Service And Troubleshooting Guide - Wohler ATSC-1 Manual

SERVICE AND TROUBLESHOOTING GUIDE

General Mechanical Observations
Elimination of cabinet and component sympathetic vibrations (resonances) requires considerable attention to
mechanical details. Because of this, and the physical constraints of the speaker's acoustic enclosures, even
minor changes to any of the mechanical details of the unit can seriously impair its acoustic performance. This
especially applies to the speaker baffles. If mechanical work on the unit is necessary, be sure to make adequate
notes to permit accurate reassembly.
Unfortunately, the unusual and wholly proprietary method of magnetic shielding is usually degraded slightly by
any disassembly of the unit, except removal of the rear panel. Almost any maintenance or repair will require
removal of the cover. If an immediately adjacent video monitor shows magnetic interference after reassembly of
the unit, it must be returned to the factory to restore the shielding completely.
Refer to the annotated block diagram and the overall unit interconnect diagram following, for assistance in
tracing the signal path through the detailed schematic.
General Audio Circuitry Observations
Since a single-sided power supply is used, all amplifier sections are "biased" with a 1/2 supply reference, so
all opamp signal terminals on the main board should have a DC level of +12V, +/-0.7V. Signal inputs to the
main audio board from any of the input select circuits are via the balanced input stage, in lieu of the XLR analog
inputs on the basic unit. Signal feed points for level meters and the phase indicator are immediately after the
input stage, and before the volume control section.
The signal pick-off for the headphones is after the volume and balance controls. Analog switches mute the speakers,
controlled by circuitry, which senses connection of headphones to the jack.
The power amps are attached to an aluminum heatsink plate (which is also connected to the circuit common for
these devices). The heatsink plate forms an operational module separate from the chassis, which allows access to the
solder side of the circuit board while power is applied to the circuitry. To avoid thermal shutdown of the power
amp(s), they should NOT be operated without their tabs being fastened to the heatsink plate.
Variations in the frequency response of different production runs of drivers has sometimes required minor
adjustments in the equalization/crossover components in individual runs of units. Some of these components
may have values slightly different than those indicated in the schematic, which are the nominal ones. If any of
the drivers (speakers) are replaced, it may be helpful to change some of these components to achieve maximum
flatness of response.
The operating threshold of the woofer limiter is critical to both satisfactory reproduction of musical transients
and preventing damage to, or destruction of, the speaker itself. The side speaker output limiter circuits are
similarly important, though not as critically adjusted.
The woofer power amps are arranged in a bridge configuration; care must be taken to avoid letting EITHER
speaker terminal contact the chassis (common) OR THE GROUNDED LEAD OF ANY TEST EQUIPMENT so
as not to short out the power amps. The side speaker outputs are single-ended, so these precautions are not
necessary for them.
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