Internal Modifications

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 chas- sis.

CAUTION: When reassembling the SCM268, DO NOT OVERTIGHTEN 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, remove the spec- ified resistor and place a 10μF to 33μF capacitor in the specified pc board location (polarity does not matter). Refer to the following table:

Channel	Remove Resistor from:	Place 10μF to 33μF
		Capacitor in:
1	R18	X17
2	R28	X27
3	R38	X37
4	R48	X47

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 selecting the correct capacitor value for the de- sired corner frequency.

C = 26.5 ⁄ F

where:

C = value of capacitor in μF

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:

		ENGLISH
Capacitor Value (μF)	Low-Cut Frequency Corner (Hz)
.033	803
.047	564
.068	390
.1	265
.22	120
.33	80
.47	56
.68	39
1.0	26.5
2.2	12

PHANTOM POWER DISABLE

To disable phantom power for a given microphone input, remove the specified resistor as listed in the following table:

Channel	Remove Resistor:
1	R15
2	R25
3	R35
4	R45

LINE PAD

To insert a 50 dB line pad for a given microphone input, remove the spec- ified resistor and short the solder points at the specified pc board loca- tions. Refer to the following table:

Channel	Remove Resistors:	Short Solder Points:
1	R12, R13, R15	X11 and X14
2	R22, R23, R25	X21 and X24
3	R32, R33, R35	X31 and X34
4	R42, R43, R45	X41 and X44

HOT MIC PAD

Some condenser mics have a high output. In order to avoid overdriving the input stage, the user may need to set the input pot lower than desired. To fix this problem, the user can place an 11 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 a

0.1 μF capacitor:

15 KΩ

0.1 ΜF

2.Solder the free ends of the resistor-capacitor combination into the holes at the jumper position indicated by the following table and remove the corresponding surface mount resistor.

Channel	Remove	Insert Resistor-Capacitor
	Resistor	Combination at Jumper
1	R18	X17
2	R28	X27
3	R38	X37
4	R48	X47