BK Precision 9130 Triple Power Supply repair.

[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]

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 ?

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?

[jchw4]

Here is a panoramic photo of the digital board without capacitors (these were bulged and needed replacement as suggested on this forum).

Ignore some panoramic artifacts. The board stiched correctly, higher parts do not always look good   

[jchw4]

The three sanded chips near the main IC look like [2U11.jpg], which I believe are MSP430F1101A https://www.ti.com/lit/ds/symlink/msp430f1101a.pdf.

The three sanded chips along the left side look like [2U5.jpg, 2U6.jpg], which I believe are CS5550-ISZ https://statics.cirrus.com/pubs/proDatasheet/CS5550_F1.pdf

The main chip looks like the [main_IC.jpg]  below. I could not read its markings  We need more photos from different devices to identify it.

[jchw4]

Here is power domains map with test points for those trying to diagnose it.
This time on top of the original board photo 

[jchw4]

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

As it has not yet leaked (could not find it anywhere), I guess they won't part with it.

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.

I don't know how to find these, becase I don't think main IC has any analog input/output (maybe one for the temperature sensor if I can find it).

The three CS5550-ISZ seem to use analog refs in SOT-23 package marked R30C. Which I guess are REF3025 from TI https://www.ti.com/lit/ds/symlink/ref3025.pdf.
Why would they use another part for the main IC?

But maybe it's really a 3V regulator.  But is it 3V or 3.3V ? If we have another owners of these devices, could somebody check the voltage on this part?

The temp sensor is usually right at the power semiconductors, on the heatsink- I would look there.

I looked all around the analog board and did not find anything. I don't think there is one on the main heatsink either...
I'll try to make photo of the analog board later, it's large and the process is somewhat complicated.

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?

This is good point. If it is indeed a voltage regulator, what voltage does it provide?

[jchw4]

Here is panoramic photo of the analog board.

[fzabkar]

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.

This behaviour is consistent with a power supply supervisor, or a uP reset monitor.

I expect that "7A" is a Year/Something" date/batch code.

Those "NR78" ICs may also be power supply supervisors, albeit with a different voltage setting. Maybe the date code is "Year/Month".

Edit:

http://file2.dzsc.com/product/17/10/14/1129211_101248570.pdf
https://www.onsemi.com/pdf/datasheet/cat810-d.pdf

NRYM -> CAT809x
NSYM -> CAT810x

Note: the two datasheets have different marking codes for the same devices.

[floobydust]

I think they'd use a low cost bead thermistor but can't see anything at the heatsink (unless under it). They might be using a diode or diode string to sense temperarture.
SMD marking code NS7A might be old Fairchild N-ch mosfet NDS7002A (2N7002) but I can't confirm it, yet it makes no sense for the gate to be grounded.

[fzabkar]

SMD marking code NS7A might be old Fairchild N-ch mosfet NDS7002A (2N7002) but I can't confirm it, yet it makes no sense for the gate to be grounded.

If it were a MOSFET, then its circuit reference would be 2Q27, not 2U27, just like the BR parts nearby. Moreover, it wouldn't make any sense to have a capacitor between gate and source.

Edit:

2U14 could be an 8-bit uC.

80C51/87C51/80C52/87C52 - 80C51 8-bit microcontroller family:
https://www.nxp.com/docs/en/data-sheet/8XC51_8XC52.pdf

Page 6 has the QFP-44 pinout.

Pin 4 is Reset, which connects to the supervisor IC's output.

Pins 14 and 15 are the crystal inputs.

Pin 38 is Vcc, and pin 16 is ground.

[floobydust]

That makes more sense, because the mystery SOT23 has a pullup resistor on the output pin which you don' t need with an analog temp sensor IC.
A temperature sensor is best located next to parts that heat up, so it can see their temperature. Usually it's independent circuit not using two pins on the MCU (temp in , fan on/off out). Does the fan power transistor trace go back to the MCU then?

Another approach would be to look at other BK Precision power supplies in the family - for schematics or board pics etc. 9129, 9100 etc.

[jchw4]

I think they'd use a low cost bead thermistor but can't see anything at the heatsink (unless under it). They might be using a diode or diode string to sense temperarture.

There is nothing that looks like thermistor, I spent quite some time looking for it. There is nothing under the heatsink too.

[jchw4]

I'll check the power monitor idea and report back.

Fan does trace to the main MC, I'll try to draw some schematics.

[Fraser]

This may be of no relevance to this power supply but one of mine uses the calculated wattage to set the fan speed. There is no heatsink temperature sensor for the fan. The lab power supply knows how much voltage and current is being produced and calculates the wattage. This is then used by the microprocessor to determine how much cooling is needed from the fan and the fan speed is set. I was surprised to see such a cooling routine but it certainly appears to work. There is a safety circuit monitoring the heatsink for over temperature in case the fan port becomes blocked. For those doubting this approach, as I did, I tested my power supply response to load and sure enough, the fan has a stepped speed response and each step comes in at a set wattage. At low wattages the fan is off and the next step is cyclic fan on / fan off. Once higher wattages are reached the fan is constantly on and running fast.

As I say, this may have no relevance to the particular power supply that is the subject of this thread, but I thought I would highlight that some power supplies use unconventional fan control systems.

Fraser

[Kean]

I have a BK 9130 on the round tuit bench which I've recapped in the past but is now reporting wrong readings.
Happy to take a look at any readings you are after, but I agree with fzabkar that the component is most likely a power supply supervisor/reset chip.

[fzabkar]

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 ?

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.

When you removed this IC, the output pad should have risen to +5V via the resistor and capacitor. AISI, if the capacitor doesn't charge, then it must be leaky, or the resistor must be open.

[jchw4]

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 ?

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.

When you removed this IC, the output pad should have risen to +5V via the resistor and capacitor. AISI, if the capacitor doesn't charge, then it must be leaky, or the resistor must be open.

Your idea was right, it's likely a supervisor.

I cheked that it's output goes low when input is more than 4.7V.

When I removed the IC device did not boot until I pu some low voltage on the IC output pin, so yes, it's voltage supervisor.
Thank you!  I soldered it back.


Now, back to the fan problem.

This may be of no relevance to this power supply but one of mine uses the calculated wattage to set the fan speed. There is no heatsink temperature sensor for the fan. The lab power supply knows how much voltage and current is being produced and calculates the wattage. This is then used by the microprocessor to determine how much cooling is needed from the fan and the fan speed is set. I was surprised to see such a cooling routine but it certainly appears to work. There is a safety circuit monitoring the heatsink for over temperature in case the fan port becomes blocked. For those doubting this approach, as I did, I tested my power supply response to load and sure enough, the fan has a stepped speed response and each step comes in at a set wattage. At low wattages the fan is off and the next step is cyclic fan on / fan off. Once higher wattages are reached the fan is constantly on and running fast.

As I say, this may have no relevance to the particular power supply that is the subject of this thread, but I thought I would highlight that some power supplies use unconventional fan control systems.

Fraser

This is an interesting idea, but after I recapped the device I set it to load test. With 30V 3A constant output device significantly heats up, but I did not see fan spinning.
If they calculate wattage, it's definitely broken.

Fan only spins in the very beginning when device performs tests. Actually, as I noticed that resetting main IC spins up the fan, it might be just a side affect that FAN PWM was not initialized while booting.

I have a BK 9130 on the round tuit bench which I've recapped in the past but is now reporting wrong readings.
Happy to take a look at any readings you are after, but I agree with fzabkar that the component is most likely a power supply supervisor/reset chip.

Does it spin the fan? Could you check when it starts the fan? I think just one scenario that I could reproduce would be very useful!

The fan is controlled by the 1Q25 (TIP41C) transistor next to to it. Which is controlled by 1IC12 (TL082CN)  that you can find on my analog board photo above the transistor along the left board edge.
Here is quick schematics:

Fan (+) goes to unregulated CH1 12V (before the 7812 regulator).

1R39 input goes directly to the pin 3 of the main IC (measured about 0.4 Ohm between them). So I am pretty sure this fan is directly controlled by the main IC.

But I still don't know the soure for the temperature. I don't see any analog inputs of the main IC (so far all I traced were digital lines). It has a few digital output lines to the analog board, but I haven't traced them yet. Maybe it has digital inputs too? I did not get to the analog board yet, so I don't know.


Upd: If you want to monitor fan PWM there are two convenient vias in the top right corner of the digital board to probe:

[Kean]

I have a BK 9130 on the round tuit bench which I've recapped in the past but is now reporting wrong readings.
Happy to take a look at any readings you are after, but I agree with fzabkar that the component is most likely a power supply supervisor/reset chip.

Does it spin the fan? Could you check when it starts the fan? I think just one scenario that I could reproduce would be very useful!

I ran it for a couple of minutes with 100W+ load across Ch1 & Ch2 and the fan didn't come on.  The heatsink was getting pretty warm.
There is a good chance I have a similar fault.  I will have a quick poke around, but I need to head home for the evening soon.

[Kean]

I can confirm my fan spins for a few seconds if I power off and on again, but not if I leave it off for a few minutes.  Then it just moves a few degrees.  Fan PWM stays low even when the heatsink gets hot under load.
All my power rails seem fine, but the regulators run quite warm - which is why the caps fail I guess.

The weird problem on mine is that Ch2 voltage readout follows Ch1, and Ch2 current readout follows Ch3.  But the channel outputs do work correctly as programmed.
It is such a painful unit to work on I ended up putting it aside, but I did quite like it.

[fzabkar]

Why not just run the PSU at a very low load and use a hair dryer to warm up the components? If there is a temperature sensor, that should find it, assuming it is working.

[jchw4]

The digital parts for the three channels are not symmetric. The per-channel microcontroller MSP430F1101A for the channel 1 has additional circuitry that I believe could be used for the temperature measurement.

Assuming the SOT-23-3 parts with "BR" and "15" marks are NPN 2SC2412 (as suggested by Google) I traced it to:




I haven't looked at the analog board yet.

Hopefully somebody could suggest a suitable part number for the main IC 2U14.

Upd: Added pullup resistor to pin 37.

[jchw4]

Why not just run the PSU at a very low load and use a hair dryer to warm up the components? If there is a temperature sensor, that should find it, assuming it is working.

I tried it, but never succeeded. Either my heating was not enough (analog board is too deep and has too many components and wires), or I did something wrong.