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Old 18th May 2019, 11:37 am   #1
Whaam68
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Default 60 year old TCC Lectropack surprise

I’m just renovating a very neglected mono bloc Heathkit valve amp from around 1960 that looks like it’s been sat in a shed (rusty). I’m replacing the out of tolerance and any obviously leaking components. Of the two multi section can caps one was obviously not in a good way having leaked crusty electrolyte everywhere.
The main smoothing can however surprised me as appearing to test basically ok.
The 250uf section measures at 234uf with an esr of 0.2 ohm and v loss of 1.6%.
The 60uf section measures at 58.8uf, esr 1.4ohm and v loss 1.5%
I had already purchased replacement caps some time ago...I’ll probably just fit the new parts to save a job at a later date but am wondering if the main smoother would have soldered on? Maybe I’ll test first next time!
Interestingly all the half watt white carbon comps also test within tolerance so I’m only replacing the 1/4 watt brown carbon comps that have all drifted significantly.
Mike
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Old 18th May 2019, 12:07 pm   #2
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Default Re: 60 year old TCC Lectropack surprise

Under working conditions that cap could fail. I admit depending what it is, I do not change all smoothers etc.
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Old 18th May 2019, 12:07 pm   #3
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Default Re: 60 year old TCC Lectropack surprise

I'd still treat it suspiciously. I had a CDE electrolytic that tested good (<0.1 ohm ESR) but exploded about a week later causing even more of my hair to go white.

Those carbon comps in heathkit stuff are trouble!
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Old 18th May 2019, 12:08 pm   #4
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Default Re: 60 year old TCC Lectropack surprise

My point entirely.
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Old 18th May 2019, 12:10 pm   #5
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Default Re: 60 year old TCC Lectropack surprise

On a positive note, when it exploded it left the can pretty clean and empty!
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Old 18th May 2019, 12:17 pm   #6
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Default Re: 60 year old TCC Lectropack surprise

That you've removed it, and you already have acquired a new one, to me presents an obvious answer. Fit the new one and _know_ it's not going to fail, rather than refitting the old one and then having a niggling worry at the back of your mind every time you switch on.
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Old 18th May 2019, 12:18 pm   #7
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Default Re: 60 year old TCC Lectropack surprise

Electrolytic capacitors need to be tested at the full working voltage. You wouldn't test a steam boiler designed to work at 200 PSI at 10 PSI and pronounce it as OK because it passed the test.

Capacitors which read unusually high in capacitance value are generally electrically leaky. Yours don't show this, but you should not assume they're OK.
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Old 18th May 2019, 12:28 pm   #8
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Default Re: 60 year old TCC Lectropack surprise

Plus,if it fails at some time what else is is it going to take out?
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Old 18th May 2019, 12:32 pm   #9
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Default Re: 60 year old TCC Lectropack surprise

Occasionally, not that often, you will come across a 60 year old electrolytic cap that is nearly as good as the day it was made, it is the exception to the rule but I have seen this too.

I have cut most old capacitors open for inspection. In many there is gross corrosion of the internal aluminium can and of the interconnect links inside. Loss of electrolyte and drying out is common.

The corrosion may relate to Halide contamination at the time of manufacture in the insulation between the foils or other added insulation.

Occasionally, to my astonishment, the inside of the capacitor is moist, free from corrosion and electrically normal in all respects, uF value, ESR and leakage.

Presumably these capacitors have excellent seals to the atmosphere, and are contaminant free with high purity materials. Occasionally, like you have found, you come across this sort of thing. So if you are lucky enough to find such a part, keep it as likely it will be better than what you replace it with.
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Old 18th May 2019, 2:13 pm   #10
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Default Re: 60 year old TCC Lectropack surprise

If you decide to keep it in place: don't just apply voltage. Instead, reform it first!
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Old 18th May 2019, 4:56 pm   #11
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Default Re: 60 year old TCC Lectropack surprise

Large can electrolytics are generally OK but you need to introduce current to them slowly. Do a search for capacitor reforming. If a capacitor has lasted 60 years then it's probably a good one!
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Old 18th May 2019, 7:22 pm   #12
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Default Re: 60 year old TCC Lectropack surprise

Thanks for the insights all...I am going to replace the cap(s) as the parts are already here and these are on show up top so cosmetically it will look better with both cans matched. Point taken re testing at working voltage. I do have an old hunts crb tester that’s closer to that but it’s currently in bits!

Re the larger carbon comps if they still measure in tolerance then I was going to keep them in. In the past I’ve had a habit of replacing things wholesale but if resistors still measure ok then is there a point? The once’s that have drifted I’ll use whatever carbon or metal films I have in the box.
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Old 18th May 2019, 8:11 pm   #13
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Default Re: 60 year old TCC Lectropack surprise

If the Heathkit reses are as bad as Eddystone ones,you might have to change a few.Though depends what there tolerance is and how critical in that part of the circuit.

Mass changing is not normally recommended of components.
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Old 18th May 2019, 9:46 pm   #14
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Default Re: 60 year old TCC Lectropack surprise

I've noticed that carbon composition resistors can change value when current flows through them. It generally shows up as incorrect voltage.

I'd leave them alone so long as they test in tolerance and you don't see any strange voltage readings.
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Old 19th May 2019, 6:17 am   #15
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Default Re: 60 year old TCC Lectropack surprise

Will do, I’ll be checking everything thoroughly.
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Old 19th May 2019, 7:34 am   #16
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Default Re: 60 year old TCC Lectropack surprise

Whaam68,

I have previously repaired a few singles for people and have always found the large can caps leaky even if they measure ok on capacity. (I test them by bringing them, slowly, up to their rated voltage over a long period of time. Then measuring the time they take to dis-charge. That is if they have not heated up in the charging phase!)
The other 50uF+50uF cap in most cases has been fine... The non hi-stab resistors do drift rather...

I'm on the same 'MA-12 mission' as you right now. I'm building a stereo pair for my own use. There is plenty of room to fit the new caps in the cans if you want to keep the original appearance.
But I do turn the mounting clamp over and fit it under the chassis a la Leak. Neater if they are doing to be seen.

Alan

PS I'm still in need of an output transformer if any one has a spare?
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Old 20th May 2019, 7:54 am   #17
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Default Re: 60 year old TCC Lectropack surprise

Hi Alan.

Your work is much neater than mine! Well done it looks great. While I appreciate the work you've done there I'm not too worried about keeping the look too authentic as cosmetically mine are ratty (Gumtree shed finds) and I went for a quicker solution. I could have spent more time getting the bigger cap centered though! The 50uF can in mine had leaked electrolyte quite badly.

I may still re do replace the carbon comps as there aren't that many!

Regards, Mike.
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