Woolworths Christmas Tree Light Repair.

[nontechnicalian]

Split from this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=54943


I have had a set of 1950s Woolworth lights since my father died 10 years ago. They are the ones I remember as a child and have great sentimental value. Over the years the original figural bulbs have been replaced by ordinary cone bulbs, until there were only a few left. So this year I looked on eBay for working bulbs and bought enough figural bulbs to look like the original. However pretty much each time I turn on the bulb nearest the plug blows. (I've now lost 7 in the space of four days) This hardly ever happened before, so I guess there must be a problem with one or more of the new bulbs.
All the bulbs are original 20V 3Watt 0.15Amp types so shouldn't really blow, especially since they are on a 16 string set. (Which by my calculation means they are only being supplied at 15 Volts anyway) I've tested each bulb and they all have low resistances -is there anything else I can check? -Would welcome your advice.

[Hartley118]

These days I've mainly converted to LEDs, but I do have a clear memory of working with those vintage 20 volt 3 watt bulbs. That current rating of 0.15A is probably a nominal figure and some of the bulbs may be of a lesser rating and so dropping more than 20V which could account for the failures, particularly at the switch-on surge when the filaments are cold and hence of lower resistance.

There's also the issue that as bulbs get older, tungsten tends to evaporate from the filament - if they were clear, you'd see it as blackening of the glass. This loss of filament metal increases the resistance and hence the voltage drop when operating in a series string. To avoid losing too many bulbs, I used to have to sort out my various strings into high medium and low current ratings, even though they were nominally similar.

I'd recommend connecting 4 or 5 more bulbs in series with the 15 you have so that they're all under-run. You may have to reject one or two lowest resistance bulbs that leave too many volts to be dropped by the rest of the string. If you want to be scientific, you could sort this out with a voltmeter and measure the drop across each bulb. That would require an adaptor made of a spare bulb base and bulbholder wired together so that you can get your voltmeter prods in there. As some of the bulbs have sentimental value, you may wish to take this trouble.

As ever, there's a health warning - of course assume that any part of the string could be at live mains potential, so be careful with the voltmeter prods!

Martin

[nontechnicalian]

Originally Posted by Hartley118

If you want to be scientific, you could sort this out with a voltmeter and measure the drop across each bulb. That would require an adaptor made of a spare bulb base and bulbholder wired together so that you can get your voltmeter prods in there.

Thanks for the reply Martin, but that's a bit out of my league I think. The most techy I do is using a multimeter. I've read other threads about using an in line diode or an isolating 180V transformer, but I wondered if something like this would be good solution?

http://www.ebay.co.uk/itm/Mains-Dimm...-/310415989317

If I switch it on at the lowest setting I should avoid the initial surge that keeps blowing the first one and then put it on half way or two thirds?
What do you think guys?

[Hartley118]

Your suggestion looks to be a good way of minimising switch-on stress.

Martin

[The Philpott]

nontechnicalian I am wondering if you have a rogue bulb elsewhere in the string, one that is going through a process of arcing,welding, arcing and so on. If the filaments aren't readily visible i don't know how we are going to find it. My first thought would be to pull each bulb out separately, power it at nearly 3volts in one of those little MES black 'project' bulbholders, and give it a flick to try and flush the fault out. One could also measure the current taken, and compare.

I would suggest the bulb(s) adjacent to the 'repetitive expiring socket' would be the first ones to be suspicious of. i would tend to replace both if possible and see what happens.

if i come across any figural bulbs in my travels next year you can have them- i have no MES sets to use them in.

[broadgage]

If lamps are failing unduly frequently, despite extra lamps being inserted in a series string, then that suggests to me that several lamps are dropping much less than the correct voltage and thereby overrunning the other lamps.

Possibilities include
Some lamps are of the correct voltage but higher wattage, for example 20 volt 6 watt rather than 20 volt 3 watt. Any such lamps will be under run and last almost indefinitely, but the other lamps in the string will be over run and fail quickly. Not very likely as 20 volt 6 watt lamps are very rare.

Or some lamps are the correct wattage but the wrong voltage. A distinct possibility, since 12 volt, 3 watt lamps are fairly common and look just like the 20 volt 3 watt ones. A 12 volt 3 watt lamp in a series chain of 20 volt 3 watt lamps will only drop about 4 volts. Therefore a couple of such lamps might drop 8 volts in total rather than 40 volts in total. An extra 32 volts on the remaining lamps is significant.

Another possibility is that some lamps are the correct ones but are faulty.
Sometimes the filaments get twisted in a loop and a large part of the filament thereby shorted out. So a 20 volt 0.15 amp lamp might be turned into say a 6 volt 0.15 amp lamp. The other lamps are thus subjected to an extra 14 volts. Significant if several lamps are faulty.

A crude test is to remove the lamps and test them on a 12 volt battery. They should light but only dimly. Any lamp that lights brightly should be rejected as it is either a 12 volt lamp, or one with a partly shorted filament.

A more sophisticated test is to obtain say 15 MES batten holders and screw these to a piece of board. Wire them in series, insert the lamps and connect to the mains, Then measure the voltage across each lamp, they should all be about the same and about 16 volts. TAKE GREAT CARE IF DOING THIS. REMEMBER THE MAINS VOLTAGE IS PRESENT ON THE EXPOSED LAMP HOLDER TERMINALS.

[The Philpott]

I meant to say 20volts not 3volts. (but bearing in mind post #6 12volts seems more advisable)

[nontechnicalian]

Many thanks for your replies peeps and especially The Philpott for offering to send me some bulbs in the future if he comes across any.

The suggestion to use 16 x MES batten holders is a good one I'm sure, but at a cost of £35 (they cost £2.19 each) it doesn't really appeal. However, I am up for buying one and testing each lamp. I have an old power supply for something I've probably thrown out which lists its output as 12V 1A 12VA; would that do the trick in terms of supply to the batten holder?

Lastly what did you mean by 'give it a flick' The Philpott?

[broadgage]

MES bulb holders are available at much lower prices than you have found.
A quick look on ebay found a pack of 10 for £5-50.

Yes the 12 volt power supply that you have should be fine. Connect each lamp in turn to the 12 volt supply and observe that it lights dimly. Any lamp that lights brightly should be rejected as it is either defective or of the wrong voltage.

If possible measure the running current at 12 volts of each bulb, they should all be very similar. Reject any that draw significantly more than the average.

[The Philpott]

i meant give it a tap on the side to show up if the filament is intermittent, shorting, or has come loose from it's support leg/s. You should see a flicker or a moving shadow inside the bulb if this is the case.. or it may just extinguish itself.

Dave P

[nontechnicalian]

Originally Posted by broadgage

MES bulb holders are available at much lower prices than you have found.
A quick look on ebay found a pack of 10 for £5-50.

Apologies Broadgage, I only looked on Maplin's website for prices, but I've ordered a bulb holder online and I'll pick it up tomorrow. I will carry out the tests suggested by yourself and Dave Philpott, but I'm not quite sure what you mean by 'if possible measure the running current at 12 volts of each bulb'.

-If I set up a simple circuit with my 12 volt power supply to the batten and insert a bulb, will I be able to measure the running current by applying my mulitimeter to the terminals?

[The Philpott]

The multimeter (start with the highest current setting and work downwards if required) goes in series with the circuit, ie current (amps) measured THROUGH the circuit- {the meter becomes part of the circuit.} If you were measuring voltage you would measure it ACROSS something. I hope this makes sense but say so if not.

It should also be said that a 12volt power supply (if an unregulated type) can chuff out much more than 12v if lightly loaded. (Someone may have already mentioned this.)

[nontechnicalian]

Originally Posted by The Philpott

The multimeter (start with the highest current setting and work downwards if required) goes in series with the circuit, ie current (amps) measured THROUGH the circuit- {the meter becomes part of the circuit.} If you were measuring voltage you would measure it ACROSS something. I hope this makes sense but say so if not.

I think I need to speak up at this point Philpott!
I was going to set the multimeter on the the current setting and then put the red probe on the terminal tag the black/white striped transformer wire goes to and the black probe on the other terminal tag. -Is that right?

[The Philpott]

If you are using a modern digital multimeter it most likely has reverse polarity facility- but check the instructions. If it does have this facility it does not matter which way round you connect the meter for this current measurement, but it does matter HOW you connect it.

The best power supply would be one that has variable voltage setting, then you can ramp up the voltage to avoid blowing the bulb by applying too much voltage (see my comment in post #426)

Set up the bulb to be powered by the power supply first. If all is well, switch off then break into the Low Voltage part of the circuit at any point you choose, and with meter set at highest current setting insert the meter into the circuit. Switch on and note the reading.

If you are not too confident about this i can duplicate the test circuit tomorrow night on the bench and photograph it for you.

[broadgage]

Originally Posted by nontechnicalian

I think I need to speak up at this point Philpott!
I was going to set the multimeter on the the current setting and then put the red probe on the terminal tag the black/white striped transformer wire goes to and the black probe on the other terminal tag. -Is that right?

No, it is not right.
Connect one wire from the transformer to one terminal of the lamp holder.
Connect the other wire from the transformer to one of the test meter leads.
Connect the other test meter lead to the vacant terminal of the lamp holder.

Presuming that the transformer has a DC output, set the test meter to read DC Amps.

If the meter is a digital type, then it may give a negative reading, do not worry.
If the meter is an analogue type, and the needle moves the wrong way, then swap over the two test meter leads.

If the transformer has an AC output, then set the meter to read AC Amps, otherwise as above.

[nontechnicalian]

Thank you for that, I should have realised that's what was meant.
Bearing in mind what was said about the AC adaptor I tested its output and it was as high as 14V AC. I'm not too worried about blowing any 12V bulbs, because they shouldn't be in the string anyway, but is this a little too high for the 20V bulbs?
I have no 12 V battery to hand (its an odd size), but I could wire up a 9V PP3 if that's a better course of action.
-What do you think?

[Herald1360]

There's a 12V battery on your car. Be very careful not to short it! It won't give any electric shocks but it has the capability to deliver very nasty sparky thermal ones!

[broadgage]

Connection to a car battery is potentially hazardous for the novice due to the vast short circuit current, I advise against use of a car battery for this sort of testing.

I would avoid over-thinking this and simply use the 12 volt transformer that is to hand, connecting as previously suggested.
The 12 volt transformer may well deliver a bit more than 12 volts if lightly loaded, but I would not worry about this.
14 volts is fine for 20 volt bulbs, and almost certainly OK for a brief test of 12 volt bulbs.

[nontechnicalian]

Originally Posted by broadgage

The 12 volt transformer may well deliver a bit more than 12 volts if lightly loaded, but I would not worry about this.
14 volts is fine for 20 volt bulbs, and almost certainly OK for a brief test of 12 volt bulbs.

Well the results are in!
I went through all the bulbs one by one and measured the current going through them. Most started off at 2.5A and then quickly went down to between 0.6 and 1.0A. There were 3 above 1.0A but the top one only consumed 1.3A.
Interestingly a couple flicked around by 0.2 or 0.3A without assuming a steady state. All the filaments seemed steady though with a little flick.
None of the bulbs blew under test and all looked bright without looking over-bright, so I don't think there are any 12 Volt bulbs in there.

Not sure I've learned anything about them TBH, but I do now know how to measure a current!

I've attached a file of my setup and will post a pic of the string before we take down the tree early next year.

Any other suggestions peeps? -Many thanks for your help so far with this.

[The Philpott]

See my thumbnail pic. Noting the comments above regarding analogue(needle) meters versus digital ones (and also noting whether supply is AC or DC)

V1 is a voltmeter (not strictly necessary with your test)
V2 is also a voltmeter and is needed for comparison between bulbs.
A is an ammeter (current) and is also needed for comparison between bulbs.

The dotted lines are just for clarity, they represent ordinary wires.

Now you see what i mean by measuring voltage (across) things and amps/current (through) things

Good luck!