Autoranging DMM Probe Power

Citroënbender · 14-10-2021, 10:26 PM

What I want to do: Trace back circuits on a two-sided SMD board.

It’s all but impossible to follow them by eye, due to multi-way connectors soldered over parts of the circuits.

Is it safe to carefully probe back? I’ve looked at some of the semiconductor specs and they seem quite tolerant of boofheadry. Most of the lines I want to trace use an applied ground as a signal. Once I have a good clue on a single circuit, I only need to confirm by following back a couple of others.

slowsnake · 15-10-2021, 08:10 AM

Isn't there anything you do that is basic and easy?
What would be near impossible for most folk to achieve, well to you its like a weeks holiday on the Gold Coast!....lol.....

PS-..I read the post below, but, " what does it all mean "..lol..hahaha

Citroënbender · 15-10-2021, 08:41 AM

This is potentially the cheapest repair, that’s why I’m keen to have a crack. As I don’t do TV, it’s a good evening pursuit. Being shortsighted I can almost put my nose on the circuits and with aid of an LED torch, read the markings on most components by eye.

If I can pinpoint a faulty piece, then it’s serviceable. Most of the ICs on that board are under $4 each. A donor set comprised of transponder chip, cluster, body computer and engine management would probably be $300-$350.

J Arthur Rank · 15-10-2021, 09:51 AM

CB my good fellow, I reckon you'd have more luck asking this at the eevblog forums. Heaps and heaps of obscenely talented folks there.

ford71V8 · 15-10-2021, 10:38 AM

I see no reason why you cant follow the traces around the board.
Top to bottom layer and back again is done by little via holes. Bit of scraping back the soldermask to bare copper in areas you want to probe with a scalpel and after you can recoat with a pen / conformal coating. Can get ugly if the PCB is 4+ layer though.
No tell tale signs around the board / components?

Citroënbender · 15-10-2021, 10:45 AM

Unfortunately, no smoking gun. There was a little oxidisation haze from water drips, but it came off with careful dry brushing.

I had a quick look at the referred forum, it may be an option.

My problem is somewhere in the “middle”. I can drive the faulty circuits with actuator tests, and the inputs to the board are all correct. A number of the input status lines are being misread, and that’s the main issue.

ford71V8 · 15-10-2021, 10:59 AM

If its an older board (which it looks like from the photos in other thread) you might have some dry joints. Hit anything that looks a little dull with some flux and the iron. Maybe a cracked joint at one of the through hole / leg components? They can look good until you zoom in and find a concave blob of solder not wetting to the leg of a connector or a regulator.

J Arthur Rank · 15-10-2021, 11:54 AM

What is the board you are mucking around on mate?

**aussiblue** · 15-10-2021, 12:10 PM

What SMD board is it exactly? If it's common one someone may have already mapped it .

Citroënbender · 15-10-2021, 12:21 PM

It’s the BEM out of an ‘01 Alfa. Double layered, double sided, multi-faulted.

J Arthur Rank · 15-10-2021, 12:37 PM

Do you have the gear for reflowing/resoldering? As 71v8 suggests, it could be dry joints but I imagine you will go mad trying to redo smd stuff with a tip iron.

There are a lot of international users/watchers of Dave's forum and channel so give it a crack - maybe someone there has the same car.

**aussiblue** · 15-10-2021, 08:34 PM

This: https://www.youtube.com/watch?v=X2Y10clvbQc ?

But if it's an Alpha these sort of electrical issues are considered a feature. But tracking the circuit with a DMM should be safe,

Citroënbender · 16-10-2021, 09:37 PM

Yes AB, watched that video - his issues were similar to mine. Not a “car guy” so much as a small electronics tech.

Yesterday I traced back from the outputs to try identifying what ICs drive what - in case there was a single or pair of items at fault. Not so!

I have a second set of everything, will put that in tomorrow morning. Hopeful of a result; will get the immobiliser chip cloned if successful.

slowsnake · 16-10-2021, 09:54 PM

You fellows are very clever!

Citroënbender · 16-10-2021, 10:07 PM

For me it’s about striving for champagne on a beer budget, or distributing the same net expenditure differently.

Workshop fees would be $160 test (to ultimately condemn), $550 warranted used electronics/locks set with one flip key, $220-$400 labour and clear codes. If my budget is similar, say $50-$200 goes to that repair and the rest gets me a new timing belt kit, oil/filter, water pump and brake hoses.

**Captain Stubing** · 16-10-2021, 10:13 PM

As long as your negative probe is connected to a good earth, the positive probe is just taking readings at different points. You just need to take care not to short out close by pins… just in case!
I imagine they still space out the positive and negative pins to avoid this.

Citroënbender · 16-10-2021, 11:15 PM

I have to honestly say, on an enjoyment scale, I’d rather scrub the shower. But it’s preferable to digging post-holes in 35° heat with a crowbar and shovel.

zilo · 17-10-2021, 02:15 AM

As long as you take static electricity precautions you should be ok. Working on a concrete floor is a good technique.

A typical DMM has an impedance in excess of 10 megaohm so injected current is bugger all. A lot of modern units have continuity test settings with an audible tone correspinding to the resistance which speeds things up a bit.