Troubleshooting the 38 Special

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(1) Apply power briefly and touch all ICs. None should be more than luke
    warm. Anything else and you have a problem around the "hot" chip.

(2) When any part of the design is suspected "dead" the first step is to
    test all power supply connections (referenced to ground):
    (a) 12V at the power connector
    (b) 8V at all ICs as follows (IC/pin):
            U1/8, U2/14, U3/8, U4/20, U5/8

(3) Other DC voltages:
    (a) verify about 4V at U5 pins 3 and 5.
    (b) check the base of TR1 (2N3904, middle pin) as follows:
            about 0.6V on receive (key open)
            about 1.5V on transmit (key down)
    (c) check U2/12, U2/13, U4/1, U4/13, banded side of D6:
            8V on receive (key open)
            0V on transmit (key down)
    (d) check U2/5, U2/6, U4/7, banded side of D1 and D2:
            5V on receive (key open)
            8V on transmit (key down)
    (e) check DC voltage on banded side of D3, while varying the
        tuning pot position for 0-8V voltage range.

Any problem with the DC voltage levels mentioned means you have something
wrong in that area of the board. These tests can be done even with a
cheap analog voltmeter.

(4) Verify sidetone with key down. If you hear the sidetone then the
    audio stage is OK and most likely U4 is not faulty. In general, we
    found that the ICs are very rugged. Do not remove an IC from the
    board before you have verified all components around it first!

Basic Receiver Tests

It is assumed that the DC tests checked OK.
(1) First test that the headphone jack is compatible with the socket.
(2) You should hear the sidetone with key down. This tells us that the
    audio amp circuit (U5) is functioning.
(3) If no sidetone heard, the audio amp might still be OK:
    (a) Listen with the headphones while touching different places in
        the receive path with a metalic object (screwdriver, etc.). You
        should hear some noise in the headphones (possibly you'll pick
        up a local AM station, due to the high gain):
            U5/2, U5/6
            U2/10, U2/11, U3/4
            U3/1, U2/2
            U3/6, U3/7 (both are part of the 12.0 MHz oscillator)
            U2/1
            U1/4 (if U2/1 was OK and this one is dead, then check the
                  crystal filter)
            U1/6, U1/7 (both are part of the 22.1 MHz oscillator)
            U1/1
            banded side of D1 and D2
            antenna connector (center pin)
    The above procedure tests the receive chain "backwards". If you stop
    hearing at a particular point, check the circuit in between that
    point and the previous ("good") point.
(4) If you have a general-coverage receiver, "listen" to the 12.0 and
    the 22.118 MHz oscillators around those nominal frequencies.
    Use a short wire for the antenna of the test receiver and place it
    close to the "38 Special" board, while applying power.
    Check that the 22.118 MHz signal varies when you change the position
    of the tuning pot.
    Check that the 12.0 MHz moves down about 600 Hz with key down vs.
    key open.
    This step verifies that the oscillators are functioning correctly.
    This step can be also executed with a scope and a frequency counter
    (make sure the probe is not too capacitive).

Basic Transmitter Tests

It is assumed that the DC tests checked OK.
(1) You should hear the sidetone with key down, assuming you verified
    that the audio amp is functional.
    Lack of a sidetone may mean that the sidetone circuit is faulty
    or that U4 has some problem. Do not remove U4! Check all other
    components around it first.
(2) With a general-coverage receiver, you should hear a carrier around
    10.110-10.130 with key down. Tune the transmit trimcap TC2 and scan
    that range with a short wire as an antenna on the test receiver.
    You should use a 50 Ohm load for the "38 Special" for this test.
    In general, the sidetone on a stock kit will sound harsh when it's
    not tuned correctly, so this should be used as the first indication
    of correct transmitter tuning.
(3) Verify that the oscillators are working correctly, as described in
    the previous section (basic receiver tests).
    The correct operation in receive mode is satisfactory.
(4) All other transmit chain tests require a scope. Signal levels in the
    transmit chain are viewable on all scopes (at least 100 milivolts
    p-p)and should be easy to trace.
    You should be looking for a clean sinewave at the base of TR1 with
    key down, and check for about 2.5 p-p swing on its collector.
    On the dummy load at the antenna connector you should see a few volts
    p-p swing and very clean sinewave at the 10.110-10.130 MHz range.
    More detailed description of signals is beyond the scope of this
    basic debugging procedure, and may appear in a future article.

72,

Ori Mizrahi-Shalom AC6AN

 
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Page last updated:  April 15, 2004