Author Topic: BK Precision 1788 Variable PSU Repair  (Read 690 times)

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Offline salvagedcircuitry

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BK Precision 1788 Variable PSU Repair
« on: August 24, 2018, 11:23:39 am »
Hi! I purchased a BK Precision 1788 broken off ebay about 4 months ago with hopes of repairing it back to life.
When I originally got the unit, it showed a bunch of garbage on the display (1st image). I took the PSU apart and visually inspected it. Interestingly enough, I was the first one in. The board showed no signs of 3rd party rework, as all the connectors had silastic compound on them. I did notice that one of the rectifiers on the heatsink was manually resoldered by hand and that a mosfet on the opposite side of the heatsink was reworked. That's about it. This must have been done at the assembly plant because in order to replace either of those components, you would need to remove the (2) large filter caps and then re-apply the silastic compound around the base of the caps. 

The 1788 is part of the 1785B series. Another eevblog post mentions that the 1785/1786/1787/1788 may use the same service manual.
http://www.bkprecision.com/products/power-supplies/1788-0-32vdc-0-6a-digital-readout-dc-power-supply-w-rs232-intfc.html

After resoldering some of the main PCB solder joints, touching up some of the VFD solder joints and reassembly, the PSU now displays a solid "Error data" on the VFD. The VFD also flashes all of its indicator lights about every second during operation. 
I checked the 1788 instruction manual and it does not specifically mention "Error Data." The manual, here:
https://bkpmedia.s3.amazonaws.com/downloads/manuals/en-us/178xB_manual.pdf
mentions only "ERROR CAL" and "ERR EEPROM"

I am wondering if anyone knows what the "Error Data" error code means and if there is any way to get past that error flag.
Also, if anyone knows where to find a 1785/1786/1787/1788 service manual, that would be fantastic.

Thanks a ton guys!
« Last Edit: January 08, 2019, 03:26:18 pm by salvagedcircuitry »
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Offline salvagedcircuitry

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Re: BK Precision 1788 Repair
« Reply #1 on: August 24, 2018, 11:26:40 am »
Here is an image of the inside and the back side of the main PCB. labeled IT6830-Main-V4.1

Thanks!
« Last Edit: January 08, 2019, 03:26:57 pm by salvagedcircuitry »
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Offline salvagedcircuitry

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Re: BK Precision 1788 Repair
« Reply #2 on: August 24, 2018, 12:48:07 pm »
I am fairly certain this is purely a software problem at this point. After combing through the main PSU circuit board and finding nothing suspect, I believe this is an issue with the calibration data stored on an EEPROM or a microcontroller that needs reprogramming. I believe the problem lies in the pcb for the VFD.

Taking a closer look at the FVD PCB, there's 4 main ICs.


Surprisingly, only one of the ICs in this entire psu were sanded off. A lot of BK precision equipment features chips that are ground down to reduce copy-cat behavior. Unfortunately, it seems that one IC that may be contributing to the problem is sanded off. Regardless, there is a 6 pin .1mm spaced unpopulated header near the sanded off chip, so I think I'm going to spend some time probing around there. I am looking further into the EEprom chip now and trying to take a best guess at what the unknown chip is.

If anyone has any advice on how to reprogram a random BK precision calibration chip, I'm all ears. Thanks!
« Last Edit: January 08, 2019, 03:27:12 pm by salvagedcircuitry »
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Offline salvagedcircuitry

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Re: BK Precision 1788 Variable PSU Repair
« Reply #3 on: January 08, 2019, 03:26:03 pm »
Update: I contacted BK precision and they provided the schematic for the unit, which is awesome. It is attached. If BK wants me to remove the schematic, let me know and I'll remove it.
The ground off QFP 64 pin 16x16 package is indeed an MSP430.
I decided to test the unit and see how it would perform without the eeprom, so I desoldered the microchip eeprom and started the unit. I used a hot air station to remove the chip.
Instead of an error data, the unit displayed an error eeprom. This is not the most scientific test, but it is possible the eeprom is working fine and the msp430 internal flash became corrupt.
At this point it is obvious that the MSP430 is at fault, as that is the only main microcontroller in this unit.
I am not sure how to reprogram the MSP430 so I went ahead and just purchased an old HP variable power supply instead.
Hopefully this information is useful to someone else who has a faulty BK 1788 Power supply.

« Last Edit: January 08, 2019, 03:30:25 pm by salvagedcircuitry »
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Offline floobydust

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Re: BK Precision 1788 Variable PSU Repair
« Reply #4 on: January 09, 2019, 06:20:54 am »
"DATA ERROR" may be a corrupted EEPROM I would play with the serial port and see if you can read cal values or restore default calibration.
It looks like the DB-9 has TTL-level async serial at 9600,N,8,1 and PC1785B_1788 calibration software is used. See user manual 5.3 Restore default calibration settings
"ERROR CAL" or "ERR EEPROM" are the only error messages mentioned, but the PSU also has 16 preset settings, serial number etc so if they get scrambled I'm not sure what the error message is. There is a lot of data stored in EEPROM with a CRC for each block.

The MCU only talks to the keyboard/display controller and outputs PWM or reads its ADC, or the EEPROM. It looks very basic.
 

Offline shakalnokturn

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Re: BK Precision 1788 Variable PSU Repair
« Reply #5 on: January 11, 2019, 12:09:05 pm »
Assuming the MSP430 is still working and firmware is not corrupt, you could throw in a new/blank ("FF"ed) 24C02 see if the MSP430 will re-write some defaults to it.
I have seen it is now for sale, however I'm not in the US.
 


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