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

[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]

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?