How do YOU troubleshoot a circuit without a schematic?

[777funk]

I've been a DIY electronics fixer since I was a little kid. I'm getting better at it as I get older. But I'm still learning. I find it super useful in my daily occupation (building electric guitars) in keeping machines running and I also find it useful in keeping up vehicles. Anyways, enough yapping about all that. To the point of the thread. When something breaks and I have no schematic, after checking the fuse(s) I start with the basics:
1. Look for burnt or exploded parts.
2. Check for shorted transistors using a diode check on an ohm meter.
3. Look for loose/bad connections.

EDIT:
and if it's something I have a duplicate of (i.e. a Stereo channel amplifier, or a multi-axis CNC amp board where I have a known good example to check from) I compare ohm readings to ground at various points.

I'm curious how other people tackle the no schematic repairs.

 

[KeepItSimpleStupid]

No particular order.

Inspect is always rule #1. A friend found a sandwich or a nest of roaches in two different VCR's that his shop took in for repair.

There is always a rule which states, check the supply voltages. Use identifyable IC's to find possible pins.

Smell. Feel with fingers or temperature probe for high temperatures.

Reseat connectors.

Make sure that there is power. I remember when I was doing troubleshooting over the phone for a remote CRT terminal and I asked "Is it plugged in"? and I got a no.

I've been burnt by the "intermittant fuse" twice, so the ohmmeter test only detects a failed fuse. It does not detect a broken one 100% of the time.

CMOS devices always get the power supply shorted with power off. Fixed a few devices this way.

 

[ljcox]

CMOS devices always get the power supply shorted with power off. Fixed a few devices this way. Good advice, but I did not understand this one.

You can also look for dry joints and cracks in the PCB tracks.

For example, I once found an intermittent fault in my TV.

It was due to a hair line crack in the solder around a wire.

 

[KeepItSimpleStupid]

CMOS IC's can be non-destructively damaged by static discharges. The charge gets trapped and alters the functionality of the device permanently e.g. a bit stuck on. It may take a while to drain the charge away naturally and shorting discharges it faster. So, potentially the thing that was broken and you took the batteries out, but didn't throw it away might work after a year sitting.

I repaired an HP calculator that was stuck in commas for the decimal mode, a bicycle computer that was dragged across a van carpet and the vacuum florescent clock in one of my cars this way. The clock died after the car was jumped.

 

[ljcox]

CMOS IC's can be non-destructively damaged by static discharges. The charge gets trapped and alters the functionality of the device permanently e.g. a bit stuck on. It may take a while to drain the charge away naturally and shorting discharges it faster. So, potentially the thing that was broken and you took the batteries out, but didn't throw it away might work after a year sitting.

I repaired an HP calculator that was stuck in commas for the decimal mode, a bicycle computer that was dragged across a van carpet and the vacuum florescent clock in one of my cars this way. The clock died after the car was jumped.

Thanks, I didn't know that.

 

[ghostman11]

i wonder how much stuff has been thrown away because of things like that!! a very very useful tip to know thanks guys

 

[Mikebits]

I have learned over the years that usually without a schematic, repair attempts are often futile. Aside from a good visual inspection there is not much you can do. Often though, there are symptoms which bring light to this dark troubleshooting conundrum. Observe the symptoms; often the failure mode is after a device warms up, in such cases a can of freeze spray and a hot dryer can locate the problem. Other examples can be a unit begins to cycle up but as power hungry parts power up the unit shuts off, this could indicate a supply problem and help you localize the fault. But alas, in most cases, your best bet is to get the schematic...

 

[BrownOut]

I've been able to hold the circuit board up to the light and see buried tracks well enough to make my own schematic. I would only attempt to do that for very simple circuit boards however. Last one I tried was a simple switching regulator.

 

[Nigel Goodwin]

I've drawn out many circuits over the years, particularly amplifiers and even mixers.

Generally you get to have a good 'feel' for what goes wrong, there's a LOT of experience involved in doing repairs.

 

[ben7]

I've been able to hold the circuit board up to the light and see buried tracks well enough to make my own schematic. I would only attempt to do that for very simple circuit boards however. Last one I tried was a simple switching regulator.

Yes and depending on the circuit you can follow a signal path-such as in an amplifier-to eventually find the fault. If the signal goes in the pre-amp, and doesn't come out then you know its faulty.

-Ben

 

[BrownOut]

Or just ask the chipmonk where the fault is.

 

[ben7]

Or just ask the chipmonk where the fault is.

One of my neighbors house is infested with squirrels, she asked me if I could repair her non-functional electrical outlets, but I said no.
I am still waiting for the catastrophe to come! :/

-Ben

-

 

[ElectroMaster]

Great tips! I've actually quoted a few for the the Repair Tips sticky in the Repairing Electronics forum.

 

[gary350]

Get a parts book or do Google search. Look up the number of each part it will tell you the voltage at each terminal. Test each part with your volt meter. Everything is checking good then your find a part that tests wrong voltage you found the problem. Next question, did the part before this or after this part go bad and short or open the other part. Check the parts around the bad part chances are you will find a chain reaction killed a couple more parts but maybe not it just depends why the bad part died in the first place and what is does in the circuit.