Modifying a bench power supply

psu_photo.JPG
This post is about a small modificaton of this linear bench power supply I bought from amazon.de some months ago. For the most part I've been satisfied with it, the only complaint I had was that the refresh rate of the voltage and current displays was a bit slow. It wasn't that bad and I figured I'd just get used to it, but a month ago I saw an interesting video by mikeselectricstuff on youtube. He had bought a bench power supply and he had the same problem as me, the refresh rate of the displays was too slow. But unlike me, he didn't just whine about it, he decided to do something about it.

Inside his power supply, he found the two chips that measured voltage and showed it on an LCD. One measured the voltage at output and the other voltage over a shunt resistor to measure the current. He found the datasheet for the chips and by simply adding one resistor, he managed to increase the refresh rate. Sounded simple enough, so I decided to try the same. I of course started by opening up the power supply.
psu_opened.JPG
Then I continued to remove the display module.
psu_display_back.JPG
Those two chips are obviously responsible for showing the output voltage and current. They are manufactured by Semico and the model is "CS7107GP". After some googling I found the datasheet.
psu_datasheet.png
Unfortunately it was in Chinese. I don't read Chinese. Thankfully, block and circuit diagrams are universal.
psu_block_diag.png
The part marked in red is clearly an RC oscillator. Speeding it up should hopefully make the refresh rate higher. There are two ways to do it: either you decrease the capacitance or decrease the resistance. The easier thing is to decrease the resistance, all you have to do is add another resistor in parallel to the current resistor. By adding another resitor of same value in parallel, you halve the resistance and double the frequency of the oscillator. A simple test first, I jammed a 100k resitor on the correct pins between the chip and the chip carrier:
psu_test.JPG
By the way, In that image you can also see a fine example of the craftsmanship of the chinese workers responsible for soldering this power supply together. Pay especially close attention to the underfilling of the solder joints of the TO-92-packaged device and the savagely violated section of the DIP-chip carrier behind the trimpot. Somebody's been stabbing it with a hot soldering iron.

Anyways, does it work? Time for some videos.

Before the mod:

After the mod:

A clear improvement! The refresh rate is now higher. Next step is making it permanent. I soldered two resistors on the other side of the board. You can see them to the right of the third and sixth 7-segment displays.
psu_hack_ready.JPG
After that, I just put the power supply back together again, tested that it still works and called it a succesful modification.