Cassette Deck Magnetization

Cassettes
Demagnetizers are sometimes used on cassette decks, but they are rarely needed, and usually don't produce enough field strength to demagnetize.

Head magnetization used to occur sometimes on early decks using DC erase, for 2 reasons.
 * 1) The tape magnetized by DC erase had a very slight tendency to magnetize the record/play head(s) over time.
 * 2) The metal the heads were made from was relatively easy to magnetize.

DC erase has not been used on anything but cost-cutting decks in decades. However permanent magnet erase is found on some modern low cost decks, and does effectively the same thing as DC erase.

Tape head technology has come a fair way, and modern head materials are not as vulnerable to magnetization as was once the case. This also means they aren't as susceptible to demagnetization, and a demagnetizer is not normally capable of either magnetizing or demagnetizing a cassette deck head.

Demagnetizer Test
If you really do need to demagnetize a tape head, a hand-held tool does not normally produce enough field strength to do so.

Studios use much larger higher power demagnetizers to produce enough field to erase tapes. These items weigh several kilos and may consume 500w or so. Demagnetizing a tape head requires greater flux density than for erasing tapes.

Magnetizing a head takes more flux density than recording a tape. This has to be the case for all tape heads, otherwise they'd magnetize themselves in normal use. So if your demagnetizer can't erase a tape completely, you can be sure it won't demagnetize any tape heads.

Two retail demagnetizers were tested in 2006 and found to cause no data loss on floppy discs, and to be unable to erase a ferric cassette tape recording.

Reel to Reel
The situation is different with open reel decks, because many of these date back to the 1950s and 60s, and use more easily magnetized head materials.

Connecting a multimeter to the erase head can tell you whether it uses AC or DC erase. The better AC erase decks should not suffer any tendency to magnetization.

If you have a DC erasing deck, the deck will normally have more quality problems than simply the DC erase itself. It is possible to convert these decks to AC erase if really desired, but almost no-one would want to use a sub-optimal open reel deck for regular recording today.

8 Track
Most 8 track decks don't record, so have no erase system, and thus nothing to magnetize the head. Magnetization is unlikely in these decks.

For magnetization to occur would require:
 * 1) A susceptible head
 * 2) Use of DC erase
 * 3) A lot of recording use

Magnetization is also unlikely in recording decks where the record function is not used a lot.

8 Track demagnetization cartridges
The demagnetization cartridges sold in the 1970s are ineffective. They used small permanent magnets and lack the necessary field strength.