DC503/Repairs

Repair Issues

• If no LED or display segment is lit after turn on, check for +15 V from U500 Voltage regulator IC type 723.
• All ICs are placed in IC sockets. Contact problems can occur. Removing and inserting ICs may help temporarily.
• Check board is mounted to frame with plastic washers in between near pcb conductor tracks, otherwise a short to GND can occur.
• If the display responds properly to knob positions but not to input signals, check the signal coming out of the attenuators; on mine these switches seem to make poor contact. Also check the -22V rail.

Overheating / overvoltage

A bad TM504 PNP pass transistor damaged the -22V regulator circuit in my DC503, necessitating replacement of Q540, Q545, Q548, and R548. In spite of this exposing everything on the -22V rail to around -35V, it doesn't seem to have damaged anything outside the regulator. The only symptoms observed were R79 getting very hot and the unit not responding to input (though I _think_ the latter is due to an attenuator issue).