Re-capping, servicing old gear

[Hud]

Hi all

Just wondering after what amount of time would you expect that a racked piece of equipment (I'm most interested in preamps) would need to be re-capped or otherwise serviced? Is ten years perhaps a good rule of thumb..

If it did need re-capping, would it just sound noisy? Much thanks for any enlightenment

[Chinagraf]

Hi Hud,
it really depends on the operating temp of the gear and the temp rating of the caps. A lot of people say 10 years, but I've recapped 20 year old console channels and found no difference.
The other thing to consider is the sound of the unit with original caps. If you take an old blackface or 1073 and recap it, it will sound different to how it does with the dried out original caps.
Which sound is better is a matter of personal preference. I had a 2254 recapped and hated the sound, so I put the I old ones back in.

As far as symptoms of dried out caps, it can be hum if it is the power supply filter caps or in the signal path it can show as a loss of bottom end. Having said that, people often upgrade the cap value when recapping to lower
the corner frequency of the R/C network hpf. Sometimes this is good, but sometimes the cap value was chosen to hide some shortcomings in the bottom end of the circuit. Dropping the corner frequency can potentially
expose these shortcomings in a way the original designer never intended.

[Chris H]

A scratchy pot is often an indicator that the caps need replacing.

[Hud]

Thanks heaps for that Andy and Chris

[davek]

A scratchy pot is often an indicator that the caps need replacing.

I disagree .. a scratchy pot means the pot needs cleaning!

[Futureman]

I disagree .. a scratchy pot means the pot needs cleaning!

Often caps are used to block DC going through a pot.

DC going through a pot can potentially lead to crackle when the pot is moved etc.

So, a failing cap may potentially let the DC pass = crackle.

regards
Mike

[rob]

yep to all of the above. It is hard to put a time frame on the life of electrolytic caps, it depends on brand, heat and time. The blue phillips capacitors that were used by Neve and other UK and european manufacturers from the 1970s are notoriously unreliable. As a general rule modern capacitors seem at this point in history to be more reliable and certainly there is rarely a reason not to use high temperature capacitors today for extended life. A capacitor that has become "leaky" can cause DC voltages across pots which can accentuate any "scratchiness". Tantalum capacitors, which are not used much any more in pro audio gear, but were very much a part of 70s Neve equipment and a definite part of it's "sound" can become leaky, noisy or just fail dead short causing other circuit failures. An ESR meter ( available as a kit for around $80 ) is the go-to tool for quickly assessing age / heat related electrolytic capacitor degradation.
An interesting experience with recapping the 2254....there are only a couple of electolytic capacitors in the signal path. Most are tantalum. Replacing the tantalums with electolytics has an imact on the sound of the unit. I wonder if this is what happened?

[Chris H]

A scratchy pot is often an indicator that the caps need replacing.

I disagree .. a scratchy pot means the pot needs cleaning!

.......you learn a new thing every day.

[Hud]

Indeed. Thanks heaps for the knowledge everyone!