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Old 1st Dec 2019, 11:22 pm   #1
alanworland
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Default Quad 66 CD player

Shortly will be working on a working 66 cd player which has obvious problems with certainly one power supply capacitor!
The unit has quite a few electrolytics on the PCB some of these as small as 2.2uf in aluminium cans.
I have replacements on the way as I am of the understanding that even the diddy ones are likely to be/become defective?

Alan
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Old 2nd Dec 2019, 6:58 am   #2
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Default Re: Quad 66 CD player

The smaller capacitors contain a smaller volume of water, but their seals are scaled down by the area - not quite as big a factor, so they can have a shorter life before their ESR goes up.

Offsetting this, big capacitors are more likely to be used in places with significant ripple current and the heating accelerates water vapour loss. But in equipment which generally runs hot, you may well find the little 'uns dry out first.

I went through a Revox B261 tuner which has an awful lot of small electrolytics. The PSU was the source of a bad hum. The reservoirs were measurably past their prime, but it was a small part in the reg circuit which was the culprit. Looking around this machine, over 50% of the electrolytics had gone high-ESR. It had run 24/7 in a broadcaster's rack of hot equipment as a monitoring receiver. They had every right to show signs of suffering. It made sense to swap the lot.

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Old 2nd Dec 2019, 10:15 am   #3
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Default Re: Quad 66 CD player

Thanks for confirming what I thought! I guess while it's in bits they might as well be replaced with fresh.
It's interesting while looking through RS website the little ones seem to have generally shorter life ratings than the larger devices, except when you look at the tantalum types where no life is stated?

Alan
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Old 2nd Dec 2019, 12:12 pm   #4
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Default Re: Quad 66 CD player

Yes, that's the volume to area ratio at work.

As far as the life expectancy of Tantalum capacitors goes, it rarely gets menytioned by suppliers because it is awful. Metal cased wet slug parts are more reliable but one I need for a repair of an HP spectrum analyser is £80 each from a large UK distributor.

Aluminium electrolytics have improved a lot, use them to replace tantalum except where the tant is running at low bias voltage which is their one trick that aluminium ones can't equal.

David
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Old 2nd Dec 2019, 6:55 pm   #5
alanworland
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Default Re: Quad 66 CD player

Some of the capacitors I want to replace are small values (2.2uf) and low voltage jobs, only 16v.
I understand that when replacing I should aim for at least the original working voltage but can you go too high?
I have found some from Farnell with a good working life but they are 400v working - would they be usable?

Alan
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Old 2nd Dec 2019, 7:37 pm   #6
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Default Re: Quad 66 CD player

Yes, you can go too low with aluminium electrolytics. Ideally you want them running with a DC bias voltage on them between 20% and 80% of their rating. If you go too low, the capacitors can slowly de-polarise and need re-forming. At around 10$% of rating they tend to reduce in their voltage handling capability so if you ever gave them full whack you can push them into failure.

This is where Tantalum capacitors score, they can run with zero volts DC bias. But their other problems outdo this advantage.

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