An amplifier stored exposed to our last UK winter , maybe dropped to -15
degrees C. Loss of the negative side of the biasing area of one channel. By
a secondary route there was some negative V around , did not rise to + rail
or centre and difficult to find
as no reason to suspect a small o/c R. In an area with only -15V secondary
supply and loads of 1N4148 and TO92 around. This resistor was normal 1/3W
4K7, not a fusible resistor, metal oxide spiral construction. On scraping
off the coating , break was 1.5 turns along the 5 turns, ie not central.
Absolutely no sign of heating on original R surface or the board and no sign
of heating with the coating removed. A closely defined break , probing with
DVM. But nothing seen in the way of a crack , bubbling, corrossion or
discolour even under x30 magnification. Could not close the break by
twisting or bending the body of the R.
I've come across mechanically broken R from being dropped but never from
cold or damp.