Repairing this amp is a rather ambitious project for me, considering my limited experience with electronics. Please bear with my request for step by step explanation; your help is appreciated.

Short story

Receiver goes into ‘ASO/DC protection mode’. Service manual indicates this is caused by the primary power supply circuit. I did some voltage measurements. The connector between the standby power circuit and – I guess – ‘main power’ circuit connection is labeled as followed:

Label Measurement
D5V 10.5V
D5V 10.5V
DGND
DGND
M5V
M5V
MGND
MGND
MAINPOWER 6.1V
X2-VOLDET
PDOWN-S

This unit should run on a DC 5V. Is the voltage measured on ‘mainpower’ the culprit? How do I troubleshoot further?

Additional info
The problem first occurred when hooking up a Google Chromecast to any of the HDMI inputs. Now it occurs even when nothing is hooked up to the amp. Sometimes this happens within seconds the amp is turned on. Sometimes it takes hours.
When the amp goes into protection mode, it’s shuts off with the power button flashing in cycles of 0.5 seconds. According to the service manual this is not overheating, but ‘ASO/DC protection mode’’. Service manual trouble shooting leads to “Check the primary circuitry parts including the POWER switch (for poor contacts, etc.) and replace defective parts.” (p.34)

Service manual: https://www.manualslib.com/manual/949448/Denon-Avr-3310ci.html

 

That is one of the most comprehensive service manuals I have seen. It includes extensive troubleshooting flowcharts. Are they any help? It's a huge manual, where did you find info about ‘ASO/DC protection mode’ and/or the flashing power button?

When you make voltage measurements, be sure you are referencing the correct ground. Looks like there is at least DGnd and MGnd - without spending hours on the schematic, I can't tell for sure if these are connected together anywhere.

So what do you see between M5V and MGnd? How about D5V and Dgnd? Is there any voltage between DGnd and MGnd?

 

Look on page 182. You will find the power supply and the associated voltages.

 

Scratch those voltages in my first post. I had my multimeter in AC setting (I’m in denial of getting reading glasses).
M5V to MGnd: 5.2V
D5V to Dgnd: 5.2V
DGnd to Mgnd: 0V

Thank you for pointing me to page 182. I’ll do some further testing and compare.

The flowcharts on p.34 led me to ‘Check the primary circuitry parts including the POWER switch (for poor contacts, etc.) and replace any defective parts’.
Warning indication by the standby led is described on p. 19 (ASO/DC protection flashes in cycles of 0.5 seconds, thermal protection in cycles of 2 seconds).

 

The schematic on p. 182 leads to another pcb (‘REG UNIT’, p.115 bottom part).
There’s a number of sockets (voltage in brackets, measured against the chassis).

CW182
1. DA+5V (5.0V)
2. DA+5V (5.0V)
3. DARETGND (0V)
4. DARETGND (0V)
5. +14v (14.1v)
6. +14v (14.1v)
7. TRIG_AGND (0V)
8. TRIG_AGND (0V)
9. A+7V (+6.9V)
10. A+7V (+6.9V)
11. AGND (0V)
12. AGND (0V)
13. A-7V (-6.9V)
14. A-7V (-6.9V)
15. MUTEB7V
16. DA.POWER
17. NEWPDOWN
18. VIDEOPOWER

CW065
1. +9V (9.0V)
2. VA+5V (4.9V)
3. VA+5V (4.9V)
4. VA-5V (-5.0V)
5. VAGND (0V)
6. VAGND (0V)

CW082
1. A+7V (7.0V)
2. A-7V (-7.0V)
3. HIGHB (OL)
4. GND (0V)
5. FANON (0V)
6. THERMAL (0.1V)
7. RELAY+B (11.4V)
8. RELAY-B (-11.7V)

Seems to check out. Not sure what to do next.