iMac G4 1.25 GHz: chime and fan, but no video, USB devices unresponsiv

Machine: iMac G4 (flat panel), 1.25 GHz, 17"
Changes made:

• Opened the case, replaced the hard drive with an SSD (IDE to Sata, Sata to mSata)
• Installed 1 GB RAM, but PC3200 instead of PC2700
• Replaced the clock battery

Sequence:

1. After reassembly the iMac booted normally.
2. When starting the installer I got an "invalid memory" error. I removed the new RAM and reinstalled the original module.
3. I then tried a PRAM reset (Cmd-Opt-P-R). There was no second chime, but the iMac continued to boot normally.
4. During startup with Option held (startup disk picker), the display went off after about 20 seconds.
5. Since then the display stays black at power-on.

Current state:

• The chime plays and the fan spins up.
• No video. With a flashlight in a dark room I cannot see a faint image either.
• USB devices (mouse and keyboard) receive power but do not respond.

Already tried:

• PMU reset (done once)
• Removed the battery and booted without it: no change
• SSD and optical drive disconnected from the logic board: no change
• Original RAM reinstalled (it worked before)

Cannot test: External monitor (I only have Thunderbolt and 4K HDMI displays).

Question: Has anyone seen this? Possible causes I have read about include the video cable or connectors, a hung PMU state, or a logic board fault. Are there other tests I can do without an external monitor?