Multilayer board, track tracing?

Any tips for single ended signal injection and tracing for high resolution tracing , floppy/HD head as searchcoil ?

Reply to
N_Cook
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I would have thought that any tracks which passed internally between power and ground planes, would have struggled to radiate enough signal to be picked up in the way that you are envisioning. Also, where there are fine-pitch 'bunches' of tracks which run parallel to one another for any distance, I would think that crosstalk might become a problem to this technique. If such a board has any (internal) tracking problems, I don't think that you are going to do a lot with it anyway. Even if it is just generally faulty in some way, if the board is complex enough to require multi-layering, I think you are going to be pissing in the wind attempting a (commercially viable) repair without the benefit of at least schematics ...

Arfa

Reply to
Arfa Daily

I would second that. I doubt the time and effort spent in attemptng this is time and effort well spent. You'd be better off going for a nice walk in the countryside instead, after telling your customer this is not a viable repair. Which you or he wll end up doing anyway.

Gareth.

Reply to
Gareth Magennis

On 5/19/2009 7:42 AM Gareth Magennis spake thus:

I third that. Much as I commend the spirit of trying to fix everything yourself that is possible, some things just aren't made to be fixed. Multilayer circuit boards seem to be one of those things.

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Reply to
David Nebenzahl

You might be lucky and only power and ground are in the inner layers along with a jumper or two. A bright light is about the best you can do. If it's more than 3 or 4 layers you will need schematic in front of you.

I fourth it, that it better be worth your effort with a contract to do x number of like units unless you are independently wealthy and like profitless challenges. Alternatively, you could redesign the board as a retrofit upgrade for the whole product line.

Reply to
JB

It was worth an enquiry. Perhaps someone had hit on a wondrous new technique, i was not aware of. eg > 1MHz ultrasonic source touching the exposed metal of a trace and ultrasonic sniffer picking up the buried path. Polyester and glass damping the propogation sideways, but free passage along the metal trace. Anyone got a few tens of thousands to investigate ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

On Tue, 19 May 2009 08:15:46 +0100, "N_Cook" put finger to keyboard and composed:

I've repaired a lot of multilayer PCBs, but I've always had circuit diagrams or a knowledge of the board.

What about a VCR head attached to a ballpoint pen tube? I would think that as you rotated the pen, the max/min signal amplitudes would indicate the direction of current flow.

You could also experiment with ribbon cable. That would give you an indication of sensitivity to crosstalk, and the insulation thickness could simulate a layer of PCB.

- Franc Zabkar

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Reply to
Franc Zabkar

Yep. Just yesterday I rejected a big ol' Pioneer amp with every output transistor short on both channels, and most of the emitter Rs open. The mackled-up box that it came in from the customer, suggested that they might have obtained it from uPay or some such. With a sigh, I put its covers back on, and explained to the shop which took it in, that it was going to be more trouble than it was worth. They told me that they 'knew' this was going to be the case, but took it in anyway ...

I then went out for that walk in the country that Gareth suggested ...

A few years back, I would have fixed it come what may. Now, it seems that my time is more importantly used on jobs that actually make me some money.

Arfa

Reply to
Arfa Daily

It's a thought Franc, but you would need either a damned strong signal source (see my earlier thoughts about crosstalk in prallel track bunches), or a very high gain and very low noise amp on the end of the video head. I say this because from what I've seen of video heads over the years, they have very few wire turns on them, relying for their output on the fact that they are rotating at very high speed relative to the tape, and in extremely tight physical contact with it.

Wouldn't it be easier in the end, to just use your DMM on its lowest ohms range with a continuity beeper, to hook onto one end of a track where you do know that it finds its way to the top or bottom surface, then go hunting for the next place where it appears by initially listening for the beep, and then checking for the near-zero ohms reading that you will be getting, if you have found a direct connection ?

Either way, a very tedious process, and definitely not commercially viable unless, as someone else commented, it is an expensive board for which there is no service info available, and that someone wants to contract you to mend a large number of.

Arfa

Reply to
Arfa Daily

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You have my sympathies. After replacing $100 worth of high power transistors or hybrids, it doesn't leave much for labor. You would be better off fixing cars or washing machines.

It has been my experience with consumer equipment, that if you can fix it, the customer will either be poor and can't afford it, or will be genuinely disappointed because they really wanted the newest, latest gimmick. If you fix it, they will never return to claim it. Or in one case, brought the whole family over and while I was distracted, one snuck out with TV and paperwork and all bolted on queue without paying.

Always leave the unit disassembled or at least with the back off with wiring and circuits exposed and settle the invoice before putting it back together. Thieves don't want to get electrocuted.

Reply to
JB

On Wed, 20 May 2009 09:32:14 +0100, "Arfa Daily" put finger to keyboard and composed:

The 15" x 15" multilayer boards I used to repair typically had 300 TTL chips.

ISTR that someone once suggested to place a sheet of aluminium foil over one half of the board to test for continuity and then narrow down the precise location with successive binary searches. Alternatively you could make a chip-sized metal pad and probe an entire chip at a time.

I agree that looking for a small signal may be fruitless. My first thought was to adapt the method used in some digitizing tablets, but these use a relatively high current pulse.

As you suggest, I do all my reverse engineering with a DMM on the low ohms range. I might try the Al foil method next time, though.

- Franc Zabkar

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Reply to
Franc Zabkar

It never ceases to amaze me what antics customers will get up to. I generally keep myself 'decoupled' from the actual equipment owners by offering a trade repair service only. You don't make quite so much money as you would by charging a customer direct, but at least you don't get the hassle of having to get money out of them, or even get them to come back to collect their pieces of s**te.

Most of the places that I do work for, take a deposit which at least covers the basic service charge that they will have to pay to me, come what may. That ensures that neither they, nor I, are ever out of pocket on items like the Pioneer. Of course, for various reasons, we both occasionally 'take a hit' when one or other of us makes a bad call on ordering some parts, or placing unfounded trust in a customer, but overall, myself and all of the stores that I am doing work for - mostly family owned businesses well known in the area - have been in the game long enough to be able to make valid judgements 98% of the time.

Arfa

Reply to
Arfa Daily

JB wrote: [...]

I fourth it. I don't bother with repairs that're likely to need that kind of time & effort. You'll invariably lose money on such jobs.

Ouch! That's a new one on me. My own worst story was the time a pissed off customer pulled a knife on one of our counter staff when he came to pick up his 'warranty' repair laptop & was told that we were going to charge him for what had turned out to be a software problem.[0] I called the cops, then went to the counter to rescue the counter girl & try to calm him down until the cops got there. And no, he didn't get away without paying. ;^)

Good tip. We used to keep finished jobs on shelves about 5' behind the service counter, so people couldn't just grab & run. That obviously wouldn't be very practical with TVs, of course.

[0] When the guy first booked the machine in for warranty service, I'd warned him that the problem would almost certainly be software, & that he'd be charged if it was.
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Reply to
Bob Larter

I've actually done that, combined with putting the PCB over a strong light to make it easier to trace the tracks visually. It's a pain in the arse, but it's not totally unfeasible. The one problem is that if you're tracing a digital circuit, you're likely to get lots of beeps on power pins you hit, due to all the ESD protection diodes in the chips.

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Reply to
Bob Larter

You might want to ask your dentist, vet or clinic to try an x-ray picture of the board. If he succeeds to achieve about 30 gray-steps per layer you might be able to identify the traces with a graphic software after reading the x-ray-pic with a scanner. Traces passing under a chip will be a problem, but having done all others you could do these with conductivity checks.

Regards, H.

Reply to
Heinz Schmitz

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