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

Discussion in '"Boat Anchor" & Classic Equipment' started by KB4MNG, Aug 16, 2013.

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  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    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. K9STH

    K9STH Ham Member QRZ Page

    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. KB4MNG

    KB4MNG Ham Member QRZ Page


    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. W9GB

    W9GB Ham Member QRZ Page

    Brian -

    Heath-kit SB-303 HF receiver (ZIP of schematic GIF scans)

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

    [FONT=.HelveticaNeueUI]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)
    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
    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: Aug 16, 2013
  5. K9STH

    K9STH Ham Member QRZ Page

    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!

    Glen, K9STH
  6. KB4MNG

    KB4MNG Ham Member QRZ Page

    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. K9STH

    K9STH Ham Member QRZ Page

    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. WC5P

    WC5P Ham Member QRZ Page

    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.
  9. KL7AJ

    KL7AJ Ham Member QRZ Page

    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.)

  10. KB4MNG

    KB4MNG Ham Member QRZ Page

    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.
  11. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    As for the S-Meter; is the AGC turned on ether to fast or slow?
    I should also point out there are adjustments that effect the the operation of the S-Meter Read through the aglinment procedure. ...
  12. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    So you have done all that...
    My SB-303 had a dirty AGC switch and the S-Meter wouldn't work all the time.
    And I see you have addressed my other questions....
  13. KB4MNG

    KB4MNG Ham Member QRZ Page

    I will focus on the agc switch. Not matter what position. It will not work. I've been over the manual and it does not seem to address the smeter. Very occasionally, it will slightly move with a received signal.
  14. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    If you have a scope or analogue meter, you can get in there and watch the voltage on the wiper contact of the AGC switch, if you don't see it there then trace it back to the AGC circuit, which if remember correctly is on the audio board.
  15. K9STH

    K9STH Ham Member QRZ Page

    The problem with the SB-303 is that, unless one has the extender boards, it is VERY difficult to do much of any trouble-shooting!

    Glen, K9STH
  16. W5RKL

    W5RKL Ham Member QRZ Page

    Since the meter responds when the RF gain control is adjusted, I don't suspect the meter, D509, or Q505 on the IF/AUDIO board are at fault.

    I do, however, suspect Q506 may be bad. Q506 is the S meter amp. You said when receiving a strong signal the meter tends to "flicker" slightly. That tells me some of the IF signal is getting through Q506 to Q505.

    A replacement for Q506 is a 2N3393 or an NTE-85.

    Since the audio distortion as you called it, is a "warble", I agree with Glen, the LMO "FORK" connection between the LMO variable capacitor and the variable capacitor frame is causing that. You don't have to remove the LMO to fix this. Remove the receiver from the cabinet, slide the "RTTY BOARD" (sit above the LMO) out to expose the Metal tap on the top of the LMO. Be careful, the wires are easily broken on the terminals behind the LMO. You don't have to pull the RTTY BOARD all the way out, pull it just enough to expose the entire metal tape on the top of the LMO.

    Pull the tape back so you can see the variable capacitor shaft and the FORK connection. Set the receiver so the front panel is facing you then rotate the receiver on it's left side (RF ATTN control is closest to the bench.

    Apply small amount of contract cleaner to the FORK and SHAFT connection then rotate the LMO knob throughout it range in both direction a number of times. Press the tape back onto the top of the LMO and gently push the RTTY BOARD back into its slot, checking the wires for breaks at the LMO rear terminals and test the receiver. Note there is a round hole on the rear of the RTTY BOARD. Center the RTTY BOARD's hole over the screw adjustment on top of the LMO. More than likely you will have to repeat this cleaning a number of times before the LMO will tune without warbling. I have an SB-303 that had this exact same problem and cleaning the LMO's variable capacitor shaft to FORK connection fixed it and I didn't remove the LMO from the receiver to do it, I pealed the tape back. If you pull the tape completely off, you can replace it with Aluminum tape found at Home Depot, Lowes, or most hardware stores. the tape will conduct so it must not make contact with the foil side of the RTTY BOARD so press it firmly onto the top of the LMO.

  17. K9STH

    K9STH Ham Member QRZ Page

    The problem with just cleaning the contacts on the LMO variable capacitor is that the problem often comes again within a relatively short period of time. Of course, relatively is not a definite figure. I have seen the warble return in just a few weeks in some LMOs and in some LMOs 2 or 3-years.

    The permanent "fix" is to add a piece of braid between the shaft of the variable capacitor and the frame of the capacitor (or some nearby ground point). This is the procedure that most people use to correct the problem. The procedure that I outline in the link posted earlier in this thread is an example of how to add the braid. There are a couple of other ways to accomplish the deed. What is outlined in the link is the method that I use when working on SB-Line units for others.

    Glen, K9STH
  18. KB4MNG

    KB4MNG Ham Member QRZ Page

    Thanks Mike and Glen!
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