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MicMix STUDIO B Master-Room spring reverb

 

TRANSISTOR/IC REPLACEMENT


Transistors and IC's used in the STUDIO B Master-Room are selected from standard available types for particular parameters and cannot be changed indiscriminately. For optimum performance, replacements for these components should be ordered from the factory.

In addition, certain of these components in the STUDIO B require special adjustments if they do require replacement.

The amplifiers formed by Q21, Q22 and Q23, and also by Q41, Q42 and Q43 are current sensitive and are dependent on the gain characteristics of the IC-transistor combination. Potentiometers R32 and R49 provide only a 15% adjustment in current drain and compensation for large gain errors must be accomplished by changing resistor pairs R30/R33 and R44/R50, respectively. These resistors vary from 270 to 470 ohms and values in each pair must match within 5%. Locations for parallel resistors are provided in order to achieve required values.

For setup, utilize the specified external power supply. Remove Q21,Q41, Q81 and Q82 IC's, marking them for return to the same location. Remove Q61 transistor.

The nominal gain of Q22, Q23, Q42 and Q43 is 35 to 75 at 10 ma. Other gains can be used provided the gains of each pair are closely matched, but gain values outside this range will require different resistor values in the base circuits. (R30/R33 or R44/R50).
Using the external supply for power, determine that voltages at TP #2, 3, 4 and 5 are correct. Quiescent current should be 16 to 18 ma for the unit with Q22, Q23, Q42 and Q43 only installed.

Install Q21 and note current increase which should be 8 ma. If a current change of 8 ma cannot be obtained by varying R32 as previously described, R30 and R33 must both be changed. When replacing Q21 or associated transistors, it is possible that the current draw will be considerably more or less than the 8 ma required. Increasing R30/R33 resistance values increases the amplifier stage standing current.

Repeat the above procedure for Q41, noting that the Current change is 12 ma. The heatsinks (cooling fins) on Q42 and Q43 must be installed when making this test. Again, it may be necessary to substitute several IC's for Q41 or to change R44/R50 to obtain proper current for the stage.

With the addition of Q61 to the circuit, current should increase 9 ma. Another increase of 2 ma should be noted with the addition of Q82. Total current in standby conditions should be 48 to 51 ma and this value should increase to 65 to 70 ma under normal steady state Zero VU indication with a 1 KHz sine wave input.

 
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