HP 6114a help needed

[Joule Thief]

I love it when a plan works out. Well maybe. I spend a few hours a year helping a friend do maintenance on his lawn equipment and in return he keeps a keen eye on the dumpster at the local college.

I don't think this one actually hit the dumpster but someone took a crack at repairing an HP 6114a supply stuck in current mode. Several of the wires leading to the front of the main pcb are disconnected. I have already found the current limit control to be defective with a confirmed broken wirewound element at the low current position.

This is my first 6114a and I do not have a visual reference for reconnecting loose wires at the front of the main pcb. Continuity checks by function have lead me to a point where the majority of the wires are confirmed correct but some have me scratching my head. Service manual info on this area is sketchy.

Does anyone have hi-res photo(s) showing the color codes for the wiring on the front of the main pcb?
The area of primary interest is located between the amp and voltmeter cal pots on the front of the pcb. I have searched the forum and found 2 rebuilds with pics but neither cover the area I'm concerned with in sufficient detail. If a detailed photo is available for the AMP / VOLT switch, I'll take that too.

Thanks in advance.

[PA0PBZ]

google is your friend:

[echen1024]

If you need any photos, I can open mine up and take a few snaps for you later today.

[robrenz]

here you go

[Joule Thief]

here you go

That is the money shot right there. 

Thanks to all for the responses. This closeup answers the question of wire placement and why I was lead to believe a wire was missing. On to the VOLT / AMP switch and straightening of the front panel.

[Joule Thief]

A quick update.

The front panel is straightened and the open current control pot replaced with a ten turn replacement.

Power on, no smoke and the 6114a voltmeter reads exactly what is programmed into the decade thumbwheels. Fluke 87 confirms the voltage output is spot on (scarey how accurate it is).

BUT

any load connected +S and -S immediately changes the supply to constant current mode indication and no adjustment of the current is possible thru the current control potentiometer.

A quick measurement of the reference voltages shows the -14.7 VDC V- op amp supply down around -9 volts. A measurement across the 490 ohm zener resistor for that supply calculates to nearly 50mA flowing to the 3 op amps on the Control PCB. The + / - 28 VDC into the control board is fine as are the +16 and +6.2 VDC outputs.

Time for additional troubleshooting.

[BravoV]

<snip>... (scarey how accurate it is).

Yep, thats how good this HP 611xA series does when it comes to regulation, personally I see my 6115A as a high power "voltage reference" instead of an adjustable psu. ... LOL

Btw, I posted a stiched and colored  circuit schematic for the 6114A/5A -> HP 6114A/5A schematic (stiched & colored), hopefully this will help in your troubleshooting. Its a big sized picture, suggesting you to save it locally.

[Joule Thief]

Thank you BravoV

the schematic helped in finding an unexpected surprise. The manual specs out R3 as 600 ohms. I see 800 ohms in my system.

Not sure what to think yet. I still have the -14.7 reference problem to fix and I doubt this is the cause of my -14.7 supply reading -9.6.

[DrJoe]

Thank you BravoV

the schematic helped in finding an unexpected surprise. The manual specs out R3 as 600 ohms. I see 800 ohms in my system.

Not sure what to think yet. I still have the -14.7 reference problem to fix and I doubt this is the cause of my -14.7 supply reading -9.6.

Joule - you can download the manual from Agilent and it has the calibration and troubleshooting guide with all the info you need. Look under the Library section and search for 6114.

[Joule Thief]

Thank you DrJoe. 

I do have the manual from the HP site, as BravoV states, the schematic is a parsed into several pages making flipping back and forth page to page a little difficult to follow His "stitched" schematic solves that problem.

Seems Google is truly my friend in regards to my 600 ohm resistor and reference voltages problems.

This website has answered a few questions.

http://www.amplifier.cd/Test_Equipment/Hewlett_Packard/HP_other/6114A/6114A.html

Seems the 800 ohm resistor is correct and the -14.7vdc  reference voltage issue has been experienced by others.

Hopefully I will have a few more hours to invest in troubleshooting this supply in the coming week. 

[Joule Thief]

Success!

After blocking out all the distractions, turning the tide on the appliance revolution currently taking place in the house (water heater, water softener and evap fan in the frig all in the last 2 weeks), some bench time revealed a burned trace on the power pcb.  This trace hangs very close to the SCR overvoltage trigger transformer and was not immediately evident due to the angle I was viewing the supply on the test bench.



The burned trace connects the current sense resistor (upper right corner) to the front panel + output jack.



A resistance check of the series pass transistors showed no abnormal readings and +V to -V rail resistance check did not show any indications of a short on the power rails.

I wired in a temporary jumper wire from the sense resistor to +V jack and the supply sprung to life. Current control now works as anticipated and supply is stable and without any release of magic smoke after 2 hours of bench testing at full load.

Next up is the continuing investigation of the -14.7 reference voltage, still low @ -9.6v. Could I be lucky and have it just be the Zener?

Today was a big step forward to recovery of this "dumpster dive diamond in the rough".

[Joule Thief]

The -14.7 reference voltage issue is now resolved. I did not have a 14.7 volt Zener but with the specs showing +/- 5% on 14.7volts, a 15 volt Zener was substituted as a test.

The reference voltage came up to -14.9 volts and stayed solid for a few hours.

After recapping the 4 main power supply electrolytics (1 of the 490uF caps read 29pF on the LCR meter) I re-calibrated offsets on the control board and the 6114 is now 99.9% finished. What's left?

Damn slight buzzing in the power transformer. Tightening the mounting hardware decreased the buzz somewhat. The buzz is not loud and is certainly drowned out by the sound of the air conditioner in the workroom, but when all is quiet the buzz can be heard.

For now, the $30 I have invested in this supply has proven to be worth the effort.

Several discrepancies were found for parts values and designations called out on the schematics versus what was actually present on the power pcb. I will complete the list and update this thread with the information for future reference.

My thanks again to all those who made contributions and recommendations in this thread.

[Joule Thief]

Adding power pcb to front panel wire color coding

[BravoV]

I've been following this good repair thread, great job you've done there, especially on spotting the pcb trace problem.   

From your last photo there, I guess you will spend some quality time with brush to clean the dust. 

After recapping the 4 main power supply electrolytics (1 of the 490uF caps read 29pF on the LCR meter) I re-calibrated offsets on the control board and the 6114 is now 99.9% finished. What's left?

If you have a logging multimeter, try burn in "with load" to see it drifts, my 6115A basically is rock solid under 48 hours burn in with a "dynamic load" using a 5.5 digits logging multimeter, its quite amazing to see how stable it does. 

[Joule Thief]

It is amazing how photo flash lights up dust specs ! 

This is an early on photo of how the wiring looked was when I first received the supply. The original owner disconnected several wires on the power board in their attempt to repair the supply. 

I have since given the power board an IPA rubdown and rinse. 

The best DVM I have is a 5 1/2 digit Fluke 8800A. I am just a home hobbyist trying to rebuild a workbench and wouldn't really know (or appreciate) if the supply drifted in the 3rd decimal place! You are correct on the quality feel and build of this supply.

Next I will have to acquire a DSO with a noisy fan to hide the buzzing transformer. 

Thanks for the kind words.

[robrenz]

Just a thought on the buzzing transformer. I use Loctite 290 in my shop, it is a wicking style of material designed to put on after fasteners are assembled. It is also used to fill casting porosity so it really is good at filling minute cracks and pores. So if you scrape any varnish or sealant off the edges of the transformer where the lamination edges are. Then wet those surfaces with the 290 and replenish where it sucks up the material. let that sit for a day or so and make sure any areas that suck in all the Loctite are replenished. Wipe off anything left on the outside after a couple of days (It only cures where it is confined).  That should solidify the laminations to remove any noise coming from lamination vibrations.

[saturation]

Thanks for this thread and the input from other 6114a owners.  I think you may have uncovered a common fault in this design and worth keeping an eye out for when used for prolonged periods or pulling near its maximum current.

Its more than coincidence that a fault on this 6114a:



... is similar [ identical ?] to the fault from the Geman 6114a, the trace heading to the current sense resistor, R15 on board A1.



It may be prudent to provide a fan for forced cooling in this area.  When I was breaking the units in, the general region of R15 does get a bit too warm for my liking [ i.e., near 50^oC].  A simple fan is cheap insurance.