HeathKit SB-303 Diode Specs

Discussion in 'Radio Circuits, Repair & Performance' started by W8RLC, Jan 22, 2020.

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  1. W8RLC

    W8RLC XML Subscriber QRZ Page

    I am currently restoring a HeathKit SB-303. The receiver is missing Diode D513, G.E. STB-620 or IG1270, on the IF/Audio PCB. This is part of the S-Meter amp circuit. This diode is Stabistor. If you have the spces for this diode or have a source for it. Please advise. Also will consider purchasing the entiere IF/Audio PCB if necessary.
  2. K9STH

    K9STH Ham Member QRZ Page

    From what I can determine, DZ-513 in the Heath SB-303 (I do have an original manual for the SB-303) is basically a high stability zener diode. A stabistor is a tunnel diode used, in this case, as a voltage reference that has a very stable level.

    During the 50-years since the SB-303 was designed, component tolerances have become much better and normal zener diodes have much closer tolerances. The voltage measurement, when in standby, shows to be -1.8 VDC on the emitter of the transistor and 0 VDC when operating. Considering that there is a 270-ohm resistor from the 10-VDC "rail" and the stabistor (zener) and then another 270 ohm resistor between the stabistor and the transistor emitters, I suspect that the zener action is somewhere around 3-volts.

    If you cannot obtain the correct part, I would try using a normal zener with voltages between 2.0 and 3.3 volts. I really don't think that this voltage is all that critical. However, by trying different zener diodes you should be able to find the range in which the circuit works properly. Also, even better would be for someone who has the extender cards for the SB-303 who could actually measure the voltage present on the stabistor during receive and then the proper zener value would be known. Unfortunately, most of the SB-303 extender cards are long gone!

    Do not expect all that good a performance from the SB-303. It was Heath's first attempt at an all solid-state amateur radio band only receiver and there are all sorts of problems with the receiver. Things like the front end badly overloading cause all sorts of problems. Both the original SB-300, and the much improved SB-301, are considerably better performing than the SB-303.

    Yes, I did own an SB-303 and still own an SB-301. In addition, over the years, I have worked on a number of SB-300, SB-301, and SB-303 receivers and am quite aware of the performance of these receivers.

    If you do not have a copy of the complete manual for the SB-303, I strongly suggest getting such a copy. There are a lot of things involved with the SB-303 and having the manual definitely makes things easier.

    Glen, K9STH
  3. W8RLC

    W8RLC XML Subscriber QRZ Page

    Hi Glen,

    Thank you for the feedback. I am aware of the reported shortcomings of the SB-303 therefore, I am not expecting it to preform as well as the the previous SB-300 series receivers.

    Another has also suggested a zener as well. You are correct it would be a very big help if someone that has an operating SB-303 measure the voltage at the junction of the 270 resistors and the diode while operating.

    Once I find a good resolution to this problem I will post the results.

    Bob, W8RLC
  4. K1APJ

    K1APJ Premium Subscriber QRZ Page

    Well, don't know if this will help, but here goes-

    The same diode is used in the referenced IG-1271 function generator, The schematic of the IG-1271 gives voltages in the circuit using the 56-61 (D14). Looks like about 1,34 volts. That is probably the "magic," 1.34 is below the range of zener voltages.


    GL es 73 de K1APJ

    P.S - The SW-717 manual shows the 56-61 as a 1 mA, 1.3V stabistor.

    I'd probably just put a couple 1N4148 in series and see what happens.
    Last edited: Jan 22, 2020
  5. W8RLC

    W8RLC XML Subscriber QRZ Page

    Thanks Andrew, that is were I thought the voltage for the zener would be. But would still be very helpful if the actual operationing voltage was known.

    Bob, W8RLC
  6. K9STH

    K9STH Ham Member QRZ Page

    To use as a pseudo zener, I would use 1N400X diodes. They are even cheaper than the 1N4148 diodes.

    I keep 1N4007 diodes in stock since those work fine at all voltages up to 1 kV. But, for this situation, any diode from 1N4001 through 1N4007 would work fine.

    Glen, K9STH
  7. W8RLC

    W8RLC XML Subscriber QRZ Page

  8. W8RLC

    W8RLC XML Subscriber QRZ Page

    Andrew that last bit of information was very helpful. Glen thank you for the suggestions if the 1N4830 does not pan out I'll have to try the two diodes as a substitute.

    Bob, W8RLC
  9. KB7WG

    KB7WG Ham Member QRZ Page

    Perhaps I am misreading this, but doesn't the two drops of D14, bias the drops of Q22BE and Q24BE for a linear output.
  10. K1APJ

    K1APJ Premium Subscriber QRZ Page

    Yes, of course, but this thread is not about THAT circuit. We were trying to identify the voltage of ZD513 in THIS circuit, with no voltages marked.


    The circuit you analyzed above is a different circuit with the same component (56-61) used as D14. The voltages marked on that schematic provided a clue as to the nature of the 56-61 diode.

    Thank you for your input.

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