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Old 10th Apr 2021, 4:48 pm   #1
Niechcial,Steve
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Default DC Mains on Pre-War TV's

Recently I came across a fascinating item for sale on Ebay. It was a service manual for a power converter unit for Philips TV sets enabling use on DC mains and it is dated 1936. It wasn't expensive so I bought it.
It basically describes a vibrator system feeding a full wave double-wound transformer. The input voltage goes from 110v to 245v. Unfortunately, the manual doesn't give an output voltage. However, whatever the output voltage is, it is not 'standard' AC mains because the TV set itself has to be the 'universal model' i.e one fitted with a different mains transformer. 'Standard' models could be converted to 'Universal' by changing their mains transformers.
There was a lot of DC mains around in London in 1936. All TVs of course had mains transformers so I guess other manufacturers had similar provision. Does anyone know anything about this? Also intriguingly, this service manual starts out ' Instead of the former method of construction of Universal Receivers, use is now made of a vibrator and transformer'. What could the 'former method' possibly be?
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Old 10th Apr 2021, 4:55 pm   #2
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Default Re: DC Mains on Pre-War TV's

Fascinating find! I've never come across a vibrator-PSU intended for anything other than 6 12 or 24V DC input.

I'm not aware of any pre-WWII TV valves that were designed-from-the-outset for series-heater wiring which would have been the obvious way to do DC-only supplies. Indeed, some of the first-generation of post-WWII supposedly-intended-for-series-connection TV valves had heater-cathode insulation that could at best be described as 'problematic' - yes, Brimar. I'm looking at you.
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Old 10th Apr 2021, 5:33 pm   #3
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Default Re: DC Mains on Pre-War TV's

I doubt many, if any, valve line- ups had consistent heater current throughout. You could of course get round that problem with shunt resistors but you would still have needed a mains transformer to get the 4 or 5 KV for the tube, and in most cases, high enough voltage (in excess of mains voltage) for the HT lines
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Old 10th Apr 2021, 7:18 pm   #4
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Default Re: DC Mains on Pre-War TV's

Was a rotary converter used to operate a pre-war TV on DC mains? Basically a DC motor and AC generator on the same shaft.

DFWB.
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Old 10th Apr 2021, 9:36 pm   #5
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Default Re: DC Mains on Pre-War TV's

Several of the sets here can be used with a converter and some do mention "rotary".

Peter

Last edited by peter_scott; 10th Apr 2021 at 9:42 pm.
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Old 11th Apr 2021, 5:49 am   #6
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Default Re: DC Mains on Pre-War TV's

AFAIK, some early TV sets DID work directly from DC mains, series heater chain as in a valve radio, with HT straight from the mains.
EHT either flyback derived, or a step up transformer supplied from a valve oscillator.

I recall reading on this forum of a VALVE inverter being used to supply an "AC only" TV set from DC mains. Possibly home made rather than a ready made product.
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Old 11th Apr 2021, 8:15 am   #7
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Default Re: DC Mains on Pre-War TV's

As far as I know, Valradio offered a valve-based inverter (using KT55s) for powering tape recorders from c.1960. Whether it had something similar for TV receivers I don’t know. But it had been in the vibrator-type DC-to-AC converter business since the late 1930s, offering a wide range for radio, audio, and TV applications, with DC input voltages from 6 to around 250 volts. By the late 1960s I think it had moved across to transistor inverters.

Re “home-made”, I understand that P.J. Walker of Quad developed an oscillator-with-a-Quad-II-pair combination to power turntables and tape decks for his public demonstrations.


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Old 11th Apr 2021, 10:11 am   #8
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Smile Re: DC Mains on Pre-War TV's

Hi,
I have a valve inverter that was given to me by a seafaring colleague years ago.
It wasn't big enough to work a television, but just a record player or small radio.
It has six PL81s in parallel push-pull driven by an ECC81 oscillator. There's a dropper for the heaters.
I must dig it out and see if it works. But I'd have to rectify and smooth the mains first before inverting it back to AC.
Cheers, Pete.
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Old 11th Apr 2021, 11:07 am   #9
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Default Re: DC Mains on Pre-War TV's

As David has mentioned the preferred method of conversion would have been the rotary converter. I have some EMI TV data somewhere that actually mentions this.

Radiograms were often run this way from DC mains. I'm not sure of the make but it might have been McCarthy [David tells me it was an ACE] that produced a radiogram with an electronic converter mounted in the base of the cabinet utilizing two 50CD6G power line output valves.

During the early to mid 1960's I had a mate with a shop in York Road Battersea London. He was on DC mains making it very difficult to demonstrate equipment especially record players etc.

He had two very large what looked like soundproofed [!] rabbit hutches in the back yard that made quite a racket giving out a crude AC to power the display area. It was far from perfect probably due to the lack of maintenance of the motor generators. They gave a very smooth output when properly maintained.

The shop was demolished soon after to make way for local authority flats. I think that was probably the last domestic DC in the London area. John.
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Old 11th Apr 2021, 2:35 pm   #10
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Default Re: DC Mains on Pre-War TV's

Circuit diagram of the gram motor AC supply generator in the Ace RG354U.
Two 50CD6G power tetrodes were employed in this application.
To supply a pre-war television set a purely electronic DC to AC mains converter would have been completely out out the question. No doubt such a device could have been made but imagine the huge valves needed to supply for the power demands of a TV receiver?
The answer was an electro-mechanical solution, the rotary converter.

DFWB.
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Old 11th Apr 2021, 2:51 pm   #11
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Default Re: DC Mains on Pre-War TV's

I seem to remember that in that ACE radiogram, the radio/amplifier was an AC/DC mains design, heaters in series, rectify the mains to get the HT+. So the converter only supplied the turntable motor. Also there was no rectifier for the anodes of the converter valves, there didn't need to be as the converter was only used on DC mains.

Did any company ever make a turntable where a 50Hz or 60Hz mains synchronous motor was powered from a transistorised converter, allowing it to be used in Europe or America? The reason I ask is that I have a Tektronix PDP11 (by which I mean a Tektronix computer with a DEC PDP11/03 or PDP11/23 CPU board in it) where the 8" floppy drive spindle motors are powered by a transistorised circuit running off the 350V DC in the switch-mode power supply. Thus the drives run at the right speed with no modifications no matter what the mains frequency is.
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Old 11th Apr 2021, 4:19 pm   #12
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Default Re: DC Mains on Pre-War TV's

The Linn Valhalla PSU is an example that drives its motor via an output stage running from rectified (or voltage doubled) mains.

Re. rotary converters, there is a significant operational difference between a rotary converter (one machine with both AC and DC external connections) and a motor-generator (two machines - motor and generator - coupled together).

For DC-AC conversion, the motor generator is more flexible and adjustable. The frequency can be set by adjusting the motor field current and hence its speed, while the output voltage can be adjusted independently via the generator field. There is interaction, but in principle both parameters are adjustable.

In the rotary converter, the input and output voltages are inextricably linked because the two windings, or two connections to the same winding, operate within the same field system. Adjusting the field alters the frequency but there is no easy way to adjust the voltage independently within the machine itself; a separate tapped autotransformer would be the simplest method.

I don't know whether it has any bearing on the choice of mains transformer in sets used with a rotary converter, but it seemed worthy of mention. I was wondering whether in the days of frame scan being mains-locked a converter outputting a wild frequency would have been prone to generating hum bars that moved in response to beam current. I suspect a rotary converter might have the edge on frequency stability vs. load.
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Old 12th Apr 2021, 10:10 am   #13
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Default Re: DC Mains on Pre-War TV's

Thanks everybody- fascinating debate. Like David, I don't think an all electronic converter would have been an option for a pre-war tv due to the high loads. It sounds like the majority would have gone for a rotary converter as any standard set would work on these. The downside I guess would be a place to put one where the noise is not too intrusive
My Philips converter manual mentions 'additional parts'. These are most Cs and Is presumably to suppress interference from the vibrator. But one item also mentioned is a motor from the Garrard RC5 or Garrard U5. Presumably this applied to Philips TV radiograms. But why would you need to change them? I can only think that the frequency of this converter was different to standard 50 hz AC . It's frustrating that it doesn't need give details. Let me know if anyone is interested and I'll scan it and post it
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Old 12th Apr 2021, 11:00 am   #14
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Default Re: DC Mains on Pre-War TV's

I wonder whether the requirement for the specific mains transformer in the set for use with the vibrator follows from it outputting a non-standard frequency rather than a non-standard voltage. Perhaps the universal type has lower loss or more iron? Perhaps the vibrator unit has a tuned output filter that might reveal the frequency?

No hurry at all but it would be nice to see the circuit as I'm interested in early power supply arrangements generally. In the collection here there are both vibrator and rotary units for public address amplifiers. Cinema amplifiers were also sometimes equipped with rotary converters energising the amplifier's normal mains transformer from either DC mains or the 110V house DC, at 60Hz in the case of USA-made units.
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Old 12th Apr 2021, 5:24 pm   #15
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Default Re: DC Mains on Pre-War TV's

Quote:
Originally Posted by TonyDuell View Post
I seem to remember that in that ACE radiogram, the radio/amplifier was an AC/DC mains design, heaters in series, rectify the mains to get the HT+. So the converter only supplied the turntable motor.
They made record decks with DC motors. The famous Garrard RC75 was made with a brush type motor. I was asked to have a look at one a few years ago - I hadn't come across one like this before. I don't know what the original arrangement was, but there was a large(ish) rheostat in a cage on the underneath of the deck plate, so the motor must have been some lower voltage than you'd normally expect. The motor wouldn't run properly at any voltage and seemed to have shorted turns somewhere on the armature, so just wasn't worth messing about with. I told the owner that if he could source a good AC motor for the deck then I'd fit it for him. He did come up with a replacement motor - from the States, it was 110/120 volts 60 Hz, so after re-linking the windings on the motor for UK mains, I swapped the pully from the old DC motor to correct the speed and the deck worked perfectly. The old motor I think was AC/DC, as there was no rectifier fitted anywhere. This was over ten years ago, so struggling to remember the finer details.
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Old 12th Apr 2021, 6:47 pm   #16
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Default Re: DC Mains on Pre-War TV's

It's almost certain all-electronic converters were made for special applications before WW2. All the necessary components were available, high power valves, transformers etc.

DFWB.
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Old 27th Apr 2021, 5:45 pm   #17
Niechcial,Steve
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Default Re: DC Mains on Pre-War TV's

Here are scans of the circuit
Attached Files
File Type: pdf Converter p.1.pdf (1,007.4 KB, 37 views)
File Type: pdf Converter p.2.pdf (1,012.6 KB, 36 views)
File Type: pdf Converter p.3.pdf (759.0 KB, 32 views)
File Type: pdf Converter p.4.pdf (905.7 KB, 28 views)
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Old 27th Apr 2021, 9:21 pm   #18
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Default Re: DC Mains on Pre-War TV's

I have a couple of early 1950's Garrard catalogues. The RC 75 (and RC 80 and some, but not all, other models) were available with five different motors:

* AC only: dual voltage range, 100/130V and 200/250V, 50Hz as standard, pulleys for 40 and 60Hz available on request;

* DC only, 6V;

* DC only, 12V;

* DC only, dual voltage range, 100/130 and 200/250V;

* AC or DC, dual voltage range, 100/130V and 200/250V, 25 - 100Hz.

[Lynmouth had a 100V, 100 Hz public supply until its power station was destroyed in the disastrous 1950's flood ]

No mention of the need for any rheostats etc for these models, only that

" Units fitted with universal or DC motors will require an extra depth of 1 1/2".

However, the description of a universal AC/DC, dual voltage, motor for 78rpm only, does say that it incorporates an internal rheostat.

The DC & universal models were more expensive, approx £5 for the 6V & 12V models, £6 for the DC, and £8.50 for the universal, on top of the £17 cost of the AC-only model.

Last edited by emeritus; 27th Apr 2021 at 9:48 pm. Reason: typos, 78rpm motor note added
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Old 27th Apr 2021, 9:33 pm   #19
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Default Re: DC Mains on Pre-War TV's

Quote:
Originally Posted by broadgage View Post
AFAIK, some early TV sets DID work directly from DC mains, series heater chain as in a valve radio, with HT straight from the mains.
Ferguson 941T is an example from around 1949.
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