Require service as in it's readings are off, or as in it is not putting out light, or no numbers to read. These are quite different issues.
Burned out bulb; well look in the base first; there was a time macbeth's shipped with a spare in the base; alas mine has a holder with no spare bulb.
Readings off - has it worked for you in the past? I had a spider set up home in one of the filter holes in the rotating filter turret a year ago.
I had not used it in some months, and the weird numbers for magenta readings had me totally perplexed until I pulled the turret out and looked though the filters visually.
I have the owners manual for a TD504 that tells you how to adjust potentimeters on the back to correct it's gain and curve response. Mine are on so I have never tried to adjust it per these instructions.
Dead photmultiplier tube; give up, and buy another used one off the *bay. The fix of your unit will be much more expensive.
I have yet to buy a spare bulb, in case my tube burns out first.
You may want to consider an analyser/timer in lieu of a densitomiter if shopping for a new unit. Some (Lici/Jobo) will do color.
