BK Precision DC Electronic Load - Model 8540 Repair

[bostonman]

I have a piece of test equipment (BK Precision DC Electronic Load - Model 8540). The knob doesn't change the display numbers (but it does show numbers when first powered - probably just a startup sequence) nor do the buttons work.

Since I don't have the schematics, I'm randomly testing various stuff. So far it seems I have 5v, +12v, and -12v. Looking at the board behind the buttons and knob, all the traces seem to go towards the M430F4250 IC chip (I can't tell exactly because the seven segment displays cover the traces, but the only IC in this area on the other side of the board is this).

I've looked for the datasheet, and it seems I find MSP430F4250 instead.

Does anyone know if this is the same IC? Also, this IC seems to perform a few functions including driving the seven segment displays. From what I can tell, the segments work, so I'm wondering if this IC is only partially damaged, or if I'm just on the wrong path.

[rsjsouza]

MSP430F425 perhaps?
https://www.ti.com/product/MSP430F425

The MSP430F4x family has an integrated LCD controller.

[Kim Christensen]

It's the same... They are just marked without the "SP"

You problem with replacing it will be that it needs to be programmed. A new one will be blank, unless you order the part from BK.

[bostonman]

I didn't realize this would need to be programmed prior; I assumed it was RAM that held information from an onboard chip.

This isn't good news then.

The PCB for the display only has four or five chips (this being one), another is a EEPROM I believe, a HEX invertor, and another that's listed as an LCD driver or something like that.

About two-weeks-ago I posted a message about this unit in the repair section, so I'll avoid trying to duplicate messages.

Now it seems this won't be an easy fix since this PCB seems to have the necessary voltages, the display works, but the knob and buttons don't work. I was hoping to spend a few bucks on a gamble and replace the chip, but seems I can't do it easily enough.

[Kim Christensen]

Just because the M430F4250 appears to be "dead" doesn't mean that it is. Here's some things that could do that:

It's clock crystal is bad. Program won't run without a clock.
It's reset pin is being held low by an external circuit.
The power supply voltage is too low, keeping it in Brown-out reset.
Some combo of inputs has the program in halt waiting for it to clear. (Unlikely though)

The datasheet can help you with troubleshooting the first 3 in the list.

[bostonman]

Actually, that's a big help.

The unfortunate part is the lack of space due to ribbon cable length restrictions. Either way, I'll work on taking this approach.

[bostonman]

BTW, one reason I assumed the chip is faulty is because the numbers are displayed on the seven segment displays. It was my assumption that if any external pieces were not working (crystal or voltages), then the display wouldn't work either. Obviously I could be wrong.

Tonight I dove slightly deeper. I took a 9v battery and connected it to the input terminals. The display read the correct voltage, and, as confirmed before, none of the buttons work except one button (mode button I believe); so five out of six buttons don't work.

I measured the crystal and got the correct 6MHz and the reset line was sitting at 3v.

I'll dive deeper into the datasheet, but trying to take measurements is difficult due to size and ribbon cable length limits. Due to one of the six buttons working, I find this a bit confusing because, again, they all seem to go to the same location.

[Kim Christensen]

All you can really do at this point is trace the button wires to the corresponding chip pin and see if they toggle the voltage there or not. Start with the working mode button to verify that it measures as expected. Then trace out the others and test.

[HoinJoin]

Will it be MSP430F425IPM, or MSP430F425IPM, I found it on a website(https://www.ntcomponents.com/Datasheets/IntegratedCircuits/msp430f425.pdf), hope it can help you
In this article, I found several data sheets that cannot be found on the market, and they are quite reliable.

[HoinJoin]

I think you're right 

[bostonman]

All you can really do at this point is trace the button wires to the corresponding chip pin and see if they toggle the voltage there or not. Start with the working mode button to verify that it measures as expected. Then trace out the others and test.

I plan to dig deeper, but taking measurements is a challenge. The buttons is a rubber keypad that sits in slots on the front panel held in place by the PCB. The back side of the buttons are those little round black conductive things that touch the PCB points. Then the traces go across the board, under the seven segment diplays, and (I assume) through vias to the opposite side into this IC chip (since it's the only IC in this area).

Did I miss something on the datasheet, is having the reset line sitting high an issue?

[Kim Christensen]

Did I miss something on the datasheet, is having the reset line sitting high an issue?

High is good.  Low would be in reset. Besides, like you noticed, the chip must be running if it displays the voltage of the 9v batt. Does the working mode button do what it's actually designed to do, or does it behave like a different button?
It's unfortunate that there's no service manual because then figuring this out would be relatively simple. (repair might still be difficult)

[bostonman]

I believe the 'mode' switch is working correctly, but haven't read the manual to confirm.

I'll dig deeper into this issue at some point. My first approach was to use a theoretical analysis since measuring is extremely difficult with power applied. Since all the buttons and dial led to this one IC, I thought to gamble with replacing it, however, since it's programmed, that somewhat eliminates the gambling approach.

[bostonman]

Just for reference, here is a picture of the front and back side of the board. The ribbon cable end is where the seven segment displays are mounted.

[Kim Christensen]

Notice that all the switches except SW4 (Mode) have a common trace between then? Hmmm...

Also, I'm curious what the markings on R24, R100, R101, R103, R104 & R105 are. Are they all the same? Or all different?

[bostonman]

I'll have to look closer.

SW4 looked like the trace took the long way back to that IC, but maybe I'm wrong.

[bostonman]

I dove deeper.

SW4 (the 'mode' switch), the only switch that works, doesn't connect the same way as the other five. SW4 is directly fed by 5v, and, when it makes a connection, is fed into the M430F4250 (the IC that I originally questioned whether it's good). It goes through two points, one is a via to the opposite side, through a 10ohm resistor in parallel with a capacitor to ground, and up to the IC.

The other long trace goes under the display, through a trace, connect to three resistors in parallel, and eventually to other stuff including C102 (capacitor near the ribbon cable). I'm assuming this is a more elaborate circuit as it needs to switch relays (as I hear them clicking when I push the 'mode' button.

The other five switches that don't work all go through vias to the diodes and resistors next to each other (R100, R101, etc...) and then into CH453D IC (16-Bit LED Nixie tube Driver and Keyboard Control Chip CH453).

First piece of curiosity is the buttons and how they make a connection. I measured the black dot on the backside of the buttons and they are resistive. I'm assumed they are 0ohms and connect the middle copper spoke wheel to the outer copper circle, however, that doesn't seem to be the case.

Are these capacitive connections or a known resistance that acts as a voltage divider so the IC knows which button has been pressed (my meter seemed to move around when I tried to measure the resistance so I don't know if it was moving due to them being capacitive or not getting a solid connection)?

As for the CH453D IC, it seems the five non-working buttons that go through the diodes and resistors and connect to this IC are connected to pins 1, 23, 24, 26, and 28. Looking at the datasheet, these pins have multiple purposes - from the datasheet:

Segment drive of LED Nixie tube, active at high level, Keyboard scan input, active at high level, built-in pull-down resistor

I guess the question is whether the chip is getting SCL and SDA, but trying to measure anything is way too difficult.

The only oddball issue I see is the buzzer appears to have been replaced due to flux in that area whereas no other portion of the board has flux. The buzzer gets 5v directly and the negative side is driven by a transistor that seems to be controlled by the M430F4250 IC. I'm sure this isn't an issue, but the thought had initially occurred that maybe the buzzer is trying to pull too much current from the IC (but it is driven by a transistor).

[Kim Christensen]

First piece of curiosity is the buttons and how they make a connection. I measured the black dot on the backside of the buttons and they are resistive. I'm assumed they are 0ohms and connect the middle copper spoke wheel to the outer copper circle, however, that doesn't seem to be the case.

It's not unusual for these carbon pads to measure 20-100 ohms or so. Perfectly normal.

I guess the question is whether the chip is getting SCL and SDA, but trying to measure anything is way too difficult.

The CH453D IC also gets the data for the LED display via those two lines, so those are probably OK.

As for the CH453D IC, it seems the five non-working buttons that go through the diodes and resistors and connect to this IC are connected to pins 1, 23, 24, 26, and 28. Looking at the datasheet, these pins have multiple purposes - from the datasheet

I found an English version of the datasheet. So it looks like DIG15 (Pin 22) doubles as an interrupt pin to the MCU when a key is pressed. It should go low every time a key is pressed. (You'd probably need a scope to see it toggle). Also, the common to your switches should connect to one of the DIG0~DIG7 (pins 4-8, 12-14) on the CH453. The good news for you is that you could replace a faulty CH453 because it doesn't need to be programmed. Not sure if it's faulty though. A stuck key could have caused others not to work. I assume those were all OK with no gunk under them. (Your PCB looked pristine though)  Or a broken trace between the keys common line to one of the DIG0~DIG7 lines would stop them working too.
Here's an excerpt about the keyboard scan:

Code: [Select]

CH453 periodically inserts keyboard scan during the display driver scan. During keyboard scan, the pins
DIG7~DIG0 output high level in sequence from DIG0 to DIG7, and the remaining pins output low level.
The outputs of the pins SEG7-SEG0 are disabled. When no key is pressed, SEG7-SEG0 are pulled down to
low level. When a key is pressed, for example, the key connecting DIG3 and SEG4 is pressed, SEG4 detects
high level when DIG3 outputs high level. In order to avoid error code caused by key jitter or external
interference, CH453 performs two scans. Only when the results of two keyboard scans are the same, the key
will be confirmed to be valid. If CH453 detects a valid key, the key code will be recorded, and active low
keyboard interrupt will be generated through INT# pin. At this time, the MCU can read the key code through
the serial interface. CH453 does not generate any keyboard interrupt until a new valid key is detected.

INT# is used for keyboard interrupt output and is at high level by default. INT# outputs keyboard interrupt
active at low level when CH453 detects a valid key. After the MCU is interrupted, it performs a read
operation to CH453, and CH453 recovers the INT# to high level and outputs the key code from SDA. The
MCU gets a byte of data from SDA, among which the lower 7 bits are the key code.

When the X_INT bit is 1, up to 15-bit LED Nixie tubes are supported. DIG15 pins are used for keyboard
interrupt output, whose functions are the same as the INT# pin.


[bostonman]

I didn't get a chance to check the common connection between the button pads and wherever it goes, but will hopefully today.

The issue with trying to measure while pushing a button is that the PCB doesn't have standoffs for the keypad to rest on. It requires the PCB to be pressed against the backside of the front panel after the buttons are inserted into the front panel cut outs. Doing this obviously eliminates the ability to measure anything.

I attempted to touch the PCB keypad button section with the multimeter lead to simulate a key being pressed, but it wasn't easy to do (I attempted it on the working 'mode' button keypad spoke wheel.

Also, don't forget, the dial doesn't change the front panel numbers either; unless it's dependent on a particular button being pushed first.

Once the dial is removed, the shaft can slide out the backside. The bottom of the shaft is just copper plating that sits inside the potentiometer on the PCB. That pot (maybe it's obvious from the picture) has various copper pads. From what I can tell, the base of the pot is soldered onto the PCB with no electrical connection. There are three leads that solder onto the PCB, one is ground and the other two sit at equal voltages (I measured them but forgot the voltage). I'm assuming it's nothing more than a simple wiper and without the shaft inserted into the pot, the voltage sits in the middle of the two points.

[Kim Christensen]

Also, don't forget, the dial doesn't change the front panel numbers either; unless it's dependent on a particular button being pushed first.
Once the dial is removed, the shaft can slide out the backside. The bottom of the shaft is just copper plating that sits inside the potentiometer on the PCB. That pot (maybe it's obvious from the picture) has various copper pads. From what I can tell, the base of the pot is soldered onto the PCB with no electrical connection. There are three leads that solder onto the PCB, one is ground and the other two sit at equal voltages (I measured them but forgot the voltage). I'm assuming it's nothing more than a simple wiper and without the shaft inserted into the pot, the voltage sits in the middle of the two points.

I don't think it's a potentiometer at all. It looks like a quadrature encoder. It outputs two square waves 90 degrees apart when the shaft is turned.(Switching to ground with pullup resistors) By measuring the phase difference, the MCU determines the direction. Frequency of pulses determines speed.
So it's possible that this is also attached to the CH453D as two switches. Be interesting to see where those two connections go.

[bostonman]

Not being a potentiometer would make more sense since it has a resistor or two on the back side.

From what I can tell, the middle pin (as previously mentioned) is ground and the other two pins go to separate 5.1k resistors, and, oddly enough, the other side of both resistors are connected together. That side of the resistors go into two pins on the M430F4250 IC.

I measured the pins at the encoder and I didn't see a waveform. I saw a 4v DC voltage, and, as I turned the knob (I stuck it in and held it in place while turning) the voltage would go to 0v periodically. Eventually I could slowly turn the knob and get it to sit at 0v or 4v, however, at no point did I see a middle voltage or a waveform.

As for the buttons, the inner spoke on all the buttons (except the 'mode' button) are common and go to pin 4 of the CH453 and one of the seven segment display pins.

The outer spoke of each button (except the 'mode' button) is connected to individual diodes. From what I can tell, separate pins on CH453 connect to a resistor (I previously listed which pins are connected to which resistors), the other side of the resistor goes into the anode of a diode, and the cathode connects to the outer spoke.

I also measured the SDA and SCL on CH453 and saw action on both lines (I don't know if it was correct, keeping the probe steady was difficult enough).

Attached are scope captures (I forget if the scope capture is for SDA or SCL, but they were both similar). The file names are somewhat self explanatory, but I'll explain them:

Inner spoke is the measurement on the inner part of the button.

Outer spoke is the measurement on the outer part of the button.

SDA or SDL is the two lines on the CD453 IC.

Anode of diode is the anode side of each diode for each button.

[Kim Christensen]

I measured the pins at the encoder and I didn't see a waveform. I saw a 4v DC voltage, and, as I turned the knob (I stuck it in and held it in place while turning) the voltage would go to 0v periodically. Eventually I could slowly turn the knob and get it to sit at 0v or 4v, however, at no point did I see a middle voltage or a waveform.

The "waveform" would be created as you turned the knob... So that makes sense. You seeing it toggle between 0v & 4v is the low frequency "waveform" I was thinking about.

As for the buttons, the inner spoke on all the buttons (except the 'mode' button) are common and go to pin 4 of the CH453 and one of the seven segment display pins.

Now, the fact that there is no waveform here, looks like a fault with the CH453 IC. Pin 4 of the CH453 (Inner spoke) should be going high regularly to "scan" the keypad. So you should see a waveform/pulses here. If it's staying low the entire time, there'll be no way to detect a button press.

Note: There is a possibility that the MCU is not enabling keyboard scan in config via the SDA/SCL lines for whatever reason. But unlikely.

[bostonman]

Is the waveform on the outer spoke doing the scanning and feeding it back when a button is pushed?

[Kim Christensen]

The waveforms you're seeing on the "outer spokes" are from the "segment drive of the display LEDs"... They are bidirectional multiplexed lines which change to inputs to check for keyboard presses. Then they change to an output to drive the LEDs. They switch rapidly back and forth between these two modes while multiplexing the LEDs which is the waveform you are seeing.

From the datasheet:

5.3. Keyboard Scan
CH453 keyboard scan feature supports an 8×8 matrix 64-key keyboard. During keyboard scan, pins
DIG7~DIG0 are used for the column scan output, and SEG7~SEG0 pins have internal pull-down resistors
for the line scan input.
CH453 periodically inserts keyboard scan during the display driver scan. During keyboard scan, the pins
DIG7~DIG0 output high level in sequence from DIG0 to DIG7, and the remaining pins output low level.
The outputs of the pins SEG7-SEG0 are disabled. When no key is pressed, SEG7-SEG0 are pulled down to
low level. When a key is pressed, for example, the key connecting DIG3 and SEG4 is pressed, SEG4 detects
high level when DIG3 outputs high level.

[bostonman]

I'll plan to order a new CH453 soon. Most likely I won't get it until late next week which means I won't get to replacing it until next weekend.

Usually I see an IC fail in a more catastrophic way, so this will be interesting if one pin/function of the IC is the culprit.

The fact the inner spoke waveform was collapsed had me questioning what was going on, but, as I mentioned, I "assumed" once a button is pressed that it completes the circuit and sends the waveform on the outer spoke back to the IC.

Maybe this function of the IC got damaged because someone pushed two or more buttons simultaneously (or one got stuck and another was being pressed) and the driver couldn't handle the low impedance.