SB-200 Bias Correction & Soft Keying: Complete Installation Guide Based on CTR Engineering / W8JI Specifications

 SB-200 Bias Correction & Soft Keying: Complete Installation Guide

Based on CTR Engineering / W8JI Specifications

This guide upgrades the Heathkit SB-200 bias supply to eliminate IMD ("splatter") and adds a safe keying interface for modern radios.

1. Required Equipment & Tools

Item Specification Purpose

Soldering Iron 40W+ with chisel tip For heavy gauge wiring and ground lugs.

Multimeter Digital (DMM) To verify bias voltage and idle current.

Wire Strippers Standard For 18-22 AWG wire.

Heat Shrink Assorted sizes Insulating high-voltage joints.

Desoldering Tool Solder sucker or braid Removing old components.

Insulated Screwdriver High-voltage rated Safe chassis adjustment.

Dummy Load 50Ω, 1kW+ Safe testing without antenna.

2. Parts List (BOM)

Ref Des Qty Part Description Digi-Key / Mouser Part # (Example) Notes

D1, D2 2 Zener Diode, 3.9V, 5W 1N5335B-TP (Mouser) Critical for bias lock.

C1 1 Capacitor, 0.1µF, 200V+ EF1104K-ND (Film) or Ceramic Bypass for Zeners.

R1-R4 4 Resistor, 33Ω, 1W, Non-Inductive 33QBK-ND (Carbon Comp) Grid stoppers (One per tube).

Q1 1 Opto-Isolator / Buffer Kit SK-201 (Harbach) or CTR-RB-1 Recommended: Buy pre-built buffer.

Wire 1 ft TFE/Teflon Wire, 18-22 AWG M22759/11-22-9 High temp/ HV insulation.

Terminals 5 Crimp Lug, #6 or #8 Ring WM1000-ND Secure chassis grounds.

Note: If building a discrete buffer, you will also need a high-voltage N-MOSFET (e.g., VN10KM) and a 5mm LED.

3. Part A: Bias Supply Correction (Zener Mod)

Objective: Replace the lower bias resistor with Zener diodes to lock the grid voltage, preventing it from shifting ("waddling") during transmission.

Step-by-Step Instructions

Safety First: Unplug the amplifier. Wait 15 minutes. Discharge HV capacitors with a grounded screwdriver.

Locate Components: Turn the chassis upside down. Find the Bias Supply area near the power transformer and relay. Locate resistor R16 (33Ω) which connects the relay coil/grid circuit to the negative return.

Remove R16: Desolder and remove the stock 33Ω resistor (R16).

Prepare Zener Stack:

Take two 3.9V 5W Zener diodes.

Twist their leads together in series: Anode of D1 to Cathode of D2.

Orientation Check: The remaining Cathode (Striped) of D1 goes to the Grid/Relay side. The remaining Anode of D2 goes to the Negative Return/Ground side.

Install Zeners: Solder the Zener stack in place of R16.

Install Bypass Capacitor: Solder the 0.1µF capacitor directly across the entire Zener stack (from the top of D1 to the bottom of D2).

Install Grid Stoppers:

Go to the tube sockets (inside the RF cage).

Cut the wire at Pin 4 (Grid) for each tube.

Solder a 33Ω 1W resistor directly to Pin 4.

Reconnect the circuit wire to the other end of the resistor.

ASCII Schematic: Bias Correction

       TO TUBE GRIDS (Pin 4) & RELAY COIL

                |

                |

          .-----+-----.

          |           |

        [ R1 ]      [ R2 ]  (Existing Upper Divider Resistors)

          |           |

          +-----+-----+

                |

                | (Point A: Connect Zener Cathode Here)

                |

          .-----|<|-----.  <--- D1 (3.9V 5W Zener, Stripe to Point A)

          |           |

          |     +     |

          |     |     |

          '-----|<|----'  <--- D2 (3.9V 5W Zener, Stripe to D1 Anode)

                |

                | (Point B: Connect Zener Anode Here)

                |

          .-----||-----.   <--- C1 (0.1µF Bypass Capacitor)

          |           |

          |           |

        GND         GND    (Chassis / Negative Return)

LEGEND:

[ R ] = Resistor

|<|   = Zener Diode (Stripe = Cathode)

||    = Capacitor

4. Part B: Soft Keying (Relay Buffer)

Objective: Isolate the modern radio from the SB-200's dangerous -120V keying line. Recommendation: Use a pre-built board (Harbach SK-201 or CTR Buffer) for reliability. The diagram below shows the connection logic.

Step-by-Step Instructions

Mount Buffer Board: Secure the buffer board to the chassis floor using a screw or double-sided tape, away from high-voltage points.

Input Wiring (To Radio):

Run a shielded cable from the buffer board "Input" terminals to a new RCA or 1/8" jack on the rear apron labeled "PTT IN".

Connect Shield to Ground, Center to Input+.

Output Wiring (To Amp):

Locate the original "Key" jack wires on the SB-200 (usually going to the relay coil circuit).

Disconnect these wires from the front panel jack.

Connect them to the buffer board "Output" terminals (Polarity usually does not matter for the switch side, but follow kit instructions).

Power Wiring:

Connect the buffer board +V and -V (or GND) to the SB-200's internal -120V DC supply points (often available at the relay coil terminals).

Note: The buffer uses this high voltage internally to switch the relay; it does not output it.

ASCII Schematic: Soft Keying Interface

       MODERN TRANSCEIVER

       (Safe PTT Output)

              |

              | (3-5V Logic / Ground Closure)

              |

        [ Rear Panel Jack ]

              |

              +---(Shield)---+

              |              |

              | (Center)     |

              |              |

       +------+------+       |

       | INPUT       |       |

       |             |       |

       |  BUFFER     |       |

       |  BOARD      |       |

       |             |       |

       | OUTPUT      |       |

       +------+------+       |

              |              |

              | (Switched -120V Internal)

              |

       .------+------+------.

       |             |      |

     [ Relay ]    [ Coil ]  |

       |             |      |

       +-------------+------+

                     |

                   GND

NOTE: The Buffer Board internally switches the SB-200's -120V using the safe 3-5V signal from the radio.

NO high voltage ever reaches the transceiver.

5. Testing & Calibration

Step 1: Bias Verification (No RF)

Plug in the amplifier (Standby mode).

Set Multimeter to DC Volts.

Connect the Black Lead to Chassis Ground.

Connect the Red Lead to Pin 4 (Grid) of a tube socket (carefully, while powered).

Reading: You should see approx -60V to -70V.

Idle Current Check:

Switch to Operate (Relay clicks).

Watch the front panel Plate Current Meter.

Target: 90mA – 110mA total.

Adjustment: If current is too high, add another 3.9V Zener in series. If too low, remove one.

Step 2: Keying Safety Check

Turn Amplifier OFF (but plugged in).

Set Multimeter to DC Volts (200V range).

Measure the voltage at the new "PTT IN" jack on the rear.

Result: Must read 0.0V. If you read -120V, STOP. Your wiring is incorrect and will kill your radio.

Step 3: On-Air Test

Connect Dummy Load and Transceiver.

Transmit SSB voice.

Monitor plate current: It should swing smoothly with your voice and return instantly to the stable idle point (90-110mA) when you stop talking.

Listen to your signal on a receiver (or WebSDR): It should be clean with no "splatter" or distortion.