@Justin re:"don't understand..."
As a student, I bought a JVC KD-720 deck to go with my budget Hi-Fi. Now, 35 years later, it is the last original component of my system, and still works pretty well.
It was at the time the "Best Buy" budget (~£100) cassette desk, and (IMHO) did a thoroughly good job of recording and replaying music. The main problem was that it did not have a 'proper' FeCr setting, and pre-dated metal tapes, so could not use them (the Cr02) setting was also not really calibrated for psudo-chrome tapes like the TDK SA). It also only had a 'normal' tape head (not a Sen-Alloy one), so I was always expecting the head to wear, but even now, it's not too bad (apparently, early generation chrome and metal tapes were very abrasive, I guess that missing these tapes out prolonged the life of my deck).
Even this budget deck had a wow and flutter of 0.2% DIN, and a frequency response of 40-14000Hz with ferric tapes, and 40-15000Hz on chrome dioxide tapes.
I nearly always used TDK AD-90 ferric tapes to record, because they were the sweat-spot of the TDK range, providing good frequency balance (although slightly bright), good frequency extension (the brightness extended the upper ranges while minimising tape hiss) at an affordable price. They also used to be about 47 minutes long (useful for both sides of a long album) and very well made with friction reducing embossed teflon sheets either side of the spools. The Audio Society used to buy them by the case-full, and sell them at near cost to the members. TDK SA tapes were better but much more expensive, although I believe that the calibrated tape brand for JVC decks was supposed to be Maxell.
Anyway, my deck always gave exemplary performance. You could tell it was a tape from the hiss if you listened hard, but otherwise it sounded very good and I had very few tape jams or mangled tape in this deck. The sound of it persuaded several of my friends who had the "cassette tape is no good" attitude to change their mind and invest in a decent deck. It also helped having a reasonable music source to record from (turntable in the day).
To prevent jams, always wind the tape from end-to-end in a single operation and leave them on the leader before storing them. This prevents 'ridges' which increase the drag, especially on the take-up spool. If the take-up spool cannot be turned by the motor (remember it has a slip clutch so that it can cope with the varying diameter of the spool as the tape is wound on), then the capstan and roller will continue to feed tape in until it loops back on itself, and gets mangled. This was a common problem on C120 cassettes, because the nearly full spool was very heavy.
If it does look like this is happening, immediately re-wind the tape as best as you can before attempting to eject the tape. If you just eject the tape, you are absolutely guaranteed to break or stretch it, and will end up fishing out short lengths of broken tape forever.
I still keep iso-propyl alcohol around as a general cleaning fluid after using it for so long to clean the tape heads with cotton buds.