Carvin R1000 DC on ouput

[markorock37]

Just replaced the outputs and drivers on Channel 2 on a Carvin R1000. It is on the light bulb limiter. I have -0.4V on channel 1 output, and +2.4V on channel 2 ouput. I replaced D207,208, and 214. D214 is showing 2.4V on each side. Not sure where to check next or where this voltage is coming from.

[markorock37]

R1000 Schematic
https://cdn.shopify.com/s/files/1/0476/5297/files/R600_R1000_REV-H.pdf?2188

[Loudthud]

Have you tried replacing the dual opamps, A200 and A400 ? Are they in sockets ? Check the resistors on their outputs, R221 and R421.

Note: a TL072 probably won't work in this circuit, 5532 have higher output current capability.

[markorock37]

I haven't since they are not in sockets. I have some 5532's I can try. Of course I'll check those resistors first.

[Kaz Kylheku]

Note: a TL072 probably won't work in this circuit, 5532 have higher output current capability.

I've seen amps with an almost identical topology use TL072, like Marshall 8008. Op amp drives a kind of complementary pair voltage-amplification stage, whose bases are tied to a fixed voltage.  The base voltages are derived from the lower voltage power supply, same as the op-amp; the collectors are on the higher voltage rail.

That point between the emitters of Q202 and Q203, driven by A200B, probably has a decently high impedance (at least from a DC perspective), because the transistors are current sources.

[Kaz Kylheku]

The working-condition DC offset is a tad on the high-ish side at -0.4V.

Let's think about what might cause that one?

The op-amp input stage is in an inverting configuration. For that it has resistances on the input/feedback side: 22K || 22K -> 11K. That assumes something is plugged in which is DC coupled with an output impedance of zero. If we assume nothing is plugged in, the input resistor doesn't count and we have a 22K resistance.

The opposite side of the op-amp is just tied to ground without any resistance.  So due to the input bias currents of the NE5532, there will be an output offset (I mean, in addition to the tiny output offset voltage that is characteristic of the device).

The input stage is DC-coupled to the next stage that serves as the differential stage for the power amp. So any output offset will be amplified. The gain is 10. There are no bypass caps in the feedback circuit that would reduce the gain to unity at DC; the amp has DC gain. So any offset voltage from the input stage gets multiplied by -10. The feedback circuit of that op-amp is impedance-balanced with regard to that op-amp's opposite input.

If there is -0.4V at the amp output, it must be that the input stage has a 0.04V output offset. (That seems high if attributable only to input bias current: it would be caused by 1.8 uA of input bias current through the 11K resistance, which is high. The 5532 is supposed to have a worst case 1 uA input bias current over its entire operating temperature range).

[markorock37]

R221 tested good, replaced A200 5532, showing same results.

[Tassieviking]

I haven't since they are not in sockets. I have some 5532's I can try. Of course I'll check those resistors first.

I hope you are installing sockets as you go, be silly not to do it.

[markorock37]

Yes I definitely put a socket in.