Icom 745 / Yaesu fl1200b

Discussion in 'Amateur Radio Amplifiers' started by VA7WCJ, Oct 11, 2019.

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  1. VA7WCJ

    VA7WCJ Ham Member QRZ Page

    So let me start by saying I'm unemployed but work side jobs to survive. I recently came across a couple of deals I couldn't say no to, Yes it's a disease .
    I purchased a good condition FL1200B linear amp and an MFJ tuner for basically 1/2 price. I have an old Icom 745, and an SGC SG-2000 that I wish to use with this amp. The 745 has outputs for controlling an amp, but from what I have read I can't use that setup because of the high amperage requirements of the amp. So I have seen a cable online that uses a relay to trigger the amp on transmit, but for the life of me can't find a schematic to build my own. As I have mentioned, I have barely any budget for my radios, and spent all my savings on the amp and tuner. If anyone can send me in the correct direction in regards to the wiring, I'm certain I can build a cable, instead of $80-$100 canadian on a $8 relay.
    73, Art
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Use a 12Vdc (coil voltage) low-current (coil current) relay of any brand or type: It only needs to be SPST but it can be DPST, DPDT or anything as long as you can access two connections: A normally open one, and a common.

    Connect one end of the relay coil to your station 12Vdc power supply. Connect the other end to your "amplifier keying" (ground on transmit) line from the 745. Use the "contacts" (which close on transmit) to key the FL-2100B using the E and RY screw terminal connections.

    To make the IC-745 internal keying circuit last longer, wire a diode (like a 1N4001 or almost any rectifier rated maybe 100V and 1A, or more) across the external relay coil in reverse polarity to the applied voltage: i.e., the cathode connects to the +12Vdc and the anode connects to the other end of the coil that goes to ground on transmit. This will help quench the kickback energy stored in the relay coil.

    Total cost should be about $5 or so.

    That's all these "external keying line" circuits do, usually. Well, they might also provide cables and stuff.
  3. VA7WCJ

    VA7WCJ Ham Member QRZ Page

    Thank you for the quick reply. I had wondered about that configuration based on the manuals for the 2 units, but couldn't find the GND on transmit, so you have saved my bacon. And thanks for the tip on longevity. I shall do some shopping in the next few days!
    73, Art.
  4. VE3TMT

    VE3TMT XML Subscriber QRZ Page

    I've been doing the same think for years with many different radios. Most radios have either a TX switched +12V or switched to ground line.

    As Steve said, wire the coil of the relay to these points and the normally open contact and common on the relay to the RLY and E terminals on the amp.

    I actually put the relay inside my 2100B and my RLY terminal is now for a switched 12V line.

  5. G3YRO

    G3YRO Ham Member QRZ Page

    I also put a small/fast 12V relay inside my FL2100 (when I had one) . . . but actually derived 12V from the amp to feed the relay . . . so all you needed was a normal low current ground to operate it, which ANY rig would happily then do.

    Roger G3YRO
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    This is what I do with all "old" amps.

    Some (actually, many) don't have 12Vdc available anywhere inside the amp, but a simple voltage doubler from a 5V filament line provides it and only takes a few components. Small relays can draw <50mA coil current, so a doubler woks fine and it needn't be regulated in any way.

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