pgena
Member
My vintage MC275 suddenly developed a 120Hz Hum (ca. B-flat, an octave below middle C). I replaced the filter caps with the same brand that I had installed 10.5 years ago (C-EC100-100-500 from Antique Electronic Supply). After removing the old ones, I verified that the A sections on all four filter caps had failed! I wonder if this is typical (or a chain reaction?). Full disclosure, this amp has been used for the upper end in a bi-amped system (MC2105 on bottom), so perhaps a slow decline was less noticeable? Plus, both amps power a highly efficient JBL Paragon.
The MC275 now sounds exceptional again (full-range evaluation temporarily on additional vintage JBLs) with no residual noise. BTW, in the past I had replaced many of the resistors, and the caps row is all new, except for four Black Beauties and 2 Black Cats, @ .22uf. Similarly, I had long since eliminated the selenium rectifier and beefed up C28 and just now rechecked all the diodes. Tubes are all Golden Dragons.
The DC voltages have never been up to the published specs. and remain that way (i.e. -59dcv instead of -61 and ca. 435dcv rather than 450). Also, as mentioned by others in previous posts, like many factory-wired units this MC275 deviates from the circulating schematic. That is, R60 (10k 2w) connects from C26b to C26a instead of C25b. Also, R64 was 62k ohms, which I now swapped out for 68k ohms. The line voltage switch is at 117vac; the reading from my dedicated AC line is 121.5. Setting the switch to 125vac gives a slight boost: e.g., -63 instead of -61. I opted for the former switch setting.
Despite the lower voltages, the amp still sings! However, I’d appreciate any thoughts on the voltage deficiencies.
The MC275 now sounds exceptional again (full-range evaluation temporarily on additional vintage JBLs) with no residual noise. BTW, in the past I had replaced many of the resistors, and the caps row is all new, except for four Black Beauties and 2 Black Cats, @ .22uf. Similarly, I had long since eliminated the selenium rectifier and beefed up C28 and just now rechecked all the diodes. Tubes are all Golden Dragons.
The DC voltages have never been up to the published specs. and remain that way (i.e. -59dcv instead of -61 and ca. 435dcv rather than 450). Also, as mentioned by others in previous posts, like many factory-wired units this MC275 deviates from the circulating schematic. That is, R60 (10k 2w) connects from C26b to C26a instead of C25b. Also, R64 was 62k ohms, which I now swapped out for 68k ohms. The line voltage switch is at 117vac; the reading from my dedicated AC line is 121.5. Setting the switch to 125vac gives a slight boost: e.g., -63 instead of -61. I opted for the former switch setting.
Despite the lower voltages, the amp still sings! However, I’d appreciate any thoughts on the voltage deficiencies.