Thread Tools
Jan 19, 2015, 09:15 PM
Registered User
Thread OP

Voltage adjust pot for 438202-001 HP Power Supply

I have a HP 438202-001 power supply up and running. It's small and quiet.

Output voltage is 12.2 VDC and I'd like to bring it up to around 13.5 or so.

Of course, there's no schematic that I can find.

There are 3 small pots on the board - one toward the input side, one toward the output side, and one in the middle of the board.

Can anyone say for sure which pot is the output voltage adjust and what voltage can normally be obtained from this supply? How has the supply worked out for you? Have you found any documentation/schematic for it, and where?

Thanks to the Board for the ideas and inspiriation on this project.
Sign up now
to remove ads between posts
Jan 23, 2015, 06:09 AM
Registered User
Thread OP
I've been reading the build threads here and searching to no avail.
Jan 23, 2015, 01:39 PM
Registered User
I don't know, but I would suspect the one in the middle or toward the output side. What I would do is first carefully mark the existing position of each pot, then connect a multimeter to the output and slowly adjust one and note the effect. If it doesn't adjust the voltage, set it back to exactly where it was and try the other. DO NOT randomly start turning adjustments without marking them first. I've repaired a lot of electronic gear and it seems like the first thing a lot of people do when something breaks is fiddle with every adjustment they can find. That can turn a simple repair into a writeoff.
Jan 25, 2015, 06:12 AM
Registered User
Thread OP
Good comment, but the people that need that hint the most probably never bothered to research it to start with . (And they never mention to the repairman that they've had it apart - it just 'doesn't work'... .)
These pots are the smallest that I've seen, and there's not much room for error, much less marking.
The supply is audibly silent (neat-o) but generates a significant amount of radio frequency noise - not a whine, but broad-band static. I built a set of toroidal filters for the output that helped some and the next step is likely to be some big electrolytic capacitors.
Thank you for replying.
Anyone else?
Feb 06, 2015, 06:04 PM
Registered User
Thread OP
I still need help raising the voltage on my PS.

While my question hasn't been answered, I finally found a post from BryanPerth at referencing this supply. He doesn't say how he determined the function of the pot he's tweaking, and I'm afraid of messing up the oscillator. I'm hoping he replies to the PM I sent.

This supply is known as:
Model PS-2122-1CB-LF
HP part nr. 438203-001
HP Retail P/N 498152-001
HP Generic P/N 490594-001
Regulatory Model HSTNS-PL11

The output of the board resembles, but the board where the voltage adjustment is shown differs significantly.

There are also five, not three pots as I earlier stated.

The case of the 'large' pot on the far left is 3/16" square. The other 4 are much smaller.

FWIW - Adding a 470 microfarad electrolytic and a 0.01 microfarad capacitor across the output made a huge difference in the amount of radio frequency noise generated. Larger electrolytics had no effect.

Please help!
Last edited by Hammerpower; Feb 07, 2015 at 04:19 PM. Reason: added comment about cap mod
Feb 07, 2015, 04:05 PM
Frankenstein recycled packs
rampman's Avatar
Originally Posted by Hammerpower
There are also five, not three pots as I earlier stated.

The case of the 'large' pot on the far left is 3/16" square. The other 5 are much smaller....

Please help!
last week I got the exact same power supply as your link and also the exact same as you are working with. The one similar to the linked power supply had the same pot set up and I was able to raise the voltage some.
the power supply exactly like yours did not adjust up. I tried every one of those pots and got no difference in voltage.

Mar 22, 2015, 04:50 AM
Registered User
Thread OP
I thought I'd give it another try to see if anyone knows how to get another volt or so out of this beast.
It's dead stable at 12.2 VDC and runs quietly, but around 13 - 13.5 VDC would be sweet.
Oct 24, 2015, 07:41 AM
Registered User
BryanPerth's Avatar
Hi Hammerpower,
Ive PM'd you - sorry I never saw the message icon :-(
Apr 10, 2017, 10:15 AM
Registered User
its the pot on the left, to the right of the 14 pin horizontal dip.
It will increase up going ccw to a max of 12.8 VDC.
I am looking for a schematic of the trim resisters around the pot which set the fixed range.
I have had some luck bumping it up more by paralleling the small chip resister on the left of the pot, there may be others.
Cant tell exactly what I may be doing without a schematic.
Hope this helps some, and if anyone finds a schematic, I would be interested.
Nov 14, 2017, 06:28 PM
Registered User
jdowdle - Have you played more with this powersupply? I would love to get 13.7V out of it, but I understand that that's a longshot. Any tips om how to max get the voltage close to 14V is much appreciated!

Quick Reply

Thread Tools