Not sure if this is the right place to post this, but can anyone tell me what a good Sony cassette deck might be that's capable of recording Type II (CrO₂)... and do any of them have optical input?

I am not aware of any walkman recorders that would have featured an optical input, but I use what many would consider to be one of the best Sony walkman recorders - a WM-D6C to record with TDK SA tapes. The D6C features quartz locked tape speed for recording and also pitch control for playback if required from other decks. The combination playback record head is of laser amorphous type.

One minor drawback is, that the bias by default is not optimally set for TDK cassettes, but rather Sony cassettes, however this can be manually adjusted once the unit is dismantled if accurate bias is demanded.

Didn't know that about the bias

Bias is like an electronic bribe to the tape’s magnetic particles—without it, analog tape would sound like a digital children’s toy. It is basically a minimum distortion adjustment—sort of like tuning an old-fashioned AM radio; too little bias makes the sound bright on top and fuzzy on the bottom. Excessive bias results in dull and dirty recordings. Bias is a Radio Frequency (RF) signal—anywhere from 60 kHz (for cassette decks) to 432 kHz (for the Ampex ATR-100 Series)—that is mixed with the audio signal. As with digital sample rates, the higher the bias frequency, the better (for lower high-frequency distortion).

The correct amount of bias current is determined by the size of the gap in the Record head. The “complications” are tape oxide type, tape speed and the type of distortion being minimized. Magnetic tape does not have the same sensitivity at all frequencies, but when adjusting bias current, high-frequency output is most affected, hence its use in achieving the most precise adjustment.

If you followed the first two parts of the series, by now you should have aligned repro and/or sync (Playback) with the alignment tape. Now, put up a blank tape, set the oscillator to 10 kHz and press Record on all tracks. On a three-head machine, if the high-frequency tones waver more than 1 dB, do not pass Go! Don’t even waste your time until the problem is solved, either by thorough scrutiny of the tape path or by having the heads relapped. Remember that many narrow-format machines have only two heads—Erase and Record/Sync—so when you see “PAR,” it means “Playback After Record,” or PITA (Pain in the Ass).

The adjustments, in order, will be:

• Bias (using 5 kHz, 10 kHz or 20 kHz)
• Record Level (1 kHz)
• HF EQ (level): 10 kHz and 15 kHz
• Low Frequency Adjustment: Bass Sweep from 250 Hz down…

Note: On a three-head deck, record bass sweep while monitoring via Playback head. Align the low-frequency EQ until the last peak and dip fall on equal sides of “0 VU,” then select a low frequency that falls on “0 VU.” Print that tone on tape and note it on the box. Include the bass sweep if the tape becomes a mix master.

On a two-head deck, record bass sweep, “PAR,” noting the peaks and dips, then select a low frequency that falls on “0 VU.” Print that tone (on tape) and note it on the box.

well I have always adjusted bias by recording from a blank 'spot' on the "FM" dial and listening and then adjusting the 'bias' on the front of the deck untill the most of the noise has gone away - have I been doing this correctly ?

Been using a Teac V-2RX deck and TDK Ma or Sa Tape depending ....

With different type tapes which use different metals, you have to compare the source, that why I only use Discrete 3-Head tape decks. My favorite the Nakamichi CR-7 and like most three head deck you can monitor what you are recorder to compare it to the source. Would love to find a good condition Nakamichi Dragon, but price and very hard to find parts for, as the NAAC stop working correctly due to crack plastic and tracks the tape incorrectly.