Electronics > Metrology

HP 6114A PSU hack and test + repair


Hi, in this article I want to talk about some modification that I’ve done to my HP 6114A.

For who doesn’t know the HP 6114A is a high precision power supply, 0 to 40 V, 2 A at 40W.

I bought it from Ebay for 55€ + 8,90€. The meter :bullshit: was broken and the PSU was emitting a very loud buzzing but it was quite spot on!
I started by repairing the meter, the spring was broken but I managed to repair it. The meter lost precision but it works and it’s good enough for indication.
It took me a while to discover that the buzzing wasn’t coming from the transformer but from the yellow 1uF caps but after change it’s perfectly silent!

I started thinking at some mods, I love them all so I order them by chronology:
-Having sense terminals only in the rear so I added two binding post on the front for the sense, to short sense and force I used a relay switched by a swich on the front (the switch in the middle). The relay it isn’t a special relay so it will generate some tens of uV between the contacts so when you need a very stable voltage at low current (some mA) you take the voltage directly from the sense binding post (note: to reduce noise you should add a filtering capacitor also between the sense binding post).
-Obviously I substituted the 1 turn CC pot with a 10 turn one!

-Added a normal switch, Low voltage and 0V: During use I noticed that every time you have to bring the output to 0 V you have to bring all the decades to 0 (consuming the already old decades and wasting time) or disconnecting the bananas (not good especially if you are using the 4-wire configuration). While searching a microswitch I noticed that I had only one 3 position switch (on1 – of – on2). Adding a low voltage range with 1uV resolution, why not?!
The wiring is quite simple (also messy due to the lack of space): To decrease the range you need to increase the value of R24 (32Kohm): 320Kohm = 4V max, 3,2Mohm = 400mV max and so on. I decided to build the 400mV range. A 40mV range would be cool but I noticed that the last digit was wasted due to noise (<1uV) and a 32Mohm precision resistor would be expensive.
The 3,2Mohm(-32Kohm) resistor consist of 5 619Kohm 15ppm resistors + 1 100Kohm 10 turn trimmer mounted on the decade’s PCB.
The range switch: -In the 40V position shorts the 3,2-0,032Mohm resistor to get the normal current/voltage.
-In the 400mV position the switch is open so you got 3,2Mohm in series with the decade an a reduced current.
-In the 0V position the switch short circuit the decade so you get a true null.
The “zero-volt” circuit remains the same so it’s very difficult to adjust it to 0uV, the power supply needs almost 3 hours to be stable down to the uV. In some weeks the 0 drifted about +- 15uV, good.
To calibrate the 400mV range I used a friend's multimeter. After a month it’s still spot on down to 4 digit.
Downside: I’ve calibrated the 0V for the 400mV range so the 40V range has a 500uV offset, not a big deal.
Stability test 10V: HP 6114A vs LM584L, Aneng 8009 as null meter. After warm up they match very vell (10uV = 1ppm!) (at the end of the graph I’ve opened the window in fact there’s a small voltage drop).
These modification probably reduced some of the noise specs but i found them extremly usesful!

There's still one problem that I can't solve (I'm an elettrical enginering, not electronic one;)): when I turn off the power supply it sent out a 40V peak (duration 300msec). Ovp protection and CC do not trip, maybe some old caps?

(I'm not english, forgive writing mistakes;)).



Lazy schematic (only 40/0,4/0v mod):

Hi! News! I finally found out the turn off problem! The reference supply's capacitor C2 was death! (500pF instead of 490uF).
After adding a good 100V 680uF cap the problem was gone! The output slowly drop down to 0 instead of jumping to 40/50V :D

So I changed C1, C2, C3 and C12 (also death)

After one month still work perfectly. :D


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