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The Tascam 4-tracks you can get new for $150 or less are no great shakes, being the Porta 02mkII and the MF-P01. These are entry level, single speed, no-noise reduction models. The 02mkII records 2-tracks simultaneously and the MF-P01 records 1. They're about as stripped as can be, with no input-side mixing, no EQ, and no NR. The sound is not impressive, but they do what they do, and they fill the niche of entry-level recorders, aka "scratch pad" recorders. It's the most inexpensive way to get your ideas down on 4-tracks as possible, with no frills, no ups and no extras. Although I have both of these machines, and they're functional and fun, they're by no means meant for hifi audio or great production value. I have many, many Tascam 4-track Portastudios, and the Porta 02mkII and MF-P01 are the bottom rung of machines, not nearly as great as some of the high end Tascam Portastudios available.
Having said that, if you can score the guy above's 464 for $150/delivered, then I'd say that's practically the best thing going, and to do it NOW. The 464 is a high end, high speed, dbx NR model that includes 4-track/simul recording capability, 4-XLR mic inputs, and 3-band (mid-sweep) EQ on each of 4 channels, with an additional 8 basic no-frills inputs as extra "aux receive" type inputs. I usually don't mention the 464 first in any of my posts, but it's a fine machine, that's well worth the $150 asking price. Shipping on that unit alone will be around $20 or more. I do have the 464, and it is a high-end enough of a unit to deliver excellent sound quality and good production value, considering it's only 4-tracks,... it's GREAT!
I'll go on to mention several other high-end Tascam Portastudios on 4-track cassette that are very nice, indeed.
The 414mkII is the highest model that's still available as new stock from retailers, for about $250. This one is a little more stripped than the very highest of the Portastudio models, but is still quite good on features and sound quality. It has 2-XLR inputs and a deicated guitar input. It has 4-full mixer strips with 2-band FIXED EQ, with an additional 4 "stripped" inputs. It also records up to 4-tracks simultaneously at high-speed & with dbx NR. As used Ebay items, the 414mkII goes for about $75-$125, depending on the day. It's a decent choice for a mid-level Portastudio. I have this one, and I like it for what it is. It's a bit smaller and more portable than the highest Portastudio models.
The 424mkIII was the newest and highest-end Tascam Portastudio, but has recently gone out of production and off the retailer's shelves. It's now an Ebay-only item. You may get the 424mkIII off Ebay on many days for $100-$150, depending on the day, and it's well worth it. Many of the Ebay'ed 424mkIII's are still like new with little or no useage. The 424mkIII has all of the best features, including 6-full channel strips, 4-XLR inputs, 3-band EQ, High and Normal tape speed and dbx NR. There's another 2 "raw" inputs for efx returns. It's a very nice package, that's well worth getting at today's Ebay prices. Sound quality and production value is excellent.
The 424mkII is the previous generation of the highest end unit. It has 4-full channel strips with XLR's and 3-band EQ, but is otherwise comparable in features to the 424mkIII. The 424mkII offers good features & excellent sound quality and production value. Ebay cost on this item is appx $85-$125 on most days. (If you like the 424mkII, you may also want to check out the 488mkII, a cassette based 8-track Portastudio).
There's the Tascam 644, which is another level of high tech altogether. It features a full 8-channels with 3-band EQ, plus (mixer) dual-function that handles up to 16 inputs, total. There are only 2-XLR mic inputs on the last 2-channels. The tape speed is Normal/High, and dbx NR is standard. There is a Midi-Tape-Synchronizer section that's built into this unit, for those sequencer and MIDI-inclined home recordists. There's also an electronic LCD menu routing and switching section, that enables an 8x4 assign matrix, and is way more high tech than any of the other 4-track Portastudios. (The 644 is comparable in features to the 688, which is another cassette based 8-track Portastudio).
There's the vintage '85 Tascam 246, which is the pinnacle of the VU-meter style designed Portastudios. It sports 6-full channel strips with 2-band sweepable EQ, a 4-buss assign matrix, which is a cut above all the other 4-track Portastudios, with exception of the 644. The 246 has the requisite High/Normal tape speeds and dbx NR. The cue monitor is stereo. There are handy return-to-zero and loop functions that are handy. The only modern feature that the 246 lacks is XLR inputs, with the 6-main inputs being 1/4" phone jacks. Otherwise, the 246 is a topnotch package, almost second to none. Sound quality and production value is excellent. The 246 goes on Ebay anywhere from $125~$175 or more, depending on the day. This one is highly recommended.
There's the more vintage but still worthy Tascam 244. It's a bit more stripped than the 246, but is still highly capable, and is feature loaded as compared to the likes of the Porta 02mkII or MF-P01. The 244 has High speed, dbx NR, 4-full channel strips with 2-band sweepable EQ, and 4-inputs on 1/4" phone jacks. Access Snd/Rcv patch points and a stereo aux buss gives ample I/O for effects patching. The 244 has a stereo cue buss as well. Sound quality and production value on the 244 is excellent.
If you have a mixer that you're already very happy with, I'd suggest looking into the Tascam 234 Syncaset. It's a 4-track cassette recorder with RCA and 1/4" inputs, to accommodate either a mixer-input or mic-inputs. High speed and dbx NR is standard. There is a basic cue/monitor mixer onboard that's in stereo. There's some handy return-to-zero and loop functions that are comparable to the 246 (and 388). This a 3U sized rackmount recorder in the VU-meter styling of the 244/246, that's a very highly regarded recorder in it's own right. Sound quality and production value on the 2345 is excellent. You'll see the 234 on Ebay for about $100-$200, depending on the day.
If I were in the market for a nice, high end Tascam Portastudio with 4-tracks on cassette, I'd probably seek them out in the following order:
424mkIII
424mkII
464
644
246
244
... or for the less common unit,...
the 234,
For 8-track Portastudios on cassette:
688
488mkII
488(std).
... are what I'd get.
Oh wait, I already have those!
One last note: I think the case for cassette Portastudio's sound quality being poor, and tape hiss being unbearable is quite overstated. Any of the high-end Portastudios above are capable of making exellent recordings.
Thanks & good luck!