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Quick couple of questions on reforming caps in old equipment. First off, this relates to old equip in general, but specifically I'm working right now on a Fisher amp (uses a voltage doubler). I've read over a number of posts on the subject, but the best I've found is advice to replace caps is they have "excessive" hum or "excessive" leakage. What's excessive? I have no hum at all on this amp, even on the phono input with my 97+db Fostex's. The cans get slighlty warm, but they are next to a warm transformer and hot output tubes. I can open the cap circuit and measure dc current flow, but again, I'm not sure what would be considered excessive. Thoughts?
On a related topic, I sometimes pick up an interesting unit on ebay for myself or a firend and then do some restoration on it. Almost always the seller will say, "Well, I plugged it in and all the tubes light up." I'm relatively sure this isn't done on a variac. So... of what value is reforming over several hours with a variac, if someone else has already applied full voltage for some undetermined amount of time? Once you hit the cans with full B+, is there any point to the slow variac method?
Thanks!
Steve
Follow Ups:
Steve,
I always replace all small electrolitics (under chassis) in vintage amps, but not necessarily the big filter caps over the chassis.
About Fisher, in the directions that come with his Fisher 400 Resto-Pack, the FisherDoc (Al Pugliese) writes:
"Generaly speaking, and contrary to popular belief, power supply electrolytics should NOT be changed unless they are leaking. They are remarkably long lived, and the more they are used, the better they behave. If the originals are in fairly good condition, the unit should be run with the existing filter caps in place. Often a flter cap will exhibit a small degree of leakage when the set is powered up. This "ripple" will often disappear as the set is used. Sets that have been idle for a year or more need to be run for a while which allow the filter caps to "re-form". If after a few months of steady use the ripple is still there, and if it's AUDIBLE, then the failing filter cap section should probably be by-passed by wiring in individual caps under the chassis, unless a suitable replacement is obtained. Generally speaking, ripple should be less than approx. 1cm with the scope set to 2v/cm.".
I hope this help
Raul
I actually lifted one of the cap leads yesterday to measure the current through it.. came out to about 40 uA. I didn't think that was too bad. One cap that does kind of bother me are the two used for the voltage doubler - and that's primarily because they seem to run so close to their max voltage. Each of the two caps are rated for 250v, but on power up, before the output tubes start to draw current, the B+ hit about 495v. I might replace those two with 350v units.
To limit the potential damage at start-up you could install an in-rush current limiter, the voltage won't change much, but the reduced current will help. They use such in some MacIntosh amps. I installed a Keystone CL-80 in my Fisher 500C. If you don't use the phono input, pulling the 12AX7s will allow the power tranny and caps to run much cooler; I did this on my 500C without any ill effects, AFAIK.
Thanks for the advice. The in rush current limiters sound like a good idea. As for the phono tubes, yes I do use that input, but in addition, all four of the input tube heaters are tapped off the cathode of the output tubes (cheap way to get DC for heaters), so removing one (of a series string) would not work.
A few thoughts on old electrolytics:The quality and longevity of electrolytics, in general improved vastly in the mid '60s. Many of that age or less are probably still OK, especially if they have been used occasionally.
The drying out is accelerated by internal heat. That comes not only from external ambient heat (such as being close to hot-running tubes) but also from the amount of ripple current a filter cap has to endure. For example, a first filter cap right across the rectifier cathodes has a very hard life - the peaks of ripple current are great. If it is a "choke input" filter, with a choke inductor preceding the first electrolytic, life is much easier on the first cap. But few circuits do that because the B+ voltage is increased by using a capacitive-input filter. The filters after the first have very little ripple current to deal with, consequently they have long, peaceful lives. But that first one should always be suspect. If in doubt, I would replace that one, just to get the newer cap technology if nothing else, and leave the others alone unless there was good reason
to replace them.I have used the Sprague capacitance tester mentioned earlier for years. That tests the cap under voltage, for dissipation as well as capacitance. Dryout is indicated by increased dissipation. There are many new small capacitance/inductance testers; does anyone know of one which also measures dissipation?
And about coming up slowly with a Variac -No point in leaving it overnight at one setting. If the caps are going to re-form, they will do it within just a couple of minutes at each lower setting.
For those without a Variac (or equivalent; Variac is a brand name), if the amplifier can be connected for 220/240 volts, put it on that connection for a while; it will run at half voltage. Transformers are sold for overseas travelers which will reduce 240 to 120 - that would accomplish the same thing. Half an hour at half voltage is more than enough. Better to bring it up in closer steps, but half voltage before full tilt is some improvement. Your old caps will appreciate it.
Thanks for your input. I did have an electrolytic fail on an old Lead Stereo 20 (circa 1959) and it caused tremendous current draw from the transformer. Fortunately, I caught it before the transformer suffered too much. But as you've noted, I have not really any problems in the 8 or 9 amps I've worked on from the mid to late 60's vintage. I think the advice to replace the first filter cap is a prudent one - I'll do that in this Fisher I'm working on. As for the other electrolytics, I've measured the current draw through them and it is very low in all cases (10's of microamps).
I think reforming refers to NOS or relatively healthy cap that has been sitting for a while and losing the electrolytic layer,not wrt those faulty ones, you're in fact inviting an explosion or so.I have read someone just start up the vintage equipment with a variac at lower voltage and put it there overnight. This is pretty dangerous without knowing the input voltage and current(not to overload the valuable transformer) and first capacitor voltage(not to exceed say 80% rated voltage)especially the power tubes may be unplugged and B+ voltage is high.
You'll notice a difference in the ac input current after reforming, the current should drop substantially. Check for such change.
Caps are designed for around 1000 hours at 85 degrees C and assuming 10 degree C lower would double the life and in reality, it has a total of 8000 hours at 50 C. Even though, the vintage caps have reached the end of life,by design and replacement by new production can cap, if available from AES is a basic necessity. Repairman would always do that for reason of liability after repair. However, there are more than frequent occasions that these caps are not available and space is at a premium for under-chassis mounting. Even if you replace them all "wholeheartedly" and would like to sell the equipment some time later, the potential buyer would mind such tampering. There are more confirmatory test to verify the serviceability of cap by ESR reading and I presume also, rated voltage test for leakage as performed by Sprague TO 6 tester, still looking for one in good condition. Price has escalated and also, I don't want to pay high shipping cost for lemon.
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,
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.
.
Hi, Steve. I would say that a cap must be replaced when it's eight years old or more. Yes, the machine could be working perfectly but one never knows when the calamity strikes. So, try to identify the year of manufacturing. A point to be reminded is that a cap dries quicker when it's near a heat source as seems to be your case. Well, anyway you know: "if it works, don't fix it" ;o)
About the Variac point, I would say that your reasoning is OK.
Keep having fun with the "irons".
Bye.
Dr. Tube.
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