HP/Agilent E3610A CC dropping

[galacticmuffin]

Hello all,

I have just obtained an HP E3610A from a neighbor who moved, and I already have an Agilent version and I love it. On this unit however, when I hold down the CC button in both ranges it starts at around 0.4-0.5A and then steadily drops down to 0A. If I release the CC button it will flash a value around the same, maybe 0.3-0.4 for a split second, and then return to zero as it should be with no load and the button being free. When I press the button again, usually it will restart the countdown. When I turn the knob it does not do anything, except for when I throw it all the way CCW and the CC light comes on gently; this happens with my perfectly functional Agilent variant so I am glad at least that still happens.

I have tested the potentiometer and it works fine, there's no bad looking joints or any leakage, and the testpoints test fine as far as I can tell from what I could glean from the schematic. The voltage output agrees with my DMM and that adjusts fine-ish. If I have it set to 12.01V, it will either flip-flop between 12.00V and 12.01V, OR 12.01V and 12.02V, but never both (it doesn't seem to be able to drift beyond 0.01V after leaving it on for a while). I am just a hobbyist, so I don't know if this has much significance, but 12V across a 0.5W 10k resistor had my DMM reading 0.115A, but the display presented 0.00A, or if I hold the button it will start counting down from around 1.2-1.4A this time.

Testpoints with Voltage knob all the way CCW, and the Current knob set to just above the zero point so the CC light does not come on:
1 & 4: 12.19V
2 & 4: 28.05V (a little high?)
5 & 4: 5.02V
8 & 4: -12.4V

I focused on the testpoints around the CC adjust circuit, but I have no idea if I am really close. Could it be U4? It is in the "CC Set Circuit" and also in the "Current Error Amplifier". Could it be one of the 3-legged things attached to a heatskink and a weird yellowing resistor blob thing? Has anyone seen this specific problem before? Sorry for yet another E3610A thread, but I don't think I was able to find a definitive replicate case.

[fmashockie]

I would start by cleaning the CC set and range buttons.  Clean them with contact cleaner like Deoxit.  Depending on what make they are, it might be hard to penetrate the switches with the cleaner.  I have a E3612A which had a similar issue (the supply would only limit to 0.060A regardless of the setting. And when adjusting the current knob, it would only allow me to set it to a max of ~0.3A on the 0.5A range setting).  I drilled a small hole on top of each switch with a dremel tool to get the contact cleaner in there.

After cleaning the switches, I would check the series pass transistors Q1, Q3, Q4, Q5, Q6, & Q7.  On my E3612A, it was either Q4 or Q5 that was short (I forget which).  But this circuit is what provides the current limiting for the power supply.

I actually did a video on this repair on my YT channel.  Good luck!

[pqass]

These power supplies are easily repairable; just a matter of tracking down the problem.

Backgrounder (just in case): The current knob sets the maximum allowable current that the load can draw. However, if the load requests more than the set value, the power supply will lower the voltage to meet that set value.  For example, if the voltage and current settings are 5V and 0.10A, respectively, and the load is a 100R resistor, then the meters will show 5V and 0.05A.  However, if the resistor is replaced with 33R, then the meters should show 3.3V and 0.100A.  With low value resistors or shorted output, this will result in a voltage of 0V (or close too it).  Same for full CCW current setting.

Given the above, 10K across 12 volts should result in 0.0012A (Ohms Law) which is too low a value to be displayed by the ammeter display; only good to 10mA increments.  Check with your DMM again. You may have misread the scale.

As for your testpoint readings, be aware when reading the schematic that it is best to think of this power supply as being a negative supply.  What I mean... when probing with your DMM consider the positive output post to be 0V and the negative output post to be variable from 0V to -15V.  Why?   Because, the bipolar power (U2, U6) supplied to the op amps is referenced to the positive output post (see the [+S] symbol, aka. TP6).   Other an TP2:TP4, all your other readings depend on the voltage knob.  Instead, you want to know: TP1:TP6, TP8:TP6, TP5:TP6, TP7:TP6.   BTW: TP2:TP4 seems reasonable for an unloaded+rectified+filtered secondary (at peak) of 28V since the supply is spec'd to provide 15V@2A.  See attached.

The 3-legged things attached to the heatsinks are U7, U6, U2 voltage regulators. Not sure about the "yellowing resistor blob" (C10 maybe?). What's the text label beside the component say?

As for the "counting down affect"... I would start with checking TP5:TP6 and TP7:TP6 are stable when the CC Set is pressed vs. not pressed. Then work your way out from there.  Like checking if the counting happens with a load attached (with full CW on current knob), and comparing with your multimeter (ammeter mode) in series.  It could be just the meter PCB or extend to the CC Error Amp and beyond.

Also, I just remembered, my E3611A needs 5-10 minutes after cold start for the internal reference (U3) to settle down (drifts downward a few 10s of mA with the CC Set pressed).  I've been meaning to change that out for a new TL431 (+ resistors) to replace the LM336 (U3).  Maybe you have an especially bad one. Does the counting down level-off after, say, 15 minutes?  The place to check is TP5:TP6.


HP E3610A Service Manual

[fmashockie]

Yes pqass makes some great points as well.  Before taking some of the steps I mentioned, in general you do want to always check the voltage rails. (-12V, 12V, and 5V which are TP8, TP1, and TP5, respectively, with respect to TP6 ground).  You can also check the display supply by measuring TP9 (5V) with respect to TP10 (ground).  And as pqass stated, the ICs attached to the heatsinks are voltage regulators (U2, U6, and U7).  U3 is a regulator as well for the 5V rail.  The yellow blob is capacitor C8 which I believe is a 0.33uF 50V polymer cap - its doubtful anything is wrong with that. 

The reason I suggested checking the snap action switches is because they can reek all kinds of havoc when they are dirty.  I have repaired a number of electronic test equipment with what appeared to be really complex symptoms and it was just a dirty set of these style gang switches.  And as I mentioned, these switches caused similar issues with the current control on my own E3612. 

I still recommend checking the series pass transistors as well.  These transistors are what regulate the voltage and current for the supply.

[nfmax]

I had a similar-sounding problem on a recently acquired E3612A. The current set reading was unstable, and didn't match what the knob setting should be. The actual displayed current value wasn't quite solid, either. I tracked it down to poor contact on the connector that carries the signals to the front panel DVMs. All I had to do was disconnect & reconnect it. I gave it a bit of a clean for good measure, and it's been OK since.

This unit worked OK before it was shipped to me.

[galacticmuffin]

Thank you all for responding, I've been a bit busy with school.

I would start by cleaning the CC set and range buttons.  Clean them with contact cleaner like Deoxit. 
...
On my E3612A, it was either Q4 or Q5 that was short (I forget which).

I actually saw this post when going through the forums before posting, and I did see the video and tried to follow along as best I could. Unfortunately I have no Q5 or contact cleaner, but I did take apart the switches and de-dust them and checked for corrosion, they seemed nice and clean and shiny. I attached a picture.

However, if the resistor is replaced with 33R, then the meters should show 3.3V and 0.100A.  With low value resistors or shorted output, this will result in a voltage of 0V (or close too it).  Same for full CCW current setting.
...
Instead, you want to know: TP1:TP6, TP8:TP6, TP5:TP6, TP7:TP6.   BTW: TP2:TP4 seems reasonable for an unloaded+rectified+filtered secondary (at peak) of 28V since the supply is spec'd to provide 15V@2A.  See attached.
...
As for the "counting down affect"... I would start with checking TP5:TP6 and TP7:TP6 are stable when the CC Set is pressed vs. not pressed. Then work your way out from there.  Like checking if the counting happens with a load attached (with full CW on current knob), and comparing with your multimeter (ammeter mode) in series.  It could be just the meter PCB or extend to the CC Error Amp and beyond.

Also, I just remembered, my E3611A needs 5-10 minutes after cold start for the internal reference (U3) to settle down (drifts downward a few 10s of mA with the CC Set pressed).  I've been meaning to change that out for a new TL431 (+ resistors) to replace the LM336 (U3).  Maybe you have an especially bad one. Does the counting down level-off after, say, 15 minutes?  The place to check is TP5:TP6.

I just want to say that I appreciate this explanation very much, I actually made an interesting finding because of this! I screwed in a 33R resistor and set the voltage to 5V, and I just kept turning the current knob until it got down to 3.3V. I would repeatedly mash (gently) the CC button and at some point it would start to say around 0.09-0.1. I then decided to just use it normally as a test and I set the CC to around 0.04 (I think it was around there, the display is not very helpful but Ohm's law was) and hooked it up to an old regen I built. It worked just as well, I even tried to force it to screech but the CC worked and the voltage dropped and it started to motorboat. TP5:TP6 was at 5V with the CC pressed and unpressed, and TP7:TP6 was -4.4V at pressed and unpressed. Also, 1:6 gave 12.16V, and 8:6 gave -12.05V. I did let the machine sit for a while before getting measurements. It is nice to know that so far the unit is mostly functional, with just the CC area not so hot.

I had a similar-sounding problem on a recently acquired E3612A. The current set reading was unstable, and didn't match what the knob setting should be. The actual displayed current value wasn't quite solid, either. I tracked it down to poor contact on the connector that carries the signals to the front panel DVMs. All I had to do was disconnect & reconnect it. I gave it a bit of a clean for good measure, and it's been OK since.

This unit worked OK before it was shipped to me.

I reconnected the display cable from both ends to make sure there wasn't anything that looked bad, and I didn't notice anything there. I assume the machine was not 100% when I got it though, because my neighbor took all his other stuff with him when he moved.

[ward9y]

Any update on this? I think another possibility is that a tiny amount of electrolyte is conducting near the CC section and wreaking havoc with the control loop. C7 and C13 in particular seem to leak often and sometimes there is no visible sign from what I've seen on YT. You might have some luck scrubbing around U4 with isopropyl and a soft toothbrush.