WHAT ARE THE DIODES D16 AND D18 IN THE SB-200 SCHEMATIC

 Based on the Heathkit SB-200 schematic and parts list, the diodes labeled D16 and D18 are 1N4007 (or originally specified as 1N2071 / 1N1319 in early manuals, which are equivalent general-purpose silicon rectifiers). 

• Function: These diodes are part of the ALC (Automatic Level Control) and metering protection circuitry.

  • D18 specifically monitors plate current for the ALC system, developing a DC voltage proportional to the RF output to feed back to the transceiver.

  • They are typically located near the meter switch and the 20nF (C12) and 2nF (often part of the meter calibration or bypass network) capacitors on the main chassis or meter board. 

• Specifications:

  • Type: Silicon Rectifier

  • PIV (Peak Inverse Voltage): 1000V (for 1N4007)

  • Current: 1A

  • Replacement: A modern 1N4007 is the standard direct replacement.

If your unit has been modified (as many SB-200s have), verify the orientation: the cathode band (stripe) usually faces toward the meter or the ALC output line, depending on the specific schematic revision.