Telltale signs of failing electrolytic capacitors.

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I help out at a vintage computer museum and some of those power supplies had early wet tantalum capacitors. Once they started leaking it becomes nitric acid and corrodes things badly. The legs remain soldered and the cap falls off. You have to scrub the boards under water before working on them.
 
The bottom line here is this......
It usually depends on the vendor/manufacturer at the time who made the capacitors for a particular product.

I would agree, but I'm not a repair tech, so I don't do this every day. Thus, I have no way to recognize which vendor/manufacturer uses better or worse stuff. As someone who owns a ton of gear from various time periods, things start to act funny. It usually starts with a blowing fuse. This usually leads me to replace PSU caps. Most of the time, this is all it needs. This has revived about 5-7 devices for me. Other times, strange problems persist, so I dig further. In two cases, a PSU recap didn't resolve the problem, but a total recap did. So now I just replace em all.

I've never bothered with testing caps. In order to properly test a cap (which is not trivial - you need to have multiple expensive meters/devices) you need to lift one leg at least. I find it easier to lift both legs and put in a new cap. Caps are cheap and replacing them can be less effort than testing them. If it still doesn't work, I've eliminated one possibility for sure.
 
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I would agree, but I'm not a repair tech, so I don't do this every day. Thus, I have no way to recognize which vendor/manufacturer uses better or worse stuff.

I've never bothered with testing caps. In order to properly test a cap (which is not trivial - you need to have multiple expensive meters/devices) you need to lift one leg at least. I find it easier to lift both legs and put in a new cap. Caps are cheap and replacing them can be less effort than testing them. If it still doesn't work, I've eliminated one possibility for sure.


The thing is..... educated troubleshooting.
That's something the typical DIY'er doesn't know or how to perform.
They haven't gone to school for it, they just "dive right in" blindly in hope they'll get lucky.
Some do, many don't.
That's the difference between me, for instance, and the diy crowd.

Even many of those youtube videos are not enough to do a proper job, simply because the authors are not trained, experienced techs.
So it's a case of "the blind leading the blind"
 
I presume by 'education' you refer to planned curricula, lectures, practicals, exams and finally, certification. However, all these are based on written knowledge conveniently printed in books by reputable authors and publishers. Diyers have all the freedoms to get hold of a couple of good books and spend some quality time studying them. Some authors abuse notation and language which is not a good sign their publications are high quality. For instance, a formula for the capacitive reactance of a capacitor is incorrectly written as:
Code:
Xc = 1/2πfC
This is incorrect, 2πfC should be enclosed in brackets. As it is, the formula is similar to:
Code:
Xc = (1/2)πfC

Lately, I was reading 'The Art of Electronics', an ebook. I am disappointed with so many grammatical errors and abuse of notation. This bodes very badly. Some examples did not even work under LTSpice!

It is far better studying a book that has undergone proper proof reading with examples which simulate properly.
 
Yep, that's me - I don't have formal education in this domain, so I fumble my way through and can usually make things work again. I've only had one device that I couldn't repair by recapping it. It might take me longer, but I learn a little along the way and avoid the repair bills.

I've often thought that if I went back to school again I'd probably be an electrical engineer so that I could better understand this fun DIY hobby ;)
 
When capacitance starts to be 20% down it's time for replacement.
So you're going to replace even new 'lytics with a tolerance in capacitance of +50/-20% which isn't that unusual?
For instance, a formula for the capacitive reactance of a capacitor is incorrectly written as:
Code:
Xc = 1/2πfC
This is incorrect, 2πfC should be enclosed in brackets. As it is, the formula is similar to:
Code:
Xc = (1/2)πfC
I'd not regard your 1st example as false as there's consensus that in a mathematical fraction anything right from the slash until the next blank is the denominator, hence to be regarded as written between brackets. Something with the meaning of your 2nd example should be written with a blank: 1/2 πfC.
Best regards!


 
At my service shop back in the 1990's, early 2000's, there was a rash of Mitsubishi large screen tvs going bad.

I repaired a few dozen ECU's from Mitsubishi cars during the same time period. All I did was change out all of the electrolytics, clean the goo off of the PC board and scrub off the green stuff. Electrolytic goo in on a PC board even at 12 volts will corrode and eat into the copper in the humid Florida air.

We have all heard about the Dell computers with bad electrolytics. I had one of the high end Dell Windows XP machines from the mid 2000's at work. I had built it into an automated testing system that ran 24/7 running a rack of test equipment and a temp chamber for IC chip certification. I had overclocked it a bit, taken it's covers off and added a fan. About half of the caps on the motherboard had vented, but the machine just kept on working. Most of those old Dells had died several years back.

Our PC support guys kept telling me to swap it out, and even brought me a new HP machine that I never used. Finally the ultimatum came. That machine, and all Windows XP machines, WILL BE REMOVED from the network on March 31, 2014. Since that had already been determined to be the last day of my 41 year career, it became someone else's problem.

I presume by 'education' you refer to planned curricula, lectures, practicals, exams and finally, certification...... i have college education in electrical engineering

I learned how to fix stuff, by fixing stuff. That kind of knowledge only comes from experience. I also have a couple of engineering degrees, both obtained after I became an engineer at Motorola.

I was surprised at the number of good electrical engineers who could not troubleshoot something, even if they were part of the design team that invented it! This became more and more true as the level of complexity in the device went up.....

Troubleshooting a device in the field is one thing, since there is the usual expectation that the device worked correctly at one point in it's lifetime, all you have to do is find the dead part.

True troubleshooting skills are tested when the factory makes a batch of cell phones or two way radios and NONE of them will operate correctly. Someone screwed up somewhere and there is a wrong part, or defect in the PC board, or..... Many times it's fixed through forensics. leave the dead devices alone and trace your way backwards through the manufacturing process to find the wrong reel in the pick and place machine.

The MX-300 series of police radios in the mid 70's were the first to use 4 layer PC boards and plug-in modules. If all new modules didn't fix it, they called me. I used to troubleshoot these with a 30 amp power supply. It was the quickest way to find out where the short was on the internal layers of the board. Sometimes flaming tantalum caps were involved too.

The real fun starts when you get 10,000 phones from China all with different symptoms from won't power on, to "works normally most of the time, but randomly resets or reboots." Now the factory is on the other side of the planet, the phones are 2 to 10 weeks old by the time the defect is discovered, so there are no forensics. These are usually due to a wrong part, or batch of defective parts, often in the RF or power control circuits. Often the Contract Manufacturing house would do an unauthorized part substitution to meet build quotas. I once found some phones where a PNP transistor was placed instead of an NPN. That same PNP was NOT used anywhere in any Motorola product produced at that plant. Oddly some, but not all of the phones would power on and work until the battery voltage dropped a bit. A wrong reel of capacitors in the RF circuits can make for a host of different issues.

Many of these issues went unsolved, the entire lot of phones were "rejected" and should have been scrapped.......unfortunately some found their way to Ebay with poorly made third party boxes and badly translated Moto Manuals. Ever see "new" Razr phones on Ebay for $20 to $50. That's where they came from. Some didn't work at all, some worked pretty good.

I gave my wife a purple RAZR phone from a batch purchased on Ebay that worked fine and lasted for several years. Some others from this batch had problems, but most at least appeared to work. It was a genuine Motorola phone made at the Foxconn factory that found its way to Ebay. It had been made about 3 months before we got them, and had the correct software flashed into it for the time it was made. The box, packing, LITHIUM BATTERY, charger and all printed material were poor counterfeits. This was the reason for some of the "flaming phones" in the early 2000's and isn't supposed to happen.

Those cheap Chinese 6L6GC's and KT88's on Ebay.......same thing, some work, some make good fireworks.
 
The Chinese invented firework, didn't they?

Anyway, nothing wrong with Chinese stuff if it's made to spec. Most people call Apple phones a high quality product, and they are of course made in China. Of course, you can also get a lot of complete trash from China. It all depends on the product and source etc.

FWIW, I've never had a Chinese "firework" tube, but the Chinese 6Z4 didn't run as long as the Soviet 6Z4P in my testing.
 
Motorola, Apple and Nokia all had phones build in the same Foxconn mega-complex in China. It was an established concept that no phones were built unless a Motorola employee from the US, or Malaysia was present. Even with a "watchdog" present funny stuff occurred with regularity.

Motorola began losing the "arms race" in the phone game in the mid 2000's and started laying off phone engineers with regularity. As the surplus of unemployed phone engineers built up in south Florida other companies came to town, set up design centers and hired them. Most notably were Blackberry, General Dynamics, and Foxconn.

It was well known that Foxconn just wanted to extract phone design and manufacturing knowledge from these people and move on, but people needed jobs and several of my friends went to work there. That gig lasted about 4 or 5 years. During that time several of my friends spent time at the Foxconn mega factory in China. Not one of them has anything good to say about the company, and most quit before Foxconn closed down their Florida operations.

FWIW, I've never had a Chinese "firework" tube,

Stan at ESRC (RIP) called me back around 2000 and asked me if I wanted a big lot of new production Chinese KT-88's, cheap.....real cheap....$3 each kinda cheap. I asked what was up and he said that selling those tubes were one of the biggest mistakes he ever made. I bit, and bought maybe 50 tubes.

Yes, one of the first tubes I plugged into my amp which worked fine with Sovtek 6550's literally exploded scattering glass all over my workbench. It did turn out to be the worse case and I found most of them acceptable for an amp of no more than 35 watts. It was also not a good idea to use them in a combo guitar amp since they were easily rattled to death. Some were indeed good tubes and I have four good ones that I use for tests that might not end well, and they are still alive 20 years later. They have been to 100 WPC on 500 volts several times without issue.

After 10 more years of making junk Shuguang got the KT88 right and their new stuff is pretty good. I still have a few firecrackers left if you want to risk your OPT's and speakers.
 
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Not really.

It would only be possible if a rail was collapsing and causing the amp to suddenly shift its DC operating point. The problem with that is there would almost certainly be other noticeable symptoms like a massive increase in hum/noise.

Much more likely to be a dry, or an intermittent semiconductor such as thr VAS stage that runs hot. Also intermittent small caps such as ceramics that might be in the signal path but that would be pretty uncommon.

Electrolytics like the old ROE brand (small signal stuff) give issues in old equipment as can tants.

Don't forget speaker relay issues either.
 
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Could be... you need to measure the rails and look with a scope to see what is happening.

If you do suspect (or want to eliminate) the reservoir caps then just tagging even a 470uF or 1000uF of appropriate voltage across them would prove the point. It doesn't need be something massive for a quick test.

Other obscure causes can be fuse holders and incredibly rarely fuses themselves where the wire is just touching the end cap within the fuse.

Voltage regulators common to both channels are another common area.
 
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