IIRC, this is caused by the 5V rail not coming up; the PSU section attempts to power up, it fails the "POWER GOOD" test and goes into shutdown and continues to cycle this way until power is shut off or the fault is repaired. If it got any further, the unit would begin its CPU/Acquisition POST and send error codes to the screen and serial port.
[EDITED TO CLARIFY]
A fault in any of the main voltage rails can cause this; look for shorts or open circuit on all main power rails. Go back through all the work you did on the DC-DC converter board and the A5 board. Look for shorts/opens/incorrect voltage on the main power rails at both the DC-DC converter and the A5 board where you worked. Look for cables plugged in wrong too.
[/EDIT]
I've attached a short dissertation on the very repair you just did, and I can eMail you the Tek Troubleshooting Oscilloscopes Training Manual (Older analog Only; will not have any digital systems troubleshooting) and Tek XYZs of Oscilloscopes Primer that are seminal reading in both servicing all 'scopes and how they work so you know how to get the most out of them.
Here is
KO4BB's archive of service manuals and resources for the 2465B/2467B. Please don't forget to give back a little via the button; hosting those documents isn't free and you can skip Starbucks or McD's for a day.
Cheers,
mnem
Needs more cowbell.