Siglent SDG 5082

[Johaness]

Hello!
I'm trying to repair Siglent SDG 5082 digital waveform generator. Not sure what have happened with it before it broke down. But now there is only either -4V DC or -11,6V DC signal on both of its outputs, depends on waveform parameters (amplitude or offset) with any type of waveforms. Actually, it tries to produce waveform, but since there is an -11,6 (-4V) offset (maximum amplitude for this model is +-10V in HighZ mode), all i can see is the small positive part of the waveform. Here is teardown video and links to some photos.
https://www.eevblog.com/2013/07/24/eevblog-497-siglent-sdg5000-function-generator-teardown/
I checked all power supply voltages on checkpoints (according to the service manual), power supplies on output amplifiers - they are all ok.
Next i tried to check signal path from DAC to the output, but it's not very obvious. I can say for now that there is kinda correct signal on Pin 2 of EL5166ISZ amplifier (no. 1 on the attached picture), and there is -4V DC on it's output Pin 6. I tried to change this opamp, but nothing changed.
It's also strange that both channels have the same issue now, although only channel 1 has been used recently and i couldn't damage channel 2 output directly.
Now i don't know what to do. As i can see, each channel has independent circuits and it's hard to damage both of them through the one channel output. So maybe there is a problem with something common for both channels? And can anyone explain me, how they produce DC offset in normal operation? I'm also wondering what is the function of circled part of the pcb?
I would appriciate any help.
Sorry for my english.
Thanks.

[tautech]

Welcome to the forum.

While these AWG's have isolated outputs they don't offer full protection against reverse voltages into the outputs and maybe the output stages have been damaged.
SDG5000 might share similar construction to SDG1000 models that are from a similar age therefore there could be something to help in this thread:
https://www.eevblog.com/forum/testgear/the-sdg1000-and-sdg800-thread/

[Johaness]

Yeah. You were right. This https://www.eevblog.com/forum/testgear/the-sdg1000-and-sdg800-thread/msg787268/#msg787268 post helped me to localize the problem. The problem was with the analog MUX/DEMUX 74HC4051D, one of them located in the circled area on the picture (soic-14) and another one on its right, so i replaced both of them and everything works fine now.

[tautech]

Yeah. You were right. This https://www.eevblog.com/forum/testgear/the-sdg1000-and-sdg800-thread/msg787268/#msg787268 post helped me to localize the problem. The problem was with the analog MUX/DEMUX 74HC4051D, one of them located in the circled area on the picture (soic-14) and another one on its right, so i replaced both of them and everything works fine now.

Excellent, well done ! 
Thanks very much for reporting back.