Valeo AC650/690 PRO scales, opamp, PIC controller fault

[fejlong]

Hi everyone,
I was just wondering if anyone has knowledge and kindness to help me repair an aircon top up machine control board.
This unit came in and the owner described that one of the scales are no longer working. It has three scales, OIL IN, OIL OUT and refrigerant TANK. So, the refrigerant tank scale is not moving at all when checked through the menu. I've checked the scale, and it's all fine, it has its voltage supply and gives signal back into the control board. Connectors on the control board to the three scales are identical, cables are interchangeable, and I did switched them over to check scales. All three scales are working on OIL IN or OIL OUT ports. None of them show any sign of life on refrigerant tank port. This tells me that scales and their cables are all fine. The problem must be on the CIRCUIT BOARD.

The CONTROL BOARD
As far as I could reverse engineer, the incoming signal from the scales are received by TLC2652 op-amps(three of them, one for each scale input) Just before the op-amp input each line is protected with bidirectional diodes(these are not shorted and diode test shows 0.547V voltage drop across them on the DMM. These are for ESD protection as far as I can understand. The output from the op-amps are taken over by an analog to digital converter TLC3548(outputs from all three op-amps are heading to this A/D through various resistors and parallel caps) and then by a pic microcontroller which does all the magic.
Because two of the ports are working( OIL IN and OIL OUT ), the pic micro is working the A/D is working( can the A/D be defective on one input? ). I decided to desolder an op-amp from a working port(from OIL IN) and solder it in place of the defective port(TANK). This made no difference and the op-amp died(as I had no ESD protection). Soldering it back into its original place made the port to behave exactly as the TANK port, not reading the scale.

Ordered three op-amps and a set of ESD gloves and leads to ground myself and all tools, mat, soldering station,etc.
Soldering in an op-amp to the OIL IN port was a success, but the TANK port is still the same, not reading the scale.

I don't know what to check further on, hopefully I have not bored you all to death with my story.

Anyone's help/advice would be greatly appreciated.

Daniel

[fejlong]

Re: Valeo AC650/690 PRO scales, opamp, PIC controller fault

To update you all i have made some further testing on the CONTROL BOARD.
On the input of the deat port(REFRIGERANT TANK)there is around 1.6V which goes lower or higher as I press on the scale.
This voltage can be measured on the output pin of the op-amp as well. And on the input pins of the A/D converter. All three input ports behave in the same way.

Can it be, that the input pin for the TANK is dead on the A/D converter?

[budm]

Re: Valeo AC650/690 PRO scales, opamp, PIC controller fault

May be uploading some good clear pictures so we can see what we are dealing with, right now we are blinded.

[redwire]

Re: Valeo AC650/690 PRO scales, opamp, PIC controller fault

Yes, I think they are pressure transducers. It could even be bad calibration or sensor settings for the one channel not the H/W.

[fejlong]

Re: Valeo AC650/690 PRO scales, opamp, PIC controller fault

Thanks redwire, it might be as you say.
Here are some pictures of the board.
it looks messy around the op-amps as I desoldered them multiple times. One of the traces come up. I had to use micro wire to re-do the connection.

[Retro-Hipster]

Re: Valeo AC650/690 PRO scales, opamp, PIC controller fault

Ah.. you said that you measured before the op amps, then after them, and then at the input pins of the PIC?

Have you checked for shorts at all between where the ground enters the chip and the input pin that is having the problems? I would expect each of these i/o pins for each of the scales to have the same sort of voltage potential differences. If you measured from something like what seems to be a common ground you might actually be getting some false readings..

If all of the inputs have the same sorts of voltage, there doesn't seem to be any low ohms shorts between the faulty pin, and the op amps behave the same all the way up to the chip I would think that there was something physically wrong with the chip or that there is some calibration in the software that has gotten badly jacked. For instance, maybe there was a calibration where you were suppose to have the scale empty but they had it full at the time and the software doesn't handle negative numbers.

[fejlong]

Re: Valeo AC650/690 PRO scales, opamp, PIC controller fault

Hi Retro-Hipster,
Thanks for your reply.
I measured before and after op-amp. But in in between op-amp and PIC there is an A/D converter. There are no shorts here.
Thanks for the tip regarding calibration. Searching the net I found a manual which sais there is a menu that allows calibration of the TANK scale, it is a bit awkward as I had to put on the scale a sample weight that measures in between 10 and 15kg for reference. Six plastic bottles of 2l water measures 12kg so i used them to recalibrate and it worked. Probably not the most accurate but at least now I know it's a calibration problem and not hardware. That was definitely the cause, they recalibrate it with the gas tank on and it went bonkers.

Now the TANK scale weight is changing. Before it was a blocked at 9750.

Thank you!