Hi all,
I don't know if any of you are into repairing Philips equipment, but if so, maybe you can use the information below.
I recently repaired a Philips FR675 and found an apparent design error in the power amplifier section. Each channel has a 2SC2240 transistor that is used as a current source of about 9 mA to 10 mA having almost 50 V across it. That gives a power dissipation of some 400 mW to 500 mW, while the 2SC2240 datasheet states an absolute maximum power of 300 mW at 25 degrees C. Not surprisingly, one of the 2SC2240's had broken down. I've replaced both 2SC2240's with ZTX458's (readily available from Farnell, unlike the 2SC2240, and rated for higher power).
The 2SA970 that is used as a degenerated common-emitter stage has the exact same problem: 400 mW to 500 mW in a 300 mW transistor. I replaced both 2SA970's with On Semiconductor 2N5401G's. Their beta is smaller than that of a 2SA970 which may cost you some loop gain, but the fT and Ccb are similar, so the stability should not be affected much. I did some quick checks with a 150 MHz analogue oscilloscope to check for oscillations and didn't find any. The ZTX458 and 2N5401G have worse noise performance than the original transistors, but as they are not used in an input stage I don't expect that to have much effect.
The pin-out of the ZTX458 and 2N5401G is different from the 2SC2240 and 2SA970, so I had to swap the base and collector connections (using some heat shrink sleeve to be sure they don't short). The bridge rectifier BR82 had also broken down, so I replaced it with a GBPC1502W (which is considerably larger and therefore difficult to mount on the PCB). Finally, I played some loud music into an 8 ohm power resistor while switching on and off the amplifier 10 times to remove oxide from the output relay contacts.
Best regards,
Marcel
I don't know if any of you are into repairing Philips equipment, but if so, maybe you can use the information below.
I recently repaired a Philips FR675 and found an apparent design error in the power amplifier section. Each channel has a 2SC2240 transistor that is used as a current source of about 9 mA to 10 mA having almost 50 V across it. That gives a power dissipation of some 400 mW to 500 mW, while the 2SC2240 datasheet states an absolute maximum power of 300 mW at 25 degrees C. Not surprisingly, one of the 2SC2240's had broken down. I've replaced both 2SC2240's with ZTX458's (readily available from Farnell, unlike the 2SC2240, and rated for higher power).
The 2SA970 that is used as a degenerated common-emitter stage has the exact same problem: 400 mW to 500 mW in a 300 mW transistor. I replaced both 2SA970's with On Semiconductor 2N5401G's. Their beta is smaller than that of a 2SA970 which may cost you some loop gain, but the fT and Ccb are similar, so the stability should not be affected much. I did some quick checks with a 150 MHz analogue oscilloscope to check for oscillations and didn't find any. The ZTX458 and 2N5401G have worse noise performance than the original transistors, but as they are not used in an input stage I don't expect that to have much effect.
The pin-out of the ZTX458 and 2N5401G is different from the 2SC2240 and 2SA970, so I had to swap the base and collector connections (using some heat shrink sleeve to be sure they don't short). The bridge rectifier BR82 had also broken down, so I replaced it with a GBPC1502W (which is considerably larger and therefore difficult to mount on the PCB). Finally, I played some loud music into an 8 ohm power resistor while switching on and off the amplifier 10 times to remove oxide from the output relay contacts.
Best regards,
Marcel