How to Remove the SWR Bridge from Your Heathkit SB-200

 

How to Remove the SWR Bridge from Your Heathkit SB-200

The internal SWR bridge in the Heathkit SB-200 is a convenient feature, but it can introduce insertion loss, become a point of failure, or limit peak power handling. Removing it creates a direct, low-loss path from the Pi-network to your antenna. 

This guide covers the removal process with ASCII schematics, formatted for easy reading on your blog.

⚠️ High Voltage Warning: The SB-200 contains lethal voltages. Always unplug the unit, wait at least 5 minutes for capacitors to discharge, and short the high-voltage capacitor terminals to the chassis with an insulated screwdriver before touching any internal components. 

1. Understanding the Stock Configuration

In the stock SB-200, the RF path flows from the Pi-network output capacitors, through the directional coupler (SWR bridge), and finally to the rear SO-239 antenna connector. 

Stock RF Path:

[ Pi-Network Output ] -----> [ SWR Bridge ] -----> [ Antenna Connector ]
                                  |
                                  +-----> [ Detector Diodes ] -----> [ Front Meter ]

2. Tools and Parts Needed

  • Soldering Iron & Solder Sucker: For desoldering connections.

  • Heavy Gauge Wire or Short Coax: RG-142, RG-400, or #12/#14 solid copper wire for the jumper.

  • Screwdrivers: To remove the chassis covers and mounting hardware.

  • Multimeter: To verify continuity after modification.

3. Step-by-Step Removal Guide

Step 1: Access the Chassis

Remove the outer cabinet and the top inner shield. Locate the SWR Bridge assembly near the rear of the chassis, situated between the large output tuning capacitors and the rear panel SO-239 connector.

Step 2: Disconnect the Detector Wires

The bridge has two small coaxial cables (usually RG-316) running from the internal diodes to the front-panel meter switch.

  1. Desolder or unplug these two small coax connectors from the circuit board or terminal strip.

  2. Tape the ends of these wires to prevent them from shorting against the chassis.

Step 3: Remove the Bridge Assembly

  1. Input Side: Desolder or unbolt the connection coming from the Pi-Network Output (usually from the junction of the loading capacitors and the band switch).

  2. Output Side: Desolder or unbolt the connection going to the Antenna Connector.

  3. Unscrew the mounting hardware holding the bridge box to the chassis and remove it completely. 

Step 4: Install the Direct Jumper

You must now create a direct link between the Pi-network and the antenna connector.

Modified RF Path:

[ Pi-Network Output ] =================================== [ Antenna Connector ]
           |                                                 (SO-239)
           |
      (No Bridge)

  1. Measure: Cut a piece of heavy wire or short coax just long enough to span the gap between the Pi-network output lug and the center pin of the SO-239 connector.

  2. Solder: Solder one end to the Pi-network output point (where the bridge input was).

  3. Connect: Solder the other end directly to the center lug of the rear SO-239 antenna connector.

  4. Insulate: Ensure the jumper does not touch the chassis or other components. Use heat-shrink tubing if necessary.

4. Verification and Testing

  1. Continuity Check: Set your multimeter to resistance/continuity. Place one probe on the Pi-network output and the other on the center pin of the SO-239. You should read 0 Ω (or very close to it).

  2. Ground Check: Ensure there is no continuity between your new jumper and the chassis ground.

  3. Reassemble: Replace the top shield and cabinet.

  4. Operational Test:

    • Connect an external SWR meter between the amplifier and your antenna/dummy load.

    • Tune the amplifier on a low band (40m) and then 10m.

    • Verify that power output is stable and SWR readings on your external meter are consistent.

5. What You Lose (and Gain)

  • Gain: You eliminate ~0.1–0.2 dB of insertion loss and remove a potential point of arcing or failure. The amplifier is now slightly more robust for high-power operation.

  • Loss: The front panel meter will no longer display SWR or Relative Power.  You must rely on an external meter or your transceiver's internal readings for tuning and monitoring.

Pro Tip: If you ever want to restore the amp to stock condition, keep the removed SWR bridge and the small detector coax cables in a labeled bag. Future owners often appreciate original parts!

Disclaimer: Modifications to high-voltage equipment are performed at your own risk. Ensure all connections are secure and insulated before applying power.

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