After replacing LD31 to revive the +18VDC rail, the amp is producing more heat than before (and the other channel) and I measure a too high voltage over the resistors R79-R81.
The voltage should be about 35 mV, it can be max 60 mV but I measure 90 mV.
There is also a 50 Hz background noise over the speaker.
All power supply voltages (18V/30V/40V/50V) are there and within range.
What could cause this problem?
Regards, Hans
Sorry, this is impossible to tell without examining the amp.
We can still service this obsolete kit, so if you wish, you can return it for inspection/repair.
Thanks for your offer to diagnose/repair the amp. Commercially obsolete, indeed, but still running and imho far better than the couple of K8040’s I’ve also built.
Got rid of the 50 Hz noise by reviewing some solder points in the power supply section and bypassing a suspect pcb connection to an earlier replaced LED.
After replacing the transistors T5 thru T8 (bias) the voltage over the resistors R78 thru R81 is now within range: 35-38 mV.
As to be expected the overheating of the amp is also over.
The K4020 is now ready for another technical life cycle 
Kind regards and thanks again for your kind attention.
Hans
Happy to read you found the cause of troubles.