Internal Modifications; Knob Assembly - Shure SCM268 User Manual

WARNING!
Voltages in this equipment are hazardous to life. No
user-serviceable parts inside. Refer all servicing to
qualified service personnel.
Disassembly
To access the printed circuit board (pc board) for internal
modifications, use the following steps:
1. Unplug the power cord.
2. Remove the knobs and retainer nuts from the front panel
(See Figure 2).

KNOB ASSEMBLY

Figure 2
3. Remove the four screws at each corner of the back panel.
4. Remove the two screws at each bottom corner of the front
panel
5. Slide the back panel and pc board out from the rear of the
chassis.
CAUTION: When reassembling the SCM268, DO NOT OVER-
TIGHTEN the knob retainer nuts. Use a minimal amount of force
to secure the nut (0.6–0.8 N m (5–7 in lb)). Damage to the internal
components will result if too much force is used.
Low-Cut Filter
To bypass the built-in low-cut filter for a given channel, re-
move the specified resistor and place a 10 F to 33 F capacitor in
the specified pc board location (polarity does not matter). Refer to
the following table:
Channel Remove Resistor
from:
1 R18
2 R28
3 R38
4 R48
To select a particular corner frequency for the low cut filter,
remove the R18, R28, R38, or R48 resistor for a given channel as
specified above. Then, in the corresponding pc board location
(X17, X27, X37, or X47), place a capacitor of the specified value
(polarity does not matter). Refer to the following formula for se-
lecting the correct capacitor value for the desired corner frequen-
cy.
26.5
C =
F
where:
C = value of capacitor in F
F = corner frequency (–3 bB) for low-cut filter in Hz
The following table lists the low-cut frequency corners for
some of the most common capacitor values:

INTERNAL MODIFICATIONS

Phantom Power Disable
move the specified resistor as listed in the following table:
Line Pad
move the specified resistor and short the solder points at the spe-
cified pc board locations. Refer to the following table:
Hot Mic Pad
Place 10 F to 33 F
Capacitor in:
overdriving the input stage, the user may need to set the input pot
X17
lower than desired. To fix this problem, the user can place an 11
X27
dB pad into the input gain stage of a selected channel.
1. Twist together the leads from one side of a 15 kΩ resistor and
X37
X47
2. Solder the free ends of the resistor-capacitor combination
English –
Capacitor Value ( F) Low-Cut Frequency Corner (Hz)
.033
.047
.068
.1
.22
.33
.47
.68
1.0
2.2
To disable phantom power for a given microphone input, re-
Channel
1
2
3
4
To insert a 50 dB line pad for a given microphone input, re-
Channel Remove Resistors:
1 R12, R13, R15
2 R22, R23, R25
3 R32, R33, R35
4 R42, R43, R45
Some condenser mics have a high output. In order to avoid
a 0.1 µF capacitor:
into the holes at the jumper position indicated by the follow-
ing table and remove the corresponding surface mount re-
sistor.
Channel Remove
Resistor
1 R18
2 R28
3 R38
4 R48
5
803
564
390
265
120
80
56
39
26.5
12
Remove Resistor:
R15
R25
R35
R45
Short Solder Points:
X11 and X14
X21 and X24
X31 and X34
X41 and X44
15 kΩ
0.1 µF
Insert Resistor–Capacitor
Combination at Jumper
X17
X27
X37
X47