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 used elsewhere in the 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.

 

[bebo]

Thanks for replying, I pulled one of the flameproof resistors out of the circuit and I think you're right. I placed it besides 1w flameproof Vs 0.5w Vs 0.25w to compare and yes they could be 0.5w.

Do you have any advice on how to determine the current limit in case I don't have the reference of those resistors? For example the mono input channel of my Amek TAC Bullet mixer (attached).

Also I noticed maybe I did a mistake buying the PSU I have, it is a GW-Instek GPC-1850D and I bought it for 150eur used, so I thought it was a good deal. I didn't realize the current indicator has just 2 decimals so I don't really know where I am between 10 and 20mA. Do you think it is so critical?

Thanks in advance, best regards.

 

[Khron]

For example the mono input channel of my Amek TAC Bullet mixer (attached).

You could have considered attaching the entire schematic of a channel. Except mind-readers (to which none of us belong), i/we can't tell much from a couple diodes and capacitors

I didn't realize the current indicator has just 2 decimals so I don't really know where I am between 10 and 20mA. Do you think it is so critical?

Nah, don't sweat it.

 

[bebo]

You could have considered attaching the entire schematic of a channel. Except mind-readers (to which none of us belong) can't tell much from a couple diodes and capacitors

My apologies, I had it on a DinA3 paper and I did a quick photo with my phone, I managed to scan it, so here it is attached.
Also at the end of the stereo input schematics I see a "power distribution" section, so I guess if I count how many IC's are they per channel and check the datasheet for their current consumption I can guess more or less how much it is. Can you recommend me any "quick way" of guessing it?

Thanks for repliying, best regards.

 

[Khron]

Also at the end of the stereo input schematics I see a "power distribution" section, so I guess if I count how many IC's are they per channel and check the datasheet for their current consumption I can guess more or less how much it is. Can you recommend me any "quick way" of guessing it?

Check the datasheet for those opamps, and the quiescent current total should be a good starting point. Driving whatever loads (from the line outputs or wherever) will need a bit more "juice" than that.

https://sound-au.com/troubleshooting.htm
https://sound-au.com/articles/troubleshoot2.htm