I don't see a diode in the RC reset circuit you put in. The data sheet shows that there is one there to bypass the resistor to let the cap discharge quicker. When you turn it off and then on so quickly, could that be causing the latch up since the cap isn't discharged below 0.2v before the power is restored? That reset chip (5-pin SOT package) must have been monitoring the power rail for stable power and with the bad cap in the power rail it never became stable and so it never brought the controller out of reset. I think if there were maybe a smaller cap and a diode across the resistor to discharge the cap quicker the thing would boot every time with no over-current. The datasheet for the controller says the reset should be held below 0.8v for at least 20 cycles. At 4MHz that wouldn't be very long at all but no harm in having it low for longer. The cap just needs to get below 0.2v before power is restored after power off. A power on reset chip can hold the reset low for as long as 250ms, so I think this reset circuit is now the trouble here.

i like the repair more then the normal tear down, where tear down is a blast thru and this is way easier to follow and more understandable also for people not really into electronics

Please Dave, don't leave one of your most exciting episodes without a sequel!

Jolly good show. I learn no matter what the end result, so keep them coming Dave. I would hate to think this footage would be dumped because the device was 'unrepairable'.

When I have power going 'somewhere' due to suspected low resistance or short (like when you had 90mA+ during the direct pull-up through the meter) I just shine the thermal camera at the board and see who's holding the line down (or up). Has saved me some serious pain :-)

It's like watching an electronic version of House xD

Great job Dave. I love the troubleshooting videos you make. Great learning experience for me.

Dave - I enjoy your repair videos, regardless of success. The process for troubleshooting and isolating issues is the fascinating bit.

this was very interesting for me, i love fault finding, and love all the steps you took but as you say, if its not going to be reliable then its still no good! fair play for all the steps you did take though and keep it coming :) loving the channel!

I love this type of investigative troubleshooting even if the end result is not what you want, it still makes a very interesting video. Most people who commented spotted the same things that I was wondering about but very few people had any useful suggestions especially concerning the high current draw. It seems to me that the current is finding a path that should not be open which points to some other component going bad somewhere else, could be a whole string of faults in there.

Very nice troubleshooting Dave, we some times cant fix everything perfect, but least you tried. Great work.

I enjoyed that demonstration of tear down security immensely ;)

You’re still a rockstar Dave! Never would’ve gotten to the resistor pull-up/pull down whatever it was.

Check the processor data sheet for the spec of the reset signal, it might have a maximum rise time which is why the simple R-C circuit won't cut it. I've had fun and games with them not resetting properly before now.

I enjoy watching the trouble shooting process even if no fix - good show mattie

thanks Dave, same unit, similar black LCD line, swapped both caps for 5v reg, and lifted reset pin which was grounded, voila it boots, made an rs232 lead for 2.5mm jack and I'm happy it's worked. Thanks for the insight, appreciate it. Dave DPS

Love the repair vids. I've seen these exact units in heaps of apartments, would imagine that there would be some spare parts for them out there.

Hi Dave. Thanks for the awesome videos. I had the same experience troubleshooting a treadmill motor board just month ago.I found the faulty components and replaced them right way. The micro-controller on board was just working ok. After a short break and back to troubleshooting i powered the board up and guess what! chip never booted-up. Next step was to check 5v rails and all was just fine. I changed the crystal just to be sure it dum works.but i had no luck. After getting the data sheet for the micro i started with the reset pin and traced on the PCB, the voltage on reset pin was just 1.3v and normally it should be around 5v, down the line it was hooked to the collector ofA337 transistor that has to pull the pin high (5v)with a resistor divider on base and the emitter to 5v. I decided to add a 10ohm resistor on base pin (A733)to 5v, to allow 5v flow from emitter pin to collector to reset, and wow the A733 started heating up really hot due to high current flowing to reset pin. I got the micro out of PCB and did a standalone test on solder-less test board and also added logic components that a micro needs to boot. Lastly i measured how much current the reset pin was consuming and wow 250mah! Really!The chip never booted but was getting kinda warm. Hmm

Brilliant fault finding vid Dave. Learned a lot from this one mate.

That moment when, out of nowhere, EEVblog gives you a close up of a dead and crushed bug.

Great video, and I couldn't agree more that the outcome is not as important as the troubleshooting steps and in spite of the fact that the device did not function in a useful way it was fun to watch and I did learn a lot. thanks Dave and please keep these repair videos coming.

that was good one, gave me some ideas what to look for in my current repair :)

Probably the voltage regulator was causing the reset chip to stay low. Put the reset chip back in and recheck.

Even when the task is not successful you can still learn lots from it.

Best video ever! Thank you so much for going through the troubleshoot steps.

If you're struggling to read part numbers that are waterproof coated, like in the video, try using light of just a single wavelength. You can pick up red, green, blue laser pointers, and so IR bulbs and UV lights can be used. Just trial and error, but it's handy to have at least one of each in your workshop. They are really cheap and can be extremely handy in reading difficult things. You'll be amazed how some things look like they have no writing on but under the correct sort of light appear crystal clear.

Win! The diagnostic vids are how the rest of us learn :)

I loved watching this video as it followed my technique but maybe in a different order . I always go for supply then reset then data to display .thanks for a great video

Its seems to be more trouble than its worth, good video

Awesome video, I wonder, for expensive electronics where space is not too much of a concern, why can't they just socket as many components as possible in order to make repair easier?

I'll call this a win. Always enjoy your repair videos, Dave.

Maybe the microcontroller's reset pin has some internal diodes for protection that were wrecked somehow and are now partially shorted out? Possibly the lower protection diode.

Thanks for the upload. i learned a lot today

1 of the only channels where i have watched every ep

Dave, you had the reset dip switch on all the time! I guess that's what is pulling the damn reset line low.

Very informative video Dave, you gave a good attempt at repairing it! Just wondering... did you put DIP switch #1 back into the correct position after you tried the reset? Also, it might have been worth seeing if the processor was temperature-sensitive, which would rule out other parts of the cct maybe.

I had to go google SCR latchup. I had no idea that was possible, but would certainly explain the overcurrent situation and the bright LEDs. The question is then, what's causing the latchup, and why/how is it doing it consistently?

"When the supply voltage is in the range where operation is guaranteed, a reset is effected by holding the reset pin level “L” (0.2VCC max.) for at least 20 cycles" Maybe adjusting the reset network as some have said. No idea about the LEDs. Bad design + software PWM? Devil's work? Would be really cool if you got it working because it seemed so hopeless at first. Then again it could very well still be.

I really enjoy the touble shooting videos!

leds on full as out put ports on micro turned on when processor locks up. try making reset line from psu input voltage.

I like it how Dave uses a different multimeter every time. (:

Dave Is there an amp IC on the part with the speaker? Could you please do a vid on that part?

Dave, did you try to toggle the reset DIP-switch again after the 'repair'? I see you left it on, while the manual stated that it should be off...

really enjoyed that video Dave, I feel your pain when you got conflicting information that led you astray. I felt I was with you all the way with your thinking and procedures so that for me is reasuring. Electronics isn't an exact science but it should be.

Excellent troubleshooting video Dave. More of these please! p.s Was that a bit of a plug for a new watch?? ;) Who can blame yeh!!

Maybe the oscillator? The processor may be picking up the frequency from inside the multimeter?

The M16C Datasheet clearly states (page 14) that "When the supply voltage is in the range where operation is guaranteed, a reset is effected by holding the reset pin level “L” (0.2VCC max.) for at least 20 cycles. When the reset pin level is then returned to the “H” level while main clock is stable, the reset status is cancelled and program execution..." So you maybe really replace that 5223 thing? It also says "Free Delay Time Setting [...] Delay Time Controlled by external Capacitor" in the datasheet. So your RC might just not meet the specs for a proper reset here. Maybe you could trace where these LEDs are going? The M16C seems to have some 8 Channel DACs and all the rated I/O current is only 5 mA so there probably is a driving transistor for the LEDs or something?

Has the capacitor on the reset circuit failed short, maybe?

I realize this is a very old video, and I don't know what the outcome of the fix was, but the number 1 dip switch which Dave mentioned as a "reset" is turned on again at 35:45.

I'm sure you probably did off camera, but did you check the supply at the processor? Possibly being powered through the reset pin because no supply at the vcc pin?

You could rig up a 555 monostable with a resistor and transistor to kick the reset, possible? It didnt go mad untill you applied that electrolitic across the supply.

Sorry if it's been covered before, but have you checked the reset line doesn't go off somewhere else?

The troubleshooting was definitely interesting.

Looks like brown out, I'd replace the IC you removed with a voltage monitor. Had a very similar experience with an entire product line one, check Microchip AN686. A quick fix would be a 555 timer to hold in reset until the power supply settles.

I wonder if there is some floating input problem when the other boards are not connected?

Great video as explains well learn skills needed from it so even though couldn't fix it I have learned a lot from it, so a bit like Apollo 13 in the fact I would call it a successful failure, as failed to work but a greater good has come out of it educating people.

Thou check voltages!! what you are seeing is the processor being powered through the reset pin, because it's 5v supply is missing.

Comms between controlling station and keypads may be using DTMF A,B,C,D codes not normally on consumer 0-9 + * + # keys.

Love this video. Big thumbs up.

Could it be a hairline fracture on the board which you "fix" when applying pressure with the probes?

Maybe the improper slow rise time on the reset pin cousing the high current start. Are the low impedance on the reset trace is still exists ?

A wild guess: First it was the ridiculous amount of ripple on the 5V rail keeping the supervisor, and later the processor, in reset. Maybe connection through DMM (in low impedance mode?) smoothed out the supply voltage enough for the processor to start up, direct jump with resistor did not work. Then when the 5V rail was fixed, the reset timing was wrong. I guess the supervisor chip wasn't there just for "good engineering" after all..?

Ive seen these processors used in electronic oven controls, and some washer/dryers, and i can say on occasion that I have seen these processor spontaneously combust.

Hi David, I noticed on the scope trace that there's a triangle ripple voltage on the 5 volt supply, have you checked the rs232 chip as it may use a charge pump to drive the 232 line, maybe a faulty 232 driver chip or charge pump capacitor?

Dave, you said in the video that the reset trace doesn't go anywhere, but maybe it goes to a via under the chip. There could be another component pulling reset low. For example, what are the optocouplers for? If I were designing this thing, I might use an opto as a means for resetting the unit remotely.

Usually reset pins / circuits inside micros include MOSFETs in their innards, maybe they got FUBAR in this exercise?

I'm patting my back for calling the brown out reset when you thought the uC was dead. Seems it is afterall but at least that got it to boot

Does the chip have a mask ROM? Might be possible to replace it with a generic one if it doesn't?

I hope you pointed to this video when you invoiced the strata manager.

In a situation like this, since you cannot get a replacement, I would whack in a PIC programmed to wait a few mSec on power up and perform a reset on the main processor. You could keep it old school with a 747541 and an RC time constant to do the same thing. There's a million ways to fake a reset on a micro.

very cool video, thanks for sharing. Too bad it's getting more and more strange along the way.

Hey, a non-fix is just as informative as a fix. For example, I had no idea there were chips specifically designed to handle the reset state of a CPU.

Not having the rest of the boards plugged in be an issue?

Hello EEVBLOG. I think I had found a similarity in a research which I had done as student. I had study the aging of the sequentially digital circuits. So I had three set of two 8bit FF register, the one part of the set is made with discrete logic MSI chips and the second one was made with a PLD. I had study the starting condition in both of this device. By had the reset off, and switch on the power for some time, store the output value of this two devices on a logger, and repeat the process. I did that for about 1 month. The result was the following. Initially both of this is start with random values, and the study of this values is give a rectangular distribution in both of that devices. But after of some days the initial values start to take more some values than others, the distribution of the random initial values goes to be Gaussian. First, that happened to PLD, and later to MSI chip (remaining, both CMOS technology chips, temperature of the both device was constant 25°C and also isolated from the outer world, actually I had it on a oven, the distribution analyses had be done with than sampler of one day). So, as a conclusion every sequential circuit had a unique tuning destination (I think you understand what I mean). And this one is actually so unique where is very difficult to make any approximation on that. Even, device with close parts numbers had totally different result. So that one my Story about my study on the aging silicon chips. For your problem with the door-telephone, you can try to apply a sequence of pulses on the reset pin of the μC. Probably it's will make it to wake up from the latching statement of the internal Software reset circuit. And some words for this BLOG. It's the first time where write to your blog, but it's one of the most beautiful blogs on the concept of electronics. Always you try to make the things Easy, and this is good, very good. One of the thing where I learn on school of the life is: "The most brilliant people on the world, was brilliant not because they know many complex things, but because they make this complex things so easy where even a 10 years old child can understand it". This I can say it with one word as "Μεταδοτικότης" or "transmissibility". As you say "Thumbs up".

EEVblog btw. I thin the marking is bv3223 which curiously leads in google search ("bv3223 datasheet") to an as1917 supervisor ic ...

Does the DIP switch go to an I/O pin? :DDDDDDDD

Hey Dave, why didn't you use a thermal camera? If the device takes more power than its suppose to, the defective component gets hot. If its the processor, you're right.

I was thinking maybe the VCC of the CPU was high impedance and delivered too little power. When you shorted Reset to VCC, the CPU probably drew it's power from the Reset pin.

As someone who doesn't know nearly enough about this stuff to provide practical advice, (though your vides are amazing and I do learn from them some.) as the 2 LED's seem to be sporatic in behavior, I'd check out the circuits they are involved in, a component attached to them may be somewhat sickly, if not dead. Least that's what I can come up with.

Dude, couldn't the "random" power-on behaviour be related to the missing reset chip? After all, it's supposed to take the CPU outta reset only when it's supply is stable.

As AIPHONE specialist GF-NS is old model GH-NS newer model and the last virsion is GT-NS but unfortunately it's totally not compatible but I think if you ask AIPHONE agent request PCB XC-884A it may be available at factory

flat i once lived at had one that didnt filter dtmf from the mic port. major fail

You left the DIP switch on! -- Nah, just kidding. IO pin and all that. That board is becoming a bit of a mess with all your mods running around! You're a trooper for taking it this far, good show Dave.

I have a bit of a silly question... I thought that it was never okay to measure resistors in-circuit, so how is he able to do that? Is it only not okay when they're in parallel with other stuff?

This means that the LCD Display is initialized. Power up the LCD the first Line is not visible but after the Command Power ON this will displayed. The Cursor is not at position 1 - left upper first line. Measure the EEPROM first that looks like a death counter. If you measure a Communication on the EEPROM first after Reset and then show nothing on the LCD you could be sure it depend on the information in the EE. If you have a younger System like this you can copy the information from the other 2nd System to this EEPROM and you'll see that is working. I think so I have a problem like this in a other System the System was from a Time Stamp Company.

At 9:11 solder joint looks cold broken?

Hy just a quick question what camera do you use ?

There looked like cracked solder joint on back of that board early on in the video.

Do you still have those magnetic current sense probes?

but on 18.30 there is some life in it as you power it up, the led start blinking on the keypad, so some life is still in the processor

Dave, an idea: Well first, great job troubleshooting, a very logic approach and you found the problem area very quickly. Looking at the data sheet, I suspect other pins in the vicinity of the reset pin may be affected. Check the voltage level on the CNVss pin and the BYTE pin, both two pins over from RESET. They need to be at a particular logic level for proper chip operation. The CNVss trace runs suspiciously close to the RESET pin trace. I don't know why CNVss and BYTE are connected through a 10k, but that means there must be a trace hidden somewhere that is connecting those to Vcc or Vss. Perhaps a dodgy trace under the uC or a defect in an inner layer is affecting that entire board area. Good luck! You better get some free rent out of the bastards if you fix their keypad. :)

Try resoldering the reset chip back down, not using the suspect trace

Just wanted to say about the reset dip switch but found out that a tone of people allready done that

This might be a newbie observation (still very new to all this) but you mentioned that the tenants names are possibly stored in the EPROM, could it have reached its maximum read/write quota and crapped out?

But what about that jumper number 1. Shouldn't it be in the other position?

"excuse the crudity of my finger" I died laughing :D

Dave mate.. Where is that reset switch now? "to reset put switch 1 in ON position for 2 secconds" .. so.. reset line latched and won't fall unless shorted to ground?.. Where is the switch? .. in ON position !!

Great video

Did you upgrade the DS1054 to 100MHz? I'll get my DS1054 next week and I want to know if it's safe to do that.

No sound coming from my intercom handset, what do you think the problem be?