I have an amp circuit board I'm trying to repair but the colors on a resistor seem bad and I can't match up the resistor. It seems like it has an orange band. I was just poking around trying to find visible problems and this resistor looked like it had been repaired (I am the only owner of this amp and it was new when I bought it). It had a very thin section and just snapped off at the stem. Anyway, it measures 146 ohms, doesn't look (otherwise) fried. Another resistor near it is 96 ohms and the colors are also pretty bad, but with it I can at least make out that it's a 100 ohm resistor.

Can you guess the colors? It looks like the tolerance band is silver or gray. (First two photos). Is it common for colors to be this bad?

I happen to have another of the same amp, so I opened it up and both of those resistors (third photo) are the same and the colors much more obvious, and they are 100 ohm (they both read 96 ohms).

What reason might the manufacturer repair the amp in the factory and up the resistor by 50 ohms?

 

Welcome to AAC!

Are you making these resistance measurements with one lead of the resistor removed from the circuit? If not, as a general rule, you can't measure resistances in circuit.

Trying to read color codes from photos is hit or miss. Tell us what you think the band colors are.

 

Those look to me like they metal oxide film power resistors. They may be a special category known as "flameproof fusible" which are designed to reliably go open circuit and not shoot flames in the event of gross overload. Metal oxide film is fairly common for power resistors in the 2 or 3 watt range, particularly in consumer equipment. They withstand short-term overload better than other film types and are cheaper and less inductive than wirewound resistors. The leads are often formed to make the resistors stand off of the circuit board surface. It is not uncommon to flatten the lead to do this, and I think that your resistor has just broken at the flat part.

I think it likely they are both 100 ohms. Note they have the same reference number with different suffixes.

The don't look to me like they have run very hot. They aren't badly discolored and there doesn't seem to be discoloration of the circuit board.

[EDIT - I got all confused. I'm old. Whadyah expect. - well actually I just wasn't paying attention. I think it is likely that the resistor is supposed to be 100 ohms, has been damaged and increased in resistance - kind of unusual - and should be replaced. Following left but greyed-out]

If I were repairing it, I would essentially extend the broken lead with some copper wire about the same diameter and solder it all back together, just being careful to keep the part body up off the circuit board at around the same height. The original lead might be copper or it might be steel. Copper puts more heat into the PCB which increases the power handling of the resistor, but makes the board hotter. Steel lowers heat conduction to the board which can help prevent overheating of the board. Always tradeoffs.

 

.. It is not uncommon to flatten the lead to do this, and I think that your resistor has just broken at the flat part.
...
[EDIT - I got all confused. I'm old. Whadyah expect. - well actually I just wasn't paying attention. I think it is likely that the resistor is supposed to be 100 ohms, has been damaged and increased in resistance - kind of unusual - and should be replaced. Following left but greyed-out]

Your response was very informative and helpful. Thanks. In regard to flattening to extend off board, since I have another of the same amp, there us inconsistency all around with this odd-ball resistor. There of the four are exactly as you described...flattened as if to raise of board. But this one that looks like a repair is 1) different ohms 2) was VERY thin where it broke (obvious hack in place solder job) and 3) where the others lay parallel to the board, on the soldered side of this resistor, it is higher so it doesn't sit parallel.

I still don't get the colors. If I look for a 150 ohm resistor orange in either the 2nd or 3rd band don't equate. The 2nd amp are clearly brown black brown gold = 100 ohms.

Are the band colors different for this resistor type vs. normal resistors? There are plenty of other resistors on the board... These ones ARE unique. They have a dull 'porous' looking surface while the rest are like most resistors I see, shiny and much more obvious band colors.

I will replace this resistor with 100 ohm and see how it goes. I created a better ground for the other amp and it could be (hopefully) nothing else is actually wrong. I did smell some electrical burning, though, on this unit. I couldn't see any other sign of damage. Looked for a popped cap or burnt transistor and couldn't find anything.