With the unit switched OFF, simultaneously press and hold the UP ARROW key and the ASSIGN key and then turn power on to the unit. The two keys will have to be pressed until the opening DRAWMER ELECTRONICS message has cleared (about 4 seconds).
A prompt will ask RE-CAL?, if the YES key is pressed then the unit will jump into the second stage of the calibration process. Any key other than YES will abort the re-calibration.
Software calibration will require you to connect two balanced cables from the output to the input of each channel. This appears rather odd to the normal use of any piece of equipment, but in this case the M500 will be testing itself and will not cause any damage. When connected up, hit any key.
The calibration process commences. After about 30-45 seconds a message appears which should give a measurement for the four detectors named A, B, C & D. You should notice the levels passed by the SLAVE processor to the MAIN processor being displayed in an ascending order. Eventually a report should be given as to the success of this first stage of calibration. The detector letters should all have the word OK against them, or, a low number eg. 1 or 2. The optimum first calibration check will display:
A OK     C OK     CAL DONE
B OK     D OK     HIT KEY
Up to two errors are permissable on any detector. 1 error is equal to 1dB of un-linearity in a signal level detector somewhere between -50dB and +20dB, obviously this is fairly insignificant! If any more than 1 or 2 errors is displayed then it is suggested that the unit be powered off and then back on again to repeat the first half of the re-calibration. If, for example, the message reads A LOW then the A detector signal level is low. If A, B, C, and D are LOW then the test tone signal is probably missing (IC38 Pin 6). Other possible messages are:
OVERLD INP DET >A<              A, B, C, D detector overload, signal level too high from IC31, 32 Pin 1, 16
OVERLD INP DET >E< (or F)    Comparator IC14 not working over 0v-5v range


If the calibration results are greatly different from the four OK's, switch off the M500 unit, check the connecting leads and try again. The same applies to Filter calibration which follows on from detector calibration. The optimum filter calibration check will display:
IC3 OK      IC4 OK  :  HIT KEY
IC8 OK      IC9 OK  :
One filter error is permissible. (1 error is equal to 3dB un-linearity in thye filter table). If more than one filter error occurs, then restart the re-calibration process by switching on and off again. You could try swapping the LPF with the HPF filter IC blocks on that channel. If this has to be done take care when removing blocks from the IC bases.