Gen1 DCDC Converter Troubleshooting and Repair

[eldenh]

I read 6.28k and 4.55k across C242 and C249 respectively. I assume those have to do with other current paths in the circuit. I've ordered a power supply with adjustable current limiting to try to test the 5v via TP201.

[kiev]

That's a good measurement that the caps are not shorted, i measured 5.7k and 4.5k on mine.

A power supply will be a good tool for this task. It will also let you decouple and adjust the input and output 12V later to detect and check circuit functions when you get it up and running.

[kiev]

Posted this schematic in the page 1 section also,

[eldenh]

The power supply finally arrived. As I considered how to hook it up I discovered that none of the GND pads on the top of the board are connected to each other or pin 1 or CN201. So it seems I must remount the board in the chassis or create a system to connect the pads. Is it possible that such isolation can be used to investigate some functions of the board in isolation?

[kiev]

It may be possible. It appears that the pads are capacitively coupled to the chassis, likely for filtering ac noise. GND1 couples the HV input after the EMI filter and is not needed for testing; GND2 couples the output 12V low side return thru some large capacitors; GND3 couples the LV section gnd plane to chassis thru a single small capacitor; GND4 couples the L201 inductor 12V output return to chassis before the current sensor.

If you wanted to just troubleshoot the LV section of the board without trying to start and run the PWM, then only GND2 would need to be connected to the 12V output return. But there are so many necessary conditions in the logic gates that i'm not sure how useful it would be. The main 12V output power return occurs at the center tap of the T103 transformer secondary, so that is where you need to be careful not to run the PWM without full continuity to the chassis ground for the entire board/system.

But if you just wanted to power up the low side supplies to check Vcc at the chips, etc. then that might be okay without the full-blown GNDs connected.

For example, 15V could be put onto the TP201 pad to measure if the 5V reference is being created by IC209. i would have the meter connected to measure before briefly touching the power supply lead to the TP201. Just a brief touch to see the result and then remove it. And have the 15V supply current very limited to just a few mA.

[eldenh]

My results are discouraging. I found 15v on the 5v buss. Q229 still checks as a good transistor, so I assume IC209 is damaged. Without being able to power up the LV section, I don't see a way to continue.

[kiev]

It's good that you found a problem with the 5V supply; You can replace the chip and move on to testing the LV logic section.

[eldenh]

When I looked for the part number in the list, the best I found was that it is an obsolete part. Where can I find a suitable replacement?

[kiev]

It looks like Arrow may have some in stock at $1.76, digikey and mouser showed 0 stock. Maybe i should buy a few also...

https://www.arrow.com/en/products/lt146 ... og-devices


It may be that you could remove the chip and then power up just the 5V buss with your power supply if you wanted to check for a short on the 5V side. Also put 15V on TP201 with the chip removed and check for some other sneak path that could be putting 15 on the 5V buss (e.g. internal short in comparator or op amp, logic gate, etc.). i think i have drawn up nearly all the chips but need to double check. Go thru the ICs in order, determine Vcc and if there are input pins at 5, 12 or 15, etc and make sure there is isolation.

[eldenh]

Thanks for finding a source. But Arrow wants $50 for shipping! I have no experience with smd devices or multilayer pc boards. Rather than learn on this one, I prefer to buy one if possible or find someone to repair the one I have. I doubt I can effectively determine what was the original fault until it is as functional as it was before my probing damaged it further.