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Thread: Need some assistance on an SB 303.....

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  1. #1

    Default Need some assistance on an SB 303.....

    I have a mint condition SB 303 I purchased about a year ago and finally starting to work on. It receives signals fine and all the controls seem to work excpet for the Smeter.

    It does not move with a received signal or the calibrator. If a strong signal is received you may see a slight movement but not much. It goes back and forth as it should with the rf control.

    I have the manual and cannot find much mention of the S meter. Its not addressed in the section of possible faults and what to look for. I adjusted the agc voltage and it is spec.

    It has never worked and I thought a cleaning-pots, board removal etc. would correct it but no luck.

    The only other possible problem is with a received signal, it seems garbled but comes in clear with fine adjustment but I will tackle that problem later.

    Any one familiar with this radio and can give me some suggestions? I have a signal generator.


    I know that this is solid state and not a boatanchor but it was built with a 401

  2. #2

    Default

    Do you have the manual? If not, get one before proceeding!

    Have you cleaned all of the plug-in connectors on the boards? You do have to remove the band switch shaft to get them out. Doing so often "fixes" a lot of problems with the SB-303.

    Also, the SB-303 is infamous for blowing one of the push-pull audio output transistors which, in turn, causes distortion in the audio.

    There are other transistors that tend to go bad in the SB-303 and that may also be the problem with the "S" meter.

    Glen, K9STH

  3. #3

    Default

    Glen,

    The distortion(kind of a warble) comes with the ssb signal but you can tune it in clear.

    Can you still get the transistors?

    I do have the original manual. The only mention of the s meter is agc adjustment on one of the boards.

  4. #4

    Post

    Brian -

    Heath-kit SB-303 HF receiver (ZIP of schematic GIF scans)
    http://www.scootworks.net/dave/hamra...als/sb-303.zip

    The complimentary output transistors (from Heathkit manual) are:
    (Q511, 417-415, Motorola MJE-371) and (Q512, 417-414, Motorola MJE-512).


    Can you still get the transistors?
    The larger Audiophile market keep audio transistors or suitable replacements plentiful.
    ON Semiconductor is the spin-off from Motorola semi-conductor/transistor division.

    ON-SEMI MJE-371 (new parts meet RoHS lead-free requirements)
    http://www.onsemi.com/pub_link/Collateral/MJE371-D.PDF
    The PNP Bipolar Power Transistor is designed for use in general-purpose amplifier and switching circuits. Recommended for use in 5 to 20 Watt audio amplifiers utilizing complementary symmetry circuitry.
    ===
    DID Heathkit accidentally transpose MJE521 to MJE512 when describing this Q512 part ??
    APPEARS that they did, since PNP MJE371 is Complementary to NPN MJE521 transistor.

    ON-SEMI MJE-521 -- MJE371 is Complementary to NPN MJE521
    http://www.onsemi.com/pub_link/Collateral/MJE521-D.PDF
    The NPN Bipolar Power Transistor is designed for use in general-purpose amplifier and switching circuits. Recommended for use in 5 to 20 Watt audio amplifiers utilizing complementary symmetry circuitry.
    Last edited by W9GB; 08-16-2013 at 10:05 PM.
    Nullius in verba

  5. #5

    Default

    OK! You have the very famous Heath LMO "warble"! This happens because the lubricant used for the LMO variable capacitor dries out and, as you tune the frequency, the capacitor rotor makes and breaks contact with the frame of the capacitor. All Heath LMOs eventually get this problem, both tube-type and solid-state. The "fix" is pretty easy but the LMO has to be removed from the radio to make the repairs / modification.

    Here are instructions as how to "cure" the warble permanently!

    http://k9sth.com/uploads/LMO_rebuild-1.pdf

    Glen, K9STH

  6. #6

    Default

    Thanks for all the information guys! I will definitely tackle the vfo situation. Would it be better to order these transistors and replace them while I have the unit apart?

  7. #7

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    You have to trouble-shoot the unit and find which transistors, if any, have gone bad. There are also other components which may have gone bad as well as other possible problems including poor soldering, etc. Remember, the receiver was built from a kit and workmanship various all over the place!

    Glen, K9STH

  8. #8
    Join Date
    Jan 2007
    Location
    République de Banana
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    1,228

    Default

    Are you using the full assembly manual, or one of the reprints? The reason I ask is, there is not really an AGC adjustment, although there is a bias adjustment which is measured on tha AGC connector. In that same section of the manual there is an S-meter zero and an S-meter full scale adjustment. As I recall, these are fairly touchy. It may take some fiddling to get them exactly right.

    If you have the full manual, start at the figure 5 foldout from page 110 go through pages 111 and 112. Usually you will find the 15 volt power supply adjustment is way off. I would probably NOT twiddle with any of the coil adjustments on something this old unless there is good reason. Reason being that the coil slugs may be stuck and might crumble when you go to turn them.
    Sebenny Trees

  9. #9
    Join Date
    Mar 2003
    Location
    3763 Lyle Avenue, North Pole, AK 99705
    Posts
    22,650

    Default

    Be SURE the preselector is properly twiddled for a peak. The front end of that rig is pretty selective. Also be sure the AGC is actually on (I think it's disabled in some modes....failing memory here.)

    ERic
    "The more you know, the less you don't know."

  10. #10

    Default

    I have the original and fold outs. I've done the agc bias and s meter adjustments with no luck. I don't believe I have checked the 15 volts.

    I know the ham that put this together. He was very meticulous and the solder work looks really good. I'll go over it really good.

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