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Central Electronics 20-A relay control help...

Discussion in '"Boat Anchor" & Classic Equipment' started by W6ELH, Dec 7, 2017.

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  1. W6ELH

    W6ELH Ham Member QRZ Page

    Hello all-

    I have a 20-A I'm trying to put into operation. All electrolytic and paper capacitors have been replaced, along with a couple of cracked or out of tolerance resistors I spotted.

    On the attempted maiden voyage, I found the relay coil to be open, but actuating the relay manually showed output on the CE MM-2 scope, so I'm still hopeful of getting this old girl back into action. A replacement coil was obtained, and when switched from "standby" to "manual," the relay now actuates as it should. However, the relay remains actuated when switched back to standby.

    The manual discusses changing the value of R-45 to control sensitivity. I presume this is for VOX sensitivity, which I have no desire to employ, but I pursued this as a starting point. I swapped in the specified maximum value of 3K without change, and just now jumped in a pot at the location and ran the resistance up to nearly 10K with the relay never dropping out of transmit despite being in the "standby" mode. The relay only de-actuates when power is switched off.

    I also find that the relay control tube, 12AT7, does not show the expected voltages per the manual:

    Pin one is: 0 VDC should be: 165 VDC (shows 365 VDC in standby)

    Pin two is: 1.0 VDC should be: 26 VDC

    Pin three is: 14 VDC should be: 27 VDC

    All others are approximately correct.

    I have swapped out that 12AT7 with a new one as well as changing out the 6AL5 - no change in behavior noted. I also looked at terminals 6 and 7 on the back of the chassis. They show the expected -100 VDC on standby and ground in transmit. Because the relay shorts pin one of the 12AT7 to ground in transmit, I'm struggling to understand how that could possibly be expected to have 165 volts on it per the schematic.

    As usual, I'm certain I'm missing something simple and obvious here, but I could use help from you "Elmers." Thanks in advance for your kindness! Cheers... Jim W6ELH

    Screen Shot 2017-12-06 at 1.00.06 PM.png Screen Shot 2017-12-06 at 1.15.55 PM.png
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    Did you check/replace C41 ?

    What is your new coil resistance ?

    Good Luck.
  3. W9GB

    W9GB Ham Member QRZ Page

    When you “swapped out” coil — ASSURE you don’t have a Mechanical or Magnetic issue.
  4. W6ELH

    W6ELH Ham Member QRZ Page


    Thanks for your thoughts, and sorry I’ve been slow to respond.

    I did hook up my meter to the two leads of the coil when it arrived. I did not record the resistance. Might have to unsolder one of the leads and report back.

    Yes, C-41 has been replaced along with all other paper and ‘lytic caps. Because I’ve been guilty of it in the past, I reviewed a “before” photo to ensure I had not installed a replacement cap with the polarity incorrect. All is good in that respect.

    “Mechanical issue?” I guess I don’t understand what to look for there. The fingers of the relay seem to move as they should, and the relay does release when power is cut.

    Again, thanks for the help fellows, and I would be grateful for any other leads to follow.

    Cheers... Jim W6ELH
    Last edited: Dec 9, 2017
  5. KK6IYM

    KK6IYM Ham Member QRZ Page


    Does the key have to be inserted and open for the relay to return to standby?

    KB4QAA likes this.
  6. W6ELH

    W6ELH Ham Member QRZ Page

    Ah! Good thinking Norm!. I tried that very early in the cycle of trying to sort this out, but haven't thought to try it again after making some changes. I'll do so tomorrow.

    Of course my troubleshooting is at the "money poking at it with a sick" level, but I surely am ready to have an 'Ah ha!' moment! I could make a bubba repair by using a momentary switch to cut power to the relay when switching to standby, but I'd really like to sort it out properly.

    How is your recalcitrant KWM-2 coming along?

    Thanks for the lead Norm... Jim W6ELH
  7. KK6IYM

    KK6IYM Ham Member QRZ Page

    The KWM-2 has been returned to life. It works well and is putting out decent power on all but 10 meters-where it is down a bit.

  8. W6ELH

    W6ELH Ham Member QRZ Page

    Good! Glad you won the battle! That one really fought you, didn't it?

    No joy with the key in the jack. I pulled the operation switch and mapped out all of the connections, ensuring they were to schematic. Also added a little extra tension to some of the spring fingers that didn't seem to move much as the switch rotor came into contact with them. I was pretty hopeful, but no improvement in function.

    Now that I have that switch all mapped out, I'm hopeful that some more cogitation will lead me in the right direction. Thanks for the help! Jim... W6ELH
  9. KK6IYM

    KK6IYM Ham Member QRZ Page

    The grid (pin 2) of the 12AT7 needs to have enough negative voltage to cut-off the tube so current can't flow above the threshold that closes the relay. Check the resistors--R41, R42, R43, R44. The key needs to be in the jack and open or the negative bias is shorted to ground through the jack. The bias needs to get from the VFO socket to R44--I think that is how the schematic shows it works. Is there a plug in the VFO jack with pins jumped?

  10. W6ELH

    W6ELH Ham Member QRZ Page


    Thank you! I'll put that info to use when I can get back on this project in a day or two. I do appreciate the help! Jim... W6ELH

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