08-11-2019, 05:40 PM
In my experience, the resistance value is irrelevant - all carbon-comp resistors drift high. Honestly, 30% is really not unusual.
Usage doesn't seem to have much impact either. I've NOS resistors that are as bad as resistors that have been in use for decades. I've no data to suggest that operating voltage or current is a factor either. Nor storage conditions.
Often these changes go unnoticed because of the circuit conditions - relatively few resistors found in transistor radios depend on exact values. Where it can matter is when a supply rail is low as a result - just as you've found here. This is a particular problem for the Hacker Hunter, for example, but I've not really found it to be a problem on many other designs.
There is some more information in this thread, from post #17 onwards: https://golbornevintageradio.co.uk/forum...p?tid=4384&pid=46475#pid46475
Not really - carbon-comp resistors are readily available. They are primarily used where surge currents are encountered - the obvious place being the 100 ohms or thereabouts that you find in capacitor dropper power supplies, and for inrush limiting in small switched-mode supplies. They look just like the old ones:
https://uk.rs-online.com/web/p/through-h...s/0386048/
https://uk.farnell.com/neohm-te-connecti...dp/1265066
I've no data about whether these will perform better over time than their older counterparts, but then you wouldn't choose them where a precise value mattered anyway...
Usage doesn't seem to have much impact either. I've NOS resistors that are as bad as resistors that have been in use for decades. I've no data to suggest that operating voltage or current is a factor either. Nor storage conditions.
Often these changes go unnoticed because of the circuit conditions - relatively few resistors found in transistor radios depend on exact values. Where it can matter is when a supply rail is low as a result - just as you've found here. This is a particular problem for the Hacker Hunter, for example, but I've not really found it to be a problem on many other designs.
There is some more information in this thread, from post #17 onwards: https://golbornevintageradio.co.uk/forum...p?tid=4384&pid=46475#pid46475
(08-11-2019, 03:43 PM)ppppenguin Wrote: You will have to search hard to find new carbon composition resistors these days.
Not really - carbon-comp resistors are readily available. They are primarily used where surge currents are encountered - the obvious place being the 100 ohms or thereabouts that you find in capacitor dropper power supplies, and for inrush limiting in small switched-mode supplies. They look just like the old ones:
https://uk.rs-online.com/web/p/through-h...s/0386048/
https://uk.farnell.com/neohm-te-connecti...dp/1265066
I've no data about whether these will perform better over time than their older counterparts, but then you wouldn't choose them where a precise value mattered anyway...
