Brian Powell Audio

Repair Information Crimson Power Amplifier PCBs

35-01 and type V/VI/VII

These PCBs were in production from 1976 to 1999 and many thousands were produced.  Many of the very early PCBs are still working well (2023) but this information may help those with faulty PCBs.

The PCBs operate on high DC voltages and only those suitably trained should be carrying out this work.

Most parts are uncritical with the exception of the TO126/TO220 driver transistors.  The output stage will oscillate unless high speed low capacitance types are used.

The PCBs can only be tested if removed from the amplifier and connected to a current limited PSU together with a signal generator and dummy load and an oscilloscope to view the signal.  Before de-soldering the PCBs, discharge both reservoir capacitors to below 0.5V each. Do this twice as the voltage recovers somewhat.

Most faults can be found by testing the transistor junctions in circuit with a multimeter set to the “diode” or “silicon junction” range.  Any suspect devices can be desoldered and further tested.  Replace any burnt resistors and check for any that have gone open circuit particularly R29 or R30 (R30 needs one end desoldering to test it).  The R29 (zobel HF load) should be replaced by a higher power type as long as it is not wirewound. The output stage wirewound resistors are 0R22. Driver transistors are critical for stability only high speed, low capacitance devices will work.  Take care when replacing output devices, to maintain isolation from the L bracket.  Test this isolation before refitting the PCB.

Test the PCB into 8 and 4 Ohms. Clamp the L bracket to suitable heatsink.  Look for any fuzzy parts of the waveform which indicate instability.  Use a distortion measuring set if possible and expect THD about 0.01%.  Set the board quiescent current using VR1 to a total in the positive rail of 20 mA. Or set the potentiometer for the lowest current for minimal crossover distortion. Reset the quiescent current if it runs up too high (25mA) after a couple of minutes.  DC offset should be below 50mV.

PCBs will undoubtedly fail sooner or later and so a speaker disconnect device (such as Speaker Protection PCB – Brian Powell Audio) should always be used should the amplifier “go DC”