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2ISA0001 Desoldering equipment or solder braid
Four 4.7-volt ¼-watt zener diodes (Microsemi DZ840731ET/R; QSC part number QD-0004.7-ZT)
Test equipment required:
Oscilloscope
2-ohm resistive load (rated for at least 1200 watts)
Wire or other means of shorting the amplifier output
Variable AC transformer (e.g., Variac, Powerstat, etc.) rated for 25A (120V) or 12A
(230V). Make sure your AC supply is appropriate for the amplifier.
1 kHz audio sine wave generator
Digital multimeter
AC current probe (e.g., BK Precision model CP-1)
Trimpot adjustment tool (non-conductive)
Note: The circuit boards in the ISA 750 and ISA 800T are double-sided, while those in
the ISA 450 and 500T are single-sided. Therefore, to minimize disassembly, this bulletin
recommends two different procedures among these models:
In the ISA 750 and ISA 800T, the zener diodes will be unsoldered and soldered from the component side. The instruc-
tions for this procedure begin below.
In the ISA 450 and ISA 500T, the new zener diodes will be soldered piggyback-style to the old ones. The instructions
for this procedure begin after those for the ISA 750 and ISA 800T.
0.3"
7.6 mm
0.4"
10.2 mm
D107 & D210:
D207 & D110: 0.15-“0.2"
3.8-“5.1 mm
Procedure: Replacing zener
diodes in an ISA 750 or ISA 800T
amplifier
1 . Disconnect the amplifier from AC and wait at least 10 minutes
for internal voltages to bleed down. Remove the top cover.
2. Prepare the new zener diodes as shown in Figure 1.
3. Remove the four screws on the bottom of the amplifier that
fasten the heat sink to the chassis.
4. Remove the three screws that attach the channel module
assembly to the chassis standoffs.
5. Lift up the front of the channel module assembly. Rotate it so
that channel 1s module (the lower one) is on top; set the
assembly down in the chassis with the heat sink resting on
the standoffs (Figure 2).
6. Disconnect the ribbon cable from the channel 1 module.
7. Locate zener diode D107. It is near where three wires are
soldered to the circuit board and secured with hot glue. If the
hot glue covers the diode, use the tip of your hot soldering
iron and a small screwdriver to melt and clear the glue away
(Figure 3).
8. Notice the orientation of D107. Unsolder and remove it; be
careful not to damage the through-plating in the holes. Install
and solder a new zener diode oriented in the same direction.
Do the same with zener diode D110.
9. Reattach the ribbon cable and turn the channel module
assembly back over so that the channel 2 module is on top.
10. Locate zener diode D207 and notice its orientation. Carefully
Tools and materials required (continued):
Figure 1. Prepare the leads of the
new zener diodes.
Figure 2. Prop the channel module assembly up on the
chassis standoffs.
Figure 3. You might need to clear hot glue away from
diode D107. Use your hot soldering iron tip.
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3 ISA0001unsolder and remove it. Install and solder a new zener diode oriented in the
same direction. Do the same with diode D210.
11. Check your work. Dont reattach the channel module assembly and heat sink to
the chassis yet. Instead, continue with the bias and current-limit calibration.
New 4.7-volt zener
Old 5.6-volt zener
s n o i t a r b i l a C t s u j d A & 0 5 4 A S I
T 0 0 5 A S I & 0 5 7 A S I
T 0 0 8 A S I
7 4 1 R s s o r c a e g a t l o v C D : s a i b 1 l e n n a h C13 1 RV2 1 . 0V7 0 . 0
4 2 R s s o r c a e g a t l o v C D : s a i b 2 l e n n a h C713 2 RV2 1 . 0V7 0 . 0
d a o l d e t r o h s o t n i t n e r r u c t u p t u O0 4 1 R & 9 3 1 R : 1 l e n n a h C
0 4 2 R & 9 3 2 R : 2 l e n n a h CA 5 -“ 4A9 -“ 5 . 8
* d a o l d e t r o h s g n i v i r d n e h w t n e r r u c C A0 4 1 R & 9 3 1 R : 1 l e n n a h C
0 4 2 R & 9 3 2 R : 2 l e n n a h CA 5 . 5 -“ 5 . 4A5 . 6 -“ 5 . 5
) S M R ( s m h o 2 o t n i e g a t l o v g n i p p i l C9 3 1 R t s u j d A : 1 l e n n a h C
y r t e m m y s r o f
9 3 2 R t s u j d A : 2 l e n n a h C
y r t e m m y s r o fV 5 . 7 3 -“ 5 . 3 3V9 4 -“ 4 4
) k a e p ( s m h o 2 o t n i e g a t l o v g n i p p i l C9 3 1 R t s u j d A : 1 l e n n a h C
y r t e m m y s r o f
9 3 2 R t s u j d A : 2 l e n n a h C
y r t e m m y s r o fV 3 5 -“ 4 . 7 4V3 . 9 6 -“ 2 . 2 6
; e r u t a r e p m e t t n e i b m a t a ( * d n a m e d C A e l d I
) t o h n e h w r e h g i h, A 4 . 0
% 0 1 ± A 6 . 0, % 0 1 ±
. V 0 0 1 r o f 2 . 1 y b r o V 0 3 2 r o f 5 . 0 y b t n e r r u c y l p i t l u m ; s r e i f i l p m a V 0 2 1 r o f e r a n w o h s s e r u g i F *
Procedure: Installing new zener diodes in an
ISA 450 or ISA 500T amplifier
1 . Disconnect the amplifier from AC and wait at least 10 minutes for internal voltages to bleed down. Remove the top cover.
2. Remove the four screws on the bottom of the amplifier that fasten the heat sink to the chassis.
3. Remove the three screws that attach the channel module assembly to the chassis standoffs.
4. Locate zener diodes D107 and D110 on the channel 1 module (the upper circuit board). Bend and trim the leads of two
new zener diodes and solder them piggyback-style to D107 and D110, oriented in the same direction (Figure 4).
5. Turn the channel module assembly over and repeat step 4 with zener diodes D207 and D210.
6. Check your work. Dont reattach the channel module assembly and heat sink to the chassis yet. Instead, continue with
the bias and current-limit calibration.
Procedure: Bias and current-limit calibration
Note: During the current-limiting part of this procedure, the amplifier will draw high amounts of AC: approximately 22 am-
peres at 120V, or 11 amperes at 230V. Be thorough, but work quickly to avoid tripping the amplifier's circuit breaker.
1 . Start with the variable transformer turned all the way down to zero, the amplifier turned off, and its top cover still removed.
2. On the amplifier's circuit boards, locate resistors R147 and R247, and trimpots R131, R231, R139, R239, R140, and R240.
To reach R147 on the ISA 750 and ISA 800T or R247 on the ISA 450 and ISA 500T, rotate the chassis module assembly
and rest it on the heat sink so the bottom circuit board is accessible. Note that the trimpots for the bottom channel
module are located on a
small circuit board next to
the top module.
3. Plug the amplifier into the
variable transformer.
4. Turn the gain controls all
the way down.
5. Turn the amplifier on and
gradually turn up the
variable transformer to
the amplifier's normal
operating voltage while
you monitor its AC current
draw (it should not
grossly exceed the idle
current figures in Table 1).
Make sure the amplifier is
functioning correctly.
Bias adjustment (perform
only with unit at ambient
temperature)
Note: This calibration must be
done with the amplifier
circuitry at ambient tempera-
ture. If the amp has begun to
warm up, you must turn it off
Figure 4. Solder a new zener diode
piggyback-style onto each old one.
Table 1.
