Trying to repair Siglent SPD3303X without schematics (Solved)

[jrgandara]

Hello guys!

I'm stuck here with a defective Siglent power supply. I live in Brazil and back in 2017 I bought this PS during a trip to the USA. It worked well until about four months ago, coincidentally right after the three year warranty expired. The problem started with the PS turning itself off and on all the time. When turned off, if I pressed the main PCB, I would turn it back on. But soon shut down again. I was unable to identify any specific part of the PCB or component that caused this. Now I can't turn it on anymore.

The so-called Siglent service manual for this PS is not really a service manual. It is just a troubleshoooting with tables to check voltages in connectors. No block diagram or schematics. I cheked all the voltages and everything is right reaching the main board. I already looked the PCB with a magnifying glass, I redid some solders joints but nothing. I tested components like diodes and transistors and everything looks normal, except for a 7915 voltage regulator that enters 26V and leaves only 11V. He's a little hot. Apparently something is drawing too much current. Without schematic gets complicated, but I will continue and follow the regulator ouput and try to identify the culprit.

I sent an e-mail to Siglent U.S. asking for help but looks like they don't provide real service manual or schematics.

We will not provide the detail circuit diagram and will provide the module to repair.

"We will not provide the detail circuit diagram and will provide the module to repair."
I asked about the price for the entire board to be replaced but days gone by and no response yet. The problem is that I need the PS working again and expect their willingness to sell me the replacement PCB, wait for delivery and pay 92% taxes sucks. If I had the schematics I could fix this easily, needing from them only the microcontroller with the firmware if was the case. As it seems that nobody has this schematic and I have no experience with maintenance, any help or tip will be welcome.

Thanks in advance.

[mawyatt]

Without a proper schematic troubleshooting is going to be difficult. 7915 is a -15 volt 1.5a regulator, so if you are measuring -26 volts input and -11v output then the regulator is likely very hot and and into current or thermal limit. Check the temperature, if it's not hot then it's failed. If it's very hot then something is drawing lots of current, try tracing the -15 volt PCB path and check the components temperature along the way...the likely culprit will be very hot.

BTW if you are measuring +26 volts on the input (should be a negative input) to a 7915 then something is causing the wrong polarity input voltage, look at the main power supply source of this negative voltage power and likely a diode rectifier(s) has failed.

Hope this helps, and good luck,

Best,

[jrgandara]

Mike,

Thank you for your suggestions!

I forget the minus signal, the voltages are not with wrong polarity. Input is -26 and output labout -11.3V.

I'm trying to find hot components and at this moment I have no thermal camera to help me on this. I ordered one to be delivered today but until now they do not delivered. I tried the alcool technic from Louis Rossmann over the circuit with no success. Interesting was when I pour alcool the PS come back to life. When the alcool evaporates, the PS goes off. Very wierd behavior.

Still looking for the hot spots...

[mawyatt]

You can use your finger as a heat sensor, but be careful not to burn yourself, don't ask how I know this

If the "when I pour alcool" causes the PS to work, then get some freeze spray and try and locate the culprit component.

Best,

[jrgandara]

My finger as temperature sensor was my first try. Nothing is hot enough to be felt by the finger. The alcohol technic is not working because this glossy green PCB has no good contrast. In black PCB like ones in the Louis Rossmmann videos are much easier to see. I'm waiting the delivery of the FLIR ONE PRO to take a better look. The freezing spray is a good idea. I will order one.

[jrgandara]

Finally I managed to fix the PS!

Siglent in the USA did not help. It's been a week and no response about the module (main board) price.

So I bet on buying a FLIR thermal camera (FLIR ONE PRO) to help me with this repair without schematics. If it worked, instead of spending on replacement card + shipping and high taxes + delivery time, the FLIR would be paid with that money and the PS will be back to work ASAP.

In the first check I noticed 3 points with very high temperatures. Two resistors and an op-amp coupling capacitor. The two resistors were in parallel for greater heat dissipation. Their values were correct and following the PCB trace arrived at the VCC of a TL072. But the IC was not heating up, just its decoupling capacitior. I removed the capacitor and bingo! Shorted. I turned on the PS without the capacitor and it came back to life. I replaced it with a 100nF.

But interesting enough, that -15V output from the 7915 regulators still at -11V.  But now the PS was alive and that capacitor looks like was triggering some kind of protection. So I checked before the 7915 and the power for this specific circuit was coming from a round diode bridge. I checked with the scope and realize that one side of the AC was lower than other. Replaced the diode bridge and the 7915 output was -15V as should. Interesting that I do not noted any strange behavior due to this secondary problem.

Well, in the end everything worked out and the PS is back on the bench for use.

Thanks to everyone for the tips.

[mawyatt]

Nice work!!

Best,

[HB9EVI]

ufb!

thanks for reporting

[testpoint1]

Flir E4 is the best tool to check it, the resolution can be upgraded to 320x240, if distance is 0.2m, you can locate one capacitor.