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This article appeared originally in QST Magazine in the January 1988 issue. It is reproduced here in HTML.

Using the SB-220 Amplifier with Solid-state Transceivers

by James W. Hebert, K8SS

Modifying the antenna relay circuit to permit operation with solid-state transceivers

The Heathkit SB-220 is one of the most popular amplifiers ever sold. It was designed in an era when most amateur equipment was based on vacuum-tube technology. Because of this, special care is needed if the SB-220 is to be used with solid-state transceivers.

The SB-220 goes into transmit mode when the hot contact at its rear-panel ANT RLY jack (J1 in Fig. 1A) is shorted to ground, actuating K1, the SB-220 antenna relay. The open-circuit DC voltage at this jack is 125-Volts; the short-circuit current is 25-milliAmperes. Vacuum-tube-based exciters usually have no trouble switching power at this level. Solid-state rigs are a different story.

My ICOM IC-740 transceiver can't switch 125-Volts at 25-milliAmperes because the maximum ratings for its amplifier-control relay are 24-Volts and 1-Ampere DC. Other solid-state transceivers likely use relays or open collector transistors of similar ratings for amplifier control. The switching problem is complicated by the fact that the SB-220 antenna-relay solenoid is not shunted by a spike-suppression diode. The transient voltage developed by the solenoid's collapsing magnetic field can exceed the supply voltage. (If you've ever gotten a poke from a relay solenoid back EMF, you know this voltage is not just theoretical!) With the 24-Volt rating of the IC-740's control contacts in mind, a direct amplifier-control connection between the SB-220 and the IC-740 seemed to invite trouble.

Figure 1B shows my solution to this problem. With Q1 and Q2 handling the actuation of K1, voltage at J1 is reduced to approximately 12-Volts. Short circuit current through J1 is about 2-milliAmpere. Because the SB-220 must be opened to make this modification, now is a good time to install an OPERATE/STANDBY switch, S1, to save switching the SB-220's tube filaments on and off.

Schematic diagram of circuit under discussion.
Figure 1. The modification to SB220 antenna relay circuit lowers the voltage at the ANT RLY jack, J1, to approximately 12-Volts in (B) from 125-Volts in the unmodified circuit (A). Current through J1 is reduced to 2-mA in (B) from 25-mA in (A). Components J1, K1, and the 0.02-µF capacitor are original SB-220 parts. Resistors are 1/4-W, carbon-film unless designated otherwise.
D1—1-A, 600-PIV diode.
D2, D3—1-A, 50-PIV diode.
Q1—General-purpose transistor
Q2—High-voltage switching transistor, Vceo = 300. ECG287 also suitable.
S1—SPST toggle switch.

There is plenty of room under the SB-220 chassis for mounting the switching components; the entire circuit can be assembled on a tie strip and mounted to an available under-chassis screw. I installed my version of the Figure 1B circuit next to the SB-220's 125-VDC supply, just behind the SSB/CW rocker switch. (Take proper high-voltage safety precautions when you make this modification. Lethal voltages exist in the SB-220.) Dress the wiring for minimal coupling to the RF circuits under the chassis and near the antenna relay. As installed in my SB-220, this circuit shows no susceptibility to RFI.

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Copyright © 1988, 2014 by James W. Hebert. Unauthorized reproduction prohibited!

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Author: James W. Hebert
This article first appeared in QST Magazine for January 1988, page 45.
The schematic drawing was made by the American Radio Relay League from the author's design.

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