Heathkit Service Bulletins - SB-400 (Complete)

Joseph W Lutz joelutz at JUNO.COM
Mon Aug 18 22:01:47 EDT 1997

                        March 31, 1966
SB-400                                          Bulletin No:
SSB Transmitter                                 SB-400-1D

                        Resistor Overheating

Replace R-71 with same value with wattage rating of 1 watt.
                        May 23, 1974
SB-400                                          Bulletin No:
SSB Transmitter                                 SB-400-1

        SB-400 & SB-401 Driver Stage Trouble Shooting Guide

It is assumed that the basic steps such as making DC voltage
measurements, checking tubes and reviewing soldering have been completed.

Normal drive to the grid of the 6CL6 is 1.5-2.5V RF on all bands.  In
some cases it will be higher.

1.  Check to see if the blocking bias is being removed from pin 2 of the
tube during the key down or tune conditions.  The bias is about -15VDC in
a key up condition; in the key down condition it is about -.5VDC.

2.  Check the cathode resistor to be sure it is of the correct value
[SB-401 -- 150 ohms], [SB-400 -- 100 ohms] and correctly installed.

3.  Check the wiring of [SB-400 - C90, A, B, C, D], [SB-401 - C4, A, B,
C, D], to bandswitch decks BS3R and BS4R.  If two of the sections are
interchanged, peaking of the coils will be incorrect and very low drive
will be present.

4.  Incorrect adjustment of the 8MHZ trap can cause suck out on one or
more bands.  To adjust the trap correctly complete the outlined steps in
the manual.

5.  Check the screen and plate voltages for the 6CL6 in the key down
position.  If 50V or better low, check the following components:  SB-400;
R-93, R-94, RFC-92, C-164, C-97 AND C-99.  SB-401; R-4, R-5, RFC-4, C-8,
C-7 AND C-58.

6.  The capacitors mounted on the driver coils should have as short leads
as practical.  All leads from the coils to the andswitch should not be
any longer than necessary.

7.  Bypass capacitors shold have short leads and be placed as close as
practical to the chassis.

8.  Check the ground of the shield for the shielded cable used in teh
plate circuit of the 6CL6.

9.  Check the value of [SB-40 R-94] [SB-401 R-5].  The value should be
6.8K and 1W. This resistor determines the drive to the final amplifier
stage.  This is accomplished due to the RF voltage drop across the

10.  Retighten hardware at the tube sockets, terminal strips and shields.
                        May 23, 1974
SB-400                                          Bulletin No:
SSB Transmitter                                 SB-400-2

                         SB-400 & SB-401 Hum Trouble Shooting Guide

1.  Disconnect one lead of the low voltage filter choke and measure its
resistance.  Its normal resistance is 120-140 ohms.  If the resistance is
below 100 ohms the choke is shorted and should be replaced.

2.  Check the following capacitors for leakage:

        SB-400                  SB-401
        C180    C183            C78    C403
        C181    C184            C79    C404
        C179    C147            C81    C402
        C182A C185              C77A  C106
        C182B                   C77B

3.  Check terminal strip and terminal board grounds.

4.  Retighten hardware at tube suckets, terminal strips, circuit boards
and shields.

5.  Check the tubes for heater to cathode leakage.

6.  Check the grounds for the shield lead of each length of shield cable.

7.  Recheck all solder connections.  Be sure that the [C79 SB-401], [C181
SB-400] mounting lug is securely soldered to the metallic mounting wafer.
                        May 23, 1974
SB-400                                          Bulletin No:
SSB Transmitter                                 SB-400-3

        SB-400 & SB-401 CW Vox Trouble Shooting Guide

It is assumed that the basic steps such as making DC voltage
measurements, checking the tubes and reviewing the soldering, have been

The following information was compiled using SB-400 and SB-401
transmitters in the 80M CW key down condition.

The AC signal voltages have been measured with a VTVM throughout the CW
Vox circuit and are as follows.  Note:  The voltages may vary some from
transmitter to transmitter.

                                SB-400          SB-401
Pin 11 of V13                   .5VAC           .5VAC
Pin 9 of V13                    25VAC           20VAC
Pin 9 of V12                    1.6VAC          1.2VAC
Pin 10 of V12                   90VAC           70VAC
Pin 11 of V12                   1.7VAC          1.5VAC
Pin 2 of V12                    45VAC           45VAC
Pin 7 of V12                    18VDC           21VDC

By tgracing the AC signal from stage to stage the point of trouble can be
isolated and steps taken to correct it.

Check for bias removal at pin 9 of V12 when the key is closed.  If the
bias voltage is not removed check the wiring of FIR LUGS 11 ans 12.  Also
check for continuity from the key jack to R127 through FIR.

A change in VOX delay after operating for a period of time can be caused
by leakage in diode [SB-400  D-2]  [SB-401  D105].  The other possibility
is a change in value of capacitor [SB-400  C123] [SB-401  C141].  Either
component could experience a change in operating characteristics due to
heat.  In such a case mount the two parts on the bottom of the circuit
board to correct this condition.
                        July 24, 1978
SB-400                                          Bulletin No:
SSB Transmitter                                 SB-400-4

                LMO Drift; Intermittent Frequency Shift

See Bulletin No:  Sb-100-13 dated July 24, 1978.
                        August 7, 1978
SB-400                                          Bulletin No:
SSB Transmitter                                 SB-400-5

             Loading Capacitor Turns as Plate Capacitor is Rotated

See Bulletin No: HW-100-16 dated August 3, 1978.
That's it for the SB-400.

73 de JOE- - - - - - - - - - -W7LPF/4 (NNN0KUU)- - - - - - - - - - - - -
- -QWCA - NCVA - SOWP - FISTSGordonsville, Va 22942 (Orange
County)Looking for:  Heathkit HR/HX-20, MR/MT-1,
Central Electronics 100V or 200V- - - - - - - - - - - - - - - - - - - - -
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