BK Precision DC Electronic Load - Model 8540 Repair - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: August 02, 2023, 02:10:09 am
content: 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.

author: rsjsouza
author_rank: Super Contributor
timestamp: August 02, 2023, 03:11:15 am
content: MSP430F425 perhaps?https://www.ti.com/product/MSP430F425The MSP430F4x family has an integrated LCD controller.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 02, 2023, 03:15:02 am
content: 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.

author: bostonman
author_rank: Super Contributor
timestamp: August 02, 2023, 03:07:27 pm
content: 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.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 02, 2023, 05:17:33 pm
content: 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.

author: bostonman
author_rank: Super Contributor
timestamp: August 02, 2023, 07:10:04 pm
content: 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.

author: bostonman
author_rank: Super Contributor
timestamp: August 03, 2023, 01:59:25 am
content: 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.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 03, 2023, 04:30:13 am
content: 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.

author: HoinJoin
author_rank: Newbie
timestamp: August 03, 2023, 05:49:28 am
content: Will it be MSP430F425IPM, or MSP430F425IPM, I found it on a website(https://www.ntcomponents.com/Datasheets/IntegratedCircuits/msp430f425.pdf), hope it can help youIn this article, I found several data sheets that cannot be found on the market, and they are quite reliable.

author: HoinJoin
author_rank: Newbie
timestamp: August 03, 2023, 05:51:54 am
content: I think you're right

author: bostonman
author_rank: Super Contributor
timestamp: August 03, 2023, 02:57:20 pm
content: QuoteI 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?

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 03, 2023, 03:39:31 pm
content: Quote from: bostonman on August 03, 2023, 02:57:20 pmHigh 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)

author: bostonman
author_rank: Super Contributor
timestamp: August 03, 2023, 06:32:46 pm
content: 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.

author: bostonman
author_rank: Super Contributor
timestamp: August 04, 2023, 02:35:24 am
content: 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.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 04, 2023, 03:26:13 am
content: 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?

author: bostonman
author_rank: Super Contributor
timestamp: August 04, 2023, 12:42:33 pm
content: I'll have to look closer.SW4 looked like the trace took the long way back to that IC, but maybe I'm wrong.

author: bostonman
author_rank: Super Contributor
timestamp: August 08, 2023, 12:21:59 am
content: 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 resistorI 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).

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 08, 2023, 03:53:02 am
content: Quote from: bostonman on August 08, 2023, 12:21:59 amIt's not unusual for these carbon pads to measure 20-100 ohms or so. Perfectly normal.QuoteThe CH453D IC also gets the data for the LED display via those two lines, so those are probably OK.QuoteI found anEnglish 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:CH453 periodically inserts keyboard scan during the display driver scan. During keyboard scan, the pinsDIG7~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 tolow level. When a key is pressed, for example, the key connecting DIG3 and SEG4 is pressed, SEG4 detectshigh level when DIG3 outputs high level. In order to avoid error code caused by key jitter or externalinterference, CH453 performs two scans. Only when the results of two keyboard scans are the same, the keywill be confirmed to be valid. If CH453 detects a valid key, the key code will be recorded, and active lowkeyboard interrupt will be generated through INT# pin. At this time, the MCU can read the key code throughthe 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 interruptactive at low level when CH453 detects a valid key. After the MCU is interrupted, it performs a readoperation to CH453, and CH453 recovers the INT# to high level and outputs the key code from SDA. TheMCU 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 keyboardinterrupt output, whose functions are the same as the INT# pin.

author: bostonman
author_rank: Super Contributor
timestamp: August 08, 2023, 01:10:49 pm
content: 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.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 08, 2023, 02:54:25 pm
content: Quote from: bostonman on August 08, 2023, 01:10:49 pmI 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.

author: bostonman
author_rank: Super Contributor
timestamp: August 09, 2023, 01:36:17 am
content: 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.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 09, 2023, 02:59:05 am
content: Quote from: bostonman on August 09, 2023, 01:36:17 amThe "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.QuoteNow, 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.

author: bostonman
author_rank: Super Contributor
timestamp: August 09, 2023, 03:13:24 am
content: Is the waveform on the outer spoke doing the scanning and feeding it back when a button is pushed?

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 09, 2023, 03:47:05 am
content: 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:Quote

author: bostonman
author_rank: Super Contributor
timestamp: August 09, 2023, 01:43:47 pm
content: 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.

author: bostonman
author_rank: Super Contributor
timestamp: August 09, 2023, 02:02:38 pm
content: Maybe ordering them will not be so easy.I don't see them on Digikey, Mouser, and Aliexpress appeared in a Google search but the page wasn't found.Most of what I"m finding related to CH453 or CH453DS1 are the PDF datasheets.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 09, 2023, 03:11:06 pm
content: Quote from: bostonman on August 09, 2023, 01:43:47 pmI think that's what the diodes are for.Quote from: bostonman on August 09, 2023, 02:02:38 pmYou might be able torequest a sample.Don't know how long that would take, etc.

author: bostonman
author_rank: Super Contributor
timestamp: August 09, 2023, 05:25:18 pm
content: I didn’t find a manufacture. The link on the datasheet brought me to the webpage with the datasheet and a Google search only led me to Aliexpress which was an invalid webpage.I’ll look more tonight; maybe I’ll have to find an alternate.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 09, 2023, 08:40:36 pm
content: Didn't you see the link for a sample that I posted above?Link to the manufacturer page:https://wch-ic.com/products/CH453.html

author: bostonman
author_rank: Super Contributor
timestamp: August 10, 2023, 12:50:13 am
content: Sorry, I didn't see that link; I was using my phone and overlooked that it was a hyperlink.I'm uncertain whether my sample order went through, but I placed it. They had an odd section for the address where it was just a single line, so I entered street, city, etc... all on one line.

author: bostonman
author_rank: Super Contributor
timestamp: August 12, 2023, 12:47:37 am
content: I got a reply from the company. They ship samples, but charge for shipping, so I’m waiting on a quote.

author: bostonman
author_rank: Super Contributor
timestamp: August 15, 2023, 03:51:56 pm
content: By any chance, is Aliexpress safe to buy these chips from as I believe I saw them at one point on the site?I keep going in circles with trying to get samples from the company in the link provided previously. Fist they offer free samples but I need to pay shipping, then they needed a shipping address for the shipping cost, then they sent a link on where to purchase them but they have quantity zero available.

author: bostonman
author_rank: Super Contributor
timestamp: August 15, 2023, 04:12:29 pm
content: Actually, maybe it was Alibaba as that's what I'm on now.They list the IC as DIP/SOP

author: bostonman
author_rank: Super Contributor
timestamp: August 15, 2023, 06:10:54 pm
content: Update: I ordered them through Aliexpress for $1.01/each plus shipping.

author: bostonman
author_rank: Super Contributor
timestamp: August 28, 2023, 01:23:59 am
content: The ICs arrived this weekend, however, I was extremely busy. A possibility I'll be have time tomorrow and will update afterwards.

author: bostonman
author_rank: Super Contributor
timestamp: August 29, 2023, 01:07:55 am
content: I replaced the IC, however, it didn't fix the problem.Performing some random (desperate) button pushing, I held down the blue button (shift button) while powering and got a different startup sequence. Upon pushing the 'mode' button, it seems this is some sort of display test where each push of the 'mode' button lights a different part of the seven-segment LED along with changing LED status lights.I then repowered while holding the 'shift' button again, and held down on/off or another button, and got a quick flash of letters. Did the same with another button and got some sort of date.It seems the buttons actually register but don't do anything during normal operation.This board only has a ribbon cable connection along with a two-pin connector (5v and ground). I tried powering the unit without the ribbon cable connected and got the same startup sequence, same ability to hold the 'shift' button and perform different things, but the only difference I saw was: the relay(s) didn't click (as I'd assume since nothing is connected via ribbon cable), and it displayed some letters (I forget what they were, but didn't seem like it was anything helpful).The main board has transistors mounted to the case. I'm wondering if it's worth really dismantling this in hopes the issue is on the main board.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 29, 2023, 04:12:45 am
content: So every button actually does something when in this mode? How about the rotary knob?The only other thing that I can think of trying is seeing if the instrument responds to remote commands. *IDN? would return the model number, etc, if the main CPU was alive and sane.

author: bostonman
author_rank: Super Contributor
timestamp: August 29, 2023, 02:40:22 pm
content: The knob doesn't seem to do a thing, but it seems almost all the buttons actually respond.I'm trying to think how the unit would be laid out. Thinking out loud, the unit displays the correct voltage should I connect a battery to the input terminals, so it can measure correctly.From the (assumed) manual display test, it seems all segments and LEDs work, and I believe every button (or at least four out of six) responds during start up, so the display board has enough brains to light everything.I'm uncertain what you meant by remote commands. I don't have any software (and limited knowledge on software interfacting) to send commands.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 29, 2023, 03:03:05 pm
content: Ah yes, I forgot about the voltage readout still working... So the main CPU is still alive.As for putting it in remote mode, it uses a TTL level serial interface. It looks like they supplysome sort of software on BK's site.Download the "LabVIEW 2022 Run-Time Engine" version if you don't already have the Run-Time on your PC.If you were really adverturous you could sendthe ASCII commandsyourself via a terminal program, but the supplied software would be easier to use. You'd need the USB to TTL_Serial interface for both methods.I was thinking that if you can't repair it, the instrument could still be useful to you in remote mode. Or the remote mode might help you diagnose the problem. (It allows you to run the self test)

author: bostonman
author_rank: Super Contributor
timestamp: August 29, 2023, 03:27:42 pm
content: I'll look into interfacing, but I may get delayed as I'm tackling a few projects simultaneously at the moment.Maybe the entire display board is fine and a reference voltage has failed on the main board. Typically during "shot gunning" a repair, I start with the obvious: bulged caps, burn marks, and anything connected to a heat sink. The main board didn't show any signs of those issues, but I also couldn't access much as I was reluctant to being removing stuff from heat sinks (or un-soldering them from the PCB).Also remember, the knob voltage changes when rotated (it sits at either 0 or 4v I think and moves as it's rotated - you said the changing states is the differential measurement taking place).I don't want to contradict myself, but it almost seems the display board could be fine and I need to dig into something like a reference voltage.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 29, 2023, 05:11:12 pm
content: Quote from: bostonman on August 29, 2023, 03:27:42 pmIt's possible that some other fault is making the display/keyboard/knob system hang up. It's always a good idea to look everywhere and avoid tunnel vision.Too bad that a schematic isn't readily available.

author: bostonman
author_rank: Super Contributor
timestamp: August 29, 2023, 06:40:21 pm
content: This is why in a desperate attempt I tried holding in buttons at power on.My other initial approach was noting all the ICs on the display PCB and looking at their functions in hopes something stuck out. I'll perform this again, but, as I agree, schematics would be great because I'm just shooting from the hip.Not knowing the flow of the functions is another hurdle. Knowing the buttons "work" seems like a disconnect between the display board and main board, but not knowing how the knob and buttons interface with the main board is difficult.

author: electr_peter
author_rank: Unknown
timestamp: August 29, 2023, 07:28:15 pm
content: Just a guess - sounds like some problem (maybe solder blob shorting something) disabled button scanning. Buttons that are somehow linked (resistance/voltage/multiplexing) tend to cause issues in other products (remotes, TVs, radios, etc.) as well, as it is tricky to debug and identify.I would suggest to get patch of steel wool, solder it to one side of buttons and use it instead of carbon pads. Should help debugging somewhat.

author: bostonman
author_rank: Super Contributor
timestamp: September 01, 2023, 03:50:31 am
content: Tonight I removed the main PCB (not that easy).The main PCB is mounted to a long square tube type heat sink (with a fan at the end). It has x4 (two on each side) IRFP250N (N-Channel MOSFETs), a LM19 (temp sensor), and I think a TIP42C (PNP) too (I don't remember which other component was mounted to the heat sink, but believe it was this).Also, several OP07C (op amp) throughout the board, a 7805 (5v regulator) mounted to another mini heat sink, a 7812 (12v regulator), and a 7912 (negative 12v regulator); both in open air.I found one high wattage resistor that looks like it took some heat. It seems to measure lower than the (I believe) 10ohms; I'm measuring 2.9ohms (capacitor seems to be good).I'm not sure I understand the circuit though.From what I can tell, the negative input on the front panel goes to the capacitor in the picture (and that pipe looking thing which I believe is a type of low ohm resistor), the other side of the capacitor to the resistor, and the other side of the resistor to ground.None of the stuff on heatsinks seem to measure incorrectly as they all seem to measure the same. All surface mount components look good (i.e. not burnt), and the only thing that took heat is that resistor.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 01, 2023, 04:45:41 am
content: Quote from: bostonman on September 01, 2023, 03:50:31 amSo the series connected R2 & C1 are connected across the large shunt? (R68) If so, and if C1 isn't shorted, the resistor sounds like it's burnt and has changed value.Did you measure C1 in or out of circuit?

author: bostonman
author_rank: Super Contributor
timestamp: September 01, 2023, 02:06:24 pm
content: I didn't have a chance to remove the capacitor and resistor, but will hopefully this weekend.The capacitor is marked 225K, from reading, I think this is 2.2uF with a 10% tolerance. The resistor should be 10ohms I believe (I used an online converter since it's five bands and less familiar with these code scheme.From what I could tell, the cap and resistor go across negative input and ground which (to me) doesn't make sense). The large metal pole (shunt) doesn't look like it connects to the positive input terminal, but I could be wrong.I don't see what purpose the cap and resistor serve since the connection between the two doesn't look like it goes elsewhere (hard to tell because the components are soldered close to the board covering traces).Keep in mind, the system reads voltage on the inputs.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 01, 2023, 05:50:55 pm
content: Quote from: bostonman on September 01, 2023, 02:06:24 pmIt wouldn't be unusual to sense the current in the negative lead since that is easier to implement.QuoteProbably a Zobel network.

author: bostonman
author_rank: Super Contributor
timestamp: September 04, 2023, 12:12:46 am
content: I traced the board a bit deeper (see attached hand drawing) and I also removed the resistor and capacitor.The capacitor measures 2.3uF out of circuit (it's marked 225k which I believe is 2.2uF) The slightly burned resistor (marked R on my drawing) measures 2.3ohms (I think it's brown, black, black, gold., brown, which is 10ohms).The four RX21 (8W) resistors I'm uncertain whether they connect together at the bottom, so I left them "open" rather than assume. Same with the MOSFETs. From what I can tell, the TL084 (JFET op-amp) goes through a 470ohm resistor and the gate of a MOSFET. Each output on the TL084 is connected the same way (i.e. each output drives a different MOSFET), and I assume the other MOSFETs are connected to the RX21 resistors (but again, I didn't want to draw it on an assumption).From what I'm seeing, the board has a power supply section where the AC comes in, goes through a bridge rectifier, and into three regulators (5V, 12V, and -12V); all measure good.Besides the MOSFETs and a few other components, the board has many OP07C ICs, many (I believe) transistors marked BR with a sideways 60, several diodes market WA with a sideways EN, a few diodes marked T4 with a sideways E7, and one LM358 over by the temp sensor.It looks like this board is all power without any "brains". If the possibility the resistor measuring 2.3ohms is causing the system to shut down, then maybe that's the culprit, otherwise, it's probably back to the display board.

author: bostonman
author_rank: Super Contributor
timestamp: September 05, 2023, 02:26:55 am
content: Quick update. I removed one side of each RX21 resistors and they all measure good (as I somewhat expected). It looks like each one goes to the source of each MOSFET.Per my previous drawing, I drew only one going to Q1 with the gate connected to the TL084, but, as stated above, each resistor goes to a different MOSFET, and each output on the TL084 controls a different MOSFET gate.I'm assuming all the transistors and op-amps on the board are for power control, modes, etc...I guess it's back to the drawing board, but feel this board isn't the culprit.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 05, 2023, 03:14:38 am
content: That all makes sense. 4 mosfet/source-resistor combos, all in parallel to share the load, with the TL084 making sure the current though each is equal.

author: bostonman
author_rank: Super Contributor
timestamp: September 05, 2023, 03:27:38 am
content: I agree.I may shotgun and test all the surface mount transistors and diodes for shorts. Tracing this board is extremely difficult as it has numerous vias, so maybe just a quick cheating method may be the only option.If nothing pops out on this board, I'll try tracing the ribbon cable connections, reassemble the unit, and recheck the display board.Just to summarize: the displays the voltage when a 9v battery is connected, 'mode' switch works in all conditions, and at least four buttons respond when held in during power on. The dial doesn't do anything and the other five buttons don't change anything during "normal" mode.

author: bostonman
author_rank: Super Contributor
timestamp: September 06, 2023, 04:44:07 pm
content: I meant to ask, am I wrong, but is that resistor useless for DC?My guess is someone inputted a large AC voltage.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 06, 2023, 05:31:28 pm
content: The burnt resistor (R2) along with C1 (RC Zobel network) are there to help with stability when driving inductive loads. You'll commonly see RC Zobel networks across the outputs of audio amplifiers. So, yes, R2 does nothing when there is a steady DC across the output, and would only dissipate power if there was an AC waveform there.What's quite common is someone using an electronic load on the output of a DC power supply to test it. Perhaps they had the load in constant current mode and the DC power supply under test became unstable and started to oscillate. This would create a large high frequency waveform across the load so that R2 would start to dissipate a lot of power. (higher frequency = Lower Xc for C1 = more current through R2)

author: bostonman
author_rank: Super Contributor
timestamp: September 06, 2023, 06:58:40 pm
content: I guess more importantly: I'm assuming this resistor wouldn't have anything to do with the non-functioning buttons and/or knob?From quickly checking, it seems all the transistors and diodes are good as I didn't find any open or shorted (not to say they are not failing in some other way).If I had to guess, this entire main board only supplies DC voltages to the unit, supplies the load via the four MOSFETs, and most else is logic. The ribbon cable looks like many pins are not used (which baffles me on why they'd use such a large connector) and/or many pins go to the RS232 connector on the back.The ribbon cable has some markings, one is VDAC which I may try tracing, but, from what I can tell, the PCB designer must have got bonus money for additional vias as this board seems to have many making tracing quite difficult.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 06, 2023, 07:09:42 pm
content: I wouldn't think so. But once you solve the button/knob problem, it'll be worthwhile to replace R2.

author: bostonman
author_rank: Super Contributor
timestamp: September 07, 2023, 01:32:58 am
content: QuoteFully agree, however, at this point, I have less faith in fixing this.

author: bostonman
author_rank: Super Contributor
timestamp: September 08, 2023, 01:09:47 am
content: Tonight I reassembled everything so I could power it.The display board only has six ICs:74HC04D (possible inverter)M430F4250 (microcontroller) - part that began this thread24LC541 or SN1747 (uncertain which is the part number) - EEPROM I think77D5C9K or HC14 (uncertain which is the part number) - HEX inverter I thinkC345 (seems to be a single buffer if I found the correct datasheet)CH453S (the IC I already replaced)Besides the three ICs (assuming C345 is a buffer) that can be measured, the others I can't really measure to confirm whether they are good. Since I already replaced CH453S, we can assume this IC is good, and I can hopefully connect to the other three ICs to confirm they work. This leaves the EEPROM and microcontroller and I don't know what signals should be on these lines; nor can the chips be replaced since they are programmable.I don't have a contraption that will hold the scope probes onto the IC pins, however, even if I did, I'm limited due to the ribbon cable.Just to recap on things: the entire display works, the 'mode' button is the only button that works when the unit is powered (and at least one relay on the main board responds), the knob doesn't respond, holding the 'shift' button during power on sends the unit into some manual display test where pushing the 'mode' button changes a lighted LED segment with each key press, some of the other buttons also cause the unit to go into some self test state if held down while powering the unit (telling me the buttons actually "work"), the front display will measure the correct voltage of a 9V battery, but changing modes while measuring the battery by pushing the 'mode' switch doesn't do anything. Also, not sure if this matters, but I haven't heard the buzzer work either, however, it may only respond to certain key presses.I don't know the relationship between the microcontroller and the EEPROM. It's possible the EEPROM handles all the buttons and knob, however, the buttons "work" as listed above. So the EEPROM wouldn't be the culprit leaving the microcontroller.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 08, 2023, 02:39:18 am
content: QuoteI would guess that the EEPROM is just for settings and calibration data. You could probably replace it with a blank one and the microcontroller would then overwrite it with default data and then complain about it being uncalibrated. (Assuming well written firmware)Why are you uncertain of the part numbers? What is written on the ICs?

author: bostonman
author_rank: Super Contributor
timestamp: September 08, 2023, 12:54:38 pm
content: QuoteA few have two different numbers, so I'm uncertain which is a LDC (lot date code) and which is a part number. Also, C345 seems to be a marking rather than a part number, however, I did seem to find a datasheet for it. I went through this when I was repairing my oscilloscope. The transistors, much like the transistors on here, had a few digits with a sideways digit or two, but they was a generic number that required cross referencing.The reason I put 'uncertain' was rather than list what I assume to be the correct part name, but seems I've found the datasheets that match the part.If the EEPROM probably handles calibration storage, and the other ICs that I can measure check good, it leaves the microcontroller. If this is the case, unfortunately I can't change the micro since it's programmed and checking whether other components are good is much too difficult. Seems at some point I may need to give up.

author: bostonman
author_rank: Super Contributor
timestamp: September 11, 2023, 02:32:02 am
content: Today I took measurements on the ICs.I want to confirm since I lack experience dealing with the finer details of high and low trigger points with digital chips.SN74LVC1G34 has 3v on the input (pin 2) and 5v on the output (pin 4). If I understand the datasheet correctly, a high level is 0.7*Vcc (5v in this case). Doing the math, anything from 3.5v and higher will be considered a 'high'; therefore the output will be high.If this is correct, this tells me the IC isn't working correctly because the output should be a low.SN74HC14 has 1v on the input (pin 13) and 5v on the output (pin 12). All other gates are basically 0 with an output at 5v or vice versa. Pin 13 is the only one sitting at a point that isn't 0 or 5v. I'm confused on the datasheet because this is a Schmitt trigger inverter. I don't understand why it's showing waveforms on page 8 that shows the input and output are a high or a low (implying it's a bugger and not an inverter), and then it shows another figure with two outputs. I don't understand at which voltage triggers the output to go opposite the input.74HC04D has 3v on the input (pin 11) and 0v on the output (pin 10). All other gates are basically 0 with an output at 5v or vice versa. According to the datasheet, anything from 3.7v and higher is a 1, so the output should be 5v, but it's 0v.My interpretation of high and low trigger levels may be wrong, but wanted to post my numbers for freed back.

author: Kean
author_rank: Unknown
timestamp: September 11, 2023, 03:48:15 am
content: The output of the SN74LVC1G34 isn't necessarily wrong, more that it is undefined when the input it between 0.3*Vcc and 0.7*Vcc. This undefined operation can end up causing more problemsAs you see 1V and 3V on a couple of inputs, I would suggest tracing them back to their source. An input of 1V is within spec, but 3V is not (assuming Vcc is 5V, as indicated by output voltage).It is quite likely a problem with the upstream components, but could also be the inputs on these gates are damaged and are loading a high level signal down.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 11, 2023, 04:37:54 am
content: Quote from: bostonman on September 11, 2023, 02:32:02 amI thinkthose diagrams on page 8are just about timing and not actual logic levels or inversion. They say this:"Phase relationships between waveforms were chosen arbitrarily"QuoteSN74HC14 datasheet:On page 5, if we look at the typical "switching threshold" values (VT+ & VT-) for Vcc = 4.5V @ 25C, we see that they spec 2.5V and 1.6V. So if we got a "typical chip" the output would only change state if the input rose above 2.5V or fell below 1.6V. Any bouncing around between 1.6V <--> 2.5V would result in no change on the output. So if the input started at 0V and rose all the way to 2.49V and then went back to zero the output would stay high. But if the input started at 4V and dropped down to 1.61V and the rose again to 4V the output would stay low the entire time.

author: Kean
author_rank: Unknown
timestamp: September 11, 2023, 05:29:14 am
content: The threshold voltages apply to the SN74HC14 which has Schmitt-Trigger inputs. The SN74LVC1G34 doesn't, so a 3V input is undefined.So I suspect either the upstream circuit outputting this voltage is faulty, or the design is bad (somewhat unlikely even for a BK Precision rebadge).

author: bostonman
author_rank: Super Contributor
timestamp: September 11, 2023, 12:38:13 pm
content: I'll look again, but the chips are tied to the M430F42250 chip. As mentioned, it's possible one of the chips is holding down the output of the micro.Also note, none of the chips are clocking; they are all DC voltages. Maybe they change state when buttons are pushed, but I can't access the button side while trying to measure pins on the other side.

author: electr_peter
author_rank: Unknown
timestamp: September 11, 2023, 07:17:45 pm
content: Quote from: bostonman on September 11, 2023, 12:38:13 pmYou can solder small wires and have easier access this way - hot glue will hold them in place. Label for each wire would be helpful as well.

author: bostonman
author_rank: Super Contributor
timestamp: September 12, 2023, 12:45:23 pm
content: From what I can tell, most signals are derived from the M430 IC.The display board has a row of pads (about five or six) that allow measuring of VDAC, IDAC (?), and whatever else. I may try soldering wires on these pads and measuring them while the display board is assembled so I can push buttons.Otherwise, I basically have to throw in the towel with this unit.Unless the signals on the ICs that I mentioned previously are questionable and/or bad, the display board only has the M430 IC remaining as a component that could be the culprit (unless you consider the EEPROM too). The main board has six or eight OP07C ICs total which I could change one at a time (or just all of them to save time) to see if any of them are the culprit. Maybe about eight transistors (none seem to be shorted or open while doing a quick measurement), six diodes (also none seem to be open or shorted while doing a quick measurement), and about six to eight surface mount capacitors (none of them look like they are leaking).Besides measuring those pads (all are zero volts without the keypad connected except one that began increasing in voltage when I connected a 9v battery to the input), and randomly replacing transistors/diodes, I'd need to hope the program on the M430 can be extracted and isn't secured, have the ability to extract the program (I only have one programmer and not sure it can handle this IC), and rewrite it to another IC.

author: Kean
author_rank: Unknown
timestamp: September 12, 2023, 12:57:47 pm
content: OK - you replied while I was writing this, but I will post it anyway.If the signals are coming from the MSP430F4250 then the 3V output would make sense as that is a 1.8-3.6V supply device.The SN74LVC1G34 powered from 5V is a bad choice as it doesn't have Schmitt trigger or TTL compatible inputs and so the 3V input is not in the valid range, but is likely not the cause of the problem.There doesn't appear to be such a thing as a 74HCT1G34 which would provide better input level range for logic level shifting. The HCT series has TTL compatible inputs which means it has a lower Vih threshold (1.6-2.0V when powered from 4.5-5.5V) so is good for the 3.3-5.0V logic level conversion. The HC series has slightly higher threshold (2.4-3.15V when powered from 4.5V) so could be marginal.There is a 74HCT1G126 which would just require the OE input on pin 1 to be pulled high to operate the same (pin 1 is NC on the 1G34). Just a thought if you wanted to rule that out and can find the 74HCT1G126 in the same package.I would be surprised if the MSP430 has not had its security fuse blown, but it doesn't hurt to try read it.

author: bostonman
author_rank: Super Contributor
timestamp: September 12, 2023, 03:29:21 pm
content: I didn't realize the M430 is 1.8v to 3.6v. This unit only has 5v, 12, and -12v, so it's either getting shifted or I'm overlooking a regulator (unlikely since I've noted all the chips).From my experience, IC chips are usually last in line of components that fail; unless they are a regulator in some sort of IC looking package.It's also rare I've seen op-amps go bad. So I'm leaning towards a bad transistor or diode. Wish I had another unit so I could swap the display board and narrow down which board the issue is on.Does anyone think the M430 can be partially working? Keep in mind, the buttons "work" if I hold down the 'shift' button at turn on and push the 'mode' button; along with other buttons performing different things if I hold them down at power on too. The only thing that doesn't work in all cases is the knob.Knowing the functions of all the chips tells me the only one with "brains" (excluding the EEPROM) is the M430, but it is functioning enough (or maybe fully functioning) to provide "test mode" support, hence why I asked if it could be partially functioning.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 13, 2023, 12:50:06 am
content: Quote from: bostonman on September 12, 2023, 03:29:21 pmYup. It's entirely possible. I've seen MCUs with just a single pin gone bad.

author: bostonman
author_rank: Super Contributor
timestamp: September 13, 2023, 02:01:24 am
content: QuoteUnfortunately the programmer I have came close to supporting this part number, but not quite. Here is the list from the manual:MSP430F413@QFN64(ISP) [ISP-HEADER01]MSP430F413@TQFP64(ISP) [ISP-HEADER01]MSP430F435@TQFP100(ISP) [ISP-HEADER01]MSP430F435@TQFP80(ISP) [ISP-HEADER01]MSP430F4351@TQFP100(ISP) [ISP-HEADER01]MSP430F4351@TQFP80(ISP) [ISP-HEADER01]MSP430F436@TQFP100(ISP) [ISP-HEADER01]I'm assuming the chip is secured and I'm unable to extract the program from it (assuming it's not corrupt due to the possibility the chip is bad), so investing much into a programmer probably isn't worth the cost.I found a programmer for MSP430 chips, but it was over $300; clearly not worth buying.

author: rsjsouza
author_rank: Super Contributor
timestamp: September 13, 2023, 02:59:56 am
content: Quote from: bostonman on September 13, 2023, 02:01:24 amThe official MSP430 programmer, the MSP-FET, is $115.https://www.ti.com/tool/MSP-FETHowever, any Launchpad can program your device (an exception perhaps to the G2ET, which only programs it via Spi-By-Wire), as they have a jumper breakout that allows connecting flying wires between the kit and the target board. They start somewhere around $15.00https://www.ti.com/design-development/embedded-development/msp430-mcus.html

author: bostonman
author_rank: Super Contributor
timestamp: September 13, 2023, 12:36:12 pm
content: I don't see any program header pins on the board. Either the ribbon cable plugged into some custom board that allowed programming at the factory after the chip was soldered onto the board or it was done prior.Sadly, I feel this repair is coming to an end. Looking at the odds, I have to hope the chip is the culprit, it's not been secured allowing me to extract the information from it, and, if it's the culprit, the chip has enough life to allow extracting data.Also, besides using a basic programmer to extract and program chips, anything more elaborate is out of my area of knowledge. Spending anything more than $10 or $20 for a programmer that does both, extracting and programming, probably is a waste. If the chip is secured (which it most likely is), then extracting the program won't happen.

author: Kean
author_rank: Unknown
timestamp: September 13, 2023, 02:30:51 pm
content: Best thing seems to be to put it aside for a while and keep an eye out for another cheap/faulty BK8540 to swap parts with.Or offer it as a challenge to another member via the buy/sell part of the forum.

author: bostonman
author_rank: Super Contributor
timestamp: September 13, 2023, 05:47:47 pm
content: I agree.If I get time, I'll try measuring voltages on the op-amps, transistors, and diodes. At least I may find something if it's a hard failure.Unless a cheap solution exists to try extracting the program from the IC chip (assuming it's not secured and/or too damaged), then most likely it will sit on my shelf for a bit.This seems like a common failure, can't believe nobody has posted anything online about repairing this.

author: bostonman
author_rank: Super Contributor
timestamp: October 18, 2023, 02:23:09 am
content: I thought it would be appropriate to change the subject to what this thread deviated to instead of the original title (M430F4250 Datasheet).Hopefully anyone who has this same issue can save some time by reading this thread and eliminating what I already tried.

author: bostonman
author_rank: Super Contributor
timestamp: October 30, 2023, 02:01:44 am
content: I have a question regarding the M430F4250.Discussion in this thread revolved around trying to read the code from this micro to burn a replacement, but readers are expensive. Since others in this thread probably have a reader/burner that can read the code, would anyone be interested in trying to read the code if I send the chip?This assumes the chip isn't protected, works good enough to extract data from, and is the cause of the unit not working.

author: bostonman
author_rank: Super Contributor
timestamp: November 17, 2024, 04:29:09 pm
content: I'm digging up an old thread I started.If anyone is still on this thread and/or remembers the issue, maybe this will be of interest. Due to hoping someday I'll come across a repair for the issue my unit has, I've kept the unit on my shelf. Very soon I plan to discard some stuff and began tinkering with this again to see whether it's worth holding onto.I kept trying combinations of holding various buttons upon powering and got into some mode where the knob began adjusting the display.From observation, most of the buttons appear to work because they allow different modes to be displayed when I hold them when powering. Also, the knob appeared to work in that mode because the numbers changed (although I've yet to recreate that same mode).The display shows memory step D always displayed. I'm wondering if someone messed up settings and saved it into this step D mode thus causing the panel to lock.I've read through the manual and I don't see a factory reset anywhere and the manual states the 9-pin D-sub connector on the rear is for factory use only eliminating any external software to try unlocking it.Long shot, but has anyone come across a way to reset the unit (although it seems just simply re-powering does the trick)?

author: bostonman
author_rank: Super Contributor
timestamp: January 20, 2025, 02:09:33 am
content: A quick update.I dug out this unit again and played with it more. It seems the front panel works for about two-seconds upon power up. This was discovered when I powered the unit and immediately pressed shift and another button.If I quickly pushed shift and mode (3A/30A) I can get the unit to go into 30A mode. If I repowered and quickly pushed shift and right arrow (recall), I can get the unit to recall setups.From what I'm seeing, something is causing the unit to lock seconds after powering. I've proved all the buttons and knob works based on pushing random buttons on power up, so I'm thinking maybe a voltage is low and causing the micro to shut down or maybe a failing crystal.I plan to take some measurements to see if the 6MHz is present.If anyone understands how the micro (msp430f4260) works and can shed light on why maybe this is going into shut down mode, I'd appreciate feedback.