Array 3711A DC Load faulty

[dundee]

Hey Guys!

I bought a broken Array 3711A DC Load on ebay.

The device is working but the readback current is wrong.

After starting the load, the readback current is growing.
It's nothing connected and the output is turned to off.

A little demonstration:


Do you know where i can get schematics?
Google did not help me

Best Regards
Steffen

[mcinque]

1. If you wait the current reading to reach (say) 500mA and you power cycle the instrument, that value restarts from zero or from something > 200mA?

2. The value continues growing indefinitely or stops at a certain value?

[dundee]

If i wait, its going up to 1,1A and is growing higher, but very slow.

After restarting the load its starting at 100mA.

[Guni]

Just for your information - maybe it help you

Array 3711A = ATTEN ATZ3711A = Tekpower TP3711A

[nctnico]

It seems you have an open connection in the current sense reading circuitry. Does the load still work?

[dundee]

Does the load still work?

Yes, the load still works!

I try to check this. Thanks!

[nctnico]

So it still regulates the correct current when in constant current mode? If yes I think the processor  sets an analog signal for the requested current which is then regulated with an analog control loop. Following the same logic there must be a signal towards an ADC by which the microcontroller can read the current. The problem must be somewhere in there.

[dundee]

Thanks nctnico,

yes - the constant current mode is working perfect.

Here some pictures:

Front Panel:


Main Board:




Would be easier with schematics. I think i have to write Array

Steffen

[bdivi]

Hello dundee,

Any progress with the schematics ? I would also like to have it.

Thanks

[dundee]

Any progress with the schematics ? I would like to also have it.

Not really... They sent me schematics for the 3700 (3711 is different).

But i found out a new interesting thing.

Currents over 4 Ampere are fine. Under 4 Amps the Display is showing a wrong value (1.2A)

Take a look at these two photos and compare:
top left = measured current
bottom right = set current

Under 4 Ampere (wrong):


over 4 Ampere (right):

[saturation]

That a bit of good news, it might mean current sense resistors for the lower ranges are faulted.  Check the line up of power resistors you can see with the top off, check too breaks/cracks in the leads or the PCB.

[dundee]

Hi saturation,
this 8 resistors are splitted in 2 serial blocks each with 4 0.1Ohm Resistors .

I calculated this to 0.025Ohm. I tried to do a 4 wire measurement and got 0.022Ohm.
Should be fine, i think.

[nctnico]

Look at the schematic. The current measurement range is switched by relay LS3. So either this relay is broken or there is something wrong in the surrounding circuitry. With the schematic you should be able to find the problem quickly.

[dundee]

Look at the schematic. The current measurement range is switched by relay LS3.

That's my problem. The schematic is for "3700". I have a 3711. There is no relay.

[nctnico]

If you trace back from the current sense resistors you might find a mosfet or analog switch instead of the relay.

[dundee]

It's a little bit hard to trace back the routes.

If i understand right, there are two current sensing ranges. One for currents under 3 Amps and one for over 3 Amps (working fine).

How is that solved?

Is the high current range done by the big "shunt block" on the main board (8 power resistors)
and the low current range by the single power resistors on the two power module PCB's? (Picture 3)

Main Board:




Power modules:


//EDIT:
Updated picture 1

[nctnico]

They probably switch the amplification factor somewhere. It looks like the idea behind the circuit from the 3700 is the same. Try to draw a schematic of the current measurement part starting at the shunt resistors; at some point it will start to make sense.

[dundee]

nctnico, your first suggestion wit the open connection in the current sense reading circuitry was right! 

I checked the 12 Pin cable from the display to the mainboard. Pin 12 had no connection.
Fault was probably an incorrectly crimped cable... Fixed it   

Also I got the schematics of the newer 3711.. (PCB Labled with 3712).

You can find them in the Attachments

Thanks to all for your help!

[nctnico]

Great to read you found it!

[atikutzu]

Hello there, could you please attach the schematic for the Array 3711A because I don't see it in the attachments. .Thank you.

[dundee]

Hello there, could you please attach the schematic for the Array 3711A because I don't see it in the attachments. .Thank you.

Hi atikutzu,
take a look two posts over yours. Post #17. --> https://www.eevblog.com/forum/repair/array-3711a-dc-load-faulty/msg674136/#msg674136

Or try another browser if you have problems to see the attachments.

[atikutzu]

Thank you! I was confused. It was mentioned as "(PCB Labled with 3712)" so I thought that I'm missing something. I got the attachments  , I'm good now. Thank you for your reply. Take care!

[OiD]

Thanks for posting the schematics!

Now all we need is the firmware for the 300W model, add the extra parts and ...