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Old 27th December 2011, 10:21 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default Soundstream Reference 405

Ive got this amp which powers and plays perfectly on the 4x channels. The sub channel has a driver problem, where on the scope only a half waveform is driving my test load.

Seeing this amp has a previously repaired sub channel I started to tinker. The previous repair may have botched up something on the sub's driver board, where the C3789 measures ~-50v DC on all legs. By comparison the other driver on the same board has rail voltage on Legs 1&3, but ~0v on the center leg. The other drivers in the other four channels are measuring the same (Rail / ~0 / Rail) on their legs.

All measurments referencing amp general ground

Is the C3789 in the sub channel defective?
Leg 1: -50v
Leg 2: -50v
Leg 3: -50v

If I lift the center leg of the C3789 I read the following:
Legs 1/2/3: -50v

Pads 1/3: -50v
Pad 2: ~0v

So, when the C3789 is fully inserted, Pad 2 measures -50v.
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Old 27th December 2011, 10:50 PM   #2
Dr Zeus is offline Dr Zeus  United States
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I found the fault withthe driver. Looks like the last time the driver / sub channel blew, it took out a few of the traces on the sub channel's driver board. Im not sure if these driver boards can be replaced, but I was able to complete the former's hard-wire repair. The center leg of the C3789 was supposed to connect leg 1 of the T0-92 transistor under the PCB, yet it wasnt. I simply ran a bridge between the two and the amp is now testing out fine.

My guess is that when the outputs and drivers go, they hurt the driver board. In the previous repair, new drivers were hard-wired in a non-factory arangement to compensate for the damaged board. The former repair must have missed bridging the gap which I found.
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