Author Topic: Agilent N3300A DC Electronic Load Mainframe  (Read 922 times)

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Offline Mad Professor

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Agilent N3300A DC Electronic Load Mainframe
« on: October 25, 2018, 06:25:15 am »
Hi All.

I have picked up a faulty Agilent N3300A DC Electronic Load Mainframe.
It is fitted with the following modules.
4x N3302A 60V/30A 150W.
1x N3303A 240V/10A 250W.
1x N3304A 60V/60A 300W.

This unit shows very little in the way of life.
When powered on nothing is displayed, and the fans do not come on.
However if you press the blue shift button then shift is shown on the display, but as soon as you push any other button it goes off.
So far I have only done a little bit of research and testing.
I have found this YouTube video: .
This shows that the unit's fan's and display should work even with all modules removed, however mine does not.
I have also been able to find the schematic here: Agilent_N3300A_N3301_mainframe_eloads_schematic_5989-6688EN.pdf
I have had a very good look around the logic board and I can not see any damaged, blown, burnt components.

So onto some basic voltage checks, and I can confirm the following.
J12 (mains supply voltage) in my case 250vac.
J8 (24volts).
U21 output (5volts).
U23 output (3volts).
U20 output (2.8volts) unless Q3's gate is pulled to ground with a resistor then I get (12volts), this rain looks to be only for the cooling fans.
U19 Pin1, my DMM shows just shy of 50Hz.
U1 Pin3, my DMM shows 40MHz.

Any help or advise would be greatly appreciated.

Thanks for your time.

Best Regards.

 

Offline Samogon

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #1 on: October 25, 2018, 08:01:58 am »
Looks like CPU is not running i would check U15 x_rst and pwrdwn pins.
 

Offline bitseeker

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #2 on: October 25, 2018, 08:32:14 am »
I concur, Samogon. Just what I was about to type.  :-+

Mad Professor, although you indicate U23 has the correct output voltage, also double check that U15 is getting that power.

Regarding the shift button, its state is temporary, not latched. It's normal that once you hit another button, it turns off.
« Last Edit: October 25, 2018, 08:34:02 am by bitseeker »
I TEA.
 

Offline Mad Professor

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #3 on: October 28, 2018, 12:09:25 am »
Thank you for your replies.

I have checked U15 (X_RST) and (PWRDWN), Both are at 3.3v.

I have also checked voltages at a number of bypass caps.
U2 bypass caps (C2-C9) 5.1v
U15 bypass caps (C28-C43) 3.3v

 

Offline edpalmer42

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #4 on: October 28, 2018, 04:26:25 am »
I'm not familiar with this unit, but since the display reacts to keystrokes, doesn't that mean that the CPU *is* running?  Does the front panel use a separate CPU from the rest of the unit?
 

Offline Mad Professor

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #5 on: October 29, 2018, 01:53:20 am »
edpalmer42: Looking at the pdf from my 1st post I would say that the front control panel does have it's own processor, and that data is streamed between the boards.

I can not see any data / activity on the RX & TX lines of the main board.
At power on the RX line go from 0v to 3.2v for 90ms then comes down to 2.1v and stays at this voltage.
The TX line go from 0v to 3.2v and stays at this voltage.
 

Offline bitseeker

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #6 on: October 29, 2018, 12:53:28 pm »
I haven't worked on one of these loads before, either. However, it's not unusual for HP gear to have a separate processor for the front panel and serial comms between it and the main processor. Looks like that is the case here, too.

It's good that those U15 pins seem to be correct. Since the processor is on the GPIB board, are you able to interact with the load via GPIB at all?
I TEA.
 

Offline Mad Professor

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #7 on: October 30, 2018, 06:36:01 am »
bitseeker: I have no way of interfacing with the GPIB port, I have never used GPIB before.

What waveform type (sine/square) and voltage levels would you expect from the SG-615PH crystal oscillator?
As stated before my DMM reads 40MHz, but I only have a 2MHz USB Oscilloscope. This is showing as a sine-wave floating above ground.
 

Offline bitseeker

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #8 on: October 30, 2018, 07:52:11 am »
The datasheet has the specs. Vout should be at least Vcc-0.4V. The H version has a 5V Vcc. The speed should be printed on the IC.

The output waveform will look like a sine due to the low bandwidth of your scope (and the amplitude will appear low). If the speed on the IC is 40 MHz, then it seems like it's probably OK.

Regarding GPIB, if you have an Arduino UNO, it can be made into a rudimentary USB-to-GPIB interface. A search online will surface several projects with free firmware.

The serial lines between the GPIB board and front panel seems odd. I wonder if the whole thing might be held up because they can't communicate with each other. On other equipment, this would display an error, but such is not possible if the front panel isn't working properly.
« Last Edit: October 30, 2018, 07:53:52 am by bitseeker »
I TEA.
 

Offline Mad Professor

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #9 on: October 30, 2018, 08:02:38 am »
I do indeed have an Arduino UNO, I did not even think that someone might have already made custom firmware to turn it into a USB-to-GPIB interface.  :palm:
I will look into this, give it a try, and report back.

Thanks again.
 

Offline bitseeker

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #10 on: October 30, 2018, 08:10:13 am »
Sure thing. I'm very curious what you find because I see these mainframes available "for parts" on a somewhat regular basis and wonder if yours may be representative of a common failure. It'll be interesting.
I TEA.
 

Offline coromonadalix

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Online nctnico

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Re: Agilent N3300A DC Electronic Load Mainframe
« Reply #12 on: October 30, 2018, 10:07:24 am »
I'd pull out an oscilloscope and probe signals around the processor. Things to look at are power supply voltages, crystal oscillator and reset line. If there are external data / address lines then check these as well for activity. If there are any components socketed then pull these out a bit and then push them back (reseating).
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 


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