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 Post subject: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Fri 01, 2019 9:42 pm 
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Location: Wilmington DE USA
One of my two MC30 amplifiers stopped working. It was original recapped probably 20+ years ago but used very infrequently. All tubes were checked and ok. After taking the bottom plate off I noticed one of the electrolytics (20 MFD 450 volt) was very distorted looking from overheating. Even so I brought up the voltage on my variac to operating levels and I was able to get a reading of 330 VDC from pos to gnd on it. Because it looked so bad I replaced it with a new 22 MFD 450 volt electrolytic and the voltage was now about 350 VDC from pos to gnd. Curiously when I checked it with my meter I got 5K ohms across it (my new one measured 3+ Megohms). So my conclusion was that it probably developed a short and that was the reason for the excessive heating even though my current readings with the old vs new electrolytic were normal (drawing about 0.4 amps). I reconnected it to my system but leaving the bottom plate off and it now works fine. I put my finger on the other large electrolytic (80 MFD 450 V) and it felt slightly warm after about 5 min. Should I replace it as well (don't have an 80 MFD but have a 100 MFD same voltage). Last electrolytic (40 MFD 450 v) was cool to the touch. Any idea what is going on here and is it usual for the newer types of electrolytics to fail.


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Fri 01, 2019 9:56 pm 
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Warm electrolytic capacitors signify power dissipated. That's usually from excessive leakage. It only gets worse and eventually could be a major failure. Replace them all.


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Fri 01, 2019 10:25 pm 
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Thanks Bob. But does that mean that the replacement electrolytics that we have all been using over the past many years are suspect to failure like the very old ones?


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Fri 01, 2019 10:50 pm 
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Radiotiques wrote:
Thanks Bob. But does that mean that the replacement electrolytics that we have all been using over the past many years are suspect to failure like the very old ones?

No. Failure mode is manufacturer-related, batch lot related, and age-related. It's not reasonable to tar all similar caps with the same brush, even the cheap Chinese-made ones.


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Fri 01, 2019 11:01 pm 
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Ok thanks. Makes sense.


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Sat 02, 2019 2:15 am 
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Location: Sunnyvale CA
Radiotiques wrote:
One of my two MC30 amplifiers stopped working. It was original recapped probably 20+ years ago but used very infrequently.


Infrequent use seems to increase the failure rate, it's like you are exceeding the shelf life. The barrier is formed by using it, frequent use keeps forming it. Just sitting seems to permit it to break down, then it fails or becomes leaky. I have seen plenty of 40-50, even 60-year-old electrolytic caps that were fine in equipment that was in frequent use, and much more recent failures in equipment that has sat idle for extended periods.

BTW, once the cap has started to leak, there's really no "reforming" them, you will frequently think you managed it, but it very often fails in a few power cycles.

Brett


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Sat 02, 2019 3:05 pm 
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There really isn't enough information here to answer the question of what went wrong. Is it possible the caps were defective in manufacture? Yes. Is it possible that they were deformed due to long periods of storage? Yes, but it would depend on how often power was applied, or how long the amplifier sat since it was last used. There is a third possibility, and that is the caps were mis-spec'd and not appropriate for that amplifier. In that case they may have already been damaged by the short periods of time the amplifier was on. Additionally we do not know the brand or series of these capacitors. Some hold up better than others.

Electolytic capacitors are like batteries; they depend on chemical action for their operation. Consequently they have finite useful lives in use or in storage. The length of time they'll work depends primarily on temperature both in use and in storage, how they are used in a circuit (i.e. applied voltage, frequency, and ripple current), age, quality of construction, and other factors. De-forming of the dielectric layer is only one problem with long-term storage; sometimes the darn things simply dry out as the electrolyte evaporates through imperfect seals. So the answer to the basic question is, yes all these electrolytic capacitors we've been installing in vintage equipment will eventually have to be replaced again.

NB: Reforming of the dielectric layer in electrolytics is a normal, ongoing process when they are in service. If a capacitor has been in storage a long time, it will recover fully if it still contains sufficient electrolyte--assuming the dielectric can reform before heating due to excessive leakage current causes internal damage. (Applying a lower voltage for a while until the leakage current drops is recommended if overheating is a possibility.) In most cases the leakage current drops fast enough, or it is limited by other circuitry, and we don't even notice it. The problems start when a capacitor dries out in storage and doesn't have enough electrolyte to function properly. In that case it may work for a while but it soon fails.

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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Sat 02, 2019 3:16 pm 
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Being a bit on the lazy side, I've developed reflexive responses to certain things....If I don't know the age or pedigree of an electrolytic, it simply goes in the trash.

(Obviously??) there are exceptions: My 1970s Tektronix scope is running on the original power supply filters. But---in the real world---electrolytics are not to be trusted.

For any component, look at the cost of a replacement compared to the cost of your time trying to determine if the old one is OK. To be sure, one's assessment of the value of their time can vary widely......;)

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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Sat 02, 2019 7:33 pm 
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Chris108 wrote:
NB: Reforming of the dielectric layer in electrolytics is a normal, ongoing process when they are in service. If a capacitor has been in storage a long time, it will recover fully if it still contains sufficient electrolyte--assuming the dielectric can reform before heating due to excessive leakage current causes internal damage. (Applying a lower voltage for a while until the leakage current drops is recommended if overheating is a possibility.) In most cases the leakage current drops fast enough, or it is limited by other circuitry, and we don't even notice it. The problems start when a capacitor dries out in storage and doesn't have enough electrolyte to function properly. In that case it may work for a while but it soon fails.


Hypothetically, yes, but in practice, usually not. Once you lose enough of it to become leaky, it doesn't generally come back. It *does* reform on startup every time, but if the dielectric has sat so long and degenerated enough, you can't just get it back.

Brett


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Sat 02, 2019 10:59 pm 
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Quote:
Hypothetically, yes, but in practice, usually not. Once you lose enough of it to become leaky, it doesn't generally come back. It *does* reform on startup every time, but if the dielectric has sat so long and degenerated enough, you can't just get it back.


Nothing hypothetical about it. The theory and operation of electrolytic capacitors was developed in the 1920s, has been thoroughly explored and documented in nearly a hundred years' worth of technical journals and textbooks, and really hasn't changed all that much today. The day an electrolytic leaves the factory, its dielectric layer begins to degrade due to chemical action. When voltage is applied, electrolysis breaks down some of the electrolyte at points where the dielectric layer has weakened. The oxygen goes into forming new aluminum oxide which rebuilds or reforms the dielectric, and the hydrogen is absorbed by the filler material and slowly escapes through the seals. This is the process of reforming, and it goes on as long as there is voltage applied. Have you ever wondered why electrolytics always have a small amount of leakage current no matter what? It may be nanoamperes or microamperes but it is always there. That's the ongoing reforming and maintaining of the dielectric layer which goes on from day 1. When the electrolyte is consumed through electrolytic action, or it leaks out or evaporates past faulty seals, the reforming process stops and the capacitor fails soon afterward.

Now if an electrolytic capacitor is left long enough without voltage across it, the dielectric layer will deteriorate enough to affect performance. Specifically the leakage current will increase, as will dissipation or power factor. Such capacitors can almost always be reformed successfully if there is enough electrolyte in them to rebuild the dielectric layer, and if the current is limited so the capacitor does not overheat and vent before that happens. Indeed, when you take a piece of equipment or an electrolytic capacitor that has been sitting around for more than six months and apply voltage, there is likely to be a bit of reforming going on in the backgound, and it may continue for as much as 24 hours.

In order for this to work, the capacitor has to have what is called "excess electrolyte" in the trade. That is simply electrolyte over and above what is needed for normal operation. It is what allows for reforming. Good ones have ample supplies of electrolyte and can go through many reforming cycles. But if there is no excess electrolyte because it was consumed in long hours of operation, evaporated, leaked, or because it was a cheap, under-filled capacitor to begin with, reforming cannot occur and the capacitor tries to use the electrolyte needed for normal operation. With water based electrolytes, the dielectric layer will break down and the capacitor gets extremely leaky or shorts out; with glycol electrolytes the capacitor goes high ESR or open circuit.

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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Sat 02, 2019 11:09 pm 
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pixellany wrote:
Being a bit on the lazy side, I've developed reflexive responses to certain things....If I don't know the age or pedigree of an electrolytic, it simply goes in the trash.
Pix, you really should test them first! Best way, chuck them into an empty metal trash can... if they go "clang" then they are faulty.
Cheers,
Roger

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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Sat 02, 2019 11:23 pm 
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On a more serious note... (what, wasn't I?)
For all vintage gear (radios, amplifiers, that we hope were properly refurbished with new film and modern e-caps at the time..) being brought back into service after a long time "on the shelf" should be powered up slowly on a variac with both voltage and current meters. This will allow the good e-caps to reform properly and help determine if there are "newly failing" ones on the B+ line.
As others have said, after being on for a while, check the e-cap temperature... it should not be warm. One exception, if the e-cap is close to output tubes or the rectifier tube it will get a little warm from radiated heat... can be confusing.
Cheers,
Roger

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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Sun 03, 2019 12:11 am 
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Quote:
eing a bit on the lazy side, I've developed reflexive responses to certain things....If I don't know the age or pedigree of an electrolytic, it simply goes in the trash.

(Obviously??) there are exceptions: My 1970s Tektronix scope is running on the original power supply filters. But---in the real world---electrolytics are not to be trusted.

For any component, look at the cost of a replacement compared to the cost of your time trying to determine if the old one is OK. To be sure, one's assessment of the value of their time can vary widely......


People love simple answers to complicated problems, like, "if it goes clunk when it hits the garbage can, it was bad." If it was only that simple! No professional engineer or technician would treat a worthwhile piece of equipment that way. They would test and troubleshoot down to the bad component(s), then replace only what is bad with factory approved parts. If factory parts were not available, the specifications of the original parts would be determined as best as possible and replacement components would be matched up on a case by case basis.

The problem is, some of the components in things like McIntosh amplifiers and Tektronix scopes were off-the-shelf, generic items that you can order from "parts-R-us." Those are easy enough to replace. But many other components in high performance equipment were custom or selected, OEM products made to the exact needs of that particular equipment and circuit. It is not often apparent from the parts list or the schematics which parts were generic and which had special characteristics. In some cases it is not possible to obtain new components that have the same characteristics as the old ones, because they simply aren't made any more. That's why you don't want to change parts unnecessarily.

Long ago the FAA did some reliability testing on radios that were repaired by "shotgunning" all the old capacitors, versus changing only the defective ones when a radio came in for service. The "shotgunned" radios had about 25 times the failure rates of the ones which were repaired as needed, causing the FAA to drop the requirement for wholesale parts replacements in aircraft radios. So replacing parts that are demonstrably bad is one thing, but replacing old parts that are still perfectly good with new parts actually makes things less reliable.

Many of the people on these forums brag about owning very high end test equipment which was once the pride and joy of sophisticated R&D labs. Yet I have not yet seen a post where somebody measures the performance of a piece of equipment prior to replacing parts, then afterward to see if any improvement has been made. For example, if you replace the filter capacitorss in a Macintosh amp properly, the hum and noise should be the same or better than what was claimed on the spec sheet when the amp was new. There should be a big improvement if the old filters were on their last legs. But if hum and noise are now higher, the new caps are falling down on the job and maybe you need to try some others.

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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Sun 03, 2019 1:00 am 
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Thanks for all the replies. It is an education in progress. Here is what I did. I replaced the faulty electrolytic (the one that appeared overheated with a resistance of 5K so it was shorted). The amp came back to life and seemed to be working fine. Then based on all the responses I decided to replaced the 80 MFD although only getting slightly warm (which was independent of being alongside the power resistors which get hot by nature). When I tested its resistance I got 7 megohms so perhaps not shorted but better to be on the safe side. I used a 47 and 33 MFD in parallel to give me the 80 MFD that the schematic required. The amp sounds great without hum or distortion the way you would expect a McIntosh to sound. After one hour the replaced electrolytics are not even slightly warm and the current is 0.4 amps so its purring away. Lessons learned - newer electrolytics may fail like older electrolytics and I may have to dispose of all those vintage NOS electrolytics that I saved for a rainy day.


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Wed 06, 2019 12:30 am 
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One other failure mode for electrolytics is the environment in which they are stored, with an added factor for how often they are used. If it was in storage for some time, or operated in high temperature and drier air climates, I'd expect the electrolytics to give up sooner rather than later. Also, as others have noted, the manufacturer of the new caps can make a huge difference in how long they last.

When you are putting electrolytics in series, or parallel, keep an eye on the voltage ratings. I generally try to avoid this whenever possible, and you may want to consider ordering the proper cap for your MAC. It's worth it. ;-) .... your two caps in series or in parallel will have different stresses put on each, with predictable results if you care to do all the calculations..... even MORE predictable if you order the proper value. :)

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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Wed 06, 2019 4:13 am 
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Radiotiques wrote:
Thanks for all the replies. It is an education in progress. Here is what I did. I replaced the faulty electrolytic (the one that appeared overheated with a resistance of 5K so it was shorted).


Hmm. 400ish volts across 5K, wow. That's *32 watts*, no wonder it got hot. Lucky it didn't blow.

Brett


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Wed 06, 2019 5:10 pm 
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yea that's a lot of wattage to dissipate. Good thing those power transformers in the MC30s are massive and can take abuse (not intentional of course)


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Wed 06, 2019 5:33 pm 
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Radiotiques wrote:
yea that's a lot of wattage to dissipate. Good thing those power transformers in the MC30s are massive and can take abuse (not intentional of course)


In fact, it is probably a lot less than that, or it would have blown something up very spectacularly. Still, this is why you don't just turn stuff on and try it to see if it works, at least not long enough for *capacitors* to get hot. I suspect that that the ESR it actually probably higher than that.

Assuming it was really 5K, I get ~80 mA. The transformer can probably handle that indefinitely - figure it's probably rated closer to 2-300 mA, just to support the loads at full power. It will get very hot, too hot to touch, if you let it run long enough at full power, although you will be deaf at the end of the session. This isn't an old radio, it can throw some real big-boy power.

Brett


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 Post subject: Re: McIntosh MC 30 amplifier with hot (temp) electrolytics
PostPosted: Nov Sat 09, 2019 12:33 am 
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Chris108 wrote:
No professional engineer or technician would treat a worthwhile piece of equipment that way. They would test and troubleshoot down to the bad component(s), then replace only what is bad with factory approved parts. If factory parts were not available, the specifications of the original parts would be determined as best as possible and replacement components would be matched up on a case by case basis.


Except for parts known to fail such as paper caps and old electrolytic caps that are by now well beyond their specified life span. To further complicate things we do not have the specs of electrolytic caps used in the older gear so how are we supposed to know if they are good when tested? If for instance I test an electrolytic cap in a working 60's amplifier with no apparent problems and the ESR reads 1 ohm. Without the datasheet on the cap how am I to know if 1 ohm ESR is good or bad?

Chris108 wrote:
Long ago the FAA did some relability testing on radios that were repaired by "shotgunning" all the old capacitors, versus changing only the defective ones when a radio came in for service. The "shotgunned" radios had about 25 times the failure rates of the ones which were repaired as needed, causing the FAA to drop the requirement for wholesale parts replacements in aircraft radios. So replacing parts that are demonstrably bad is one thing, but replacing old parts that are still perfectly good with new parts actually makes things less reliable.


With some of the techs I've seen working civil service in avionics, I can see where things would be made less reliable by shotgunning or by having them touch it period, but generally mil spec caps are very reliable and shouldn't be shotgunned.

Chris108 wrote:
Many of the people on these forums brag about owning very high end test equipment which was once the pride and joy of sophisticated R&D labs. Yet I have not yet seen a post where somebody measures the performance of a piece of equipment prior to replacing parts, then afterward to see if any improvement has been made. For example, if you replace the filter capacitorss in a Macintosh amp properly, the hum and noise should be the same or better than what was claimed on the spec sheet when the amp was new.


For tube operated equipment, the electrolytic caps tend to be bad or failing at this point except perhaps mil spec caps and to some extent caps used by high end manufacturers.

Solid state, the caps actually being bad is not nearly as prevalent except maybe the earlier solid state stuff, but often get shotgunned for the previously mentioned reason of no data for the caps.

All equipment I have replaced the electrolytic caps in has performed better.

I don't need measurements to tell me that. I can either hear it if audio equipment and radios or see the improvement if test equipment.

I did recently get the chance to see what shotgunning the filter caps does to an already working device that far as I know met spec as in the bias adjustments were able to be set per the service manual and the amplifier did produce sound without distortion, hum ETC.

Listened to a new to me Lafayette LA-375 amp for about a week with the original caps.

Recapped it and replaced out of tolerance carbon comp resistors and the amp sounds so much better.

Chris108 wrote:
There should be a big improvement if the old filters were on their last legs.


If there is any improvement at all that means either the new caps are better than the originals or the originals were not to spec in some way shape or form.

Chris108 wrote:
But if hum and noise are now higher, the new caps are falling down on the job and maybe you need to try some others.


That means the wrong cap was used. For filter caps, low ESR is the main characteristic to look for in my book.


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