HEATHKIT SB-220 Desktop Shortwave Linear Amplifier Instruction Manual
- June 5, 2024
- HEATHKIT
Table of Contents
HEATHKIT SB-220 Desktop Shortwave Linear Amplifier
SK-220 SOFT-KEY MODULE
The SK-220 is an easy to install interface module which allows your transceiver to key the SB-220 seeing only 0.7 volts at 1.5 mA. The original SB-220 circuit had +120 volts on the keying line.
Parts
- (1) SK-220 “Soft Key” Circuit Board Assembly
- (1) 6-32 x ¼” Round Head Screw
- (1) Nylon Threaded Spacer
- (1) Each – Red, Dark Blue and Black Wires
Tools
- Straight Head Screwdriver (long)
- Wire Cutters/Stripper
- ¼’ Socket w/Ratchet and Extension
- Soldering Iron
- Solder
- Flux
- Book
Your Soft Key board may not have the wires connected when you receive it. If not, strip about 1/8th of an inch back on each wire (both ends)
Insert wires from the top of the board and solder from the bottom. Dip the
tips in flux and get a little solder on the tip of the iron. Then briefly
touch the iron tip to the end of the wire sticking through the board. Do not
overheat. 
Instructions
The following instructions are from Harbach Electronics website. Any of my
comments will be in red text.
To start the installation, read these instructions very carefully.
Now unplug the amplifier and remove any input, output and control cables that
may be connected to the back of the amplifier. Place the amplifier on a book,
front panel up, and remove the bottom screws holding the feet and case in
place. This removes any pressure from the screws caused by the weight of the
amp, if you do not do this it will cause difficulty, if not damage, while
removing the screws.
Lift the case up and remove it from the amplifier.
Place the amplifier on the bench with the front panel facing you. 
Remove the perforated top shield.
Rotate the amplifier onto its side with the transformer down (it is more
stable this way). CAUTION: Do not handle the amp by the aluminum rails on the
bottom side of the amp! They bend easily.
You are now ready to proceed with the installation.
From the bottom of the chassis, locate the 6-32 nut (1/4”) that is between the tube socket and the rear panel. It secures a screw that holds the RF shield (above the chassis) to the main chassis.
Locate the screw in the tube compartment that is held in place by this nut (it is near the corner of this compartment at the rear). Loosen this screw and remove the nut and lock washer from the underside of the chassis.
Screw the threaded nylon spacer from the SK-220 kit onto the screw (freed in
the previous step) and tighten well without stripping the threads in the
spacer.
Note: Before this step remove the screw that came with the spacer before
tightening it down on the existing screw or it will not tighten down.
- Harbach’s instructions say to mount the unit next.
- Rather than work in a tight space I have changed
- The order slightly. Hold off on mounting the unit.
- Unsolder (or cut very near the solder connection) the BLUE wire that goes to the center pin of the antenna relay jack on the back panel.
Strip about ¼” of insulation from this wire. Pass the wire under the nearby
wire harness, bringing it over near the SK-220 circuit board.
Wrap and solder this BLUE wire to the terminal post on the SK-220 circuit
board. 
Mount the SK-220 circuit board onto the nylon spacer using the supplied 6-32 x
¼” screw.
Note: You may need to use the existing washer that was with the old nut,
before it will tighten down.
Position the board so that it is parallel with the fan cut out edge and the
large transistor is away from the cut out edge. 
Pass the BLACK wire from the SK-220 circuit board under the wire harness and
solder it to the ground lug on the ALC jack on the rear panel. 
Pass the DARK BLUE wire from the SK-220 circuit board under the wire harness and solder it to the center pin of the antenna relay jack on the rear panel.
Pass the RED wire from the SK-220 circuit board under all of the wiring toward
the front of the chassis to the terminal strip that holds the +110 VDC supply.
Run it through existing tie straps if possible for a neater job.
Solder this RED wire to the terminal closest to the front panel. Also
connected to this terminal are the “+” lead of a 20mF capacitor, a 22K
resistor, a diode lead and a red wire going to the antenna relay. 
Dress all wires close to the chassis.
Replace the perforated cover and top rear plate cover. Reinstall the 15 sheet
metal screws holding these pieces in place. DO NOT over tighten these screws,
as it is fairly easy to strip the screw threads in the aluminum underneath.
Use the book to hold the amplifier while you reinstall the case and feet.
Note: You may as well order the replacement feet (BF-101) from Harbach while
you are at it (if you do not mind that they are brown) they are inexpensive
and will not mash like the originals.
This completes the installation of the SK-220 “Soft Key” module. You may not
SEE any difference in the operation of your exciter or amplifier, but you will
know that you have reduced the stress on the exciter’s keying relay by many
orders of magnitude. 
SK-220 “SOFT KEY” MODULE
HARBACH ELECTRONICS
Contact Info
Jeff Weinberg – W8CQ
468 County Road 620
Polk, OH 44866-9711
419-945-2359
http://www.harbachelectronics.com
info@harbachelectronics.com