Northern Telecom DNC-50 Maintenance And Troubleshooting Manual page 15

Figure 2-3
Tape Head Location in the 1/4-shelf Disk/Tape SRU
The read/write heads are located behind
this knob, close to the back of the slot.
They are exposed slightly (for cleaning)
when this knob is pushed down.
Routine Maintenance 9
Push this knob down
to clean the tape heads
Practice 450-1011-501