4-1 General Information
The MON-10 is designed to need a minimum of maintenance for long trouble-free operation. Should repair be necessary, the technician should first read the information in the manual concerning the circuits in question, and should follow proper procedures for testing and replacing semiconductors. Logitek engi- neers are readily available at the factory to provide technical assistance both during and after the war- ranty period.
4-2 Fuse
The primary winding of the power transformer is fused with a 1/2 amp slow-blow type 20mm fuse, housed in a fuse holder on the rear panel of the unit. It should only by replaced with fuses of the same type and current rating.
* * * * * * * IMPORTANT NOTE * * * * * * *
Do NOT replace fuses without first disconnecting the amplifier’s power cord from the wall socket, as dangerous voltages are present which may cause electrical shock. Always use fuses of the same value and type as specified in this manual. Do NOT use fuses of a higher value than those specified, as shock hazard and fire hazard may result.
* * * * * * * * * * * * * * * * * * * * * * * * * * *
A blown fuse is most often, although not always, a symptom of another failure. Related circuitry should always be carefully examined after any fuse failure, especially if a replacement fuse also blows.
4-3 Power Supply
Each MON-10 contains a well-regulated power supply to create necessary operation voltages.
T1 is a center-tapped step-down power transformer which is loaded with a full-wave bridge rectifier, RT1. The DC outputs of RT1 are filtered by capacitors C11 and C12 and fed to two voltage regulators, IC8 and IC7. These regulators provide positive and negative 18 volt outputs which feed the unit’s amplifier stages.
4-4 Switching and Amplification Circuitry
Input signals to the MON-10 are selected by the in- put selector pushbuttons S1-S10 and fed directly to the LINE OUT terminals on the unit’s rear panel. The left channel of the selected input signal is also fed to IC1b, which isolate the signal and convert it to a sin- gle-ended source. The right channel of the selected input signal is similarly fed to IC2a and IC2b, which do the same as IC1a and IC1b.
The single-ended left channel output from IC1b is fed to the “L”, “L+R” and “L-R” position on the SOURCE switch, S12a and the “L” position on S12b. The sin- gle-ended right channel output from IC12b is fed to the
“R” and “L+R” position on switch S12b and the “R’ posi- tion on S12a. The right channel signal is also inverted by IC3b and fed to the ”L-R" position on switch S12b.
The output of the SOURCE switch S12a is fed through the VOLUME pot R38a and buffer amp IC6 to the J6 HEADPHONE jack tip contact. Likewise, the output of S12b is fed through VOLUME pot R38b and buffer amp IC5 to the J6 HEADPHONE jack ring con- tact. The output of the J6 tip ring switching contacts are combined by R44 and R45 to form a mono sum and fed through power amp IC9 to the rear panel AMP OUT terminal. This terminal may be connected to an external speaker, or it may be connected to the IN- TERNAL SPEAKER terminal so that the speaker on the front panel is active.
The output of S12a and S12b are also combined to a mono sum by R32 and R33 and fed to op-amp IC4b, which amplifies the signal and feeds it though the me- ter trim control R32 to the front-panel VU meter. The gain of IC4a, and thus the calibration of the meter, is set by the resistor selected by the METER switch, S11. The meter may be more precisely trimmed by adjust- ing R30 for the desired reading.
The output of IC4b also feeds D3, which rectifies the signal and feeds holding capacitor C9, decay time resistor R28, and op-amp IC4a . IC4a works as a com- parator, comparing the rectified audio signal of the peak adjust control, R29. When the audio signal ex- ceeds the threshold determined by the setting of R29, IC4a supplies current to turn on the peak led, D1.
