[Solved] BK Precision 1786 Programmable power supply - Limited to 0.4V and 0.02A - Page 1
Forum section: electronics / repair

author: mlefe
author_rank: Regular Contributor
timestamp: August 27, 2018, 01:51:50 am
content: Hi guys,Yesterday I purchased this old BK model (capable of 30V and 3A) for only USD15.After cleaning the contacts in the keypad (it wasn't responding), I thought that was it.However, when I tried to set it to deliver more than 400mV or more than 20mA I got an "ERR" in the display and it fallback to those two figures.Being a newbie, I've thought that this could be something related to the ADC, DACs or output OpAmps. After some measurements and some thought I concluded that this was software related.I founded the EEPROM and started to play around a bit with it.Using a TL866A (with the AT93C56 x16 chip selected) I was able to read the Atmel EEPROM and reverse engineer where the set voltage/current is stored, along with the 9 programmable outputs and, more importantly, where it stores the max allowable voltage and current.It has a very simple structure where the numbers are recorded digit by digit (each byte is a digit) matching the display. It ignores the comma, and records the information in pairs (first voltage, then current) so:* First 4 bytes represent the set voltage: 0104 means 10.4V (it only has 100mV resolution)* The next 4 bytes represent the set current: 0002 means 0.02A (it only has 10mA resolution)* Then you have the 9 pairs (of 8 bytes each, 4 for the voltage and 4 for the current) where the programmable memories are stored.* I don't know what's next... I guess CAL data?* In the picture you can see the position where the max voltage and current is stored (I'm sorry for the crappy pic): I've changed those numbers to 0300 (30V) and 0300 (3Amps) and it worked!I guess that this means that "theoretically" it can be hacked to deliver more than it was advertised for... I've checked and uses two Panasonic 2SD1975 which are capable of 180V, 15A (continuous) and 150W (if properly cooled) and 0.22 ohm 5W resistors for sensing.As it has no fan and the max AC delivered by the transformer is 32V, I guess it won't go very far in that matter (45V, 5A, perhaps?).Cheers,Martin.-

author: coromonadalix
author_rank: Super Contributor
timestamp: August 28, 2018, 10:38:07 am
content: I would not play to much in hacking the voltage output, you need load an line regulation.EX: if your transformer is rectified at 40 vdc, and your supply gives 30vdc max, under an good load it may drop at 35vdc, your supply will be stable and have some margin to work ... but once hacked at 35vdc out, you may began to have stability issues ... and the circuit has been calculated for the given voltages.With an schematic, you may see the difference between models, sometimes they have small parts exchanged to get / have others voltages and current values.Played like this in my kepco linear ate 75-8 supply to get near 100v at the output and reverted the changes (stability issues), but the "vout adjustment" is set at 80 volts, my main supply is 103 vdc (I have the schematics and all the models parts changes) ... it has an pre-regulator circuit as big as the output section .... but very stable under any circumstances.

author: mlefe
author_rank: Regular Contributor
timestamp: August 29, 2018, 03:35:08 am
content: Quote from: coromonadalix on August 28, 2018, 10:38:07 amI agree! I believe my next step will be to find some information on the RS232 protocol (I'm going to buy one of those Rs232->Arduino modules with a MAX3232) and see what sort of functionality I can get from it