Having spent a lifetime working in electronics, 22 years of which owning a service repair center and development company, I thought I'd seen it all. But you can learn something new at anytime.
Around six years ago, I purchased a Manhattan satellite recording receiver. Within a couple of days, I kept getting a msg on screen saying that there was no signal from the receiver, and no osd either. I returned it to the retailer, and the only other one they had was in their display, but I was happy for them to change it for that one, keeping the original packing and accessories.
It all worked fine until a fortnight ago when I started to get the same msg again after it had been on for a while. I left it off for 15 mins and tried again, it ran for another 30-60 mins then cut out.
I was pretty sure it was a thermal fault, probably in the power supply as there was nothing onscreen. So, off with the case, and out with the freezer spray! Trying first around the power supply, then other areas, it would come back to life randomly, with no conclusive area being sensitive.
I tried several times warming with a hair drier to make it go off, but it was only when the case was on that it would go into fault condition.
At this point, I took it into my workshop and examined it under a big illuminated magnifying glass looking for any poor solder joints etc. but they were perfect.
I put the case back on, and re-connected the T.v., as I have a dvd recorder, I had to reconnect this as well. Then my brain cell (the one that still works) flashed on!! I hadn't checked the HDMI lead ! I wired it up with the original cable, turned it on, and 40 mins later, off it went. I quickly substituted the dvd recorder hdmi lead and I had a perfect picture that stayed on for the rest of the evening.
Close examination of the original lead revealed nothing, moulded plugs and no amount of flexing the lead caused any fault, however, warming it with the hair drier, and off went the signal. Obviously a bad internal connection, and this was the lead that had been supplied with the first receiver, I now wonder if that is all that was wrong in the first place!
The rear panel does get quite warm and was transferring enough heat into the plug to cause the fault. Amazing how it worked ok for nearly six years.
Back in the 70's and 80's it was quite common to have all sorts of problems with cheap Scart leads, and I always carried a spare when going to sort out a customers video recorder/tv.
I REALLY should have known better and tried a substitute hdmi cable before diving in at the deep end. Silly me!

 

Just proves once again that the problem is usually the last thing you check.

 

Just proves once again that the problem is usually the last thing you check.

Yes but it should have been one of the first

 

High temperature and time does wonders (irascible at times) to solder.

I once had to diagnose and locate a short between Vcc and GND on a DEC PDP-15 mini-computer. As you can imagine, there are thousands of components wired across Vcc and GND. This one happened to be a short circuit that was out of sight. It was underneath a DIP IC package. All the electronics are mounted on DEC FLIP-CHIP cards that are installed vertically in card racks. There was a blob of solder underneath the DIP IC package that over time (20 years, i.e. the circuit work flawlessly for 20 years) gradually flowed until it shorted out the power rails.

To locate a short across the power rails I used an HP current pulser and current probe. The HP kit is worth its weight in gold.

 

I had a similar type of problem with a wire-wrapped card that was connected to a DEC PDP 11/2 system. It ran the color display. The color was necessary to see the value.

As you said, it works flawlessly for may years. Probably 8 or more. Then it developed where a particular color would not be displayed.

The commercial product was wire-wrapped except for power and ground. These were soldered EXCEPT on one IC. That one was missed.

 

"...HP current pulser and current probe."

That was indeed a fine pair of little instruments - possibly the best instrument use of an incandescent lamp since Messrs. H & P built their very first instrument.
I haven't looked for them, but I've never noticed any knock-offs on the hobby market, which I find surprising.

I had a similar problem of a short under a DIP, diagnosed with the HP probes. It may have been on a DECwriter.

One of the worst short-ish problems I ever had to run down turned out to be a solder ball making a stable but moderate-resistance connection between a data line and an address line in a video terminal. The terminal worked to some extent - I don't recall the details. Stupid solder balls! (in wave-soldered boards, vias can sometimes "spit" solder balls onto the top of the board - usually they come off in the wash, but not always)

With multilayer boards finding (and maybe fixing) a short in power distribution is horrendous.

 

I had a short issue with something. It may have been an SBC. I used something called Short Squeek or a tone ohmmeter to find it.

 

Try finding a short across a power rail decoupling capacitor.
The problem is where and what is causing the short?

 

To locate a short across the power rails I used an HP current pulser and current probe. The HP kit is worth its weight in gold.

I used to have two of the current probes.

It is such a one-of-a-kind sort of tool, I even made my own using a SMD inductor as the sensor.

 

If you can still find a pair to buy it would be worth having.

HP546A Logic Pulser
HP547A Current Tracer

 

I once had a leaky ceramic decoupling capacitor [!!?] in a battery backup for some low-power thing. I think it was a CMOS RAM for config. data. That was fun to find. It was the only time I ever encountered a leaky ceramic cap.

 

Yes but it should have been one of the first

If it was the first thing you checked then it would most likely have also been the last. Otherwise why could you go on checking?

 

I remember one time that the current probe worked especially well.

I had a logic signal that was shorted. I got out the probe and was following the trace that was shorted. Bonk. Bonk???

The probe had hit a capacitor lead. It happened that the way the PCB was laid out there were 3 holes near each other -- two for a bypass capacitor and one for a via on the logic signal trace.

The board had the bypass cap accidentally installed into the via. The spacing was correct for the cap to fit without distorting the leads.

 

Nice description of the HP design: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1976-12.pdf

 

If it was the first thing you checked then it would most likely have also been the last. Otherwise whay could you go on checking?

So true

 

Hello,

Years ago, we had troubles with a computer of an NMR (AC300).
We changed boards, many times, during many visits.(there are many separate boars for all kinds of functions).
The final solution was replacing the power connections on the bottom of the computer, as the 5 Volts was failing due to the bad connector.
Of course the computer was located at the other end of the country.

Bertus

 

Hello,

Years ago, we had troubles with a computer of an NMR (AC300).
We changed boards, many times, during many visits.(there are many separate boars for all kinds of functions).
The final solution was replacing the power connections on the bottom of the computer, as the 5 Volts was failing due to the bad connector.
Of course the computer was located at the other end of the country.

Bertus

Oh of course it had to be miles away, would be no fun if it was next door !

 

Nice description of the HP design: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1976-12.pdf

And here is the manual and an article on HP products for digital troubleshooting.


edit: added forgotten file.