HP Agilent E3610A stuck in constant current CC mode

[bluebird]

Update: CR3 is indeed shorted. Bitseeker is spot-on  . Replaced CR3 with MR854G. Seems to work fine now.

One guess is CR3.

Hi friends,

I recently purchased a used Agilent E3610A that is stuck in constant current (CC) mode. It was a good price so I couldn't pass it up thinking it was going to be an obvious fix

Here's what I've found so far (with only the power plugged in and turned on):

- Voltage knob does nothing and the voltage reading from the multimeter (+12 V to -12 V) was 1.1 mV
- The current knob works and gives a reading when I turn it (nothing was connected)
- The output + and output - wires were burnt to a crisp (I replaced them before turning on the PSU again)
- The PCB seems unmodified and is very clean otherwise
- When I push the CC/CV switch the current reading jumps (lowers) sometimes

Any suggestions what I should do at this point? Should I just buy a replacement CC/CV switch (although I don't know who makes them) and try to see if that changes anything? The switch is not a toggle switch and does not produce an audible click so it's not clear to me if the switch is the malfunctioning component or not.

Any help would be appreciated! Many thanks!

[coromonadalix]

service manual : http://www.hep.fsu.edu/~wahl/phy3802/expinfo/instruments/ps/AgilentE361xAManual.pdf

seems to be an normal switch ??  maybe a good contact cleaner  or bad solder (s) ??

[bluebird]

service manual : http://www.hep.fsu.edu/~wahl/phy3802/expinfo/instruments/ps/AgilentE361xAManual.pdf

seems to be an normal switch ??  maybe a good contact cleaner  or bad solder (s) ??

The solder joints in general seem fine. I don't see any particularly dirty contacts either. It's getting late here so I'll poke around more tomorrow.

I happen to have another identical switch from another broken E3610A. I poked around with the multimeter and they seem identical (I just checked the resistances between the contacts while pressing the switch at the same time). My guess is the switch is fine.

My noob guess is something's shorted (since the CC mode kicked in) but I cannot tell what it may be. I might have to fix the other E3610A first so I can compare them side-by-side 
Am I missing something obvious that I should look for?

On a side note, the CC/CV switch (S1 according to the PCB and service manual) is obsolete according to Keysight. They also don't provide the switches' manufacturer's part numbers so it's very hard to track them down. I believe S2 and S3 on the schematic are C&K NE-18 switches (not 100% sure about the part numbers - will update once I confirm them) but I'm not sure who makes the CC/CV switch (S1).

[bitseeker]

The CC Set button is a momentary switch used to set the current limit and does not toggle into position. It switches the supply between the meter (default) and current control (pressed) circuits. It sounds like yours is fine if it's switching as expected.

Since you mentioned that the output wires were fried, there's probably a shorted component somewhere. The series pass transistors and capacitors (electrolytic and tantalum) are usual suspects. Nevertheless, start with the reference and bias supply to verify that it's outputting the correct voltages according to the schematic. If OK, then check the main supply (between TP2 and TP4 should be 25V ±10% when set to 2A range). If OK, then check the pass transistors Q1 and Q3 for shorts. If they're OK, check for other short circuits, e.g., between +output & -output as well as +output & ground and -output & ground.

If a short is found, track down which component(s) are involved. One guess is CR3. Someone may have hooked up a circuit that back fed the power supply and caused the junction to break down and/or fuse, causing a short. They may not have realized it until seeing smoke from the frying leads going to the output connectors.

[JohnPi]

I fixed a similar one recently. This is what I would do:

Be careful probing around inside -- a slipped probe can cause more damage. It is most likely that the power stage is damaged.

See if S2A (range) makes any difference.

Is the output 0 because it is shorted or because the supply is providing no current ?

Check if R2 (the current sensing resistor) isn't open circuit. It's a large one (5 W) -- it won't be shorted, but you won't be able to measure its resistance easily (< 0.2 ohm). If you don't have a replacement, you can just short it temporarily -- but then don't short the output, and only turn on the supply when the voltage setting is 0 -- you can then raise it, but each time you turn on the supply, ensure that the V setting is 0 (because the R2 also protects the supply from inrush currents in C3).


Check TP2 vs. GND (the -ve output terminal). Should be about 20-26 V. If it is low V, then perhaps the diodes in the bridge rectifier are shorted -- nearly any diode will work, but 1N4004 or 1N5404 would be good replacements. With the PCU off, short C2 (big cap) with a 1-100 ohm R until the V is 0. If you don't have one, R1 in the schematic will discharge C2 eventually (check with DVM until it goes << 1 V.) Then check each diode individually -- you might find a shorted one.
The 4 pass transistors are soldered to the board and hard to remove :-( . Either they are bad, or the control circuit is bad. You can short Q2 base to the +ve VOUT -- and see if the current limit warning disappears.

[bluereyn]

Hello,
I also have an Agilent E3610A stuck in constant current CC mode. CR3 is shorted so that part is on order but I have a question about the CC/CV switch: Is it suppose to a momentary double pole double throw (DPDT, DP2T) or a dual single pole 2 throw? The PCB has a trace to center pole on side of one 2 throw switch and 2 traces to the "outside" legs of the other switch. Ohming the switch out with it removed , the two center poles are isolated, so are the two centers poles suppose common? Any insights would be greatly appreciated, Thanks