Heathkit Service Bulletins - SB-401 (Complete)

Joseph W Lutz joelutz at JUNO.COM
Wed Aug 20 01:07:16 EDT 1997

                        [No date on fiche]
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-1D

                            LMO Change

The [PN 110-32] is no longer available.  In order to use [PN 100-40-LMO]
it is necessary to add decoupling capacitors [See SB-101 installation
                        May 15, 1968
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-2D

                       Filter Choke Replacement

PN 46-36 filter choke 4.5H @ 120 MA is replaced by PN 46-32 filter choke
4.5 H @ 150 MA.  This part should only be changed when the original has
                        April 10, 1967
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-3D

                Improving Grid Drive On 20 Meters

To improve the 14.0 MHZ grid drive add a 100 ohm 1/2 watt resistor
between the two 10 meter coils on the driver grid side.
                        February 16, 1973
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-1

                      Carrier Null Control Failure

Recently an improved mounting method was devised for the [PN 10-147]
controls in kit models SB-102, SB-401 and HW-101.  Current production
utilizes a fiber washer for greater clearance and the case of the control
is grounded by a separate wire.  We are anxious to know if this will
reduce the failure rate.  Please make note of any change, good or bad,
and keep us posted.
                        January 29, 1974
SB-401                                          Bulletin No:
SSB Tranmitter                                  SB-401-2

                                                       Relay RL-2 Hang-up

After a period of transmission, the relay remains in the transmit
[energized] mode, rather than returning to the receive [de-energized]

This problem is caused by gassy 6146 output amplifier tubes.  When these
tubes become hot, there is enough secondary emission from the screen
grids to keep RL-2 energized.

The solution is to ground lug 7 of RL-1.  This grounds the 6146 screen
grids in receive, thus preventing the secondary emission current flow
from holding RL-2.  The SB-401 manual will be revised to reflect this
change in new production.
                        May 23, 1974
SB-401                                          Bulletin No:
SSB Tranmitter                                  SB-401-3

                           Alignment of SB-301/SB-401 As Transceiver

The following method of aligning the SB401 SSB Transmitter for use as a
transceiver withthe SB-301 SSBrecevier may be preferred by those who are
not familiar with the units.

The SB-301 shuld be aligned according to the manual.  Upon beginning
alignment of the SB-401 the heterodyne oscillator crystals, LSB and USB
crystals should be removed from the SB-301 and installed in the SB-401.
[This step is not necessary if the SBA-401-1 accessory crystal pack is
installed].  Do not connect any cables between the two units.  Proceed
with alignment as given on page 97 of the SB-401 manual.

Upon completion of the alignment, remove the crystals and reinstall in
the SB-301.  Connect all cables and set LMO switch to locked position and
funtion switch to transceive on SB-401.  The receiver heterodyne
oscillators may need to be retouched for maximum drive to the transmitter
finals, in some cases going to the top peak on the coils.

The receiver BFO coil will also have to be adjusted.  The voltage may be
measured during adjustment at the 50 MFD capacitor, C127,just to the
right of V2 on the carrier generator board in the SB-401.  This will
eliminate the need for taking the trasmitter out of the cabinet and
tipping on its side.

The two units are then ready for transceiver operation.
                        May 23, 1974
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-4

                Spotting With The SB-301 & Sb-401

In using the SB-301 SSB receiver and SB-401 SSB transmitter in
independent operation with all cables connected, some difficulty has been
experienced in spotting in the single sideband mode.  This has been in
the form of feedback, inability to zero beat a station, etc.

The feedback problem can be reduced or eliminated by dressing the red
wire between the receiver antenna jack and lug 3 of the antenna relay
away from the final amplifier tube sockets.  If feedback is still
excessive, the red wire may be replaced by coax with the shield connected
to the ground lug on the receiver antenna jack.

The accepted method of zero-beathing a SSB station is to "talk" yourself
in on frequency.  Tune in the signal for maximum intelligibility, then
with the function switch in spot, "talk" yourself in on frequency until
you can also understand your voice.  With this method the transmitter is
exactly on frequency.

You may wish to use a pair of headphones while talking yourself in on
                        May 23, 1974
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-5

                     CW Sidetone With Headphones

The attached pictorial shows a simple change of interconnecting cables on
the SB-401, SB-303 or SB-301 which will allow CW sidetone of the SB-401
to be heard on headphones while defeating the speaker.

SB401 & SB-301

With this combination, make the cable changes shown and then just plug
your headphones into the headphone jack.  The speaker will automatically
be cut off with headphones plugged in.

SB-401 & SB-303

With this combination, make the cable changes as shown and then plug your
headphones into the headphone jack.  The difference here is that you must
pull the speaker disable switch on the front panel of the SB-303 to mute
the sidetone fromthe speaker.

((Sorry, but even with magnifying glass I cannot read the connections
shown on the backs of the examples.  ALso, the rear of the 401 & 301
isn't like the ones I have sitting here? [But then, my manuals are dated
1966!] )
                        May 23, 1974
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-6

                SB-401 RF Voltage Service Guide

On the attached schematic we have indicated RF voltages as measured in an
SB-401 in the 80-meter CW key down condition.  RF voltages are provided
at the grid and plate of each stage so that it can be determined if the
problem is in the grid or plate circuit of the stage.

An 11 megohm input VTVM and Heath 309C RF probe was used to make the

The voltages willv ary from trnsmitter to transmitter and should be
expected.  Crystal oscillartor voltges willvary considerable.

This procedure traces the RF signal voltage from stage to stage.  It is
an easy method to isolate a problem stage so that steps can be taken to
correct the trouble.

We suggest starting at the carrier generator and working trough to the
final, as each stage is dependent on the previous stage.

Trace the RF up to the point it is lost, then back up one stage.  Should
one or two bands be operating properly and the rest have difficult, make
reference RF voltage measuremetns on one of the operating bands and make
comparison measurements on the inopeative bands.  In this way the stage
causing difficulty is quickly isolated.

Once the stage is isolated steps can be taken to correct the problem.
Check for wiring errors, intermittent solder connections, loose hardware
and bent switch contacts/

((NOTE:  Voltages shown on the schematic were:

1.5V  [Pin 2 of V9]

36V   [Pin 6 of V9]

36V   [Pin 5 of V10]              ))

Voltages taken on 80 M.                 260 mils plate current

125 watts output on HM-102                      Level control at 10:30

.5 mils grid current                            Measurements taken in
                                        position with exception of one at
                                        V5 (USB)
                        May 23, 1974
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-7

        SB-400 / SB-401 CW VOX Troubleshooting Guide

See Bulletin No: SB-400-3 dated May 23, 1974.
                        May 23, 1974
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-8

                Alternate Tune-up Procedure For SB-401

Before proceeding, review and understand the description for each of the
controls indicated in figure 1-2, fold-out from page 86.

Preset controls as follows:

        Function Switch         TRCV or TRAN
        Mode Switch             CW
        Band Switch             Desired band
        Meter Switch            Rel PWR
        Level Control           Full CCW
        Freq Control            Locked or Unlocked
        Final Load              50 Ohm Mark
        Final Tune              Desired Band
        Driver Tune             First Mark from full CCW
        Main Tuning             To desired frequency


If the SB-401 is used with a receiver other than the Heathkits SB-300 or
SB-301 the function switch should be at [TRANS] and the freq control at


The key should be closed during and operned after each step, unless
otherwise stated.


1.  Plug the key in the keyjack.
2.  Advance the level control to 12:00 o'clock.
3.  Close the key and peak the meter for maximum REL PWR with the final
tune control.
4.  Peak the driver tune control for maximum REL PWR indication.
5.  Peak the final load control for maximum REL PWR indication.
6.  Repeat steps 3, 4, 5
7.  Turn the meter swtich to Grid; DO NOT CLOSE KEY.
8.  Close the key and check the grid current.  If in excess of half
scale, turn the level control counter clockwise to bring the meter
reading to half scale or less.  If less than half scale but above zero do
not adjust.
9.  Tune the meter switch to plate; DO NOT CLOSE KEY.
10.  Close the key and check the plate current,  It should be between 200
and 240 MA.
11.  Repeat steps 3, 4 and 5 with meter in the REL PWR position.
12.  The transmitter is now fully loaded and ready for CW operation.
13.  For SSB operation set the mode switch to the desired SSB mode.
14.  During SSB operation turn the meter switch to the ALC positon.
Adjust the mike level control for proper ALC indication as indicated on
page 108 of the manual.
15.  Antennas which resonate to the transmitting frequency, presenting a
low SWR and an impedance of 50 ohms, should be used.
                        May 23, 1974
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-9

               SB-401 Instability and Corrective Information

     ***We suggest you check for each of the following possible causes***

1.  Intermittent, rosin or cold solder joints.

2.  Loose hardware at tube sockets, terminal strip, circuit boards and

3.  Poor lead dress at V5, V8, V9, V10 and V11.  The component leads must
be as short as possible, while dressing the part down against the

4.  Check RFC 4 and RFC 6 to be sure they are not interchanged.

5.  Coil leads to the band switch should be as short as possible.

6.  Check all edges of the final enclosure for proper grounding to the
main chassis.

7.  Check C26 [Final tune capacitor] to be sure it is isolated from its
turning shaft.  This is to prevent RF from traveling on the shaft to the
front panel.

8.  Check for good contact of the ground clip to the coil box.

9.  Be sure the shield plates are properly installed in the coil box.

10.  A 100 Ohm 1/2 W resistor should be installed between lugs 1 of coils
CM and CN.  This replaces a short length of buss wire between the two

11.  The leads for capacitor TA should be down on the chassis not run
through the air.

12.  If the SB-401 is being used with the SB-300 or SB-301 in the
transceive mode the accessory crystal pack should not be installed in the

13.  Change the driver and final tubes then reneutralize per manual

14.  Improper adjustment ofthe Het OSC coils could cause improper mixing
action, resulting in the final operating at a different frequency,
appearing as instability.

15.  Check for a good ground between the front panel and chassis.

16.  Check the SWR of the antenna system at frequency of operation.

17.  Check the COAX for leakage.

18.  Is the trasmitter properly grounded?

19.  Realign using a properly terminated 50 Ohm not-reactive dummy load.

20.  Be sure all shield and tube shields are installed.
                        May 23, 1974
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-10

                         SB-400 & SB-401 Hum Troubleshooting Guide

See Bulletin No. SB-400-2 dated May 23, 1974.
                        May 23, 1974
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-11

             SB-400 & SB-401 Driver Stage Troubleshooting Guide

See Bulletin No. SB-400-1 dated May 23, 1974.
                        July 24, 1974
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-12

                       Incorrect Resistance Reading at Terminal Strip AC

Refer to manual # 595-947-07, Page 88.  The resistance readings at lugs 2
and 4 of terminal strip AC will be zero, since the addition of the ground
wire to lug 7 of the main relay RL-1.
                        July 24, 1978
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-13

                       LMO Drift, Intermittent Frequency Shift

See Bulletin No. SB-100-13 date July 24, 1978.
                        August 7, 1978
SB-401                                          Bulletin No:
SSB Transmitter                                 SB-401-14

        Loading Capacitor Turns as Plate Capacitor is Rotated

See Bulletin No. HW-100-16 dated August 3, 1978.
That's it for the SB-401.

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