Limiting lab PSU current while fixing a mixer's input channel.

[bebo]

Hello I am repairing a Soundcraft Urei 1603 mixer.
I have disassembled a stereo channel and I am going to power it with my recently purchased laboratory PSU (GW Instek GPC-1850D). I have close to zero experience with lab PSU's. Do you recommend me to limit the current on the PSU? If so, how can I know the value?
According to the mixer's service manual the power for the channel is +16v/-16v DC and the rear panel of the mixer shows "100-240v AC - 50w 50-60Hz". (The mixer has a switching PSU).

Thanks in advance,
Kevin

 

[Khron]

Do you recommend me to limit the current on the PSU?

Yes, definitely.

If so, how can I know the value?

Well, you have those (likely flameproof, hence the "F") 10 ohm resistors in series with the +/-16v rails. Ohm's law should help you figure out what worst case currents you can expect, using no more than 0.5 watt dissipation for each of those resistors.

 

[bebo]

Thanks for you reply @Khron , they physically "look like" they're 1/4w, but there is no info regarding the wattage of the resistors in the service manual. Assuming they're 0.25w, I would go with 0.25w/16v = 15.625mA.
If I find myself in a scenario where there isn't those resistors what would be your advice?
Thanks again for your help, best regards.

 

[Khron]

Thanks for you reply @Khron , they physically "look like" they're 1/4w, but there is no info regarding the wattage of the resistors in the service manual. Assuming they're 0.25w, I would go with 0.25w/16v = 15.625mA.
If I find myself in a scenario where there isn't those resistors what would be your advice?
Thanks again for your help, best regards.

That's a reasonable enough starting point, yeah.

If later you find yourself bumping against that current limit, you could go up to 50mA or so, but whether / how "sane" that is, depends on what / how much circuitry is "downstream" of that point, of course.

 

[mjrippe]

I think those resistors are more than 0.25w - for one thing, they are through hole while everything else is surface mount. Comparing them to the 33uf/16v caps is a bit tricky. I recently installed some 2w resistors that were the same size as the 0.5w in the same circuit.

Starting at 50mA current should be safe for a mixer that size. If limiting kicks on, you can try turning it up to 75 or even 100mA for a second to see if the limiting stays on. If it turns off, you are good.