Heath SB 303 smokes the Q511 & 112 transistors

Discussion in 'Radio Circuits, Repair & Performance' started by AC1DV, Oct 15, 2019.

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  1. AC1DV

    AC1DV Ham Member QRZ Page

    My SB303 worked fine except for a moderate audio hum. So I recapped most of the electrolytics on the BFO and the Audio boards. Replaced some old out of spec resistors. Triple checked soldering for bridges and correct components. But every time I turn it on now it receives fine for about 30-40 sec than smoke comes from the Q511 & 512 and they are toast. Replaced them with NBE 184 & 185 .. and they smoked. I checked all voltages and they are all perfect. + and - 35 and 10 volts. I also did the ohm check from the book and some of the readings on these transistors were sky high. For example the 24K ohm is about 35K. I can't seem to find the problem.. Any body have any suggestions? I'm stumped.
     
  2. K6BSU

    K6BSU Ham Member QRZ Page

    I've restored two of these. The audio PA is strange. Complex multi-transistor design similar to hi-fi amps of the day. Uses PNP/NPN "totem pole" output, with the attending complex bias components.

    Here is the secret; You MUST use MJE 371 for Q511 (PNP). MUST use MJE 512 for Q512 (NPN).

    These are (sort of) matched to operate together. I haven't found any other pair close enough to balance without re-designing the bias components.

    These transistors are available from Mouser or DigiKey.

    While you are at it, you should replace Q509 2N3053 because previous problems might have destroyed this driver transistor.
     
    W7UUU likes this.
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    The audio output is just one of the reasons that I definitely do NOT like the SB-303. Internally generated noise and extremely bad strong signal problems are a couple of other reasons.

    If you really want to use one of the Heath SB-Line receivers, sell, trade off, etc., the SB-303 and replace it with an SB-301. The SB-301 is ten times the receiver that the SB-303 is. The SB-303 was Heath's first real attempt at making an all solid-state amateur band receiver and, unfortunately, there were numerous problems with the receiver.

    Glen, K9STH
     
  4. K6BSU

    K6BSU Ham Member QRZ Page

    AC1DV seems to be interested only in getting his SB303 running again. The comments about this rcvr performance is another issue, and should be the subject of another thread.

    Yes, the audio stage is strange. But it does work as long as the correct parts are used.
     
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    BSU:

    Yes, the audio circuit works with the correct parts. However, there is a good chance that one of the output transistors will fail again. This is a well known problem with the SB-303. This, by itself, is just one of the reasons that I recommend replacing the receiver with an SB-301. The other problems add to the situation and I stand behind my recommendation.

    Glen, K9STH
     

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