Electronics > Repair

BK Precision 9130 Triple Power Supply repair.

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jchw4:
I got BK Precision 9130 with dead temperature sensor, so fan never turns on.

I believe that IC 2U27 is a temperature sensor. It is SOT-23-3 chip marked "NS7A". I desoldered it and found that fan is turned on when out=3V and turned off when out=2V.

I cannot find anything like this. Comon 10mV/C or 19mV/C sensors will give 3V at \$T=\frac{(3V-0.5V)}{0.010V}=250C^o\$ or \$T=\frac{(3V-0.4V)}{0.0195V}=133C^o\$ which does not seem right.

What could it be?

Upd: Component was identified as voltage monitor, see https://www.eevblog.com/forum/repair/bk-precision-9130-triple-power-supply-(identify-component)/msg3752309/#msg3752309.
But fan still does not start, so this is now "fix the temperature monitoring" thread.

Fraser:
Just a quick sanity check….have you tested the original IC for behaviour ? Is it truly dead/faulty or is the circuit monitoring its output at fault with incorrect on/off thresholds ?

jchw4:

--- Quote from: Fraser on October 13, 2021, 12:19:54 pm ---Just a quick sanity check….have you tested the original IC for behaviour ? Is it truly dead/faulty or is the circuit monitoring its output at fault with incorrect on/off thresholds ?

--- End quote ---

This was the only component that was marked IC and was not a voltage regulator. Output was stuck at 16mV. Fan was always off.

I desoldered it and tried applying external power to the output. At 3V (measured 2.92V) fan turned on, at 2V (measured 2.0224V) fan turned off.

I did not check the component on itself (will do it later today), but my guess is still that it is temperature sensor.

Hmm. Now I started thinking that it could be 3V regulator that was used as a source of a threshold. But why whould 5V-powered MC use an external 3V source for it? (And where is the temperature sensor?)

On the other hand, putting 3V regulator there will kind of solve the problem, though only 2 levels of PWM (0.4% and 100%) is not great. But may be it is by design? I don't have another unit to compare.

floobydust:
You can ask BK for the service manual, the product is discontinued.

It might just be the voltage reference IC for the MCU A/D. I would see if other analogs move as the voltage is changed.
The temp sensor is usually right at the power semiconductors, on the heatsink- I would look there.

MSchalk:
I wonder if anybody knows what what chip that is with the markings scratched out. I agree with floobydust that it is likely an analog reference. If you look at the pinouts on TO-23 smd linear regualtors they match the inputs/outputs on the picture. I took a look at some temperature sensors in TO-23 packages, none of them that I found would make sense for their input/outputs.

EDIT: Also why would you need a capacitor on a temperature sensor?

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