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SERVICE INFO ON 300 SERIES AMPLIFIERS The 300 series has a number of ECO’s that need to be done, especially on older models (331, 332, 333): Replace power supply filter capacitors add insulators: 300 series amp capacitor part numbers (4) 240-200151 332, 333, 335, 336 (4) 630-430458 (4) 240-200153 331, 334 (4) 630-430458 (4) 240-200152 All amps (4) 630-430457 ECO # 1891 - replace 3 surge resistors on VSMB board with new type (204-110087) and add insulator (630- 430304-1). (3) 204-110087 Res. 10R (3) 630-430304-1 ECO # 3020 - replace U1 on VG boards with new type (340-780319) - this is most likely cause of protection shut down. (2) 340-780319 IC LINEAR LT1097CN8 OPAMP ECO # 2080 - replace all 68uF caps with new type (240-200013) - there are 2 on each VG board and 2 on VSMB board. (6) 240-200013 New ECO without # - replace C15, C18, C30, C58 on both VG boards with new type (245-16165). (8) 245-16165 .01UF IMPORTANT After doing all these (or checking to see that they had been done), unit should be run for 2 hours with no load and no input, after which all voltages should be checked and adjusted. THERE ARE ONLY FIVE CONDITIONS THAT WILL PREVENT A NO. 300 SERIES AMPLIFIER FROM TURNING ON. THEY ARE: 1. DC is present. If this is the case, pins 2,5,6,11,15, 16 and 18 of U9 on the VSMB board will be at 5 volts. Pins 12 and 17 will be at 0 volts (this information is on the page that follows). Please measure the voltage of these pins. 2. The unit is too hot (or a defective thermal sensor makes the unit “think” that it is too hot. If this is the case, pins 2, 5, 6, 12, 15, and 16 will be at 5 volts. Pins 11, 17 and 18 will be at 0 volts.. Please measure the voltage of these pins. 3. The AC line that the unit is connected to is more than 10 % higher than it should be (a 120 volt line is higher than 132 volts, a 240 volt line is higher than 264 volts etc). If this is the case, pins 2, 6, 12, and 16 will be at 5 volts. Pins 5, 11, 15, 17 and 18 will be at 0 volts. Please measure the voltage of these pins. 4. The AC line that the unit is connected to is more than 10 % lower than it should be (a 120 volt line is lower than 108 volts, a 240 volt line is lower than 216 volts etc). If this is the case, pins 2, 6, 12, and 16 will be at 5 volts. Pins 5, 11, 15, 17 and 18 will be at 0 volts. Please measure the voltage of these pins. 5. No. 333 and 332 units not turning on: Engineering has advised that they have discovered which components on the VSMB board are responsible for the turn on voltage being only plus 7 per cent instead of the specified 10 per cent. Replacing one resistor on this board of these units will return them to plus ten per cent. R8 on the VSMB board is now a 511 ohm resistor. This should be changed to a 432 ohm resistor, Madrigal part number 100679

From what I quickly looked at, these sample circuit (ML334) post are next to useless as they’re incomplete.

Cheers George