Repair of a Studiomaster D 1200 Amplifier


Fault - no output on one of the channels 



Removed top cover revealing a huge toriodal transformer on the right and the 2 amplifier channel assemblies on the left



Carefully separated the 2 boards


As can be seen below the individual amplifier channels consist of 2 half's, the board on the left is the main amplifier, the board on the right has the job of dynamically altering the supply rail voltages to the amplifier board as the audio signal demands, at low signal levels the supply to the amplifier board is kept at  + and - 43 volts  which would allow a maximum output of aroundt 100w into 4 ohms, when the signal level is increased  the supply voltage is increased automatically to a maximum of + and - 89 volts which delivers 600w into 4 ohms, the reason for this complicated design is to reduce dissipation in the output devices allowing the amplifier to run cooler, it does however complicate the transformer requirements, also 8 high power devices in total are still required, so the net result is an over complicated circuit arrangement that does not really achieve a lot.   


The source of the problem was found to be 2 of the amplifier output transistors and 2 emitter resistors on the amplifier board, and to of the transistors on the supply voltage control board as well as some resistors and a small driver transistor, both sets of transistors were replaced with matched pairs along with the resistors and driver transistor.

The amplifier was reassembled and tested @ 600w per channel, in addition to the above work, the bridged and power LEDs on the front panel were replaced as they were faulty.

This amplifier was made in England, the newer Studiomaster amplifiers are all Chinese made and are Studiomaster in name only, they have no connection with the original Studiomaster company before 2001 or the earlier models.


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