Unknown Part Identification - Help Needed

[ednspace]

I am attempting to repair an older piece of equipment and have found what looks like a damaged component. There are two identical sections on this circuit board and each have this component. The component designators on the board start with R and all other normal looking resistors on the board also start with R.

The one I think is good measures 25K and the one I think is damaged measures around 5K. They are light yellow discs and measure approximately one inch across and are very thin, around .065" They each have two legs. The good one appears to have the marking 40/3 and appears to be newer and to have been changed before. Both were wrapped in a heavy piece of heatshrink and dated 91/01/07 with a marker.

If I heat the good one up the resistance drops but not dramatically 25K to around 16K when too hot to touch. These components are part of an output power transistor circuit and they each have a diode in a TO16 package bent over and siliconed down to the top of these discs. The discs were mounted flat against the PCB. I assume the diodes are some type of thermal protection and are also found siliconed down to some power resistors on the same board.

I would appreciate any help identifying and finding suitable replacements for these components,
thanks for your time,
ED

Could these be some older style NTC inrush limiters or what???
The bright one with the visible markings measures 25K
The darker one that appears damaged measures 5K

[ArthurDent]

It looks and sounds like an MOV, metal oxide varistor. used for overvoltage protection. The leads are soft soldered onto the metal plating on each side of the disc and when they overheat, you may see the beads of solder ooze out near the leads and the coating discolors or burns off. The numbers may be a voltage rating but unless you know the circuit they are used in, you can't figure out what you need as a replacement. Here's a photo of one.

[ednspace]

Thanks for the feedback.
I think a MOV is likely. I wonder if there is a way I could test the good one.
It might also be possible to trace out a small section if this circuit to see what's going on.
What would the numbers 40/3 mean if I assume that it is a MOV?

The legs soldered on to a metal disc and oozing out when overheated statement appears to be dead on.

[TimNJ]

I'm not sure what it is, but I'm having a hard time believing its an MOV. An MOV should be effectively open circuit, if it's good. I would imagine 5-10Megohm DC resistance Also, seems unlikely than an MOV would be used in an output circuit.

Another thought I had was that it's a 40nF capacitor, which would typically be marked "403". Maybe this manufacturer split the 3 off. But still, shoudn't be measuring 15KOhm on a ceramic disc capacitor, if it's good.

My third guess is a PPTC re-settable fuse. Maybe 40V blocking rating, 3A trip or hold current. But again, resistance measurement doesn't seem to make sense unless they are both damaged.

[SeanB]

Disk thermistors, probably used there to do temperature compensation, and along with the diode are designed to make the output stage stable with temperature, as the bias voltage on the output stage is going to change with temperature, so this does an equal change for the bias voltage. Probably a 25k one, and replaceable with another of similar size and temperature coefficient.

[ednspace]

I thought it might be a resettable fuse also but tried to pass some current through the 25K one with no luck (makes sense).

I also posted this question to the digikey forum as they have a section devoted to component identification. The response there seems to point to the answer SeanB gave. A temperature thermistor that in combination with the diode changes the bias voltage on the output stage. I have drawn part of the circuit so far but not enough to tell completely what is going on.

It definitely is not a MOV as TimNJ pointed out it should measure nearly infinite resistance until active.
I am willing to go with the idea that it is a thermistor but now the trouble becomes where to find a one inch diameter disc thermistor and matching tempco.

I really appreciate you all chiming in and helping me figure this out so far. If anyone else has a clue please feel free to add to the thread but for now, like I said above, I am going to assume its a thermistor.

Thanks again,
Eric

[TimNJ]

An interesting read on temperature compensation for amplifiers: https://www.vishay.com/docs/29090/ntctemp.pdf

If you really want to replace it, and you are sure you have at least one good one, you can try to characterize it. In theory, you can do it with just 2 resistance values at 2 different temperatures. This allows yo to find the B or Beta value of the NTC. You will need to bond a thermocouple to the NTC and heat it up with a heat gun or something similar.

https://www.ametherm.com/thermistor/ntc-thermistor-beta

[ednspace]

Thank you for those links TimNJ.
I will look into that. Closest I have found is a 100K that is very close to the same size. I am hoping I may able to change the resistance by putting another resistor in parallel but yeah, in my heart I know it will probably never be right

Still I'll check out the links and see if I can characterize it. At least I will have a better idea of the part I am looking for.
thanks again for helping me out with this,
Eric

[TimNJ]

Paralleling a resistor is probably not going to do quite what you want. I don't know for certain but I'm not really sure the size really matters too much in this application.

You are welcome. Good luck.