Refurbishing old equipment

[veedub565]

When refurbishing old equipment from the 80's or even 90's it's commonly accepted that the electrolytics are at the end of their useful life and should all be replaced. Along with the Tants too!

I've also read that the carbon composite resistors should also be replaced. That's fine, but..

The equipment is from the late 80's, carbon composite was old technology even then. There is a mix of carbon film and carbon composite on the PCB, so if carbon film was available at the time, why also fit carbon composite ?

it just strikes me that there must be a reason why a large test equipment manufacturer, building a high precision instrument, picked carbon composite over carbon film in the first place. It may of course be down to cost, and carbon composite was used in less critical area's to save money. Or it ay be to do with inductance or voltage surge.


Thoughts ?

I know the carbon composite values drift up over time. In a 30yr old piece of equipment though they ought to be well aged and stable by now though.

[Radio Wrangler]

Carbon composite resistors were just a little bit cheaper than carbon film.

Carbon film resistors are made with a helical cut in the film which is used to change the length/width ratio to get higher values. Teh cut length could be adjusted to trim resistance for closer tolerance.

Carbon comp resistors are very noisy... and can be 20dB worse than the standard noise floor calculation would put them.

Some designers used carbon comps because they worried about the inductance of the spiral of carbon film types.

Metal film types are good for noise down at the theoretical minimum (kT watts/Hz) but also have spiral cuts. The inductance is often not much of an issue.

David