Heathkit Service Bulletins - SB-104 (Part 4)
Joseph W Lutz
joelutz at JUNO.COM
Sat Nov 1 23:55:08 EST 1997
April 22, 1976
SB-104/HW-104 Bulletin No:
Solid State Transceiver SB-104-37
Low Sensitivity On All Bands
The following performance checks will help to isolate specific problem
areas in receivers with low sensitivity:
1. Set the SB-104 fron panel controls as follows to prevent damage to
the signal generator if the transmitter is accidentally keyed:
MIC/CW Level ------------------------- full CCW
HI ----------------------------------------- out
TUNE ------------------------------------ out
2. Turn the receiver RF gain full CW. With no signal applied, set the
volume control for a noise reference level that is high enough to mask
the receiver's own hum and noise. If an IM-38 is being used to read the
audio output, set the meter to the 0.3 volt range, and then set the noise
reference level at -10 db. [Some of the following steps may require a
resetting of the reference level before measurement due to changes in
receiver noise level].
NOTE: In the follow two steps it will be necessary to isolate input
leads during
signal injection. This can be done by unsoldering input leads,
or, by
using the extender card with the pin 1 through 6 extender block
shifted
over one space to isolate pin 1 on the circuit board.
3. Isolate pin F1 on the F board and inject a 3395 kHz, .6 microvolt
signal through a .01 mfd capacitor to the pin. The output should
increase by 10 db or more. If not, the problem is probably in the IF
strip or AGC.
4. Isolate pin E1 and inject a 3395 kHz, .18 to .2 microvolt signal to
the pin. Tune the signal generator across the filter passband; the
output should peak at least 10 db or better. If not, the problem is the
crystal filter or its input and output circuits.
5. Install the extender block properly on the extender card, and put the
G board on the extender card [pin 1 no longer has to be isolated]. With
an RF probe, check the voltage at pin G6. This voltage should be
approximately .5 to .6 volts on 80M and 40M, and .4 volts on all other
bands. If the voltages are significantly lower, the problem is with HFO
output amplifier Q405 or the appropriate HFO oscillator on the D board.
6. With an RF probe, check the VFO voltage at pin G4. This voltage can
be as low as .4 volts without seriously affecting receiver sensitivity,
and should be around .45 to .50 volts. If the voltage exceeds .5 volts,
reset the VFO output level to .48 volts at the MIDDLE of the VFO range.
7. Set the bandswitch to 80M and turn off the 80M HFO oscillator by
rotating the oscillator slug until the display shows 66XX.X. Remove the
filter end of the jumper wire connecting C762 with the filter output bus,
and inject an 8.6 MHz, .8 microvolt signal to the jumper wire. The
output should increase by 10 db or more. If not, there is a problem in
the 8.395 to 8.896 MHz bandpass filter. If it is difficult to keep the
bandpass characteristic flat over its 500 kHz passband when tuning C764
and C769, the Q of the [PN 40-1805] coils is probably too low, and they
should be changed. Normally C764 will set the low end response and C769
will set the high end, and the passband will be relatively flat. The
following components make up the 8.395 MHz to 8.895 MHz bandpass filter:
L725 C764 C774 C771
L726 C766 R721 C767
L727 C773 C769 C772
If the output of this step is acceptable, reconnect the jumper wire to
the filter output bus, and re-set the 80M HFO slug to start the
oscillator. Tune through the oscillator range slowly, noting the output
noise response [adjust the VFO as necessary to avoid "birdies" and other
spurious responses]. A slight decrease in receive noise will be noticed
at one point as the slug is turned through the oscillator range. Using
this minimum noise point as the final oscillator setting will give the
best signal-to-noise ratio on 80M. The other bands can be tuned the same
way, but the effect will be less noticeable.
8. Reset the noise reference level, and tune the receiver to 3750 kHz.
Inject a 1 microvolt, 3750 kHz signal through a .01 mfd capacitor to pin
G24. There should be a 10 to 12 db increase in output. Check all other
bands at mid-band; the output should also be at least 10 db to 12 db
better. If not, the problem is most likely in the Q of the bandpass
filter toroids, or, the bandpass filters have not been properly tuned.
9. If the output in the preceeding step is accepable, insert the G board
into the chassis and watch for an INCREASE in the reference noise level.
If this increase is 3 db or more, and the signal-to-noise ratio is
equally degraded, coil L729 is no longer self-shielding and must be
replaced.
10. If the output in the preceeding step is acceptable, connect the
signal generator to the receiver antenna jack and switch the antenna
switch to "SEP'. The output may decrease as much as 2 db on 40M due to
the attenuation of the 8.65 MHz band reject filter. If the attenuation
is greater than 2 db on 40M, or more than 1 db or all other bands, there
is a problem with the 8.65 MHz band reject filter. [See Bulletin No:
SB-104-35].
11. Connect the signal generator to the RF output antenna jack, switch
the antenna switch to "COM", and repeat step 7. The difference in output
should be negligible. Any additional attenuation of output indicates
problems with the filters on the ALC board, or with the antenna relay.
------------------------------------------------------------------------------------------------------------------
May 11, 1976
SB/HW-104 Bulletin No:
Solid State Transceiver SB-104-38
Improved ALC Meter Action
The drive voltage developed at the base of Q302 is often insufficient to
swing the meter to full scale. Change R322 from 10K to 6800 ohm [PN
1-27-12] to increase the drive voltage. This change is being made in the
next production run.
-----------------------------------------------------------------------------------------------------------------
May 11, 1976
SB/HW-104 Bulletin No:
Solid State Transceiver SB-104-39
R11 Overheats
The 15 ohm current limiter R11 on lug 1 of the band switch is being
change from 15 ohm 1/2 watt to 15 ohm 1 watt [PN 1-12-1] to correct for
an overheating problem with the resistor.
------------------------------------------------------------------------------------------------------------------
May 19, 1976
SB/HW-104 Bulletin No:
Solid State Transceiver SB-104-40
SB-104 Modifications
The following list is a circuit board and chassis breakdown of
modifications that have been done to the SB-104 since introduction.
CIRCUIT BOARD A
- -NO CHANGES- -
CIRCUIT BOARD B [PN 85-1580-3]
CHANGE PART NO REASON
----------- ------------ -----------
Change R-254 to 750 ohm 1-96 Increase 11
volts
Change C-227 to 47 mfd 25-223 Stabilize 11
volts
Change C-208 to .1 mfd 27-47 Reduce Leakage
Change C-222 to .1 mfd 27-47 Reduce Leakage
Change R-207 to 1.5 meg 1-36 Reduce mic
gain
CIRCUIT BOARD C [PN 85-1420-6]
CHANGE PART NO REASON
----------- ------------ ------------
Add ground clip to 10 mtr coil
cover 258-95 Eliminate 10M osc
Add 39 pF capacitor C-373 across R-314 21-167 Lower 40M
spurious
Change C-332 to 50 pF 20-97 Increase 15M
bandpass
Change R-329 to 12K ohm 1-14-12 Lower 40M
spurious
Remove and discard C-322 Better keying
Remove and discard L-321 & C-348
Replace by attached procedure 20-149 Reduce 3.395
spurious
40-1877 on 80 meter
31-76
Change R314 to 390 ohm 1-23-12 Reduce noise
on XMT
Change R332 to 1210 ohm 1% 2-68-11 Constant
pwr output
Change C 369 to 390
pF 20-106 Improve 80M
passband Change
R322 to 6800 ohm 1-27-12 Improve ALC/meter
NEW 3.395 MHz TRAP ON C BOARD
A high level sput at 3395 kHz may be present in some units due to
incorrect tuning of the 3.395 MHz trap on the transmit IF board. This
problem can be avoided by removing the old 3.395 trap on the transmit IF
"C" board, and installing a new trap as follows:
1. Remove and discard L321 and C348.
2. In the L321 circuit board holes near the top edge of the
board install
an 8-40 pF [PN 31-76] with the slotted side toward the top
end of the
board.
3. In the next lower set of holds, install a 150 pF [PN 20-149]
capacitor.
Do NOT use the holes screend for C348.
4. Next, install a 13.25 uH [PN 40-1877] coil at C348.
5. Remove circuit board D, and pre-set the front panel controls
to the
3.5 mHz band, MIC/CW level control full counterclockwise.
6. Put the meter in the POWER position, depress the HI and TUNE
pushbuttons.
7. Turn the MIC/CW Level control full clockwise. Adjust the
8-40 pF
trimmer for a minimum [null] on the power meter.
8. Turn off the unit, and reinstall circuit board "D".
CIRCUIT BOARD D [PN 85-1418-2]
CHANGE PART NO. REASON
----------- ------------- ------------
Change C-414 to 18 pF 21-60 Reliable 20M HFO
starting
CIRCUIT BOARD E [PN 85-1581-5]
CHANGE PART NO. REASON
----------- -------------- ------------
Change R-625 to 22K ohm 1-45-12 Better BFO
Switching
Change C-622 to 22 mfd 25-212 Better BFO Switching
Change 6-21 to 100 ohms 1-1-12 Reduce Transmit Gain
Remove and discard R-613 Improve 80M Sensitivity
Change R-619 to 100 ohm 1-1-12 Decrease background
noise
VFO FILTER BOARD
CHANGE PART NO REASON
----------- ------------ ------------
Remove and discard R-1251 Lower 10M spurs
CIRCUIT BOARD F [PN 85-1462-4]
CHANGE PART NO REASON
----------- ------------ ------------
Remove and discard R-569 Lower background noise
Change R-564 to 330K ohm 1-31 Better Muting
Change R-532 to .82 ohm 3-1-2 Stabilize audio
output
CIRCUIT BOARD G [PN 85-1708-2]
CHANGE PART NO REASON
----------- ------------ ------------
Change C-703 to 105 pF 20-162 Better 80M
sensitivity
Change C-707 to 105 pF 20-162 Better 80M
sensitivity
Change C-764 to 8-40 pF 31-76 Better IF bandpass
Change C-769 to 8-40 pF 31-76 Better IF bandpass
Remove C-763 and discard
Change Q-701 to this------------417-274 Increase
sensitivity
Change Q-704 to this------------417-274 Increase
sensitivity
Change R-719 to 2.2 mfd choke 45-73 Increase sensitivity
Change L-734 to 4.5 mfd coil 40-1875 Increase sensitivity
Remove L-733 and C787 and discard
Remove and discard C-742 Improve peaking
Change C-714 to 24pF 20-77 Improve peaking
CIRCUIT BOARD H [PN 85-1422-2]
CHANGE PART NO REASON
----------- ------------ ------------
Change R-954 to ferrite bead
and wire 475-12 Reduce Harmonics
Change R-956 to ferrite bead
and wire 475-12 Reduce Harmonics
Change L-952 to 4 turns Reduce Harmonics
Change L-953 to 4 turns Reduce Harmonics
Refer to Pictorial 11-5 for PA Board
CIRCUIT BOARD J
CHANGE PART NO REASON
----------- ------------ ------------
Change R-903 to 220 ohm 2 watt 1-13-2 Reduce Driver Temperature
Change RFC 902 with 390 ohms 1-48 Reduce Oscillations
CIRCUIT BOARD K
Remove and discard L814 Reduce Oscillations
DISPLAY BOARD
No Change
DC UP CONVERTER
No Change
VFO BUFFER BOARD
CHANGE PART NO REASON
----------- ------------ ------------
Change R-1201 to 1200 ohms 1-10 Eliminate FM
Change R-1202 to 350 mfd RFC 45-82 Eliminate FM
Change R-1203 to 470 ohms 1-6 Eliminate FM
Change R-1206 to 150 ohms 1-66 Eliminate FM
Change D-1201 to 1N458 56-24 Eliminate FM
Change ZD-1201 to 7.5V Zener 56-97 Eliminate FM
CHASSIS CHANGES
CHANGE PART NO REASON
-----------
------------- ------------
Change R5 to 0.15 ohm 3-14-2 Stabilize 5V regulation
Install 4 ferrite beads on wire
connected to K5 475-12 Eliminate
Oscillation
Install .001 mfd capacitor from
F13 to F15 21-14 Reduce
"talkback"
Install a wire from F7 to asscess.
socket lug 3 SB-644
Compatability
Install a 10 mfd from C7 to Ground
[+side to C-7] 25-54 Reduce T/R
"pop"
Install 390 ohm from G5 to ground 1-48 Noise Blanker
Compatability
Install 500 mfd from J1 to Ground 25-157 Improve B+ decoupling
Install .001 mfd capacitor from
pin 10 of relay to ground [C15] 21-140 Eliminate Oscillation
Remove and discard C4 Increase power on 15
& 10 M
Install .1/100V from pin C4 to ground
Install a .01 ufd from pin C5 to ground
Install .1 mfd cap. from C4 to ground 21-195 Eliminate
Oscillation
Install .01 mfd cap. from C5 to ground 21-176 Eliminate
Oscillation
Remove wire between G2 and G3. Cut
Orange 13.8 V wire running past G
board. Install both ends of this wire
onto G2 and [S2]. Increase Sensitivity
IMPROVING CW KEYING CHARACTERISTICS
1. Remove two gray wires from lug 6 of R2.
2. Connect this 6 1/2" gray wire from S4 lug 1 [R2] to the GND lug.
3. Discard short gray wire going to R2 ground lug.
4. Connect a gray wire from R2 lug 6 to terminal strip BE lug 3.
5. Connect a 5 uF electrolytic [PN 25-149] between lug 3 and GND lug of
terminal strip BE; [+] lead to lug 3.
-----------------------------------------------------------------------------------------------------------------
Okay, going on to midnight - will continue with Part 5 tomorrow. Ciao!
73 de Joe W7LPF/4 [NNN0KUU]
QWCA - SOWP - NCVA - FISTS - RCC
Gordonsville, Va 22942 [Orange Co]
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