AMREL FEL300-1 Electronic Load Imprecision Issue

[ReedTriesToRepair]

I'm having trouble with a few AMREL FEL series electronic loads (vintage circa 2003). They all seem to have a similar issue: the internal display-shown voltages are very inaccurate (pretty much always read zero) and the shown current is somewhat inaccurate. It also, only occasionally, shows "UNR" a few seconds after activating enabling the load and goes to a total short. I'm not sure if these issues are related to each other, perhaps it is having as hard time sensing the voltage across its input terminals? My multimeter shows a realistic reading when probing the terminals 

I've read that the current readings sometimes get (very) out of cal, so that doesn't bother me for now: https://www.eevblog.com/forum/testgear/motech-amrel-pps-1002-diagramschematic/

The lack of voltage readings is making it hard to put it into CV mode, as the load just pulls max current in an attempt to reach the user-requested voltage [correction -- it only does this at low set voltages. At higher set voltages, it pulls no current at all]. I couldn't find any manuals for my load in particular, or even any in the whole FEL line of loads. Pictures attached.

Does anyone have a manual for this series of load, or some input on what might be happening? I would love to get these working again!

[ReedTriesToRepair]

I guess the pictures didn't attach.

[zepto]

They all seem to have the same issue? That usually suggests operator error.

Do you know what the buttons do? Do you know how to use an electronic load?

Assuming it is in CC?? it is hard to tell...what is the setpoint? If it is in CV, what is the setpoint?

Is it dynamic or static?

Did you try a different mode of the load? There are CV CC CR CP?

Did you see what the short button does?

Take a picture of the back.


edit: The thread you linked is for a power supply.

It has GPIB and RS232, yes? Have you tried communicating with the device?

[ReedTriesToRepair]

Thanks for your response. I should have been more clear, as not much can be assumed about my knowledge as a newer member of this forum.

They all seem to have the same issue? That usually suggests operator error.

I have three of these loads. One of them shows the correct V/I measurements on the display, but the other two are erroneous- one shows zero voltage no matter what, the other shows like 30% off of what it should.

Assuming it is in CC?? it is hard to tell...what is the setpoint? If it is in CV, what is the setpoint?

Those pictures were all in CC mode with a setpoint of 0.1A on the load. I found out I only get the sporadic "UNR" error when the load is set to <0.5A because the load gets confused, being that it is always off by ~0.5A (more on that below). When is set it to higher currents, it seems to work indefinitely.

Do you know what the buttons do? Do you know how to use an electronic load?

Yes. I have regularly used them in the past, just not these fossils.

Is it dynamic or static?

Static.

Did you try a different mode of the load? There are CV CC CR CP?

Yes, both CV and CC do not work on the most faulty load (didn't test CR or CP); when placed in CV mode, the load pulls no current from the PSU. When in CC mode, the faulty load always pulls 0.5A more current than it is set to. All modes work, though for the wrong values, for the suspected out of cal unit. And they work as expected on the functional load.

I noticed the faulty load either displays 0.0001v or 0.0056v, never any other value. Perhaps the the ADC is fried? Or maybe it only has like 2.5 digits of accuracy instead of the shown 4 digits and is just fluctuating between the closest integers.

I've not tried remote operation as the front panel seems to work fine, and local control is how I normally would use the loads. I attached your requested photo of the rear as well as a photo of what the working unit shows on the display when in CC mode with a 1A load setting.

Thanks

[zepto]

There is this epidemic of people eager to fix things without understanding how to operate them. It's the wrong order of operations. I apologize for incorrectly assuming that was the case here.

I haven't used this load before and I'm certainly not an expert in electronic loads. I would double check in the menus to make sure there aren't any weird configuration settings (overvoltage, overcurrent, power limit). I would also make sure that you are displaying the current and voltage readback (instead of setpoints or limits or some other qty). I asked for the picture of the back because there are sometimes jumpers/connections that can alter the way these loads operate.

CR and CP modes might be worth testing to be thorough. CR is probably the least likely to battle a power supply control loop. Does pressing the short button do anything? I do not know if this will damage your power supply.

Without schematics, I don't have much more to offer you. You could look for internal fuses and signs of internal damage. A fuse might explain some of this behavior.

Having a working one may not trivialize the troubleshooting process. I doubt the ADC is bad.