Why is my vintage boss dd-3 very quiet on the repeats even when level is turned up full?

SOURCE: My boss DD-3 makes a

Check your power source or battery. The sound is probably due to the internal processor recycling due to watchdog timer. If power is OK, then service will be necessary to repair the problem.

SOURCE: Boss digital delay pedal DD-3 won't stay on

I have seen a somewhat similar problem before. A switching diode had become faulty and needed replacement. I have no way of telling you where to look but if you have a volt ohm meter you can test them individually without removing them. The diode you are looking for would be in the circuit which connects to the switch. Use your probes to get a reading in one direction(one probe on each end of the diode) and then a reading with the probes reversed. If the readings are close or infinite in both directions you will want to unsolder the diode (making careful note of its orientation) and test it again. If the readings are still the same or close you will need to replace the diode. I forgot to mention to use the 2000 ohms resistance scale or the diode test function if you have one. Hope this helps.

SOURCE: My boss dd-3 pedal only works with a 9v battery.

You will have to test the adapter with a volt ohm meter to make sure it is putting out the proper voltage. It should be as close to 9 volts as possible. Also make sure that thehole in the plug is negative and the outside is positive. Is it the right adapter? I don't have a schematic for this model but www.schematicheaven.com has one for a DD-2. Hopefully they haven't changed too much. If you decide to repair it yourself, check the adapter first then check for a bad solder joint where the adapter plug is soldered to the circuit board. Hope this helps somewhat.