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Old 14th Oct 2017, 1:00 pm   #1
crusher19860138
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Default AC/DC sets

I have always wondered how AC/DC sets work with the supply reversed?
For example, a Bush DAC90A can work on either supply either way connected,i always wondered how DC supplied at components like capacitors the wrong way (polarity) didn't cause problems?
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Old 14th Oct 2017, 1:01 pm   #2
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Default Re: AC/DC sets

Rectifier, and only one way with DC.

Lawrence.
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Old 14th Oct 2017, 1:28 pm   #3
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Default Re: AC/DC sets

Yes, if you plug them in with reverse polarity they just won't work, though the heaters will be powered.
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Old 14th Oct 2017, 4:40 pm   #4
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Default Re: AC/DC sets

With regard to mains input - both AC line and return are reversing polarity fifty times a second ......

But as Paul says ..... reversing DC mains input polarity is another issue ........
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Old 14th Oct 2017, 5:28 pm   #5
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Default Re: AC/DC sets

Quote:
Originally Posted by crusher19860138 View Post
...a Bush DAC90A can work on either supply either way connected,i always wondered how DC supplied at components like capacitors the wrong way (polarity) didn't cause problems?
No it can't. It can work on an AC supply either way round, but on a DC supply only one way. In fact the instruction book specifically says, if it doesn't work on DC, swap the mains plug over.

True DC-only sets almost certainly had paper HT filter capacitors. They wouldn't mind what polarity they saw, so no protection against reverse polarity was necessary. Valves will only work one way round, but they're not harmed by reverse polarity so again no protection is necessary. Cathode bypass capacitors could be electrolytic, because the associated valve would stop any reverse voltage of these.

A transistorised set, now, is another matter. Reverse the supply polarity of this and you've likely killed it. But a valve set - just turn the plug around!

Last edited by kalee20; 14th Oct 2017 at 5:29 pm. Reason: Typos
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Old 14th Oct 2017, 5:56 pm   #6
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Default Re: AC/DC sets

AC/DC television receivers were designed to work on 200/250v Ac or DC.

When used on low voltage DC mains it was necessary to short out the rectifier to prevent the voltage drop across the rectifier.

The service manual usually mentioned that it was necessary to fit a non reversible mains plug and inform the customer of the need for this.

Reversing the plug under these conditions would of course result in a blown mains input fuse as high value electrolytics were employed. John.
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Old 14th Oct 2017, 6:04 pm   #7
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Default Re: AC/DC sets

Interesting comment on the need to short the rectifier for low voltage DC mains use. I had noticed that many early 1950's Bush AC/DC TVs were specified for 200- 250V AC, but only for 230 - 250V DC. I had always wondered why.
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Old 14th Oct 2017, 7:20 pm   #8
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Default Re: AC/DC sets

I never personally encountered DC mains....we were on 200Vac until the mid 1960's. However my brother remembers when one local area was on 200Vdc and it was common to short out the rectifier as the sets were not happy on the low DC mains. It was worse during the winter when the mains would occasionally dip to 190V. Without the rectifier shorted, the sets would often display a small picture. When the area was converted to to 240Vac they had to make sure that the rectifier was back in circuit.....
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Old 14th Oct 2017, 8:34 pm   #9
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Default Re: AC/DC sets

What's more, with DC mains I read somewhere that houses on opposite sides of the road could have opposite polarities.
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Old 15th Oct 2017, 11:50 am   #10
crusher19860138
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Default Re: AC/DC sets

Really interesting comments,especially about the DC supplies to houses!
I wonder how many areas really had DC mains by the time DAC90A's were made?
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Old 15th Oct 2017, 12:27 pm   #11
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Default Re: AC/DC sets

There have been a number of threads about DC supplies he is one such

http://www.vintage-radio.net/forum/s...ad.php?t=41380

Cheers

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Old 15th Oct 2017, 5:17 pm   #12
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Default Re: AC/DC sets

Quote:
I wonder how many areas really had DC mains by the time DAC90A's were made?
Probably didn't matter, an AC/DC set was a lot cheaper to make than an AC only one and would work on DC by default.
 
Old 15th Oct 2017, 6:19 pm   #13
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Default Re: AC/DC sets

There were certainly more sets capable of DC operation than there were DC supplies to power them. Even the clock radios of the day (with the noticeable exception of the Goblin TimeSpot) almost invariably consisted of an AC synchronous clock, with its timer contacts switching a transformerless radio chassis.

None of which is surprising, really; a big resistor instead of a transformer, half as much hookup wire for the heater connections and a rectifier valve that might be a few pence cheaper represented a significant cost saving, and the increased operating cost just to heat the dropper resistor would not be blindingly obvious -- especially not when the main source of illumination was tungsten filament light bulbs. The ability also to operate on DC was just a Brucey Bonus, at least unless you were unlucky enough to live in a DC area.
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Old 15th Oct 2017, 6:53 pm   #14
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Default Re: AC/DC sets

Its true houses on one side would have been wired with red positive main and the other side black negative main I read in an old electrical book that it was important to not get into the habit of assuming the black wire was at 0 volts when you had DC mains because it would be at some voltage NEGATIVE to earth
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Old 16th Oct 2017, 10:40 am   #15
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Default Re: AC/DC sets

I remember DC semi-detached houses near Mansfield where there was one 2-core lead-sheathed cable which ran up the wall to a big cast-iron junction-box; it was "centre-tapped 400V" and each house was wired between one of the two wires and the lead sheath.

The power came from the local pit, the houses having originally been built in the late-1940s by the Coal Board. Pit workers got their electricity free [just as they got a free coal allowance for heating]. There was much nuttering when they got changed to grid-supplied AC and had to have a meter for the leccy!
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