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Heathkit Service Bulletins: GR-78 1970-84

Discussion in 'Amateur Radio Modifications' started by AudioShockwav, Mar 24, 2011.

  1. AudioShockwav

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    February 9, 1970
    GR-78 Bulletin No:
    Transistor General Coverage Rcvr GR-78-1

    Rocker Switches

    [PN 60-45] S.P.S.T. switch is being replaced by [PN 60-48] D.P.D.T.
    switch with improved contacts. Four of the six lugs are removed when it
    is used as a S.P.S.T. switch. The new improved contact switch [PN 60-48]
    will also be used to replace the [PN 60-32] D.P.D.T. switch.
    August 11, 1970
    GR-78 Bulletin No:
    Transistor General Coverage Rcvr GR-78-2

    GR-78 Modifications

    Band F Oscillator Problem: OSC. BOARD
    -------------------------------------- ------------------
    1. Remove existing osc. FET and discard [PN 417-207].

    2. Replace with MPF-105 [PN 417-169]. SS hole will not be used.

    3. Remove and discard R300 [100 ohm] and run white wire directly to hole

    4. Remove and discard R305 [10K ohm] on foil side of board.

    5. Remove and discard R304 [10K ohm] across Band F coil L306. Replace
    2.2K ohm 1/2 W [PN 1-44].

    6. Drain voltage will now be 9.6 volts DC or slightly higher.

    1. Remove and discard R203 [1000 ohm 1/4 W] and replace with 5600 ohm
    1/4 W
    [PN 1-26-12].

    2. Remove and discard R204 [5600 ohm 1/4 W] and replace with 22K ohm 1/4
    [PN 1-45-12].

    ***Oscillator trimmers will have to be re-adjusted slightly.

    Replace C402, 68pf with [PN 21-21] 200pf disc. ceramic. This change

    Replace C419, .05mfd disc with [PN 20-102] 100pf mica. This change
    provides improved AGC and also improves distortion.

    Also replace C511, 10mfd electrolytic on terminal strip with [PN 25-123]
    2 mfd electrolytic.

    To improve passband, make the following changes:

    Replace C406, 100pf mica with 3300 ohm 1/2W resistor [PN 1-14]. Stand
    resistor on end.

    Remove and discard C413, 100pf mica. Replace with 3300 ohm 1/2W resistor
    and .05 mfd disc in series. Stand resistor on end.

    While using whip antenna indoors and power cord plugged in [charging].

    1. Remove and discard C508 and C509 [.001 mfd line by-passes].

    2. Remove existing 2 wire AC receptacle and replae with 3 wire
    [PN 432-76]. Run wires from new receptacle to existing terminal strip.

    3. Replace existing 2-wire line cord [PN 89-3] with 3-wire cord [PN
    March 23, 1973
    GR-78 Bulletin No:
    Transistor General Coverage Rcvr GR-78-3


    PROBLEM: Squealing - howling - general instability.

    SOLUTION: Replae tuning capacitor C501, [PN 26-123] with [PN 26-143].

    This change will go into effect with level 11 production having series
    numbers [11----] or higher.
    November 20, 1974
    GR-78 Bulletin No:
    Transistor General Coverage Rcvr GR-78-4

    Low Battery Life and Inoperative "S" Meter

    This problem can be caused by the "S" Meter terminals shorting to

    To correct this problem, perform the following:

    -- Straighten the meter terminals and then install a 1 1/4" X 1 1/4"
    piece of insulating paper [PN 75-108] on the rear of the meter.
    April 16, 1975
    GR-78 Bulletin No:
    Transistor General Coverage Rcvr GR-78-5

    FM Interference on Band F

    Install the following change to minimize this type of interference:


    1. Install ground spring clip [PN 258-183] between tuning
    and chassis. Scrape both capacitor and chassis to assure

    2. Change R304 across Band F oscillator coil from 5.6K ohm to
    2.2K ohm [PN 1-44].
    September 27, 1976
    GR-78 Bulletin No:
    Transistor General Coverage Rcvr GR-78-6

    Band F Too Broad at Low End

    A resistor change was made in this unit in Bulletin GR-78-5 to minimize
    FM interference on band F. The resistor, R304, was originally a 5.6K ohm
    and was changed to a 2.2K ohm. This lower value resistor has caused some
    units to tune too broad at the lower end of band F. If this complaint is
    received, R304 may be changed back to the 5.6K ohm to correct the

    Check the receiver for FM interference before returning the unit to the
    November 9, 1976
    GR-78 Bulletin No:
    Transistor General Coverage Rcvr GR-78-7

    Cannot Align Band E and F

    If Band E and F cannot be aligned at the low end, change:

    C315 from .1 uf to .05 uf [PN 21-143].

    Be sure to check all bands for oscillation after this capacitor change.
    November 10, 1976
    GR-78 Bulletin No:
    Transistor General Coverage Rcvr GR-78-8


    If the sensitivity of the receiver is low and no background noise can be
    heard, check for one or more open ceramic filters [PN 404-399]. This
    defect is usually caused by excessive heat applied during soldering.

    Sometimes pushing on or wiggling the suspected filter will cause the unit
    to operate - unless another one is defective.
    September 20, 1979
    GR-78 Bulletin No:
    Transistor General Coverage Rcvr GR-78-9

    Oscillator Will Not Align

    If you cannot achieve a high enough frequency on the high end of bands E
    and F, and also, cannot lower frequency enough on high end of band A; do

    - Remove jumper wire from circuit board to top of trimmer capacitors
    C305, C306,

    - Install a 1.5 pf [PN 28-4] or 2.2 pof [PN 28-1] phenolic capacitor in
    series, as
    necessary for proper tracking.

    - Also, install a 24 pf capacitor [PN 20-77] in parallel with C301 if
    February 29, 1984
    GR-78 Bulletin No:
    Transistor General Coverage Rcvr GR-78-10

    Calibrator Inoperative or Intermittent

    Disconnect the lead of C504 going to connector CH on the large circuit
    board. Bend the capacitor 180 degrees and reconnect the free lead to the
    center lug of connector CF. This moves the output lead of C504 to G1 of
    Q201, increasing the level of the calibrator signal and reducing the
    oscillator load.
    This is all that I hold on the GR-78.

    Joe W7LPF/4


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