ICOM 720 A connect to Heathkit SB 220

[TRI International]

I'm trying to connect my Heathkit SB 220 to my ICOM 720A transmitter without burning out the radio. I did hear that it appear that I can connect then to each other but Im looking for a 2nd opinion.

Thanks in advance

[RobRich]

The SB-220 has a stock key line at 120vDC. Many "modern" radios will not handle it.

The 720 is rated at 24vDC max on the key line. The 720A is likely the same.

There is a soft key mod for the SB-220 that drops the key line to a few volts or less. There is a chance it might already be done due to being a popular mod. An example:

https://harbachelectronics.com/shop/heathkit-sb-200-sb-201/sk-201-soft-key-keying-interface/

If not present you will need either the soft key mod installed or use an amp key line buffer interface. Most buffers are pretty much an external relay. There are various models from Ameritron, RadioDan, etc. Dropping a quick search I also noted W2ENY has a cable with an integrated relay for the Icom 720 and similar.

https://www.eham.net/reviews/view-product?id=4770
https://www.eham.net/reviews/view-product?id=9957
https://w2eny.com/icom_24_relay/

[TRI International]

Thanks I do have a Ameritron arb 704 but it shows that you connect it by connecting from the multi port to the back of the radio using a din to din. The 720A doesn't have a spot for that din. I will check again later.
If you happen to have a picture diagram of how to connect the 2 with the ameritron I would be grateful.
Thanks again

[RobRich]

That 25vDC key line limit spec aside, it looks like the 720A does not actually have a dedicated send jack. I do note the below manual has an accessory jack pin diagram and descriptions.

https://www.universal-radio.com/catalog/hamhf/720Arear.jpg

Looks like accessory pin 6 delivers 8vDC when transmitting, so I suspect that should suffice for activating your buffer. That pin and a ground to the radio RCA jack on the buffer, then a RCA cable between the buffer and amp. Configure the ARB-704 jumpers for a postive send setup. Manual below.

https://cdn.shopify.com/s/files/1/0289/7782/3843/files/ARB-704.pdf?v=1586534038

[TRI International]

Rob here is what I have so far, I have the Ameritron 704 connected to the Heathkit SB-220 amp. (ACL to ACL then AMP to Relay. I'm trying to connect the ICOM 720a to the Ameritron.

[RobRich]

Take a look at the 720A manual linked earlier. Forget the din cables. It has a layout diagram showing what the various pins on the 24-pin accessory jack do. ;) Pin 6 supplies 8vDC positive when transmitting, so you can use that to trigger the ARB-704.

Got an old RCA audio or video cable? Cut one end off. Perhaps blob some solder on the bare wires. Jam the wire going to the center of the RCA connector into pin 6 on the Icom accessory port. Jam the other wire going to the outside of the RCA connector into whatever pin supplies ground on the Icom accessory port.

Connect the RCA connector side of the cable to the radio RCA port on the ARB-704. You will need to set the jumpers in the ARB-704 to a positive setup, which detects when the key line goes above 3vDC. The jumper settings are in the earlier linked manual.

Connect the ARB-704 to the SB-220 ant relay jack with a RCA cable. Connect power to the ARB-704, and you should be in business. :)

BTW, you can connect the ALC line if desired, or just be careful not to overdrive the amp.

[TRI International]

Thanks for the information this radio doesn't have the accessory socket installed wow. Happy 4th and enjoy the rest of your day.

[RobRich]

To cover the bases, there are a couple of other options even if missing the 24-pin accessory jack for whatever reason.

One would be to find the control line to the PA board inside the radio, then branch off a drop to an RCA jack or whatever on the back of the radio. In turn that would connect to your ARB-704. If you are not comfortable digging into the radio, a radio tech at an amateur or even CB repair shop probably could do it in a few minutes if interested. I doubt it would cost much.

The other option would be to key the ARB-704 from your mic, handswitch, footswitch, etc. Again might take a few minutes for a tech to wire up.

[TRI International]

Thank you you have provided some great options to look into. I'll see what we can work out here.