troubleshooting method for micro-controller board ?

Well .... if the 5V supply went seriously high, the chances are it's all fried. :-(

Have you tried powering the pcb from an external stabilised 5V power supply ?

The very first thing to look for is a clock oscillator (and valid reset signal). Get the data sheets for those 40 pin chips, identify which one is the micro and scope the relevant pin(s).

What are the 40 pin chip numbers btw ?

Graham

it's all fried.

there is a 630 mA Busman type fuses on all the lines i hope that saved things ? but the fact that i am getting a short detected between 0V and 5V line has me worried maybe thats good i do not know hopeful someone can give me an method for finding problems ?

power supply ?

valid reset signal).

is the micro and

one is the Phillips MAB 8051 micro, then SRM 2017 Static RAM, and NEC 8547E7 (D23256) ??

Clearly there is one or more shorts somewhere on the board. Your mission therefore is to determine what is shorted, and where it is.

If the ICs are socketed, I'd probably pull them (making careful note of what was where, if it's not clearly labeled on the PCB). This avoids any (additional) damage to parts that may be quite hard to replace. From there, checking components in the on-board power supply section might be reasonable, as would checking the resistance between permutations of the various power supply rails and ground for shorts (preferably with something like an ESR meter that won't damage semiconductors, especially if the ICs can't be removed.) Shorts between rails could be caused by things like failed de-spiking capacitors. If everything looks OK with ICs removed, maybe power up the board without them and see what happens; if the power supply is stable and current draw reasonable, the problem would seem to be with one of the removed ICs, or with a circuit driven by one of them.

(It's not a bad idea to figure out what the output interface circuitry looks like that's driven by the ICs before powering it up without them; maybe you'd be wise to tie some of the outputs to a fixed level rather than letting them float.)

What's the purpose of this board? What sort of board-level I/Os does it have? Based on the atypical power supply voltages, I'm guessing it must drive or interface with some external devices; maybe those output circuits are damaged, or the things driven by them or the wiring to them.

Just some thoughts; hopefully they're helpful.

--
Andrew Erickson

"He is no fool who gives what he cannot keep to gain that which he cannot
lose."  -- Jim Elliot

First thing you should do is check for shorts across the various power rails. If a short is found, using a sufficient resolution low-ohms meter might let you located the offending component. Sometimes the bypass caps can go short on order gear like this, replace them all if needed.

Dave.

Or worse. I find many times they go short with no visible evidence at all.

I was pushing the boss for a thermal imaging camera for this very purpose, just apply power till things get warm enough in the right spots to tell what is shorted.

I thought I had put forward a very good argument for saving time during repairs that are relatively rare where we were, but he still sadly didn't go for it... Oh well, I tried.

--
Linux Registered User # 302622

Sometimes it's possible to isolate sections of the circuit fairly easily, a jumper link that can be removed, or in more drastic situations, a trace can be cut. It doesn't take too many cycles of that to narrow things down.

Keep in mind that 5V TTL boards can pull quite a bit of juice, on larger boards it can appear to be a nearly dead short when using an ohm meter between Vcc and Gnd.

Second that on the caps, I recently repaired a Volkswagen ECU that had a shorted electrolytic capacitor, I think that was the first time I'd seen a lytic shorted, regulator was getting hot which clued me in. I've found shorted tantalums in several things.

I'm afraid fuses are merely for preventing fires, not preventing damage (except for the case of 'extreme' fuse types like the ultra-fast types).

No, I'm afraid that's actually bad news. I assume therefore it literally 'wont do anything' at present and any power supply connected to it goes into current limit ?

OK. That's a real easy one to check AND replace (at least with a modern CMOS version). Data sheet here.

If it was working I'd check for activity (pulses/wvaeforms) on the Xtal1/2 pins, ALE and PSEN first.

OK. Not sure how easy that would be to replace. Data here for the pinouts..

These are 'commodity' parts that are easily replaced. Not a problem here. First you need to get the 'big' digital parts functioning.

General tip. Don't apply power to any high power I/O switching chips like the L298 until the digital stuff is back to normal again.

Here are some ideas. You say you have a short across the 5V logic supply. Firstly you need to clear that and find the cause. Are the chips socketed ? If so it's easy to remove them and check for individually shorted chips. Check across the power pins. Normally these are opposing corner pins like pin 20 and

40 on the 8051 for example ( or 7/14 8/16 and 10/20 on 74 series chips) but best to get data sheets for anything unfamiliar. If you're lucky you may find one (or two or threee) failed 'sacrificially' - saving the others but you need to find the failed chips and get replacements for them.

If they aren't socketed now is the time to do this ! Removing ICs from PCBs without damage is a skilled task so I would sacrifice anything that's easily replaceable by cutting the pins at the IC body with fine cutters and removing those pins one by one from the PCB. That way there's very little chance of doing much damage. Be careful to fit sockets that firmly grip the leads especially if this is some industrial controller used in areas where there's significant vibration. That means DON'T use turned pin sockets, the pins will 'walk out' of them.

How about you give is a complete list of the ICs on this pcb, plus make a scan of it and post it where we can see it ? That'll help enormously.

Graham

Currently the only diagnostic tools at my immediate disposal are

Fluke 177 DMM, a Tek 2236 oscope and a usb/PC based all purpose logic anal/spectrum anal/oscope

i am hopeing these will be sufficient to solve / track this fault(s) if low ohm meter is not terribly expensive then i'll try to get one if it helps more than my current tools

You're missing a hammer.

That might be a lot funnier if we weren't talking about 32v on a 5v line, but at this stage, a hammer certainly seems a viable tool.

Last time I had nearly 12v on a 5v line, (most of the board was 5v logic) I didn't waste time diagnosing. It was faster to change all the silicon and worry about shorted tantalums later. Checking the semis afterwards, I found nearly half of them were stuffed, and the other half I wouldn't have trusted anyway.

That was at nearly 12 volts. Not 32.

In your shoes, (depending on the scale of the baord and availability of replacement boards) I'd either outright replace it, or, if this is the only one left in existence I'd turf the silicon, and hope the EPROM or flash or wherever the code is stored still works.

--
Linux Registered User # 302622

[trimmed lots of helpfull advice]

responses to helpfull questions in separate post

plus make a scan

enormously.

hi Graham thanks again for the help,

i posted pics of the board on (alt.binaries.electronics.schematics)

there was no obvious visual evidence of any smoked parts that i could see , only the middle busman fuse was blown after the short

and that seemed to be related to power going to the L298

thanks for all the help, robb

Tough problem with all the ICs soldered in.

Chances are that the 32 volt supply is only used for the L298s, as you suggest. It also appears that you can unsolder these without damaging any other parts on the board. (Even if you have to cut them out, they're still available and easy to replace.)

If your short doesn't disappear after pulling the 298s, try unsoldering any of the caps near those chips.

Leave unsoldering any ICs as your very last resort.

I'm now bowing out of this discussion and leaving it to the experts.

Good luck, Robb.

Tom

Funny. That's where I'd START !

Graham

Look for date codes. If its only a couple years old, some smart EE might have actually put Transient Voltage Suppressors in some important places to guard against incompetent operators. Can anyone rattle off three TVS brand names from memory he can look for?

He probably means to avoid damaging them by poor soldering skills, but who knows.

are

purpose

get

well i purposefully left the hammers out because i did not realize their usefull ness in this situation.

i actually have quite array of hammers 5 oz. to 5 lb rubber, brass, nylon etc

i just hope the time comes that i do not need to use them.... on the man or the machine

on a 5v line,

no it is still funny, in nervous and tearful kind of laugh

was 5v logic) I

silicon and

uhggg what a project my small project has become

were stuffed, and

availability of

is the only

or flash or

i really hope i actually make it to those problems considering what i have read so far.

thanks for the help and ideas, robb

i

short

as you

damaging

out,

experts.

Thanks Tom, i really appreciate any comments and help

i unsoldered one of the L298 and it has no shorts so now the next then i will do a trace of the 5V lines until i can learn something usefull about the interconnections.

i hate working blind and re-tracing things when i could just look on a schemat.

so thank you , robb

I suggest you remove the 5V voltage regulator.. That'll remove the connection from the power supply to the digital section and NOW check for a short across the 5V and ground.

The regulator chip may have an internal short you see and it is at least easy to remove from the pcb.

Graham