David Partridge wrote:
sadly they did make that very mistake quite a few times! Witness the 7904
vertical board ... R724 and R725 (499 Ohm 1/2W) always run way too hot
(burnt fingers hot) and are often well off value from overcooking ... the
PCB gets well toasted too! I normally replace them with two 1kOhm 2W metal
film in parallel and they still run pretty warm.
So, while I'm in here, and planning to replace these two resistors, should I increase the wattage? I expect that these are carbon composite resistors (but certainly don't know that for sure) and what I have ordered are supposed to be metal film 1/2 W resistors. Maybe I should order a kit of 1 W metal film resistors as well and use them instead?
Part of me thinks that it won't make much difference: these are underneath the metal shield over the HV section, so they have restricted air-flow under any circumstance. Will larger components really make much difference in their heat dissipation if they are trapped in a hot box?
Also, these resistors have likely been cooking for over 30 years, and they've only drifted by about 20%, so either they're not getting too badly cooked, or they're made of sterner stuff.
-- Jeff Dutky
