Agilent 6612C repair help

[Rycroft]

Hi all,

I recently acquired a Agilent 6612C power supply that was advertised as for parts or repair. It has an unregulated output of about -0.375V no matter what value you apply. I saw a similar fault a guy on Youtube fixed by simply replacing a fuse - I'm not so lucky. All fuses are intact. I've checked voltages and all the points I can find are good.

The unit does not show any errors so I'm hoping there is no faults in the control circuitry.

Now the only thing I'm thinking before going through checking all the regulators and output transistors is the transformer wiring. The unit on the back plate shows it was originally 120V. The manual has a wiring diagram for changing to other line voltages (I need 230V). So my question is looking at the attached wiring diagram, is that a jumper on the right side of the 230v example? I'm 99percent sure it is but before I let out the magic smoke I thought I better ask the wiser-than-me. It looks like the wiring on the left side has been changed and looks right for 230v. The only thing that has me questioning it is that the unit powers up and I'm getting good voltages on the test points?

Thanks
Rick

[duak]

Do you have this troubleshooting manual?  It covers the family from 6011C to 6014C

http://ridl.cfd.rit.edu/products/manuals/Agilent/power%20supplies/CD1/Service/6611Cser.pdf

Best o' luck

[coromonadalix]

page 34 35  show you some voltages references  do they correspond ??

and page 42 is the transformer wiring

[Rycroft]

Yes, thanks guys. I have the manual and the voltage test points are good.

I'm just not sure about the tranny wiring. Mine, right now, does not have that jumper (if that is what it is) on the top 2 terminal that it appears to be on the 230v diagram. All I can think is that there is a secondary that's not connected and it's not one of the test points in the manual. I guess I could jumper it with a fuse to be safe.

[Ordinaryman1971]

Post the picture of the board please. I think there is two versions of this board with through hole and SMD components.
If you could please list the fuses you were able to find and check on.
So.... looking at the transformer, looking from the front, the wires on the left set correct primary.
You say, you’ve checked them all and they are connect and according to the diagram you’ve poseed?
I guess it had to be right because you’ve powered it up and it didn’t... explode.
Go down the fuse list and see if you got all of them... are they through hole or SMD,
One of them.. the F309 I believe the 4A fuse is different, all other ones are green, this one is orange. Make sure you got this one too... it may be a culprit.

[Ordinaryman1971]

When you have it set to 110 there are two orange jumpers used. For 220 or 230 you use just one of them the other one is ... spare in case you need to rewire it back to 110.
I don’t think this transformer would work without this orange wire so... you have it somewhere there.
Check the wiring and make sure it’s wired correctly.

[Rycroft]

I'll post a pic tonight when I get home.

I've tested the 7 fuses noted in the manual. 4 are through hole and 3 are smd. All buzzed out.

Looking from the front the wiring on the transformer looks good on the left side. It's just the top 2 terminals on the right that are my concern.

[Ordinaryman1971]

It’s a place where you store the “spare” orange jumper for wiring to 110V, those lugs are not connected... storage

[Rycroft]

Did some more digging around. The secondaries are slightly low but the DC values at the test points are good so I don't think there is an issue there.

The supply has the 760 option. A relay array that isolates and reverses polarity, if wanted. To see if there was an issue with the 760 board  I pulled the connector and put jumpers across the pins on the main board. I've seen in videos that units without the 760 board looks to have 0 ohm resistors across the pads. Same results and the control panel recognized that the board wasn't there by not having the setting options in the menu.

One strange thing I've found is that in the manual where it has bias and reference supply voltage listed it says to reference the + rail and - rail to the - output post. This results in all 0volt readings. If I reference from the secondary common then I get the voltages expected on the rails.

[wictor]

Have you checked that sense wires are connected?

[Rycroft]

"Have you checked that sense wires are connected?"

Do you mean at the front binding posts? No, but I have the sense and output terminals at the back linked.

[gamalot]

You are welcome. 

[Rycroft]

Thanks gamalot,

Yes, that's how mine is wired. Ordinaryman pointed out that the jumper on the left was plugged into terminals with no connection. It's just a place to hold the spare jumper. I just don't have the jumper so all is good there.

Time to start testing regulators and output transistors. Can I do this with them in circuit?

[Rycroft]

A bit of progress. There is a SCR crowbar across the output. The SCR is shorted and I have removed it. Hoping I'd get something without the SCR in circuit I fired up the power supply. Same results. I'm hoping the software is seeing the SCR is not there or not functioning and when I get a replacement installed it will work.

[Ordinaryman1971]

Take the board out of the enclosure. It's just few screws that are holding it in place. You will have to remove the leads from the transformer too and please look at the bottom side of this board. Look for broken or burned out traces... and one more time... you've checked all the fuses, I mean, all the fuses?

[Rycroft]

Thanks Ordinaryman,

I've checked every fuse I can find and they correspond with the manual. I've checked all the output transistors and the regulators and they seem fine in circuit. A nice touch by HP/Agilent is there is a cutout under the chassis where you can get to the pins of the transistors for easy removal without taking the board out.

I will take the board out and check if there is anything obvious as you suggest.

I also did some voltage tests on the signals from control board to the power board. The OV-SCR, crowbar signal, is going high as soon as the output is tuned on. So something is triggering that. There is no OV condition so I'm unsure what's causing that. Reading the manual when that condition occurs the SCR is activated as well as the bias to the diode gates for the 15v bias to the output control amps is cut off. But there is no change to that signal from the control board. But the 15v bias is not on the diode gates. I'm thinking this 15v bias is broken somewhere, hopefully under the board as Ordinaryman suggests.

[Rycroft]

I've pulled the main board out and nothing looks burnt. I checked all the electrolytic and tantalum caps. I replaced a couple electrolytics that are suspect with high esr but I didn't expect that to help, and it didn't.

I'm puzzled with some of the voltages between the control and main boards. So I put the boards back in to do some more voltage checks and touched a few opamps around where I'm thinking could be suspect and, yup, some finger-sizzling super hot MC34072 opamps.

There are a few of them on the main board so I think I'll just replace them all.