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I'm repairing a receiver from the 1990s that has a few burn marks on the PCB where a few transistors overheated. After replacing them and any other offending circuitry I'd like to be able to know if the issue has been resolved -- i.e. I'd like to know if the PCB burns again.

I don't have access to a thermal camera and I don't feel like using the back of my fingers to test for overheating. Is there some way to cover up or clean the PCB so that I'll retain this indication?

PCB burn

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  • \$\begingroup\$ I'd check if those transistors are the problem first (especially check for dry joints in the area). PCB discoloration due to components running hot is more common then you'd might think but it does not have to be the source of a defect. \$\endgroup\$ – Unimportant Feb 14 at 15:18
  • \$\begingroup\$ @Unimportant Thanks for the tip! I should open up more working receivers to get a baseline instead of all these broken ones. \$\endgroup\$ – calcium3000 Feb 14 at 15:41
  • \$\begingroup\$ That's paper-based phenolic PCB material so very tiny (because of flammability concerns) pieces of paper should suffice. \$\endgroup\$ – Spehro Pefhany Feb 14 at 15:53
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    \$\begingroup\$ If it heats up immediately, covering the board in freeze-spray or just put it in the freezer to cover it in frost and observe what melts first when you power it up is a cheap but effective method too. \$\endgroup\$ – winny Feb 15 at 12:40
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Non-reversible temperature-sensitive labels are available which will change colour irreversibly if they experience a defined temperature. These for example are 14 mm diameter which should be small enough to attach near one of your transistors, or even on the transistor itself.

Another option could be to slip a small piece of heat-shrinkable tubing over each transistor, maybe using a dot of cyanoacrylate glue to hold it in place. Standard heatshrink typically shrinks at somewhere between 70 and 120 °C, which is probably the range you would consider as overheating. If you observe that the tubing has begun to shrink, you know it's seen a temperature somewhere in this range. Note that anything that impedes airflow over the transistors could cause them to run even hotter, though.

Finally if you're concerned that these transistors run hotter than they should, you could fit them with clip-on heatsinks anyway as a precautionary measure. This will increase the stress on the solder joints and PCB tracks though, in case the equipment is likely to experience mechanical shock or vibration.

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    \$\begingroup\$ +1. The heat-shrink tubing is a neat idea for detecting overheating, and one I had never considered. \$\endgroup\$ – Spencer D Feb 15 at 3:41
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    \$\begingroup\$ You can even put a piece of heat-shrink between the legs, in it heats up, it gets smaller, so you should be able to detect it if you measure it \$\endgroup\$ – Ferrybig Feb 15 at 13:29
  • \$\begingroup\$ The heat shrink tube over the transistor will help the transistor to overheat faster and earlier by thermally isolating it. \$\endgroup\$ – Uwe Feb 16 at 21:11
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You could use a blob of wax on the components that you suspect are getting hot to see if it deforms due to the high temperature. Waxes come in many different melting points. For example, beeswax melts at about 62 °C.

If something is getting really hot, you could use hot melt glue, which melts at about 110 °C.

As user71659 pointed out in the comments, calibrated melting temperature crayons are available in temperatures from 40 °C and higher. See here and below.

Enter image description here

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Use a sharpie and draw some arrows, or circle the affected areas. Then use a thermal sensor from Home Depot to check temperatures. I have one like this, I think made by Ryobi, is great for all kinds of measurements around the house (Found with search for "infrared thermometer"):

https://www.homedepot.com/s/infrared%2520thermometer?NCNI-5

Might even be able to get a phone app that shows hot spots. Google "phone infrared camera app" I think most are an infrared camera that displays on your phone, I don't how good they'd be for smaller area views. The $35 infrared gun with laser pointer I know works well. Better than burning a finger! (done that too ...)

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  • \$\begingroup\$ Ah, I'll check those out. Probably good for a quick home energy audit, too, eh? Re: burning finger - not to mention poking around 60 VDC or better! \$\endgroup\$ – calcium3000 Feb 14 at 16:07
  • \$\begingroup\$ The infrared camera method is probably better for larger areas like around, doors, windows, etc. Many states even offer free audits, paid for as part of your monthly bill. In my state, they'll come around and seal any holes in ceiling, wrap the water tank, gave us inserts to go behind wall switch/outlet covers, faucet aerators, checked gas furnace condition, discounts on programmable thermostats (or replaced them, I don't recall which), replaced door seal around a couple of outer doors, and other energy audit stuff (recommendations on additional ceiling insulation, etc). MASSsave.com for us \$\endgroup\$ – CrossRoads Feb 14 at 16:18
  • \$\begingroup\$ Those "gun thermometers" are a FANTASTIC thing to invest in - and very cheap. They are useful all over the place, all the time. \$\endgroup\$ – Fattie Feb 14 at 18:08
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Most error locators use alcohol to find the error. If the alcohol vaporises then there is a hotspot. Alcohol does not conduct electricity, and it cools down the components.

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