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Old 13th Aug 2022, 12:38 pm   #1
Helder Crespo
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Default Bush DAC90/90A smoothing capacitors.

Split from this thread:-

https://www.vintage-radio.net/forum/...d.php?t=193491


Quote:
Originally Posted by Station X View Post
This thread prompted me to turn on my Bush DAC90A and I can report that there's no hum whatsoever. This set is still running on its original smoothers and hasn't been modified in any way. Of course the waxies have been replaced.
Does anyone know exactly why so many 70+ year-old electrolytics, e.g. the large dual can smoothers in the DAC90A(32+16uF) and TV22 (250+60uF), can usually be reformed and work very well, whereas more modern caps less than half that age are usually beyond hope? I read somewhere on how the use of latex as a sealant could have a detrimental effect on the lifetime, but can't remember much more.
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Old 13th Aug 2022, 12:55 pm   #2
bionicmerlin
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Default Re: Bush DAC90/90A smoothing capacitors.

Very good question is it down to quality? I’m sure others will have options Andy
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Old 13th Aug 2022, 1:09 pm   #3
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Default Re: Bush DAC90/90A smoothing capacitors.

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Originally Posted by bionicmerlin View Post
Very good question is it down to quality? I’m sure others will have options Andy
You say "quality" but that's all part of the question - when you drill down to the workings of an electrolytic capacitor, what makes "quality" ?
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Old 13th Aug 2022, 1:30 pm   #4
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Default Re: Bush DAC90/90A smoothing capacitors.

Quote:
Originally Posted by stevehertz View Post
Quote:
Originally Posted by bionicmerlin View Post
Very good question is it down to quality? I’m sure others will have options Andy
You say "quality" but that's all part of the question - when you drill down to the workings of an electrolytic capacitor, what makes "quality" ?
Some hints on the comments to this interesting video, but again, they're hints:
https://www.youtube.com/watch?v=GMNQDoFpYQY
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Old 13th Aug 2022, 1:39 pm   #5
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Default Re: Bush DAC90/90A smoothing capacitors.

Not all old electrolytic caps are good... I had the 3-capacitors-in-1-can smoother in my Barker88 go into meltdown a while back..



And prewar electrolytic caps were well known for drying out to the point where you could tell if they were any good by how heavy they felt!!
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Old 13th Aug 2022, 2:39 pm   #6
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Default Re: Bush DAC90/90A smoothing capacitors.

I suspect it's down to size vs capacitance. Old capacitors were huge; modern ones use more exacting designs to cram more capacitance into a smaller space. I suppose there has to be a trade-off.

I usually try to reform big can capacitors unles there's visible signs of leaking, bulging seals or corrosion.
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Old 13th Aug 2022, 5:44 pm   #7
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Default Re: Bush DAC90/90A smoothing capacitors.

Interesting topic. Not all old electrolytics remain serviceable. In fact I replace a great many double can caps when restoring vintage radios. However, certain sets seem to have particularly reliable electrolytics. The DAC90A has already been mentioned; never changed the main can in one of these; and the cans in most Grundig sets are super reliable, only ever changed a couple.

It must be down to the quality of the construction. Perhaps they are less prone to drying out in the first place?
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Old 14th Aug 2022, 9:55 am   #8
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Default Re: Bush DAC90/90A smoothing capacitors.

I think it is all down to the seal. Hermitically sealed capacitors that are now 70+ years old are often 100%. Early paper box as mentioned dried out fairly quickly either going O/C or S/C blowing up the rectifier and often the mains transformer. Very few if any old radios still have this type in place unless they have been restuffed.
I have many television receivers with their original main cans in place that are OK. I don't bother to reform and many of these have not seen a volt tickle them for 50-60 years. The smaller wire ended types with poor sealing are mostly O/C. John.
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Old 14th Aug 2022, 11:26 am   #9
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Default Re: Bush DAC90/90A smoothing capacitors.

I have a fairly straightforward approach, some may say Cavalier... When it comes to replacing smoother and reservoir capacitors.
If it is obviously corroded, bulging or leaking I replace it. Beyond that I give it some volts about half of it's rated voltage then after a few mins try it on full H.T. Any signs of hum and or getting hot I replace it otherwise I give it a long run and see what happens.
I have found usually from the outset the cap will either be good or duff. Maybe I am lucky but I have not yet had a capacitor that seemed ok to later fail.
This may seem a bit of a heavy handed approach in so much as I don't give them much chance to reform but I would rather reject a capacitor that may have been coaxed back by long reforming than risk using it.

I have found that unless corroded and physically leaking DAC90A capacitors are usually OK. I have found a higher rate of corroded and open circuit ones in DAC90s.
Multi cans on 60's & 70's TVs I have found mostly OK. But the ones in my TV22 were completely shot maybe because of poor storage conditions but that said the LOPT was OK with no symptoms of being previously damp?

The smaller multi caps in 60's stuff like record player amplifiers either appear absolutely fine or completely dud I haven't had any half measures.
Should I be more cautious? I do wonder now what I would find if I tested a capacitor that appears to work fine for ESR and leakage on a tester?
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Old 14th Aug 2022, 11:36 am   #10
Neil Purling
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Default Re: Bush DAC90/90A smoothing capacitors.

You can have a capacitor that does not look in distress but seems to have lost capacitance.
I had a Ferguson FM-only set which had 60 volts of ripple on the HT line. The main can (Dubilier Drilitic) had four or five sections. The set got dumped. It was a cheapie on a PCB which was well-cooked around the o/p valve too, a UCL82 or 83 which was also 'gassy'.

Ended up with an Ekco, which was still on it's original can and not a trace of hum.
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Old 14th Aug 2022, 11:43 am   #11
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Default Re: Bush DAC90/90A smoothing capacitors.

Another, albeit somewhat crude method of reforming old electrolytics that look good (no leaks or bulges) that I have successfully used is to apply mains to the set for a few seconds (10 seconds or so, after the rectifier valve has warmed up) then switch off. After a while, say five minutes or so, do the same again. Repeat this, gradually increasing both the 'on' and 'off' times over the course of the day or several days. Keep feeling the cap for heat build up, and listening for a reduction in hum level. If it's getting hot the on time is too long and/or the off time is insufficient. It's more of an art than a science hence the difficulty in stating exact on and off times, but it can often work and it's something of a short cut if the caps are difficult to get at, you're feeling lazy, whatever etc. But if they look worse for wear then I don't even bother, I just replace them. You can often revive a barely breathing patient, but you can't bring something back from the dead; it's the same for electrolytic capacitors.
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Old 14th Aug 2022, 4:11 pm   #12
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Default Re: Bush DAC90/90A smoothing capacitors.

I too have had good luck in most Bush valve radios that have passed through my hands. HT caps seem to re-form and work very well with no problems most of the time.
With Philips sets however, I think the opposite is true!
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Old 14th Aug 2022, 5:08 pm   #13
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Default Re: Bush DAC90/90A smoothing capacitors.

I too do the "a few bursts of mains-voltage spaced an hour apart" thing when waking-up an old radio with HT capacitors of unknown provenance.

Then I go for it with unlimited mains - any capacitors that get hot are clear failures and I don't want them in my house/shack/workshop.

I wonder if there's a significant difference in the survivorship and longevity of electrolytics fitted in low-budget AC/DC radios where they may have been subjected to radiated heat from ~capacitor-cooker~ dropper-resistors, as opposed to similar parts fitted in AC-only gear?

Of course there were plenty of AC-only radios where the smoothing-cans were sited an inch away from a murderously hot-running rectifier-valve...

Also, the duty-cycle needs to be considered: is the capacitor part of an on-for-8-hours-a-day radio or was it fitted to an upmarket radiogram that might have only been listened-to for an hour a day?

[Disclosure: In my postgrad 'twenties I designed a bunch of software for analysing pharmaceutical drug-trials and pig-genetics! so I often tend to look at these issues from a rather abstract statistical/mathematical perspective]
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Old 14th Aug 2022, 6:02 pm   #14
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Default Re: Bush DAC90/90A smoothing capacitors.

In general, I find the multisection caps to be pretty reliable in radios, even in TV sets, the failure rate is fairly low.
The biggest factor is how well the set was stored, I have a few 70-80 year old sets still running perfectly well with the original electrolytics in place! These sets had obviously been kept indoors all their lives.

It is a very different experience with sets stored in damp conditions for years.
They are much more likely to be open or short circuit, even sets stored in a dry loft can have problems due to wide temperature variations over time.


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