Loud Buzz on powing up XR1212

[RepairIT]

Hi I have a XR1212 mixer, on powering up the mixing desk with non-powered speakers plugged IN I get a very loud Buzz on both channels, If I power the XR1212 mixer ON first then plug the non-powered speakers note ( master volume are at zero)
into the XR1212 mixer works perfectly, I have changed the two preamp chips IRS2092S (IC101 & IC 201)
and the four mofset FS4227 (QR203,QR204,QR103,QR104) but the problem still persists, I have tested the Trica TR100 (MAC08BT1G) thinking this may be the issue, but my test show
it seems to be working correctly, this issue was reported in 2018 by Jamesjr54. but he had no replies to is issue. I wonder if anyone as had the same issue, it would be interesting to find and fix this issue on this old XR1212 mixing desk. I have got the PDF schematics. But with my limited knowledge of the circuit I am not quite sure what’s causing this issue,
The XR1212 Mixer has been used for many years & Gigs and this is the first time this issue as happen the band always plug speakers & equipment in first before powering the XR1212.
Any views or info will be most welcome.

[j0nsnell]

"I have changed the two preamp chips IRS2092S"
They are not 'pre amp chips', they are the class D convertors, oscillators and drivers.
Hum is usually caused by the power supply, depending upon the frequency of the hum.
If the amplifier starts to oscillate and work before the power supply has settled down and the main smoothing capacitors are fully charged, hum will be present at start up.
Most manufactures delay either the start up or the loudspeaker connection at start up. Part of the protection circuit.
I would monitor the +-80volt supplies firstly and then determine which way to go.

Check there is no DC from the preamp side hetting through to the input of the power amplifier.