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Old 4th Feb 2020, 3:23 pm   #1
Ken Taylor
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Default DAC90A Electrolytic capacitors

Hello all you DAC90A enthusiasts!

I am about to start work on one of these sets and I have read many of the postings of others and seen the impressive results obtained. I hope I can match them.

One question, I notice that most of the finished projects appear to have the original reservoir/smoothing capacitors in place (C19/21 in the Bush manual). Are they particularly reliable? I do not see any bulges in mine and will bring the mains up gradually with the variac. Date code on the can is Jul 53, they show a high but falling resistance with the ohmmeter but my capacitance meter does not read that high.

Thanks
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Old 4th Feb 2020, 5:11 pm   #2
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Default Re: DAC90A Electrolytic capacitors

Mine i9s still going strong.

However, bringing up with a Variac does not achieve very much. The rectifier will do nothing until the voltage is high enough to heat the cathode sufficiently, which will be about 80% of full. So you won't be able to bring up slowly over some hours.

Personally, I'd get a 1N4007 or similar and use this as a temporary rectifier to bring up HT on the electrolytic over a few hours, monitoring the DC voltage. You only need to clip two leads onto the can. Start at 50V and gradually increase to 250V. If the can is still cold at this, then it's probably OK.

Then you can disconnect your temporary rectifier leads and bring mains up as normal.
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Old 4th Feb 2020, 5:27 pm   #3
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Default Re: DAC90A Electrolytic capacitors

My DAC90A still has its original smoothing can.
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Old 4th Feb 2020, 6:43 pm   #4
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Default Re: DAC90A Electrolytic capacitors

That is a very good point - and I should have thought of it. Neat idea with the temporary diode, thanks for the suggestion.
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Old 4th Feb 2020, 6:46 pm   #5
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Default Re: DAC90A Electrolytic capacitors

I have as far as I remember only had to change one smoothing cap in all the many I have had through the workshop.

BVWS sell them though if needed.
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Old 4th Feb 2020, 7:02 pm   #6
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Default Re: DAC90A Electrolytic capacitors

I have repaired many of these radios and never had to change the smoothing capacitors. They really do seem to be reliable even after all these years.

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Old 4th Feb 2020, 10:00 pm   #7
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Default Re: DAC90A Electrolytic capacitors

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Originally Posted by HamishBoxer View Post
I have as far as I remember only had to change one smoothing cap in all the many I have had through the workshop.
Likewise....and the only one I had to change was in a particularly bad set that had been plugged in by an 'enthusiastic' person as he remembered his dad using it. He was lucky they didn't explode since the rubber wiring was so rotten, a wire from the dropper was touching an HT wire with the result that mains AC had be shoved up the HT line! Had it not been for the cloud of smoke that ensued from the dropper and a few resistors and of course the mains decoupler resulting in a very hasty unplugging, the caps would have exploded rather spectacularly! As it was when I got it, the bottom of the smoothing cap was bulging badly.

Amazingly enough, when it had been rewired and recapped, including the smoothers, it was one of the best DAC90A's that I had repaired.

In general, the smoothing caps have a relatively easy life since the HT is fairly low so hopefully all you need to do is a gentle reform.
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Old 4th Feb 2020, 11:37 pm   #8
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Default Re: DAC90A Electrolytic capacitors

I have a DAC90A in my 'work in progress' pile. Having, towards the end of last year, replaced all the waxies and some out of tolerance resistors, basically all I have to do is to retrieve an unused pin from a valve base to replace one on which the solder terminal broke off short. The reservoir and smoothing caps should be OK, though I was thinking of changing them purely because by now they're at least the best part of 70 years old, but, in the light of what others have said, I'll power it up via my lamp limiter, and check that it is working OK before changing the caps, though surely they'll have lost capacitance by now.
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Old 5th Feb 2020, 8:19 am   #9
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Default Re: DAC90A Electrolytic capacitors

I would use a 20k resistor in series with the supply to the cap. This will allow the capacitor to automatically reform at its own rate. The voltage drop across the resistor caused by the initial high ESR will slowly drop to near zero as the capacitor becomes fully charged.
If the final current (V drop/R of the series resistance) at the full working voltage of the cap is less than 0.5mA then I class it as more or less being reformed.
(Obviously the cap has to be isolated from the rest of the circuit.)

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Old 5th Feb 2020, 7:12 pm   #10
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Default Re: DAC90A Electrolytic capacitors

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Originally Posted by Station X View Post
My DAC90A still has its original smoothing can.
As does mine and the set is in use almost every day - thank goodness for BBC Radio 4 Long Wave.
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Old 6th Feb 2020, 12:16 am   #11
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Default Re: DAC90A Electrolytic capacitors

There has been no need to change old cans on sets here either, unless they are of a brand other than TCC or Plessey.
Paul Stenning has a circuit somewhere on this forum for making a capacitor reformer, it is a crude circuit but has proved invaluable when it comes to reforming cans.
The only time I have ever had to replace a can (capacitor, to clarify things) is when it either says Dubilier or Sprague on it!
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Old 21st Mar 2020, 11:08 am   #12
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Default Re: DAC90A Electrolytic capacitors

Well I took a distillation of the advice offered in the forum and finally got round to reforming the electrolytic yesterday. I used the sets HT smoothing resistor (10k but measures 11k) is series with a 1n4007 and gradually increased the variac voltage over about 10 hours. The final current was 4mA for the 32uF section. Would this be considered to be reformed?

A further observation - connecting a DMM on ohms across the 32uF section I notice the resistance gradually decreases from several megohms. Across the 16uF section, which I so far have not touched, the resistance increases. A test new 33uF also shows resistance increasing. All caps were discharged first.

Ken

Last edited by Ken Taylor; 21st Mar 2020 at 11:24 am. Reason: Further information added
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Old 21st Mar 2020, 11:43 am   #13
Graham G3ZVT
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Default Re: DAC90A Electrolytic capacitors

That sounds reasonable to me.

You may want to consider fitting a 1A BS1362 fuse in the plug which might mitigate the effects of any catastrophic failure. They are available, but perhaps not on the High St.

If you think about it, old radios such as these have no fuse protection, and when they were in normal use, many homes did not have fused plugs*, so they were at the mercy of what ever was fitted in the fuse-box.

*and even when they did, they invariably retained the default 13A fuse. The mains dropper would be more effective as a fuse!
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Old 21st Mar 2020, 12:09 pm   #14
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Default Re: DAC90A Electrolytic capacitors

Quote:
Originally Posted by Glowing Bits! View Post
Paul Stenning has a circuit somewhere on this forum for making a capacitor reformer, it is a crude circuit but has proved invaluable when it comes to reforming cans.
You can find that here, with several other useful projects - lamp limiter, audio output meter, battery radio power supply, etc:

https://www.vintage-radio.com/projects/index.html

The link below also has many interesting projects designed by Paul.

Paul explains that 'Between 1991 and 1997 he designed various electronic projects which were published in several UK hobby electronics magazines including Electronics Today International, Everyday Practical Electronics, Electronics - The Maplin Magazine and Electronics in Action'.

He adds the caveat that: 'due to component obsolescence, advances in PC operating systems and hardware, changes to video broadcasting methods, changes to electrical safety regulations and other technological advances I do not expect many of these projects to be usable or practical now. The projects are made available for historical reference only, not for construction'.

That said, they're interesting projects and well worth a browse:

https://www.vintage-radio.info/projects

Hope that's useful info.
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Old 21st Mar 2020, 12:18 pm   #15
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Default Re: DAC90A Electrolytic capacitors

Bush radios nearly always used smoothing capacitors made by BEC (British Electrolytic Condensers) made in Acton. These are by far the best quality components and very rarely fail if kept within their ratings. Bush also kept the working conditions for all components well within their limits - an ideal formula for good reliability.

BEC were taken over by Plessey, and the early Plessey branded can type electrolytics were equally good - maybe still made in Acton. The later Plessey devices, especially the plastic red/black cased devices were rubbish. Cost cutting as usual.

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Old 21st Mar 2020, 4:09 pm   #16
Ken Taylor
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Default Re: DAC90A Electrolytic capacitors

Thanks for those comments, I feel more confident to carry on. Good point about the 1A fuse, I'll see if I have one.

And David G4EBT, thanks for the link. I expect this to be a one off project so not worth building a proper capacitor reformer.

Last edited by AC/HL; 21st Mar 2020 at 5:44 pm. Reason: OT aside edited
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Old 21st Mar 2020, 7:45 pm   #17
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Default Re: DAC90A Electrolytic capacitors

As has already been pointed out, certain brands of capacitors usually work after 70 plus years, Sprague & Dubilier are usually binned, they are as bad as Hunts!
If the OP does not have a suitable fuse, 2A fuses are readily available from auction sites, just be sure the shop itself is an electrical factors, this is important with regard to the fuses themselves being genuine.
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Old 21st Mar 2020, 10:23 pm   #18
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Default Re: DAC90A Electrolytic capacitors

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If you think about it, old radios such as these have no fuse protection, and when they were in normal use, many homes did not have fused plugs*, so they were at the mercy of what ever was fitted in the fuse-box.

*and even when they did, they invariably retained the default 13A fuse. The mains dropper would be more effective as a fuse!
I've generally maintained that an AC/DC set is safer than an AC-only (except to work on!), with mains transformer.

I have had two old sets with mains transformers, fail due to transformer - internal shorted turns - the transformer gets hotter and hotter, smokes, spits out wax. Quite likely to catch fire - though I have been there and powered-down.

Whereas, the AC/DC set, it's inconceivable that the dropper can go short-circuit. But if it fails open, the radio will just stop working.

True, the AC/DC is more likely to have an across-lines capacitor. But again, if this fails, it's usually an abrupt explosion, and then business as usual.
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Old 27th Mar 2020, 7:21 pm   #19
Ken Taylor
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Default Re: DAC90A Electrolytic capacitors

Capacitors now reformed and the radio works fine. I am fitting a mains lead as I do not have a proper connector and looking out for a 1A fuse.

Thanks again for all the advice.
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Old 18th Apr 2020, 1:51 pm   #20
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Default Re: DAC90A Electrolytic capacitors

Just in case you are still looking for 1A BS1362 fuse then there are a number on ebay - you need to make sure that you choose a plug top fuse (BS1362) as there are some listed for other purposes. I put these into all of the sets I have repaired and I now have a good selection of 3/5/13 amp fuses that were previously in use

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