Heath SB 303

Discussion in 'General Technical Questions and Answers' started by AC1DV, Oct 5, 2019.

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  1. AC1DV

    AC1DV Ham Member QRZ Page

    I'm working on an old SB303. A couple of issues. While I was checking the resistors on the BFO and Audio boards I noticed at least one or 2 resistors that were not matching what the schematic called for. Does anyone know of any factory changes for this rig? Or know were I might find this info.
    Also in doing the tests on the BFO board the all check out good (all be it VERY high) except the +10 volt. The Diode and resister in question were fine. Were would I test the +10 and +30 volt lines?
     
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    What resistors and what values?

    Do you have the complete SB-303 manual?

    The actual power supply is contained on the power supply / BFO board. The 10-volt line starts at the junction of ZD-601 and C-617. The 35-volt (not 30-volt) line starts at the junction of D-602, D-603, and R-614. All of these are on the power supply / BFO board.

    There are wires, going to other boards, that have the 10-volt, 15-volt, and 35-volt voltages. The 10-volt lines are green wires, the 15-volt lines are orange wires, and the 35-volt lines are red wires.

    Don't expect all that much performance from the SB-303. It was Heath's first attempt at a solid-state, amateur band only, receiver. The receiver badly overloads in the presence of strong signals, most examples have a high internal noise level, and the audio output transistors have a high failure rate.

    Make sure that there is a 47-ohm "dummy load" (a 47-ohm resistor soldered between the center pin and shell of an RCA phono plug) installed in the RCA connector marked "LMO Out" on the back of the receiver unless the receiver is connected to either an SB-400 or SB-401 transmitter with 24-inch long sections of RG-62/U coaxial cable. If this "dummy load" is not installed, there will be all sorts of problems with the receiver.

    Glen, K9STH
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page



    ^I must agree. I didn't build one from a kit, but did buy one second-hand from the builder, who was a Navy electronics technician who did a very good job of assembling it (Jim, K3JH) and I couldn't find anything wrong with his assembly. It worked, but was quite a disappointment compared with my SB-301, which I did build from a kit and definitely outperformed the '303.

    Sadly, the '303 was used in a portable operation (Field Day) by a local group in NJ c. 1980 or so and they had some nearby lightning that took out the '303 and caused it to be "dead." Their tube gear kept working but a few SS rigs died, and that was one of them. Took me a month of part-time screwing with it to bring it back to life, as there were three different issues that took time to find. I sold it right after that!:p
     

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