dmp
Well-known member
I'm racking up a pair of RCA BA-73A amplifiers.
I completely re-capped both of them and had to replace some bad transistors. The 2N526 were bad in both. Once those were replaced, one of them was very noisy. I found all four of the 2N220s were noisy on the one unit by replacing with BC327 silicon transistors, one-by-one, starting at Q1. So the unit was then working well with 4 silicon transistors. The other unit was working with all original Germanium 2N220s.
Strangely, the BA73A with the 2n220 has some 120 Hz noise. The one with the silicon transistors does not and just shows uniform noise. This is testing with the units in the same position in the rack.
Then I got some NOS 2n220 and replaced the silicon transistors in the one unit, testing the noise after replacing each transistor. When I replace Q2 with the 2n220, the unit starts showing the 120 Hz noise.
I tried a different 2N220 in Q2 and switched back to the silicon and confirmed the noise shows whenever a 2N220 is in the Q2 spot.
Any ideas why this would be?
I'm using a toroid transformer with a typical 2 rail voltage rectifier, to get -30v (setting +15 to ground and the -15 to -30v). Then hooking up pin4 (-30v) for the BA73s and pin6 as ground. Pin 18 goes to the chassis ground. The PSU ground is going to the chassis ground. All the grounds in the BA73s test OK (cable shields, transformer cans)
When I recapped it I used 100uF for C4, C7, and C20 so I don't think it could be noise coming from the PSU.
I completely re-capped both of them and had to replace some bad transistors. The 2N526 were bad in both. Once those were replaced, one of them was very noisy. I found all four of the 2N220s were noisy on the one unit by replacing with BC327 silicon transistors, one-by-one, starting at Q1. So the unit was then working well with 4 silicon transistors. The other unit was working with all original Germanium 2N220s.
Strangely, the BA73A with the 2n220 has some 120 Hz noise. The one with the silicon transistors does not and just shows uniform noise. This is testing with the units in the same position in the rack.
Then I got some NOS 2n220 and replaced the silicon transistors in the one unit, testing the noise after replacing each transistor. When I replace Q2 with the 2n220, the unit starts showing the 120 Hz noise.
I tried a different 2N220 in Q2 and switched back to the silicon and confirmed the noise shows whenever a 2N220 is in the Q2 spot.
Any ideas why this would be?
I'm using a toroid transformer with a typical 2 rail voltage rectifier, to get -30v (setting +15 to ground and the -15 to -30v). Then hooking up pin4 (-30v) for the BA73s and pin6 as ground. Pin 18 goes to the chassis ground. The PSU ground is going to the chassis ground. All the grounds in the BA73s test OK (cable shields, transformer cans)
When I recapped it I used 100uF for C4, C7, and C20 so I don't think it could be noise coming from the PSU.
