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flex 6300 and keying sb-200

clint yates
clint yates Member ✭✭
. i was wondering if i have to worried about keying a heathkit sb-200 amp? will the keying circuitry handle the sb200? thanks


  • Doug Hall
    Doug Hall Member ✭✭
    edited February 2017
    If the SB-200 is stock then you should not connect it directly to the TX key output of the 6300. The SB-200 presents around -125VDC on the key input jack which is beyond the capability of the TX output on the Flex. You either need an amplifier buffer of some type (such as the one from Ameritron) or you need a softkey modification installed in the SB-200. The softkey mod from Harbach gets good reviews.
    Doug K4DSP
  • George KF2T
    George KF2T Member ✭✭✭
    edited May 2019
    Recommend the isolation switch for sure.
  • Lee - N2LEE
    Lee - N2LEE Member ✭✭
    edited February 2017
    What radio were you keying the SB200 with before ?
    I think its safe to say that any modern rig should use a buffer to key a tube amp of this vintage.

    Here is an MFJ but there are others.

  • clint yates
    clint yates Member ✭✭
    edited March 2016
    thanks, after i posted i checked the keying current with an ampmeter  and it was 2 amps. according to flex 140 milliamps is maximum
  • George KF2T
    George KF2T Member ✭✭✭
    edited December 2016
    Good thing you didn't test it out with the Flex! Wow, that's a lot of current.
  • Doug Hall
    Doug Hall Member ✭✭
    edited July 2015
    Either the SB-200 is broken or the measurement was incorrect. There is no way a properly working SB-200 would allow 2 amps to flow through the keying circuit.

    The issue with the SB-200 and the Flex is not the current, it's the voltage. 
  • Jay Nation
    Jay Nation Member ✭✭
    edited August 2016
    The radio might not survive either the voltage or the current. Buffers ie;

    http://www.hamgadgets.com/UKA-2  $19.00
    http://www.hamgadgets.com/UKA-3  %22.00

    might be cheaper than the shipping costs to have the rig repaired.

    My UKA-2 assembled in 15 mins, and has always just worked.
    The Defective TX1 relay jack on my 5000 is the direct result of my googling for advice about the need for buffers with a Flex. Seems the 3000s are happy, with Heathkits the 5000s are not. I stopped reading the advice too soon, I guess.image
    I now buffer the Heathkit, It doesnt hurt it  any, and it keeps it from hurting anything else I connect to it. BTW the previous owner of the amp mentioned that the Harbach Soft Key mod had been installed by the owner he purchased it from.
    Safe or Sorry, You get to chose!image
    73, Jay - NO5J
  •   VE6KWA
    VE6KWA Ken Member ✭✭
    edited May 2020
    I've been using a Heil FS-2 foot switch for years. Built in Staggered switches,  keys the amp & radio in perfect sequence every time. No Hot switching... No mods to any of your amps and about $40.00  the last time I looked.  The ultimate simple solution... in my view.

  • clint yates
    clint yates Member ✭✭
    edited March 2016
    Doug you are right..it was 2milliamps..the DC voltage across the lead was 1 volt..I find that odd..I did find out the soft start mod is installed in the radio.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited March 2017
    Here is my advice.  Always use a buffer relay regardless of the amp.  It will be some of the best $40 insurance you ever buy for your radio.

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