Diagnosing a bad Harbach soft key modification in a Heathkit SB-200 amplifier

 Diagnosing a bad Harbach soft key modification in a Heathkit SB-200 amplifier involves verifying whether the modification is present, checking for correct wiring, and testing the keying behavior with and without the mod engaged.  The Harbach soft key was designed to protect modern solid-state transceivers from the SB-200’s high negative keying voltage (approximately –120 VDC) by interposing a relay or FET. A faulty installation or failed components can cause keying issues such as the amp staying keyed, not keying at all, or damaging the transceiver. 

Step-by-step instructions

Tools and Materials

Multimeter (capable of measuring DC voltage and continuity)

Dummy load (for safe testing)

Shorting plug or mechanical switch for keying jack

SB-200 schematic (preferably with Harbach mod annotations)

Insulated tools and safety equipment (high voltage is present) 

Prerequisites

Ensure the amplifier is unplugged and discharged before opening the chassis.

Confirm whether the Harbach soft key mod is installed (look for a small PCB near the keying jack or antenna relay connections).

Have access to the original SB-200 schematic and, if possible, Harbach mod wiring instructions. 

Diagnostic Steps

Visual Inspection

Locate the Harbach board inside the SB-200 chassis.

Check for obvious signs of damage: burnt components, cold solder joints, loose wires.

Verify that the keying jack (“Ant Relay”) is wired to the Harbach board and not directly to the original keying circuit. 

Check Keying Voltage

With the amplifier powered on and warmed up, measure the DC voltage at the keying jack (RCA “Ant Relay” jack).

Without the Harbach mod, you should see approximately –120 VDC.

With a functioning Harbach mod, the voltage presented to the transceiver should be near 0 V, and the internal relay/FET should switch the high voltage internally. 

Test Keying Function

Use a shorting plug or mechanical switch to simulate keying.

Observe whether the amplifier keys and unkeys properly:

If the amp stays keyed, the Harbach relay or FET may be stuck closed or miswired.

If the amp does not key, the control side of the Harbach circuit (transistor/FET) may be failed or incorrectly connected. 

Bypass Test

Temporarily disconnect the Harbach board:

Disconnect the wire from resistor R16 from the Harbach board.

Disconnect the wire from the “Ant Relay” jack to the Harbach board.

Connect R16 directly to the “Ant Relay” jack (splice or extend wire if needed).

Use a mechanical switch or shorting plug to key the amp (do not use a solid-state transceiver in this configuration).

If the amp keys correctly in this configuration, the issue lies with the Harbach board. 

Check Harbach Board Power and Ground

Verify that the Harbach board is receiving its supply voltage (often derived from the filament choke or another low-voltage source).

Check ground connections, especially the green wire to the filament choke, as poor grounding can prevent proper operation. 

Component Testing

Test the relay or FET on the Harbach board for proper switching.

Check polarized components (diodes, capacitors) for correct orientation and functionality.

Replace any suspect components, especially if both Harbach boards (if multiple mods exist) show identical failures. 

Reintegrate and Retest

After repairs, reconnect the Harbach board.

Test keying again with a shorting plug before connecting a transceiver.

Once confirmed working, test with a solid-state transceiver, monitoring for proper keying and absence of high voltage at the key line. 

⚠️ Caution: Never use a modern solid-state transceiver to key an SB-200 unless you are certain the soft key mod is functioning correctly. The –120 VDC present on the key line can destroy transceiver keying circuits. 

Heathkit SB-200 Harbach soft key modification troubleshooting

What parts are in a Harbach soft key?

How do I test the Harbach board relay?

Where to buy SB-200 soft key replacement parts?

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Primary Cause: Insufficient RF Bypassing

The most likely culprit is a lack of adequate RF bypass capacitors on the Harbach board wiring. When RF enters the keying circuit, it can rectify in the FET or control transistor, creating a DC bias that keeps the switch closed even after the keying signal is removed. 

Specific Components to Check and Modify

1. Missing or Inadequate Bypass Capacitors The most common fix reported by users involves adding 0.01 µF (10nF) ceramic disc capacitors:

Install a 0.01 µF capacitor from each signal wire (excluding the ground wire) to the chassis ground, mounted as close to the Harbach board as possible. 

Install an additional 0.01 µF capacitor directly across the Drain and Source terminals (the two outer legs) of the FET on the Harbach board.  This prevents RF from sustaining the FET in the "on" state. 

2. The FET (Field Effect Transistor)

The FET itself may be marginal or damaged by previous RF exposure. If adding bypass capacitors does not resolve the issue, the FET (typically an N-channel enhancement mode MOSFET) may need replacement.

RF energy can cause the FET to enter thermal runaway or latch up, effectively welding the circuit closed electronically. 

3. Main T/R Relay Contacts (Secondary Possibility)

While the symptom points to the soft key, do not rule out the main T/R relay (K1) inside the SB-200. 

If the T/R relay contacts have carbon buildup or pitting, RF can arc across the open contacts, creating a conductive path that mimics a "stuck" relay.

Action: Clean the main T/R relay contacts using a strip of printer paper with a drop of 91% isopropyl alcohol, or replace the relay if heavily pitted. 

Recommended Troubleshooting Steps

Add Bypass Capacitors: Before replacing components, solder 0.01 µF ceramic capacitors from each wire (except ground) entering the Harbach board to the nearest ground point.  Add one directly across the FET's outer terminals. This resolves the issue in the majority of reported cases.

Inspect Grounding: Ensure the green ground wire from the Harbach board is securely connected to the filament choke or a solid chassis ground. A poor ground prevents RF from draining properly. 

Clean Main Relay: If the problem persists, clean the contacts of the internal SB-200 T/R relay to rule out contact welding or arcing.