I’m currently repairing a device and I’m trying to understand what this circuit board does and how, and whether it is causing the malfunction. While doing that I stumbled upon this resistor whose color code does not agree with my multimeter. I’m measuring 152.1Ω, but the way I’m reading the colors it should be the very common value of 69.1MΩ. If I reverse the order, I get 1.51Ω/15.1GΩ/151GΩ, depending on whether the second color from the left is silver, grey or white. Black would give me 151Ω, but it definitely is not black.
The device this circuit board is from is pretty old. I don’t know how old, but there is exactly one IC on it, with a datasheet published in April 1974. The relay in the background has 1979 written on it; I’m not sure if that is supposed to be a year.
Any ideas? Am I reading the colors wrong? Do I trust the multimeter or the markings regarding the intended value of this resistor? Have you seen resistors whose color codes have changed over decades of use?
Markings say 150 ohm (brown, green, brown, silver) with 10% tolerance (if that’s silver on band 4), so tolerance range would be up to 165 ohms and down to 135 ohms. Your reading of 152.1 is within tolerance. If the 4th is silver then the fifth is TCR (temperature coefficient resistance), blue = 10 ppm/⁰K.
Based on the colors and my complete lack of knowledge, that’s a Pride transistor.
Not necessarily the color code that changed. Even though resistors are resilient, they may fail and change their value. Given that the device is 50 years old, it might happen.
69 MOhm is not very likely in a larger housing. Resistors can shift in value but they rarely break without any visual damage.
I would first check for broken solder joints on parts that get warm and check if the relay is performing as intended.
Broken solder joints is a very good idea, thank you. I already found one earlier, but I haven’t resoldered them all yet.
The relay is not performing as intended. The circuit board is controlling an electrolysis reactor which works fine until apparently the pressure switch decides the H2/O2 pressure is high enough and it tries to turn the reactor off. At that point the relay goes brrr, switching on and off very fast, producing a lot of sparks, and it stops when the fuse blows. Deactivating the pressure switch “fixes” the problem, but that is dangerous in itself.
Sorry for blowing up your thread but as for repairing it, might not hurt to draw up a schematic based on the board and try to ascertain exactly what the resistor is doing and why it is in the range it is in.
If it were me, I would separate out the pressure signaling system and see on which side the anomaly occurs. If you provide a solid signal and it still oscillates, the problem is probably in the main board. On the other hand, if a solid signal produces a one-off effect, the issue is in the pressure sensing module. Something is causing it to oscillate.
Thanks for your tips :) I already finished drawing the schematic, this resistor was the last part I hadn’t clearly identified. Now I just need to understand it :D
If I had to guess, 151 with silver being the tolerance range and blue the temperature coefficient.
Is this a thing? Have you encountered resistors with 5 color bands without a multiplier?
Personally? No. Mainly because while I have had to ID and work with resistors at work a few times, I am a hobbyist, and I haven’t worked on anything older than the 90s.
Again, from an amateur perspective, the color codes we’re familiar with wasn’t globally adopted until 1969. We don’t know where or when this resistor was made. It even looks different from the others. Considering that the reading is very close to one of the interpretations you have from the coding, I would guess it’s non standard but close and probably within the intended spec. It could very well be just the manufacturer’s coding, not our standard. Again, grain of salt, etc.
But check this out (third heading down):
Color Code Exceptions
The color coded resistors are inserted in circuit boards, and the PCB assembly manufacturing is finished by thru-hole pin soldering.
Reliability band:Resistors that are produced according to military specifications, sometimes include an extra band to indicate reliability. This is specified in failure rate (%) per 1000 hours of service. This is rarely used in commercial electronics. Most often the reliability band can be found on four band resistors. More information about the reliability can be found in the US military handbook MIL-HDBK-199.
Single black band or zero-ohm resistor:A resistor with a single black band is called a zero-ohm resistor. Principally, it is used as a wire link that functions to connect traces on a printed circuit board (PCB). Using the resistor package allows the same automated pick-and-place machines to place the components on a circuit board.
5 band resistor with a 4th band of gold or silver:Five band resistors with a fourth band of gold or silver form an exception and are used on specialized and older resistors. The first two bands represent the significant digits, the 3rd is the multiplication factor, the 4th is the tolerance, and the 5th is the temperature coefficient (ppm/˚C).
Deviating colors:For high voltage resistors, the colors gold and silver are often replaced with yellow and gray. This is to prevent having metal particles in the coating.
You could test the other resistors to see if they match their ratings. The high value could suggest that it’s burnt to a near open circuit.
With all other resistors on the board, my measurements match the color code. This one is the only exception.
I don’t know what you mean with high value though, 151 is not really close to an open circuit :/
Shit I flip the values of the ohm meter and color value.
Well my second option is to replace the resister with a new color code matching resistor and see if it works.
A high value resistor should work
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multimeters come in handy
As stated I measured; the question is how and why the measurement is so far off from the encoded resistance, and why those values are so strange no matter how I read them.
oh, sorry for not reading through all the way
