Mark Zacharias wrote in message
eb.com...
"N_Cook" wrote in message
...
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.



Maybe just age or cheap source of parts. In the past year or so I have had
a
bad 56 K resistor, a bad 39K resistor, and a bad 18K resistor, all in
amplifiers, none burned or stressed. Two of them were in units about 25
years old; the other about 10 years old. Caused by your cold winter?
Perhaps, perhaps not. Co-incidence doesn't necessarily mean causation.

Mark Z.



I expected to see a manufacturing void in the MO or something like that, so
some thermal stress met the camel's back.