Electronics > Repair
HP E3610A Power Supply - Current not displaying / Voltage not changing
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F4:

--- Quote from: DavidAlfa on December 07, 2024, 11:28:14 am ---Check the electrolitics, very common failure.
Do you have oscilloscope? If so, check how it looks before and after the regulators.

--- End quote ---
Yep, all caps replaced. Looks like it was damaged in the past from electrolyte.
pqass:

--- Quote ---The resistor, yes, the schematic says 1k, the resistor measures 320, I read somewhere it was by design on the older revision, perhaps.

--- End quote ---
Either value doesn't matter; 7mA vs 22mA through zener U3 is fine to produce a stable +5V reference for R15.


--- Quote ---Yes, these 2 have been replaced. All electrolytics have been replaced. ...
Bad shape, both fuses F1 and F2 were blown, all kinds of exposed and deteriorated traces...

--- End quote ---
Good. 
Hmm....  sounds like you're in for a treat.   ;D


--- Quote ---Removing the leg of C4 had no impact, the behavior reported before, the .5v full cw, -.5v full ccw, remains the same. I did not test further, as I also notice U6 getting quite hot.

The other thing I see is that 26v is at the output terminal now. I recall measuring 15v here in the past, not sure when this change happened, though.

--- End quote ---
By lifting CR4, we removed current limiting capability entirely.  We needed to do this to determine if the unit can even do simple voltage regulation first. So never mind TP7 output for now.

I don't know why the -12V regulator (U6) is getting hot; there's only the op amp supplies and Q2 collector connected to it.  I suggest you remove U4 and U5 for now (and with CR4 lifted, they aren't needed to test the voltage regulation loop).

Because you've reported that +26V appears on the output, that means that R3 is pulling up on Q1,Q3 (darlington) bases to turn them fully ON (ie. no voltage regulation).   So Q2 is off/broken or there's a broken trace somewhere from Q1,Q3 bases to U1 pin 6 (op amp output).

See attached simplified E3610A voltage regulation loop pix and online simulation here.  I've removed all protection diodes or tweak components that aren't relevant to understanding how the loop works.

The service manual schematic is written for all three models in the series but for the E3610A many components are jumpered or unpopulated.  Specifically, unpopulated: Q6, Q7, Q4, Q5, C22, C23, R43, JP1,  jumpered: R38, R39, R44, R45, JP2.

Get voltage regulation working first without any load; looking for trace breaks or open/shorted components in the loop.  Then, after the loop is working, add a load and see if it still maintains the same voltage (DON'T draw more than 2A).   Then we'll work on the current limiting parts (putting back CR4, U4, U5).
F4:

--- Quote from: pqass on December 08, 2024, 05:07:32 pm ---
By lifting CR4, we removed current limiting capability entirely.  We needed to do this to determine if the unit can even do simple voltage regulation first. So never mind TP7 output for now.

I don't know why the -12V regulator (U6) is getting hot; there's only the op amp supplies and Q2 collector connected to it.  I suggest you remove U4 and U5 for now (and with CR4 lifted, they aren't needed to test the voltage regulation loop).

Because you've reported that +26V appears on the output, that means that R3 is pulling up on Q1,Q3 (darlington) bases to turn them fully ON (ie. no voltage regulation).   So Q2 is off/broken or there's a broken trace somewhere from Q1,Q3 bases to U1 pin 6 (op amp output).

See attached simplified E3610A voltage regulation loop pix

The service manual schematic is written for all three models in the series but for the E3610A many components are jumpered or unpopulated.  Specifically, unpopulated: Q6, Q7, Q4, Q5, C22, C23, R43, JP1,  jumpered: R38, R39, R44, R45, JP2.

Get voltage regulation working first without any load; looking for trace breaks or open/shorted components in the loop.  Then, after the loop is working, add a load and see if it still maintains the same voltage (DON'T draw more than 2A).   Then we'll work on the current limiting parts (putting back CR4, U4, U5).

--- End quote ---

Thank you for the assistance and the diagram. I've had a chance to work on this unit some more, I thought I had it licked but it insists on being stubborn.

In going thru the traces I decided to replace R29 and R30, which looked worse for wear.  After removal I started to test continuity and saw that pin 7 and 8 of U5 were incorrectly repaired. The trace from R28 was attached to pin 8 instead of 7.

I put everything back together to see if this remedied everything, it did not. On the plus side, the output at the terminals is now -17v across both the + and - terminals. Since its a 15v supply, this is progress.  Unfortunately it does not respond to the knobs for either voltage or current.

Q2 was replaced a while ago, but put a new one in just in case, same behavior.

U6 remains hot. I double checked the caps for polarity. U6 is drawing current, would the CC set be the area drawing that current it? U6 was also replaced previously, by the way.

 

I tested the TO3 transistors (Q1,3) in circuit with the diode setting, I got proper beeps from them with testing across CBE. I also went thru and took out and tested all the black DALE resistors, all looked good. Should I pull Q1/3 and test it further?

nvmR:
I have a similar puppy, misbehaving in the voltage reading. It may sound trivial, but double check the cable between the main and the display panel. Mine was finicky.
Also, check AC at the output. I once saw one that like to give some 3V AC along with the required DC. fun fun fun
pqass:

--- Quote from: F4 on December 15, 2024, 01:05:11 pm ---In going thru the traces I decided to replace R29 and R30, which looked worse for wear.  After removal I started to test continuity and saw that pin 7 and 8 of U5 were incorrectly repaired. The trace from R28 was attached to pin 8 instead of 7.

--- End quote ---

R29 and R30 are just used to limit the current to the CV and CC LEDs.  U5 (LM393 comparator) drives their output (pin 1 or 7) to -12V if +Vin (pin 3 or 5) goes below +9.8V (center of R25+R41 divider); lighting up the LEDs at 24mA (24V/1K, hence the 1W resistors)!!!   Whichever the Current Error Amp or Voltage Error Amp is controlling the output voltage (via CR4 or CR5 switching), the other will output close to +12V which will disable its corresponding LED.

R28 is in the CREF parts; you probably mean R29 was incorrectly tied to pin 8 (+12V). That would always keep the CC LED off.

Given potentially many issues with this board, if you haven't already done so, clean the board with generous amounts of IPA to remove all gunk/film. Also inspect all obvious (non-factory) changes or damage/cracks.


--- Quote ---I put everything back together to see if this remedied everything, it did not. On the plus side, the output at the terminals is now -17v across both the + and - terminals. Since its a 15v supply, this is progress.  Unfortunately it does not respond to the knobs for either voltage or current.

--- End quote ---

If the output at the terminals is -17V (assuming your DMM on DCV leads were reversed), then this is an improvement. The resistor divider formed by R15 (14.3K) and R37 (50K front-panel voltage pot) would produce 17V output IF the wiper on R37 was open/disconnected or stuck at max range. 
Q: With the unit powered-off, if you isolate R37 from the circuit (disconnect wire from PCB to one pot terminal), does it still vary from 0 to 50K (wiper still attached to one pot terminal)?   Put the wire back onto the pot terminal once you've checked R37s resistance.

This next test assumes that one CR4 lead is still lifted off the PCB, thus current limiting won't be affecting the output voltage and only the Voltage Error Amp is controlling the output voltage.

Q: Although the voltage setting knob has no effect, can you apply a small resistance to the output terminals (try in the range of: 330R, 220R, 100R, 50R) and does the voltage stay the same?   ie. can it regulate regardless of attached load?   Don't use <20R @17V as that is 850mA and >10W.


--- Quote ---Q2 was replaced a while ago, but put a new one in just in case, same behavior.

U6 remains hot. I double checked the caps for polarity. U6 is drawing current, would the CC set be the area drawing that current it? U6 was also replaced previously, by the way.


--- End quote ---
U6 (-12V regulator) being hot is unusual.  I can barely feel it in my E3611A.

Q: Let the unit cool down then remove all three ICs U1, U4, U5. Turn the unit back on (output terminals will be at 26V, that's okay). Does U6 (-12V regulator) get hot?
If no, then turn the unit off and put back U1 and turn it back on. Does U6 get hot?
If no again, then the issue is the other U4 or U5 or in their parts of the circuit. Leave U4 and U5 out for now and concentrate on getting voltage regulation working.
If U6 gets hot with only U1 installed it could be U1 or Q2 again. Or, maybe something else (non-factory) pulling on -12V.


--- Quote ---I tested the TO3 transistors (Q1,3) in circuit with the diode setting, I got proper beeps from them with testing across CBE. I also went thru and took out and tested all the black DALE resistors, all looked good. Should I pull Q1/3 and test it further?

--- End quote ---

Troubleshooting should be methodical. After checking the obvious stuff like (a) all power supplies producing the correct voltages, (b) obvious damage areas or faulting components (bulging caps), then you need to isolate sections and work your way from the (measured) good parts until you find the unexpected.  Yes it's good to replace problem components like electrolyic/tantalum caps and swapping ICs for known good ones, but just random checking or replacing rarely solves the problem(s).

This is why I've asked you to lift one end of CR4, in order to isolate the current limiting parts from controlling the voltage at the output terminals. So we can concentrate on fixing the voltage regulation first.  Sometimes schematics are hard to read, so I hope I've given you enough clear information on the Voltage Regulation loop for you to find the "unexpected" in it.   For a detailed explanation of how the voltage regulation loop works see a previous post of mine here.


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