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Old 13th Nov 2019, 11:25 pm   #1
1100 man
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Default VE301W Radio. Dropping the voltage to the mains transformer.

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Originally Posted by Station X View Post
I've had the set running for several days with a 2W 390R wire ended resistor connected in series with the flying lead going to the mains transformer's voltage selector. The resistor is situated adjacent to the voltage selector in free air. The resistor gets warm, which is what it's intended to do. There's room on the chassis for what is effectively a heat sinked 15W resistor. It will dissipate far less heat then the dropper resistor in an AC/DC receiver. It will also provide a rigid point on which to terminate the wiring and make the mod easily reversible.
I had considered using a resistor in series with the primary of the mains transformer myself and as Graham says, this works perfectly well.

My only concern was what would happen in the event of the transformer developing shorted turns? I had planned to mount a thermal fuse on the transformer to open if it got too hot, but a series resistor would be seriously overrun under fault conditions. I guess a 2W one would just produce a lot of smoke and go open before too long?

What do others think? Having had a mains transformer catch fire in a '50's radio, I would like to guard against such failures if possible!

All the best
Nick
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Old 14th Nov 2019, 12:39 am   #2
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

Using the two watt resistor sounds good. Its cheap and relatively effective.

Joe
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Old 14th Nov 2019, 1:24 am   #3
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

There are specific fusible film resistors made to safely go open-circuit under overload- as many manufacturers used and use them in the supply feeds to modules, circuit sections, even individual ICs, they are widely available inexpensively in various values and wattages. In fact, many film type resistors will go open circuit fairly rapidly under overload but some types like carbon film with attractive lacquered coating can produce a picturesque curl of flame as they do so- types described as fusible film safety resistors are designed to avoid this disconcerting feature!

It can take a little thought and judgement, though, to select a value/rating that will be reliable in normal use but go open-circuit quickly under overload when there is something less drastic than a severe short.

In the past, where a rating of 2 to 5W or so was required, it was almost instinctive to use a small wirewound for long term-reliability- IMHO now that film resistors are available with these power ratings, I like to use these in vintage equipment as these should fail fairly quickly under overload, whereas some wire-wound types will run for a long time at high enough temperature to ignite their surroundings. I prefer the slight chance of the odd nuisance failure to incandescence!
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Old 14th Nov 2019, 10:11 am   #4
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

There are two issues here. Dropping the voltage and protecting the transformer. I think it should be borne in mind that as originally constructed all that protected the transformer was a 0.5A fuse on the primary side.
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Old 14th Nov 2019, 11:31 am   #5
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

I do tend to 'overthink' things, it's true! However, the VE301 that I'm doing is for a friend of mine, and while he is sensible, it will be outside of my control how it gets used.

What I'm trying to guard against is creating a situation where, under fault conditions, large amounts of noxious smoke or fire could result if it gets left on unattended.

That's exactly what happened to the radio I mentioned, left unattended (bad idea), the downstairs filled with smoke. When I finally got to the radio and turfed it into the back garden, it was still pouring thick grey smoke!

80 year old radio's with mains transformers are not my usual fare: I'm more at home with live chassis TV's with mains droppers, so the potential failure modes & dangers are very different!

A fusible resistor would have been my first choice, but when I looked a while ago, I could not find a supplier of such things. It would be better protection, in the event of shorted turns, than a fuse.

If anyone can point me to a suitable supplier, that would be great.

We have had many debates about how to make old equipment as safe as possible in the modern world, and it is not a simple subject!

All the best
Nick
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Old 14th Nov 2019, 11:42 am   #6
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

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Originally Posted by 1100 man View Post
What I'm trying to guard against is creating a situation where, under fault conditions, large amounts of noxious smoke or fire could result if it gets left on unattended.

That's exactly what happened to the radio I mentioned, left unattended (bad idea), the downstairs filled with smoke. When I finally got to the radio and turfed it into the back garden, it was still pouring thick grey smoke!

All the best
Nick
That comment is now available for every ambulance chasing claims lawyer in the world to see.

It implies that vintage radios fitted with mains transformers are inherently likely to burst into flames.

I'm beginning to think I should remove the dropper resistor altogether and advise the OP not to use the set on a UK mains supply. Perhaps I should also remove the three core mains lead and solid state rectifier with its dropper resistor?
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Old 14th Nov 2019, 6:09 pm   #7
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

Graham,
1100 Man's experience was with a different radio, under different circumstances.
I would think it overly risk-averse (to put it politely) to abandon this job having got so far.
Surely a suitable safety arrangement can be created?
There is a wide choice of fusible resistors available, see for example the R.S. website. Some simple calculations will give you a suitable value and wattage.
The chances of any serious fire would, I think, if the job is done with due dilligence, be reasonably remote. Tony.
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Old 14th Nov 2019, 6:40 pm   #8
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

Compared to several decades of domestic TVs safely used by millions with blisteringly hot droppers, hardboard backs and crowded circuitry including sweaty LOP valves, a modest radio with a single wisely positioned extra fusible film resistor is a negligible hazard, really. A component that combines the function of dropper and safety fuse sounds ideal in the circumstance, and it will also give a degree of limiting to initial heater over-current for scarce and elderly valves. An excellent use for those dual/quad solder tags on a ceramic pillar with a 6BA threaded base that probably populate many oddments boxes.
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Old 14th Nov 2019, 8:48 pm   #9
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

Why not fit a couple of thermal fuses (possibly rated for different temperatures) in intimate contact with both the transformer windings and the dropper resistor? That way, if the power transformer develops a shorted turn or something bad happens on the HT supply line causing too much current to be drawn, you will be sensing the inevitable temperature increase in two places.
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Old 14th Nov 2019, 8:57 pm   #10
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

We're entering a whole new world here. The implication contained in this thread is that all mains transformers in vintage radios need some form of protection to prevent them causing fires, not just those in VE301W's.

Were this any other radio capable of operating on 230-240VAC without modification I doubt the matter would ever have been raised.
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Old 14th Nov 2019, 9:33 pm   #11
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

This has got me thinking, by arguably extending the life of the radio long past what would be expected of it the original transformer is now subject to more years operating past the 70 or 80 years it's done so far which would increase the chances of its failure over the coming years compared to the chances thus far, the transformer develops a non fuse blowing fault and the fault causes very localized heating causing spontaneous combustion and it catches fire, so does the house.

Mr Trawlawyer wouldn't be able to contain himself in court, "Mr Experienced Long Time Repairer are you aware that insulation can have a tendency to break down after many many years" (Gulp) "Errrrrr sort of possibly"....."Answer the question yes or no"......check mate.

On the other hand you could forget about that and do what many would do, that's publish and be damned so to speak.

Lawrence.
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Old 14th Nov 2019, 9:49 pm   #12
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

Here's the dropper resistor mod as implemented at present. The 15W 390R wirewound resistor hardly heats up at all.

I'll be replacing it with a 390R 2W flameproof resistor mounted on a tag board with long leads.
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Old 15th Nov 2019, 12:09 am   #13
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

Hi Graham this is great, well done keep going, fantastic work! Whilst I do obviously realise that what has been talked about here is important safety wise and could be quite serrious, I think maybe we have just been spooked a little bit over the top.

You have lowered the mains voltage via the 390R 15W resistor and presumably the 0.5A fuse will remain for added protection. I don't see any reason to remove the 3 core mains cable modification, I can't see how there could be any problem with that whatsoever.

Graham, you've given this wonderfullradio set a new life, to be enjoyed by its owner for many years, you've also given this radio your most expert care and devoted attension.

Obviously on handing back the finished radio, to what undoubtably will be a very proud and grateful owner, I know it will be explained in great detail the things NOT to do.

Example, DO NOT leave the set unattended whilst in use, always unplug from the mains supply when NOT in use, always leave adequate ventilation arround the set. It's just common sense, Great work Graham you're doing brilliant, nothing going to go wrong

Paul
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Old 15th Nov 2019, 12:55 pm   #14
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

Surely what Graham has done on this set is only what any repair shop would have done.

He’s shown due diligence and clearly not been negligent. He may have replaced the rectifier and mains lead with more modern parts, but original parts were difficult to obtain (rectifier valves, capacitors) or clearly inferior (rubber mains cable).

It may be worth warning the owner that a set of this age should not be left unattended as you might with a more modern set, but wouldn’t that be akin to a garage repairing a Ford Anglia, and having to warn the owner to be careful in slippery conditions because it didn’t have ABS brakes?

Stuart
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Old 15th Nov 2019, 2:21 pm   #15
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

I second everything that Stuart has said in post #14, and also think some of the previous comments, whilst undoubtedly well-intentioned, may have over-emphasised the risks involved. However I wonder why the 15 watt 390 ohm resistor isn't perfectly adequate as a permanent dropper. As Graham has pointed out, it runs cool and the chassis should/will act as a substantial heat-sink. I admit that, without checking the data sheet for those resistors, I'm not sure if they're flame-retardant, but as, in this case it is being run well within it's ratings, how can it be any kind of risk or problem? I know it's not quite the same thing, but some years ago I re-built two output meters with 25 watt versions of those resistors, all mounted on the metal cases (inside of course!) of the meters, and have never had any trouble with them.
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Old 15th Nov 2019, 2:28 pm   #16
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

The perceived problem is that if the transformer goes faulty an awful lot of power (heat) could be generated in the transformer itself and the dropper resistor, possibly leading to a fire. Therefore the lower the wattage rating of the dropper resistor the better, as it will give up the struggle at an early stage and go open circuit acting as a fuse.
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Old 15th Nov 2019, 3:47 pm   #17
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Default Re: VE301W Radio. Dropping the voltage to the mains transformer.

With the extra surge protection of 390 ohms in series with the transformer it may be possible to reduce the mains fuse to 250mA slow blow.

Al
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