HP E3615A power supply slow ramp up

[gaminn]

One way to narrow-down the problem is to divide the path into two to determine which half has the issue.  For example, if you disconnect Q9's emitter (or remove Q9 entirely), and you still see a ramp up issue on the output posts, then you know the problem is downstream of Q9; if not, the problem is upstream.  Of course, since you've disconnected the voltage setting parts, you'd expect the output voltage to be close to the source voltage with the difference being does it still have the ramp up of few hundred mV over a minute.  Eg. use two multimeters with one red lead on drain (TP1), another red on source (right side of R1,R2), with blacks on -OUTPUT, to determine if there's narrowing voltage difference (TP1=23.0V and R1 right=22.2V increasing to 22.6V) or stable difference?

Thanks, I reall appreciate your help. I removed Q9 from the PCB. Now the voltmeter read approx 1.1V (no matter what the set voltage is), ampermeter reads 6.2A and there is smoke coming from R58 resistor. To me it seems that all the current goes via CR20 (overvoltage protection circuit). I removed also R101 to disable overvoltage protection and now the voltmeter reads 34.7V without any ramp up when turned on. I replaced Q9 by some random PNP transistor (BD244C), but that doesn't help at all, still long ramp up.

[pqass]

Ugh!  Sorry about that.   I forgot about the crowbar (CR20). 
I hope R58 (0.2R) didn't cook too badly. You should probably leave R101 disconnected for now.

However you did reveal some information... that the output remained at 34.7V and didn't ramp up from this value after power-up.  Also, since you replaced Q9 (and earlier U9), you've confirmed that the problem must be upstream of U9B inputs.

I don't recall if you measured U9C pin 1 (output) if that was experiencing a ramp-up after cold power-up. If you see it moving from 1.1V to 1.25V (WRT +OUTPUT/GND) then that would explain a movement from 4.3V to 5V on the output; with Vset knob set for 5V. 

[gaminn]

I don't recall if you measured U9C pin 1 (output) if that was experiencing a ramp-up after cold power-up. If you see it moving from 1.1V to 1.25V (WRT +OUTPUT/GND) then that would explain a movement from 4.3V to 5V on the output; with Vset knob set for 5V.

Yes, U9C.1 measured multiple times - e.g.  https://youtu.be/Vd9mJ_f1A_Q - stable. So the issue is between U9C.1 and Q9. U9 and Q9 already replaced. What else could it be?

I connected 5 ohm load to the power supply output, there is still the ramp up (5V voltage set).

I disconnected CR21 to disable constant voltage control circuit and connected back CR22 to enable constant current control circuit. Connected 5 ohm load, set voltage/current to 5V/1A. There is no ramp up with CC. Another confirmation that the constant voltage control circuit causes this issue. But how?

[gaminn]

I created an ugly external circuit for constant voltage current control. It consists of R62, R82, R87, R87, C36 and one opamp (for U9B). It interacts with the power supply like this:

U9C.1 of the power supply goes to R82 from the external circuit
-OUTPUT of the power supply goes to R62 from the external circuit
+OUTPUT of the power supply goes to R87 from the external circuit
+15V,-12V of the power supply goes to +VCC,-VCC of the opamp from the external circuit
cathode of the CR21 goes to the output of the opamp from the external circuit

And now I have a good working power supply without any ramp up.

[gaminn]

Now I left out R62 and R82 from my external circuit and connected R62,R82 midpoint from the power supply to R88 resistor on the external circuit - ramp up is there again. Something is wrong with R62, R82 or the connections around R62, R82.

[pqass]

Something is wrong with R62, R82 or the connections around R62, R82.

Check the traces (for micro fractures) and re-flow solder (or attach bodge-wire) from R62,R82 to the points where you connected to your external circuit.  Scratch the solder mask to get to the bare copper.  My E3611A has some sketchy looking traces; looks like corrosion where they meet the pads.   

What a bugger!

[gaminn]

I further isolated the problem. I got rid off my external voltage control circuit, now the original voltage control circuit on the power supply's PCB is connected. When the midpoint between R62, R82 and R88 (I consider it midpoint even though there is also S2 switch between R88 and R62,R82) is on the pcb, then there is the ramp up. When I make the midpoint outside of the pcb, then there is no ramp up.

Now it is simple - connect the yellow alligator clip (it connects the midpoint to the PCB) - there is the ramp up. Disconnect the yellow alligator clip - there is no ramp up.

Now, what is the issue with the traces? The midpoint goes also via S2 and is connected to A12 (A3) and A13 (A4) terminals.

EDIT: I noticed I can further isolate the issue by switching S2 to the other position (switching it off). Now when the midpoint created by me outside of the PCB is connected to the side of the S2 switch that goes to R88, CR27, C37, then there is the ramp up. When the midpoint is connected to the other of S2 that goes to R82, R62, C29, there is no ramp up.

EDIT2: Now I have the solution to solve the ramp up issue of the power supply by making the midpoint outside of the pcb (I don't use switches and terminals on the back of the unit). But still I would like to know what causes the issue. However, now I don't know how to find the cause other than cutting physical pcb trace between R88 and S2 in half and isolating the issue further. Visually, the trace looks ok everywhere to me.

[gaminn]

I noticed R88, CR27, C37 trace should go to A12 (A3) and A13 (A4) terminals on the back of the unit according to the service manual pdf, but on my pcb it goes to A10 (A2), A12 (A4) and A13 (A5) terminals. Hopefully that's the only difference, at least I don't see the trace goes anywhere else, potentially to the point that can cause all the trouble.

[gaminn]

The power supply works now but I'm not very confident it won't break again. I know that the problem with the ramp up was the midpoint (R62, R82, R88, S2) - when it was connected to the PCB trace between S2 and Ax terminals there was the ramp up, when the midpoint wasn't connected to that place, there wasn't the ramp up. For this purpose I cut the midpoint trace (R62,R82 to S2 and S2 to R88) at many points to localize the part that causes the issue. Then I started to "repair" the cut trace and unfortunately I finished with fully repaired trace connected to the midpoint without any ramp up. No idea what happened...

When I put the power supply together I noticed CC works only when I moved front panel or touched the cables inside the power supply) and that there is poor load regulation in CV mode, ripple of almost 50 mV, etc. I touched all cable connections with soldering iron, desoldered and again soldered S2 switch. Don't know what happened but when I put the power supply together for the second time, it worked.

Next time I will throw the power supply away immediatelly and will buy another one...

[pqass]

I'm glad it worked out in the end.
I hope it was at least a learning opportunity in power supply design, failure analysis and troubleshooting.
It's sad that the trend has been toward less durable, un-repairable, and disposable products.
I understand that time has value but rather than throwing things away, consider selling or giving them away when there's still significant value remaining.