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Old 15th Jul 2021, 11:31 am   #1
DonaldStott
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Default Understanding Dual Can Electrolytics

Currently restoring a Bush DAC10 and as part of that I removed the Dual Can Electrolytic for testing out of circuit.

In the past these have been replaced if excessive hum was present but in general those in both the Bush DAC90A and DAC10 have been fairly resilient, certainly good performers for their age!

The dual can in these sets are 32μF / 16μF and I'm looking to understand when the measured values are indicative of problems? I appreciate that there are those that would encourage capacitor reforming but not sure when that would be a viable option?

What I have for the 32μF is:-

52.75μF
ESR : 1.0Ω
VLOSS : 1.8%

And for the 16μF:-

49.1μF
ESR : 1.7Ω
VLOSS : 4.2%

All measurements done with my MK-328 Tester - not the best I know!

Not sure what those numbers are telling me but more than happy to be enlightened - thanks in advance.
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Old 15th Jul 2021, 11:39 am   #2
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Default Re: Understanding Dual Can Electrolytics

Excess measured capacitance is generally an indication that the capacitor is electrically leaky. The only sure way to check for leakage is at the full working voltage.
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Old 15th Jul 2021, 11:45 am   #3
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Default Re: Understanding Dual Can Electrolytics

Have you tried a reading from the +ve terminal of the 32uF section to +ve terminal of the 16uF section, in ohms? (and not using ground at all).

In times past I've had an iffy cap which read a little high on a capacitor meter from ground to 16uF +ve & ground to 32uF +ve. Turns out the cap was resistive from the 16uF section to the 32uF section.

I'm wondering if you're reading the 16uF plus (through a resistive path) a bit of the 32uF section, and vice versa.

The solution for (in my case) a very early black DAC90, was to re-stuff the original can. When in use the radio when from a gentle hum initially to a speedboat once warm(!)

Mark
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Old 15th Jul 2021, 11:53 am   #4
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Default Re: Understanding Dual Can Electrolytics

The circuit detailed here http://www.vintage-radio.com/project...-reformer.html is useful not only at reforming, but also measuring leakage levels.

If capacitors like this don't reform to show very low leakage, then its time to replace them.

If they are showing such wildly varying capacitance measurements, then that could suggest there is excessive leakage. Leakage adds parallel and series resistance to the capacitor, and this trips up a multimeter or analysers method of measuring capacitance. I cant remember what that method is now, but I guess you're applying a voltage to the capacitor and its taking longer to become charged due to the added resistance...

EDIT: Actually thinking about it, some leakage doesnt show until higher voltages are applied, so emphasis on the "COULD" there. The best solution IMO is to build that leakage / reformer circuit. I use it on such capacitors when powering up a new piece for the first time. I think the likelihood of them reforming and being usable is much greater with that than just applying limited power via a lamp limiter, or lower voltage from a variac. They need something close to working voltage to reform, but in a current controlled way

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Old 15th Jul 2021, 11:55 am   #5
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Default Re: Understanding Dual Can Electrolytics

FWIW I just checked a couple of "pulled" capacitors on a Peak atlas ESR60.

16uF 450V tests as 16.88uF ESR 0.36R.
32uF 450V tests as 30.34uF ESR 0.23R
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Old 15th Jul 2021, 12:00 pm   #6
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Default Re: Understanding Dual Can Electrolytics

Even a slight, harmless leakage (the sort that is acceptable, even normal in an elderly HT electrolytic) is likely to badly mislead a moderrn electronic tester and make it read high, so don't take the readings as gospel. If in doubt, replacement modern HT electrolytics aren't a fortune and are widely available thanks to the ubiquity of the off-line switch-mode power supply technique. Current modern HV electrolytics have sufficiently low leakage that they can hold charge for a hazardously long time though!
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Old 15th Jul 2021, 12:27 pm   #7
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Default Re: Understanding Dual Can Electrolytics

Regarding general replacement of such parts (where leakage is at acceptable levels), opinions will vary. Personally, if it's is going to get very regular usage over long periods of time, like the radiogram in my living room, I would replace them regardless. Same if I was working on something for someone else. Otherwise, I keep the originals if they reform well.
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Old 15th Jul 2021, 12:30 pm   #8
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Default Re: Understanding Dual Can Electrolytics

In all my years of servicing in the trade and at home I don't think I've checked the performance of an electrolytic capacitor on an item of test gear designed for checking capacitors more than a couple of times, that would have been on a Hunt's bridge back in the 1960's.

Excess ripple is usually caused by excessive load current/current flowing through the capacitor (easy to measure) or loss of capacitance, also easily checked by doing a temporary bridge with another capacitor, you don't even have to solder it in place for the test, I rarely did, my favourites for bridging kept on the bench or in the bench draw or in the tool box were 0.001uF, 0.01uF, 0.1uF, 16uF, 50uF etc.

Lawrence.

Last edited by ms660; 15th Jul 2021 at 12:39 pm. Reason: extra info
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Old 15th Jul 2021, 2:30 pm   #9
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Default Re: Understanding Dual Can Electrolytics

Quote:
Originally Posted by mark2collection View Post
Have you tried a reading from the +ve terminal of the 32uF section to +ve terminal of the 16uF section, in ohms? (and not using ground at all).

In times past I've had an iffy cap which read a little high on a capacitor meter from ground to 16uF +ve & ground to 32uF +ve. Turns out the cap was resistive from the 16uF section to the 32uF section.
Thanks Mark - connected up my DMM as suggested across both +ve terminals but no idea what is going on!

Resistance reading rose rapidly to 14.6MΩ and then dropped slowly to around 10.8MΩ ?

It's slowly increasing again after about 10 minutes and is currently at 12.36MΩ ...

I'm no stranger to re-stuffing but just thought I'd try and get a better understanding of dual cans:-

https://www.vintage-radio.net/forum/...d.php?t=171884

When I have time I may attempt the Electrolytic Capacitor Reformer suggested by Adam (PsychMan) but would need advice on where to source those small meters!
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Old 15th Jul 2021, 5:10 pm   #10
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Default Re: Understanding Dual Can Electrolytics

I only built the first version without a meter and just have some crock clips I connect to a multimeter. I did intend to find milliameter, but so far havent bothered . It works great though, sometimes I leave it on overnight if the leakage is still gradually coming down. Its quite satisfying to potter about and come back to it and check it

I dont think those readings are enough to worry about. As I learned recently, the resistance of your skin can provide 10M + resistance between 2 points on digital multimeter. Perhaps its coming from the cap where the tags are mounted?

Adam
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Old 16th Jul 2021, 5:35 pm   #11
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Default Re: Understanding Dual Can Electrolytics

The 'generic' capacitor-testers work by charging the capacitor from a constant-current source until a certain voltage is reached, then discharging it through a constant-current-sink until another voltage is reached. The testert imes how long it takes for each of these parts of the cycle and from that works-out the capacitance.

[this is known as 'dual slope' because it does both a charging and discharging measurement].

Of course if your 'capacitor' is leaky the leakage resistance slows the speed at which the constant-current-feed can charge the capacitor to its specified voltage in the first part of the test, and accelerates speed in the second discharge phase.

Leading to deeply-untrustworthy readings!

My approach is that in the time taken to unsolder the connections and then do a test, I can unsolder the connections and install modern trustworthy components, then knowing the radio can be used - and left operating unattended without the risk of it going into meltdown.

[OK, if you must, leave the old 'can' in the chassis hole for cosmetic purposes and wire the new parts to a tagstrip below the chassis: persoannly if doing this I'll remove the old can and leave the open hole, if only to improve the underchassis ventilation. I'm not big on original-appearance]
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Old 16th Jul 2021, 5:38 pm   #12
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Default Re: Understanding Dual Can Electrolytics

I see from Post 4 link that the Thread link in the "Update 2 - 275V/350V Output Version" does not work.

http://www.vintage-radio.net/forum/s...ead.php?t=6239

David
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Old 16th Jul 2021, 8:51 pm   #13
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Default Re: Understanding Dual Can Electrolytics

Quote:
Originally Posted by DonaldStott View Post
When I have time I may attempt the Electrolytic Capacitor Reformer suggested by Adam (PsychMan) but would need advice on where to source those small meters!
I've made the Practical Electronics Magazine Capacitor Leakage Tester & Reformer - I did it in this thread. I've used it a lot in the last few months to test old capacitors from equipment, reformed some and put them back, while able to discard those that failed at working voltage. Easy, and quite satisfying!
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Old 16th Jul 2021, 9:33 pm   #14
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Default Re: Understanding Dual Can Electrolytics

Quote:
Originally Posted by G6Tanuki View Post
... I can unsolder the connections and install modern trustworthy components, then knowing the radio can be used - and left operating unattended without the risk of it going into meltdown ...
... and we are back once again to the difference between Repair (what matters most is reliability) and Restoration (what matters most is originality).

Fortunately the forum caters for both points of view .

Cheers,

GJ
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Old 24th Jul 2021, 4:43 pm   #15
DonaldStott
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Default Re: Understanding Dual Can Electrolytics

Getting this Thread back on track and away from the perennial Repair vs Restoration debate, can I ask again what, if anything, these figures are telling me - good, bad or indifferent: -

What I have for the 32μF is:-

52.75μF
ESR : 1.0Ω
VLOSS : 1.8%

And for the 16μF:-

49.1μF
ESR : 1.7Ω
VLOSS : 4.2%


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Old 24th Jul 2021, 4:50 pm   #16
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Default Re: Understanding Dual Can Electrolytics

I think finding a way of testing for leakage at working voltage is the only way to get anything meaningful. Then you'll know what those figures mean, and whether it correlates with Graham's post #2 that (quite a lot) higher capacitance than nominal means high leakage. At the working voltage it can also be seen whether reforming is possible.
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Old 24th Jul 2021, 5:10 pm   #17
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Default Re: Understanding Dual Can Electrolytics

In some respects modern capacitance meters are one of the worst things ever invented. People don't seem to understand that failcapacitors because of electrical leakage which can't be checked except at the full working voltage.

You wouldn't test a steam boiler designed to work at 200 psi with a 2 psi hydraulic test would you?
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Old 24th Jul 2021, 5:11 pm   #18
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Default Re: Understanding Dual Can Electrolytics

Quote:
Originally Posted by Station X View Post
In some respects modern capacitance meters are one of the worst things ever invented.
and then some.

Lawrence.
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Old 24th Jul 2021, 5:30 pm   #19
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Default Re: Understanding Dual Can Electrolytics

Quote:
Originally Posted by DonaldStott View Post
... can I ask again what, if anything, these figures are telling me - good, bad or indifferent: -

What I have for the 32μF is:-

52.75μF
ESR : 1.0Ω
VLOSS : 1.8%

And for the 16μF:-

49.1μF
ESR : 1.7Ω
VLOSS : 4.2%
Simple
Bad, replace them/it.

Alan
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Old 24th Jul 2021, 5:37 pm   #20
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Default Re: Understanding Dual Can Electrolytics

Might I suggest that the OP tests a few new capacitors so that he can be sure his meter is accurate, bearing in mind that even new caps have a wide tolerance.
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