Old 80's Tascam 20-Ch Analog Mixing Desk.

[DrNomis_44]

Hey Everyone,


Geez, you wouldn't believe what a mate of mine will be giving me next week, an old 80's Tascam 20-Ch Analog Mixing Desk, my mate said that it may need some restoration work, but it does have the power supply that normally comes with it, want to know what it looks like?....here's a pic of it:




I'm not sure exactly what the model number is, but it should be fairly easy to work-out, hopefully I'll be able to find a user and service manual for it online.

[Simon Barden]

It looks fairly modular, but you'll still have fun taking it apart and cleaning pots and faders. Some recapping probably necessary as well.

20:8 model, so really for use with an 8 track rather than as a live desk, though you obviously can do that as well using only 2 of the output channels.

But it's got the meter bridge, which is useful.

[Simon Barden]

M520 would seem to be the model. One on Reverb here:

https://reverb.com/item/7613662-tasc...pply-and-cable

[DrNomis_44]

Hey cheers Simon, I can confirm that it is indeed a Tascam M-520, what's more, the service/user manuals are available for free on the internet as a pdf download, my mate is giving the desk to me....for nothing.


I wonder how hard it'll be to connect it up to the mic/line-ins of my Focusrite Saffire Pro 40?, the Saffire Pro 40 has a total of 8 mic/line ins.

[Simon Barden]

Well, it's got 8 main line-outs, so it should just be a case of 8 TRS to TRS cables, though an 8-way loom may be more convenient. And then you'll want your outputs 3-10 connected back to the desk if you want to use it as a proper analogue mixer.

But the mic inputs on your Focusrite are probably better than the ones on the desk, which will invariably add more noise.

Adding a mixer to a digital recording setup isn't normally a wise move unless you really want to mix in an analogue way. But then you really need a fair bit of analogue hardware like compressors and reverb and gates etc. otherwise you are just using it as an analogue summing box.

[DrNomis_44]

Well, it's got 8 main line-outs, so it should just be a case of 8 TRS to TRS cables, though an 8-way loom may be more convenient. And then you'll want your outputs 3-10 connected back to the desk if you want to use it as a proper analogue mixer.

But the mic inputs on your Focusrite are probably better than the ones on the desk, which will invariably add more noise.

Adding a mixer to a digital recording setup isn't normally a wise move unless you really want to mix in an analogue way. But then you really need a fair bit of analogue hardware like compressors and reverb and gates etc. otherwise you are just using it as an analogue summing box.

I do happen to have an ARX 6-Gate, and a DBX 166XL Compressor/Limiter/Gate, and apparently some studios are getting back to using analog gear, so it would be nice to have the option to be able to mix in an analog way, digital plugins can sometimes be a bit of a cpu hog.

[DrNomis_44]

Update:

I've just been able to find some more documentation for the Tascam M-520 online, including some circuit board layouts, I've noticed that most of the circuitry in the M-520 seems to be using 455X series Op Amp ICs, like the JRC4558, which I'm familiar with, since the JRC4558 Op Amp is an inherently noisy one, I'm wondering if I will be able to replace the stock 455X series ICs with maybe TLO7X series ICs to reduce the noise levels, since the TLO72CP is a low-noise JFet Dual Op Amp IC?, I'll see what 455X series ICs the M-520 actually uses and see if I can find IC Datasheets for them online.


I've just gone through all the documentation I was able to find for the Tascam M-520, and it looks like I've got all the documentation I'll ever need to give it a good service and restoration job......fun times ahead!!!!

[Simon Barden]

I’d do the minimum possible to start with, and only change things if there is a real problem, rather than making work for yourself.

I’d start by measuring the power supply voltages and noise levels, as noise there will be passed on to the whole mixer.

Whilst the op-amps may be noisier than more modern ones, I’d measure the noise on a channel or two as a baseline. And change a component at a time and remeasure to see what difference (if any) your change made. The mic preamp is going to have the most gain, so that may benefit from quieter opamps (if it uses them rather than discrete transistors). Most opamps will be operating at pretty low gains, so you may not hear any benefit after making the mic-pre quieter (much like fitting the quietest 12AX7 you have as the first valve in a guitar amp as that makes the most difference).

When measuring, don’t forget to fit an XLR or jack with a terminating resistor across it to simulate a mic or preamp output, as any open/unconnected input will be noisy.

[DrNomis_44]

Found this cool video on youtube, here's a guy showing how to put-together a Dub Reggae track, and, you can clearly see the Tascam M-520 studio mixing desk he's using to mix all the instrument tracks into a stereo mixdown, what a Fat analog sound he's getting there, I like it!!!

https://www.youtube.com/watch?v=z-c9KR5SBi8

[DrNomis_44]

I’d do the minimum possible to start with, and only change things if there is a real problem, rather than making work for yourself.

I’d start by measuring the power supply voltages and noise levels, as noise there will be passed on to the whole mixer.

Whilst the op-amps may be noisier than more modern ones, I’d measure the noise on a channel or two as a baseline. And change a component at a time and remeasure to see what difference (if any) your change made. The mic preamp is going to have the most gain, so that may benefit from quieter opamps (if it uses them rather than discrete transistors). Most opamps will be operating at pretty low gains, so you may not hear any benefit after making the mic-pre quieter (much like fitting the quietest 12AX7 you have as the first valve in a guitar amp as that makes the most difference).

When measuring, don’t forget to fit an XLR or jack with a terminating resistor across it to simulate a mic or preamp output, as any open/unconnected input will be noisy.

Cheers Simon, I will definitely do that because I want to restore the mixer to a fully-working condition so that it is working well enough to be usable in a studio session.

I've got a good Digital Multimeter (has a capacitance function), and a 20Mhz Dual Trace Analog Oscilloscope I can use as test equipment while working on the desk, I may need to buy a Function Generator on eBay for use as an Audio Signal Generator, I've also got a Semiconductor Analyser as well.

What value of Terminating Resistor could you suggest using?