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In Reply to: Reforming caps posted by SteveBrown on December 27, 2003 at 14:25:44:
Here's my logic:* If the electrolytic is much over 15-20 years old, I just replace it as a matter of routine during initial restoration.
* I do this mainly because I would rather not have to hunt down a power transformer which is usually the victim of excessive current drain in the cap. In extreme cases, of course, you'll get enough of a short to blow the fuse, but the more insidious problem is too much current drawn through the power transformer, heating it up to the point where the insulation on the windings breaks down and a bigger problem occurs. Since I tend to lean towards the older stuff that's at least 30 years or older, this is my general rule.
* If you do have to establish priorities for which caps to replace, here's my general rule of thumb:
- High voltage power supply electrolytic filter caps: any one that is tied across the B+ and ground. No questions.
- Coupling caps in the audio chain that have more than 50V across them during normal operation. These are the inter-stage caps, generally and can be small value caps at times.
- Bypass caps in the power supply. Smaller value caps but have high voltages on them during operation.
- Any other electrolytic cap no matter the voltage across them.Hope that helps.
Cheers,
Follow Ups:
so even if the amp is dead quiet during idle you would still recommend replacing the electrolytics in the power supply? I always thought that if there was no hum then the caps are fine and do not need to be replaced. Is this wrong? Just want to know for sure as I have a Fisher integrated that I'd like to keep around for a while. Thanks.
Hi Bill,I wouldn't get the two symptoms mixed up, here. The item under discussion is that old electrolytic caps dry out, becoming more and more like a resistor as they do. This causes more current to be drawn through the power transformer. Failure modes are hot cap cans to the point where they may pop/explode, hot transformers to the point where the insulation on the windings melt, or resistors may burn out (depending on the individual circuit).
The fact an amp is noisy is not necessarily a symptom of the electrolytics getting old. Diagnosis of this symptom should not be directly associated only with old electrolytics. Treat them as mutually exclusive until diagnosis confirms otherwise.
Hope that helps.
Cheers,
David
I'm still not sure from reading the above posts. Can an electolytic cap dry out (becoming more like a resistor) and still not cause any noticeable hum in the amplifier? My amp is dead quiet but is it still possible or even likely that the power supply caps need replacing? I'd rather not mess with it but if it's prudent I'll replace the electroytics, just not sure if it's necessary at the moment. Especially if I continue to use the amp on a routine basis which I guess is good in terms of keeping the electrolytics in tip top shape. Any clarification would be greatly appreciated. Thanks.
Speaking specifically of power supply 'lytics, I am of the opinion that they should be replaced as a matter of maintenance and safety. Since they are not in the audio signal path, they will not affect the quietness of the amp - tube amps in particurlar. By quietness, I mean the amount of hiss/noise from the amp when its hooked up to a set of speakers. For the safety and operational reasons, this is a prudent thing, IMHO.Other 'lytics can be found in the audio signal path, however, unless there is a large voltage across their terminals during operation, in which case, I would consider them part of the maintenance of the amp.
Generally, audio chain caps are not the first place to look for noise from the amp - tubes,transistors, IC/chips are the first especially in older gear.
To say again: Replacing large power supply 'lytics is a good idea for safety reasons alone. But there is no hard-and-fast rule on this as you can see from the multitude of posts on the subject here at the AA.
Hope that helps.
Cheers,
... tried to run a piece of vintage tube gear for any amount of time with the old caps something eventually blew; even if there was no noticeable noise or heat prior to the event. Sure, it likely won't happen every time but I don't want to roll the dice on not being right that time.
In the '50s, when I was just a pup, I mixed some recordings at the Cimarron Ballroom in Tulsa. The main PA was fed by two early McIntosh amps - 50 watts each I think - the ones that were transformer coupled interstage. (That's an interesting McIntosh story in itself.) Many today probably have never seen those very early ones. They are shown in Allied catalogs of the era.Anyhow, one day I was startled to see that one of the McIntoshes was gone, replaced by a Bogen or some other amp of lower pedigree. I asked the bandleader who was also the owner and he explained that one of the transformers had blown up on one of the Macs; the evidence could be seen sprayed and dried against the inside of the rack door. The engineer who maintained the place had replaced it with another amp.
Then I informed the owner that the residue on the inside of the door was electrolytic capacitor paste, not transformer tar. And that transformers don't 'blow up' in any event, at least not the size and type used in audio amplifiers.
Well, that changed things just a bit. The next time I went to the booth, there was the mising McIntosh amplifer, looking fit, on the floor. I re-installed it and became the maint. eng. as well as the mixer.
It is easy to blow an electrolytic deliberately - just connect it across the power line. The ripple current will be tremendous, will quickly over-heat the cap and thar she blows in just a minute or two.
But very dangerous - don't try this at home.
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