Hi, I'm trying to repair a C-Audio RA500 amplifier (one dead channel). In fact, I have two identical units with the same problem, so I'd really like to figure this one out. I could just throw parts at it, but I'd prefer to understand the design, and that's why I'm asking for help. I'm hoping someone here is familiar with the design - I'm not a pro and have no relevant education, I just enjoy solving problems.
I can't find a schematic for this unit, and apparently it was manufactured by some other company and is not similar to other C-audio designs.
It's a single rack unit stereo amplifier. Output devices are K135 and J50, and the majority of op amps are TL072CP
I'm puzzled by the voltages I find inside this unit.
The outputs from the primary windings connect to the main power section symmetrically at +-64V, all seems well in that area.
However the secondaries confuse me: using the transformer center tap as ground reference - the output from the secondary rectifier is 24V+ and 0V. The rectifier diodes test fine. My first thought is to start looking for a shorted component, however looking at the traces - the negative output of this rectifier goes right back to the center tap on the transformer. To me this is strange. Knowing that the TL072's need positive and negative supply I check their voltages:
one of them gets +24V and 0V, most of them get 0V and -15V.
Right now I lean towards thinking that the +24 is only used by the protect and limiter circuits, and that the TL072's (except the one recieving +24) is powered by the primary power rail, tough some drop down resistors or regulators that I have not located yet. Possibly the positive failed and that's why I have -15V and 0V.
OR is the input stage supposed to be asymmetrical? Perhaps the center tap is not the correct zero point for this part of the amplifier? If so, the voltages may actually be +7.5 and -7.5 and everything is fine in that section too...
There's also a set of 1n4004s very close to the inputs, that seem to be doing absolutely nothing. No AC or DC touching anything. Weird.
I can't find a schematic for this unit, and apparently it was manufactured by some other company and is not similar to other C-audio designs.
It's a single rack unit stereo amplifier. Output devices are K135 and J50, and the majority of op amps are TL072CP
I'm puzzled by the voltages I find inside this unit.
The outputs from the primary windings connect to the main power section symmetrically at +-64V, all seems well in that area.
However the secondaries confuse me: using the transformer center tap as ground reference - the output from the secondary rectifier is 24V+ and 0V. The rectifier diodes test fine. My first thought is to start looking for a shorted component, however looking at the traces - the negative output of this rectifier goes right back to the center tap on the transformer. To me this is strange. Knowing that the TL072's need positive and negative supply I check their voltages:
one of them gets +24V and 0V, most of them get 0V and -15V.
Right now I lean towards thinking that the +24 is only used by the protect and limiter circuits, and that the TL072's (except the one recieving +24) is powered by the primary power rail, tough some drop down resistors or regulators that I have not located yet. Possibly the positive failed and that's why I have -15V and 0V.
OR is the input stage supposed to be asymmetrical? Perhaps the center tap is not the correct zero point for this part of the amplifier? If so, the voltages may actually be +7.5 and -7.5 and everything is fine in that section too...
There's also a set of 1n4004s very close to the inputs, that seem to be doing absolutely nothing. No AC or DC touching anything. Weird.
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