switch current. It's the spark of the arc that cleans off the contact face and switches have a 'minimum current' they're supposed to switch in order to keep them clean over the long haul.

    When circuits don't switch current, the contacts build up a film and this film can be burned off (gently, please).

    You can use a 9v battery (or suitable power supply), a couple hundred ohm resistor, and alligator clips to supply enough voltage and current to make the switch work a little. Flip the switch a few times to clean up the contacts. I've had to do this with switches in logic circuits over the years.
  9. N3DT

    N3DT Ham Member QRZ Page

    I'll try that with my old HP rebuilt. See what happens, thanks. It's such a nice piece.

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