i just found a dream amp NIB/NOS, a Meridan 551. It's the amp that got me into the hobby during a dealer demo 25 years ago. If NIB is to be believed, should I be concerned with it's electrolytics? The amp could be up to 30 years old with un-used caps.
The caps may (may!) be OK - it is not unusual to see perfectly working old caps if they have not been abused by high temperatures (this includes temperatures in storage)
30 years is a long time in storage and one recommendation I got way back was to charge old caps slowly the first time after a long pause (that is: do not just switch the power on)
This may be a little bit cumbersome to implement when the caps are mounted in the amplifier - but using the lamp trick (putting an incandecent bulb in series with the power cord) could be a way. I would recommend to use a rather low powered bulb - say 30W or less - to limit the inrush current.
Of course a vario-transformer would also work 😉
Or in other words: try it out - but be careful on the first power up.
I would likely myself test the amplifier without expensive speakers connected from the start.
Cheers,
Martin
30 years is a long time in storage and one recommendation I got way back was to charge old caps slowly the first time after a long pause (that is: do not just switch the power on)
This may be a little bit cumbersome to implement when the caps are mounted in the amplifier - but using the lamp trick (putting an incandecent bulb in series with the power cord) could be a way. I would recommend to use a rather low powered bulb - say 30W or less - to limit the inrush current.
Of course a vario-transformer would also work 😉
Or in other words: try it out - but be careful on the first power up.
I would likely myself test the amplifier without expensive speakers connected from the start.
Cheers,
Martin
It's a question of WHEN the capacitors will fail, not IF---30 year-old capacitors will almost certainly fail. It's just a matter of when you want to replace them---now, or after they fail and cause further damage.
I'm with Martin. 30-year-old electrolytic capacitors may need to be reformed, but once that's done they will almost certainly work fine. If they don't, they have probably dried up, which almost never causes further damage. Even 70-year-old electrolytic capacitors most of the time work fine after being reformed.
Electrolytic capacitors necessarily have a large dielectric surface area which is achieved by creating a very uneven surface over one plate. This 'insulator' is extremely thin, and therefore, by its very very nature it leaks appreciably compare to other insulators. Besides this, an electrolytic capacitor is essentially an electrolysis cell in series with a leaky capacitor. Electrolytics naturally leak, to expect zero leakage is beyond their ability. I am still using an electrolytic capacitor I bought when I was still an adolescent, and I am in my fifties. Electrolytics are much like underground water reservoirs: one does not rebuild a reservoir because one has water evaporation.
Your analogy may fail at one point: electrolytic capacitor leakage current increases drastically when the capacitors are stored for decades without any voltage across them, but it goes back to normal when they are reformed by very slow charging. I don't think empty underground water reservoirs have such behaviour, but then again, I don't know anything about underground water reservoirs.
i suggest you use a series lamp tester, Simple Series Testing Board Kaise Banaye | How To Make An Electric Series testing Board In Hindi - YouTube
Analogies are analogies, their function is to help communication, which means, they have to be treated only as examples to help understanding.
Don't underestimate what can happen to an underground water reservoir. There are many instances in which it can fail. Concrete bottoms may crack when they are kept dry for many years, and there is also the possibility of slight ground displacement from seismic activity and from material thermal expansion differences.
I think, the simplest approach to reform an electrolytic capacitor is to increase the voltage across it in small steps taking account of the leakage current at each step. If the leakage is deemed appreciable, the voltage is kept across the capacitor until this leakage drops sufficiently.
Reforming only deals with the chemical state of the aluminum oxide. One such voltage increase cycle repairs that problem for years. I'm not so picky about measuring etc, I just put a resistor limited 12 v battery charger on the cap with clip leads and come back in a few minutes. Even 450 v caps charged to 2 v with a DVM ohms scale, don't blow up on real voltage.
The other problem is aging by oxygen of the rubber seal on the cap. Oxygen never sleeps, and there are different grades of rubber bought by different manufacturers. An unused old amp may have entirely enough water for a couple of dozen hours, but if the rubber is cracked and the temperature is warm, the water can evaporate out. Unlike the resevoirs used for analogy, you can't put more water in an electrolytic cap. Neither can you replace the rubber seal. Many an enthusiast on organforum bought a relic, played it happily for a few weeks, then post to complain the sound is going away. Water lost = high ESR.
ESR meters are $120 and often require removal of the cap to check it.
Cheapest way to check rail cap condition in an amp is to check maximum wattage available. Ie what AC voltage will it put out on the rated impedance (usually 4 or 8 ohms)? Cheap DVM produce random numbers on music, they are designed to read 50 or 60 hz. A cheap analog VOM works fine ($30). 20 vac scale works up to 100 W 4 ohm, 50 W 8 ohm. Else 50 vac scale. You'll need a music or signal source. I use a FM radio on a highly compressed pop station. Turn up until music sounds dull (clipping), then back off slightly. Read peak voltage available. P=(V^2)/Z where Z is speaker impedance. Speakers that read 6 ohm resistance are usually 8 ohm impedance. P<rated P, you need new rail caps. Get the others on the same order, the manufacturer usually uses the same grade rubber (service life) on all the caps. I buy caps >3000 hours service life caps so I don't have to replace them every 8 years as I had to when I bought TV parts store caps (500 hr? 1000 hr? atomlytic datasheet 1968 had no service life rating).
Good fortune.
The other problem is aging by oxygen of the rubber seal on the cap. Oxygen never sleeps, and there are different grades of rubber bought by different manufacturers. An unused old amp may have entirely enough water for a couple of dozen hours, but if the rubber is cracked and the temperature is warm, the water can evaporate out. Unlike the resevoirs used for analogy, you can't put more water in an electrolytic cap. Neither can you replace the rubber seal. Many an enthusiast on organforum bought a relic, played it happily for a few weeks, then post to complain the sound is going away. Water lost = high ESR.
ESR meters are $120 and often require removal of the cap to check it.
Cheapest way to check rail cap condition in an amp is to check maximum wattage available. Ie what AC voltage will it put out on the rated impedance (usually 4 or 8 ohms)? Cheap DVM produce random numbers on music, they are designed to read 50 or 60 hz. A cheap analog VOM works fine ($30). 20 vac scale works up to 100 W 4 ohm, 50 W 8 ohm. Else 50 vac scale. You'll need a music or signal source. I use a FM radio on a highly compressed pop station. Turn up until music sounds dull (clipping), then back off slightly. Read peak voltage available. P=(V^2)/Z where Z is speaker impedance. Speakers that read 6 ohm resistance are usually 8 ohm impedance. P<rated P, you need new rail caps. Get the others on the same order, the manufacturer usually uses the same grade rubber (service life) on all the caps. I buy caps >3000 hours service life caps so I don't have to replace them every 8 years as I had to when I bought TV parts store caps (500 hr? 1000 hr? atomlytic datasheet 1968 had no service life rating).
Good fortune.
Last edited:
Hi,
How would you process with the lightening bulb main cord please ?
power on without a load ? Needs a load for the rest of the amp caps ? How much time ?
Due to the ripple so heat, should we not change at least the main smoothing cap of an old machine ? Questionning myself as I have found a 40 years amp that is working... if you see the size of the Nichicon 8000 uf/40V caps... Huge, 3 to 4 times bigger than the nowadays equivalent 😱 Pioneer SA-506 (NEC B616A/D586A pair)
I'd pop the hood and look for obvious signs of capacitor leakage. If none are found, power the amp up on a variac. Gradually increase the mains voltage going into the amp and keep an eye on the current draw. Run it at, say, half the nominal mains voltage for a few minutes. If the current draw is still reasonable, increase gradually to the full mains voltage.
If the current suddenly shoots up, something is wrong. So turn the power off and figure out what went wrong.
If you run the amp with the cover off during this test, I recommend that you wear eye protection just in case a cap decides to go flying.
I have electrolytic caps manufactured in the early 1990s that I use for prototyping from time to time. They've been stored at room temperature for most of their life (and a few years in an attic as well). They still measure the 1200 uF stamped on the can.
Tom
If the current suddenly shoots up, something is wrong. So turn the power off and figure out what went wrong.
If you run the amp with the cover off during this test, I recommend that you wear eye protection just in case a cap decides to go flying.
I have electrolytic caps manufactured in the early 1990s that I use for prototyping from time to time. They've been stored at room temperature for most of their life (and a few years in an attic as well). They still measure the 1200 uF stamped on the can.
Tom
Like this:
Powering Your Radio Safely with a Dim-bulb Tester
Start with a 15-25 W bulb and work your way up. It needs to be an incandescent bulb. What you're after is basically a non-linear power resistor.
Tom
Concur with tomchr power up routine is the safest. However, I've plugged a number of 30-35 year old Peavey products in the strip, turned on mains power, & operated for a few days. If they sound "polite" I measure voltage on speaker & probably pull the rail caps if watts low. Polite means no hammer peaks of piano music or no crecendos on orchestral sources. Polite sound indicated the mains caps were limiting the AC voltage out whatever the input voltage was, on my ST70 again, 8 years after new B+ cap. (in 1978)
Some of these ~1990 amps have played the radio or TV sound for hundreds of hours after purchase without symptoms & without new caps. Just depends on the state of the seal in the cap, cracked or not. None of the 1966-68 Hammond or Wurlitzer organs sounded good at all with original caps. The 1980 Allen organ my church inherited went silent in a service for the previous owner, due to high ESR main caps. No explosions except the AC motor cap in 1 hammond H182, repaired badly by dealer service before I bought it.
Last edited:
Question
Carefully reforming caps that see rail or regulated DC voltage is logical. And in discussions like this one, I think it is assumed that some DCV will be present.
But what about caps that have little (if any) DC voltage across them during normal operation? Can AC music signal alone reform an old electrolytic cap?
Carefully reforming caps that see rail or regulated DC voltage is logical. And in discussions like this one, I think it is assumed that some DCV will be present.
But what about caps that have little (if any) DC voltage across them during normal operation? Can AC music signal alone reform an old electrolytic cap?
I'm not sure at all if this is the correct answer, but it's the best I can come up with:
Without forming, you still get an aluminium oxide layer thick enough to handle 1.5 V...2 V of voltage. This is what happens with the cathode foil of a normal aluminium electrolytic capacitor: it isn't formed, but it still gets an oxide layer, albeit a very thin one. As far as I know, that's the reason why normal aluminium electrolytic capacitors can handle 1.5 V in reverse and be used for small AC signals without any DC bias.
Presumably the anode oxide layer never degrades so far that it can't handle 1.5 V anymore. An old electrolytic capacitor can therefore always handle at least as much voltage in the right as in the wrong direction and still be used for AC coupling.
It's all conjecture, so please ignore this reply if anyone who really knows about electrolytic capacitor chemistry chimes in.
Without forming, you still get an aluminium oxide layer thick enough to handle 1.5 V...2 V of voltage. This is what happens with the cathode foil of a normal aluminium electrolytic capacitor: it isn't formed, but it still gets an oxide layer, albeit a very thin one. As far as I know, that's the reason why normal aluminium electrolytic capacitors can handle 1.5 V in reverse and be used for small AC signals without any DC bias.
Presumably the anode oxide layer never degrades so far that it can't handle 1.5 V anymore. An old electrolytic capacitor can therefore always handle at least as much voltage in the right as in the wrong direction and still be used for AC coupling.
It's all conjecture, so please ignore this reply if anyone who really knows about electrolytic capacitor chemistry chimes in.
reforming caps...this topic have been discussed to death in our boards....reforming caps site:www.diyaudio.com - Google Search
if that were the case, you would have been better with film caps....
electrolytics or ecaps are always "formed" at the factory with a rated working voltage in dc, so if not biasing them with dc voltage, then you are better off using film caps...
Yes, but I don't know any brand that spends the money for a film input cap. I've never worked on one that cost high $$$$. I've worked an Allen amp from a $30000 organ, and they didn't use a film input cap.
As far as forming from factory, caps now have a "shelf life" rating. After the shelf life presumably the cap need reforming. I let my stock caps go beyond typical shelf life, so I check them with DVM (charging to 2 v) before use. The 2020 DVM seems to not use 2 v for ohms scale, so that won't work anymore.
Last edited: