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SB-220 Schematic by N0SS

Discussion in '"Boat Anchor" & Classic Equipment' started by K5DBX, Jan 20, 2011.

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  1. K5DBX

    K5DBX Ham Member QRZ Page

    Many of us out on the web looking for a schematic for the SB-220/221 schematic, Tom Hammond, N0SS has graciously done a redraw of this iconic amp.

    However, there is one crucial and very important error in this drawing in the power supply section, where the 10 amp curcuit breakers are placed AFTER the voltage selection barrier strip instead of properly placing it BEFORE the barrier strip. This error in the schematic will still leave voltage energized inside BOTH transformers if jumpered for 120v operation, and provides NO interruption of voltage as designed.

    Anyone using this drawing for a basis for power supply wiring should be made aware of this potentially lethal error.
    Last edited: Jan 20, 2011
  2. WA7PRC

    WA7PRC Ham Member QRZ Page

    Good catch. But, it wasn't Tom/NØSS who made the error. It was Heathkit who made the error. From the same manual, the schematic and assembly drawing (attached), you can see the circuit breakers are not wired the same.
  3. K5DBX

    K5DBX Ham Member QRZ Page

    Thanks for showing that. This is the print I have from 1976.
  4. W9GB

    W9GB Ham Member QRZ Page

    David -

    Tom does accept and receive e-mail messages.
    N0SS at ARRL dot NET

    I have regularly sent him data on amplified Motorola speakers,
    especially when I find paper documentation of later production changes on the same model number!!

    Have you sent him this observation on the Heathkit SB-220 schematic for correction or adding the additional notation on his redrawn schematics???

    I would have taken that step FIRST, before posting here on QRZ -- THEN linking to the corrected information so the entire amateur community
    could immediately benefit. That is my standard operating procedure.

  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    The documentation I found came from the 'net (unknown year). My SB-220 manual (©1969) is the same. Apparently, they fixed it somewhere between those years.

    Good idea!
  6. K5DBX

    K5DBX Ham Member QRZ Page

    Before this thread goes south, which was not my intent, let me state this. I have sent Tom an email before posting here, and this ISN'T a slam on Tom or anyone for that matter. Tom faithfully reproduced the original drawing from Heath, and graciously has spent his time and energy in that effort. He should be commended.

    The only thing I was concerned with was safety. And somewhere between the SB-220 and the SB-221, I think maybe Heath realized that.

    The purpose of this thread was to simply to inform of potential hazards on one of the most popular second hand amps out there in the "boat anchor" market.
    Last edited: Jan 23, 2011
  7. K5DBX

    K5DBX Ham Member QRZ Page

    Well Bryan, I'm seriously considering changing the line protection from a thermal device (circuit breaker) to a fused configuration. I can understand the thought process of placing the breakers after the voltage selction barrier strip, from a current perspective, using a single 10 amp breaker-per-side solution for both 120v and 240v operation and placing it after the voltage selection barrier, and that's okay until you actually have a problem with the transformer in 120v operation. If the breaker trips on one side, the other side is still engergized. By using a fused solution before the terminal strip would allow for fast-acting fuses and allow one to use 10 amp fuses for 240v operation, and 20 amp for 120v operation.
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