Density Adjustment in HP Laserjet 1320 - HP LaserJet 1320n Printer

Firstly I doubt that economode is set by default, but maybe the printer is in
economode for some reason.
Secondly I would normally expect the problem to be the toner cartridge but
assuming you have checked this my suggestions are as follows.
1. Run two internal prints from the printer.
a. By pressing the green "go" button briefly.
b. By holding the green "go" button for 5 seconds.
The first print should give you an indication of the print quality, if it
is good then the printer itself is probably behaving normally.
The second print will tell you exactly how the printer is set up. This
includes whether economode is on or off and what print density it is set for
(default is 3, range is 1-5).
2. If economode is on or the print density is lower than 3 the easiest way to
fix this is to do a cold reset on the printer as follows.
Important note: This will reset the printer to factory defaults, the only
setting that is critical is the jetdirect setting. Since you stated that your
printer is a 1320 and that you are running DOS I am assuming that it does not
have an "n" (for network) suffix eg 1320n. If I am right in this assumption you
can ignore the jetdirect comments above, if it is a network printer the second
printout from 1a should provide the jetdirect information which you may need to
keep for future reference.
Cold Reset Procedure:-
a. Turn the printer power off
b. Press and "hold" the green "go" button whilst turning the printer on.
c. Continue to hold the go button for at least 5 seconds and until the amber
attention light turns on (triangle with ! in the middle), this should occur
before 15 seconds elapses.
d. Release the go button.
Factory defaults should now be reset and you can check using 1a whether any
changes occurred.