Hmmm, not sure but OK i will do re-check of the connections and post the photos during the weekend. I've soldered short leads to measuring points Q2S which is same as T1 1(4), Q2G and AD22.

Here i am not understand what are you talking about "oscilloscope inputs closed" i am still a newbie in oscilloscope measurements and it's features, but i want to learn it all.

OK no problem, until the weekend i am not able to make any measurements because i have all "toys" off hand.

Unfortunately my plans was yesterday suddenly changed and i am not back at home this weekend so i have no access to maky any measurements

OK good luck.

Hello after som time...many thanks for your detailed explanation, i need some time to absorb and understand all what you wroted and make my own measurements to see differences between these modes.

Thank you very much. This weekend i will not be at home but next i will and check the measuring AD22 point.

Hello,

good to hear that another S750 unit is brought back to life

For the audio processing board, the broken C28 i think that will have some affect, if you are enough courageous you can try it what will it does, but not recommend, you can damage some other parts.

I can measure the value but only next weekend (not this comming, but next), i havent the board by my hand now.

I recommend you to scrape off all glue from the board, i know it is hard but not impossible, just a lot of "carefully ant work"
The glue can make conductive phantom traces after some time in next use.

Till now i scraped about 7 pcs of these and another 3 pcs are waiting for it.

Good luck.

Hello, i've desoldered the caps today and measured both for you.

C28...10,357 nF...so it will be 10nF nominal.
C85...10,732 nF...si it will be 10nF nominal too.

After checking this size of capacitors package 0.8x1.6mm it is rated for 16V so i think it will be enough.

Hmmm...very hard to determine what is going on. Carefully check the audio processing board with magnifier glass for some short circuits or any other visible problems possible caused by scraping off the old glue or soldering new components. I can take detailed photo during this weekend and post here to help you.
Do you have checked the voltages of PSU?
If you disconnect audio processing board flat grey wire from woofer amp board will the buzzing sound dissapear?

What for the other AMP board it is working properly?

These were ceramic caps, so they are non-polarized.

Here are the detailed pictures of the same board as you have but this is with new caps. Check the traces if you have accidentally scraped off some. Measure with continuity tester the resoldered input and output connectors if there are is conductivity to the board or nearest part.

Hello, will you repair it by yourself?

It seems to be a stanby power section problem. Check D9, 10, 13, D26.

Hairdryer treatment is effective for electrolytics, but only for a while. TOP243Y is suspicious, but near impossible to diagnose, just very hard to replace. Can you measure capacitance and ESR of C62 and 63?

This post has been edited by MichalD: Feb 16 2020, 06:25 AM

Don’t think so. I think it only just works for caps.

Hm great knowledge, i wasn't thinking that the capacitor will be totaly dead and it will cause this problem. Maybee capacitor's ESR dramatically increases and it's capacity dramaticly decreses, so there is no power for standby section at all.
Try to use Rubycon's BXW capacitor.

First two caps loosing their capacity and ESR is increasing becuase of heat of the heatsink and therefore the standby circuit is not werking properly. Replace it for new LOW ESR one 105 °C rated for long term use. 5000 hrs or more.
Reseting is common issue on all units, you can fix this by following steps described upper in my topic posted Mar 13 2018, 10:20 PM "RELAY ISSUE (CONTROL POD RESETING) - SOLVED"

Hello, don’t know what to measure on amp board, but they are very rare to fail besides big cap, small ceramic cap and smd cap. Try connect only 2 IC amp board and measure voltage under load where was 95V with no load. After then connect second amp board with 3 ICs an measure voltage on both amp boards under load. I think the PSU can’t deliver enough power to supply both amp boards.

Hello, problem with the standby power section, can you measeure the output voltages? Check C6, C61, C8, D10, C63, D13, C62, D26. Capacitors need to be checked for capacity and ESR.

PSU board.