Tascam 388: Pre and EQ mods...

[shedshrine]

Yeah, I know, leave well enough alone. However, if anyone with the know how and experience has made any mods that they feel markedly improved or extended the performance of the pres and eq on these beasts, I'm (we're?) all ears.

The mixer sounds great already, and I have a standalone pre or two, I just like the whole idea of maxing out a peice of gear's potential. Is there anywhere in the 388 circuitry where the beancounters cut corners to meet their price point back in the 80's that can be "remedied"? Can't hurt to ask, eh?

[shedshrine]

In case you've ever wondered what it would be like to electronically trick one of these out, this guy did it...
(info from unit currently on ebay)

"Your one-time opportunity to own
a professionally (and intelligently) upgraded 388"


"You pretty much know what a Tascam 388 is, if you're looking at this page. Eight analog tracks on affordable and available 1/4" tape; with a full mixer included.

There's a lot more to it than that. There are mic preamps on every channel. The three eq controls on each channel are ALL sweepable. You can plug in to the channel or straight to the bus (more about that later). You can come right out of the tape preamp if you want, evading another pass through the channels, for higher fi. You can sync two 388s together. You can return to zero or to a cue point. There's an effects bus, a selectable pre/post aux output, and a separate monitor mix. And more, plenty more. This is not just a big Portastudio -- it is a real comprehensive tool for real work in real production facilities.

So what makes this particular 388 better than the others?

It has been Pooged. Pooge is an expression devised by Walt Jung, the engineer who researched and publicly exposed BOTH of the two most significant bottlenecks in quality audio in the chip age: Slew Rate limits and Dilectric Absorption. In this 388, all the channel op amps have been upgraded to 5532s, which are a fine-sounding audio chip whose imperfections are, for an IC, remarkably tubelike. And every one of these 5532s have been bandlimited (~70 kHz) to maintain closed loop performance to the limits of the chip itself. With expensive mica capacitors. You'd be surprised how seldom this is done, and how often manufacturers allow op amps to go into open-loop operation, which sends all kinds of spurious trash down into the audio.




Furthermore, each coupling capacitor position has been meticulously recalculated (considering every possible load on every output) so as not to have an unnecessarily large (and overly problematic) cap at that particular point. And each of those caps has been replaced by a Nichicon Muse high-fidelity cap (except a couple of positions where a mylar film capacitor would fit). AND, each of those new capacitors has been bypassed with a WIMA polypropylene capacitor, calculated to carry the majority of the signal, leaving only the bass to the Nichicons.

And by the way, those 388 preamps sound great, with fat and juice that (pardon me for saying it again) is almost tubelike.

And by the way, the local power supply capacitors on the cards have been significantly increased with larger high-quality Muse components as well.

And by the way, all this work had been done by an annoyingly meticulous tech who became a tech because nobody else could do work that was acceptable to him, him being me.

The 388 will come with an original owner's manual with all setup information, full schematics (with a couple of corrections by Yours Truly) and circuit board layouts.

AND....THERE IS MORE....

I'll also put in a couple of auxilliary boxes I made for this machine. One of them has a 1/4" phone jack and a bunch of output tails that plug into the bus inputs available at the back, so you can 100% bypass the channel and go right to the r/p cards. The switch selects which bus you want to connect to. The cables are unshielded, presuming that you are using solid-state (~150 ohm) sources like op amps). They are, if you care, 99.99% pure copper in polyethylene insulation.

The other box is a little oscillator that plugs into, and is powered by, the external control jack on the 388. It permits a wider range of speed adjustment than the narrow range provided on the front panel.

"Well, does that mean that I could crank the speed up to 11.25ips (between 7ips and 15ips), tweak the r/p trimpots to flatten out the frequency response, and get much better sound quality?"

My lips are sealed.

Happy Bidding!

PS I'm sorry to bloviate like this about this great machine but it really is a great machine.

PPS "We had this old Tascam from the Eighties with a reel-to-reel tape recorder inside the mixing board..." Billie Joe Armstrong talking about the Foxboro Hot Tubs album

PPS I forgot to mention that the chips in the r/p cards, the dbx, and the unbalanced output were upgraded as well. (I didn't use the balanced output.) I used some fancier chips in the areas that would see the whole mix. I forget exactly what's where. There are some 2134's, even a couple of 4562's (most recent design, relatively expensive, extremely clean) between the busses and the output."

[Zjcc56]

hey, everybody

a couple questions!

does anyone have a HQ version of the manual because cant zoom in and see the value of a resistor on the headphone amp....that brings me to my next question

the resistor labeled R120 is completely fried its charred and grey and i have no idea what the hell happened it has an ! next to the part saying it needs the exact same part to replace it with...in the manual it says its (i think) 33ohms carbon nonflam? like non flammable?

what would cause this? should i be checking other parts?
it says its for safety and i guess someone was saved from something

and last does anyone know more about these mods? they look pretty cool!

ok thanks,
im kind of scatterbrained so if i missed aything tell me!

cheers
Cory

[sweetbeats]

Well, I asked that same kind of question in this thread here and you can read some of the responses for yourself and maybe better form your opinion.

R120 is a 33ohm flameproof resistor. I'd have to pull out a 388 headphone amp PCB to see the wattage rating of it as the parts list is not clear in the manual, but if you replace it just get one that is the same physical size...

R120 limits the current seen by the 6V regulator that powers the headphone amp. I have no idea what would cause it to fry but I wouldn't be surprised if something else is cooked in the amp.

Does everything else work on your 388?

Any fuses blown?

If everything else works then I'd wager that your damage is limited to the headphone amp. Maybe somebody did the big no-no that applies to a number of Tascam's products from that era and plugged a mono plug into one of the stereo headphone jacks. That could cook things.

I say replace R120 and see if it works. If it doesn't, then we take it from there.

[EDIT]

BTW, I personally don't think it is worthwhile to go hog-wild on modding a 388...I believe Teac engineers knew their stuff and built solid performing relaible devices that, IMHO, sound better than anything you can get in even close the price range today...I'm talking about used Tascam product vs. new import mixer. Components were selected as a holistic system to work well together. There are many times where you can make gains by modifying electronics, but in general I don't feel Tascam stuff fits much into that category. Having clean power is a big deal both for noise-floor and protecting the longevity of all the components downstream and so I think it is a good idea to recap the power supplies with good quality 105-degree caps on this vintage of gear, but I've got first-hand experience upgrading opamps and it is that experience that has left me completely content with leaving such things alone in my 388. I'm recapping the whole thing with good quality caps, but I may or may not be able to hear the difference and its more of an insurance policy against damaging something with an old cap going bad. So, YMMV. Someday, since I have a parts 388 now, I may upgrade the opamps on it to see what kind of performance difference there is, but honestly it usually seems to work out, in my experience anyway, that the very thing I liked about something is gone after changing opamps...those slower, noisier and higher distortion opamps give it character. Notice that the modified 388 referenced above utilized a relatively slow opamp chip, the 5532. Still a very good chip but if you are just looking at numbers (noise, slew-rate, distortion, etc.), it is dated. They were used throught the 300-series mixers which sound great. So the person who did the modification stuck with a more classic chip anyway. Modding is neat and fun, but is in no way a guarantee of better results, and opens the door to irreparable damage.

[Zjcc56]

all right... all the fuses are good....
i replaced with a 33ohm 1/4 watt and it smoked up
i tried a 33ohm 1/2 and turned it on....so far so good i plugged in my head phones and i heard sound! but only on the right side and really distorted then i noticed some smoke....yup the 1/2 watt was smoking too. so i turned it off it made a laser like sound...i turned it back on and listened it went from left to both then to right....interesting...it still sounded really bad!

so ...any ideas???

thanks so far btw

[sweetbeats]

what wattage was the original resistor?

[Zjcc56]

the resistor was the same size as a 1/4 watt

[sweetbeats]

Okay. if it cooked that then there is something wrong that is drawing more current than should be, and by putting the 1/2 watt in there you are risking frying other stuff that's being saved by the sacrifice of the 1/4 watt component.

[Zjcc56]

soo..i have no idea what could be going on

[sweetbeats]

This is totally not what I'd do because I couldn't stand it not being right, but do you have an external headphone amp you can use? It is possible that somebody at some point plugged a mono plug into one of those headphone jacks which the manual clearly states is a no-no and now a transistor or one of the IC's is cooked or maybe even a current limiting resistor. The reality is that the headphone amp in most of the vintage Tascam gear is little more than adequate. IMO it is fine for functional monitoring but not for real critical listening or mixing via headphones (which is a last resort anyway AFAIC...I've done plenty of it but it is not a replacement for using monitor speakers to mix...) Anyway, my point is that it gets complicated now with that headphone amp and unless you are a stickler for things being original or "right" just because, it might be a good plan just to forget that onboard headphone amp and get a good quality external amp and connect it to one of the stereo out jacks...

Do the MONITOR OUT jacks work as well as the monitor source select switches and the monitor level control when using the MONITOR OUT jacks??

Outside of that I'd start by getting a magnifying glass and a good LED flashlight and looking real close at everything on that PCB that is in the headphone amp section...looking for broken connections, possible short curcuits, other fried components...anything. In particular I'd look at U104 which regulates the power that powers the opamps U101 and U201. U104 makes sure that U101 and U201 only see 6V. I'd also look closely at U101 and U201 themselves, and R106 and R206 which I think limit the output of U101 and U201. You could desolder one leg each of R106 and R206 and measure their resistance. If it is inifinite the component is cooked. If it is far off the 1ohm spec it would be good to put new parts in. I'm no engineer...just a hack with enough knowledge to be dangerous but that's where I'd start. I would also disconnect J102 (the plug that goes to the actual headphone jacks) and then measure to make sure there isn't any short between the left and right channels or with the ground conductor. This would be easy to do by plugging any good TRS cable into one of the headphone jacks and then setting your multimeter to continuity or resistance and checking between each of those three conductors on the tip of your TRS cable. As long as J102 is disconnected from the headphone amp PCB there should be no continuity between any of the three conductors. If everything checked out there as far as I could tell without powering the amp section then I'd try replacing R120 again with a fresh 1/4W 33ohm resistor and then isolate the headphone/monitor amp PCB...that means disconnecting all possible external connections to audio, jacks, etc. SO...pull J101 (connects to the monitor switch and monitor level controls), J102 (connects to the headphone jacks), and J103 (connects to the monitor out jacks). Now the headphone and monitor amp section is only connected to power. To get really fancy desolder the output terminal of U104. It might be good to check the output voltage of U104 and if you desolder the output terminal from the PCB then current has nowhere to go and R120 should stay intact so you can ensure that U104 is putting out right around 6 volts. Set your multimeter to DC volts, one probe to ground and the other to that output terminal of U104 and turn on the 388. Does R120 smoke? Is U104 putting out 6 volts? If R120 smoked or if U104 is outputting something other than 6 volts I'd start by replacing that. If R120 stays intact and U104 is behaving to spec then shut the 388 down, resolder the output leg of U104 and then power it back on. Does R120 smoke? If not, then without more sophisticated test equipment the next thing I'd do is connect J103. This shouldn't make any difference because it is driven by a different section of the PCB, but worth checking. If R120 still lives then I'd plug in J101. If R120 smokes then I don't know what but something would then likely be funny with the little PCB that has the monitor source select switches on it or further upstream. If R120 is still good then go ahead and plug in J102 with the headphone jacks, assuming the jacks tested okay above...if R120 then cooks there is some kind of short or something in the headphone jacks that didn't get picked up in your cntinuity test of the jacks...bang your head against the wall.

So, some crude ideas, but something to do. Hey, you like that burnt electronics smell right?

Try out whatever is comfortable for you, ask any questions of anything that didn't make sense, and let us know what happens. Good luck.