Help with Old Musical Christmas Lights

[SLE_71R]

Hello all,

I happened upon this forum while searching for some information on vintage Christmas lights. Seems there are many knowledgeable folk here, so I was wondering if anyone might be kind enough to help me.

The lights in question are a set of 120, musical with controller and powered by a 24V transformer. They have been in use each year for 20+ years, and last year a bulb finally went. Unfortunately, I have no information which tells of what voltage/wattage/current bulbs this set uses.

I replaced the blown bulb with one rated at 2V 0.29W. It was brighter than the rest, but also a clear bulb (this is a multicoloured set). It ran ok during the month of December last year, but wondering if other bulbs may be being over run this way.

With the failed bulb, 15 go out, and the rest remain lit. Based on this, would it be correct to assume that this set is wired in 8 groups of 15, each individual section being wired in series? And given the 24V transformer, each bulb may be 1.6V (24/15)? Or is this unlikely, since the controller lies between the transformer and bulbs, and is probably supplying an altogether different voltage?

Some digging online reveals that Woolworths looks to have sold a very similar set (120 musical with controller), and it is stated on the packaging for those that replacement bulbs are 2.4V 50mA 0.12W. This could be useless information though, as that is ultimately a different set. 2.4V bulbs seem much more common than 1.6V bulbs.

Is there a way I can work out what the exact bulb specs are for this set? I tried to work up a diagram showing how they are wired, but it soon became clear this wouldn't be an easy task given the number of twisted wires. If exact specs cannot be deduced, I suppose it would be preferable to have the replacement bulb be over run a bit, as opposed to other original bulbs - would this just be a case of trial and error, seeing what brightness results with different bulbs?

Lastly, upon opening the controller I've been greeted with a bulging capacitor. For years, when powered on loud-ish buzzing would emit from the speaker, before quieting down after some time. The other, smaller caps aren't bulging, but would it still be advisable to replace these too?

Attached are some photos - any advice much appreciated. Thank you!

[Glowing Bits!]

If I remember correctly, the bulb should be 2.4v 0.29w, or something like that anyhow.
Ebay sells plenty of bulbs for these low voltage sets.

[SiriusHardware]

I wouldn't bother with the other caps but certainly replace the domed one and it looks as though the bridge diodes D1-D4 have been running too hot for too long, part-failure of one of those could be the real reason the capacitor is distressed - so at a minimum, replace the capacitor and the diodes as a set. When fitting the replacement diodes mount them raised up a bit to allow airflow underneath them so they can run a bit cooler.

The question of how the lamps are wired is a bit of a puzzle, your photo of the control unit shows five connections to the lamp array, one marked 'C' (likely 'common') and only four outputs to lamp strings, each individually switched by transistors Q4-Q7.

Given no further information, that would suggest that the 120 lamps are in four strings of 30, but your observation that only 15 lights are off when one is removed suggests that actually each of the four strings of 30 really consists of two parallel strings of 15, so removing a bulb from either parallel leg of any of the four lamp strings only causes 15, rather than 30, bulbs to go out.

When you feed 24V AC into a bridge rectifier and smoothing capacitor, the resulting DC voltage is usually a lot higher than 24V, so, assuming the above reasoning is correct, replace the bad capacitor and stressed diodes, then measure the actual DC voltage across the new capacitor.

Your bulb voltage will then be somewhere in the area of (Measured DC Voltage, divided by 15).

[SiriusHardware]

I forgot to say that where all else fails, if you can remove a bulb from a working string of the set and extend the connections out of the socket to the bulb by whatever means necessary, then -

-Measure the voltage across the working bulb. It is likely that the bulb will be slightly overrated volts wise, so round up the measured voltage to the next commonly available bulb voltage.

-Break into one side of the connection to the bulb, put a current meter in series with it and measure the current being drawn by the bulb.

The wattage of the bulb is the voltage across the bulb, times the current (in Amps) being drawn by it.

The above measurements should be made only after the bad capacitor and stressed diodes have been replaced, and with the lights program set to static / 'steady on'.

[The Philpott]

There may be enough space to upgrade the 10volt rated capacitor to a 16volt one, at minimal cost. If you discard any bulbs save the green bases from them if they're not glued on- it's all too easy to get a packet full of suitable voltage/wattage rating bulbs then find out that the bases are different.

Dave

[SiriusHardware]

Does 10V for that capacitor strike anyone as badly underrated? If that's on the output of the bridge (input=24VAC) then....()

[Glowing Bits!]

The capacitor is likely to be for the electronics only, it is likely to have a dropper resistor somewhere in circuit.

[G6ONEDave]

The internal view pic seems to show 7 wires and 4 output semiconductors, so 2 wires for input power, 4 controlled outputs for the 4 channels and a common wire to all the channels. To work out the bulb voltage, count the total number of bulbs then divide by 4 and then measure the supply volts to one channel. Once you know what the channel voltage is divide that number by the number of bulbs in the channel. That should give you an individual bulb voltage rating. This takes it the bulbs are not wired in parallel but you will know that once you know the voltage.

Dave

[emeritus]

Even if the bulbs are glued in, it should be possible to crush the glass of the dead bulb using a pair of Mole grips (other brands of locking pliers are available) without making the plastic bases unusable. The advantage of locking pliers over ordinary pliers is that, when appropriately adjusted, great force is exertable on the glass via the base with no danger of completely crushing the plastic base when the glass breaks.

[Glowing Bits!]

The thought of a bulb breaking is enough to make me run off and hide! I have a broken bulb phobia sadly.

[SiriusHardware]

The photo of the bulb in the OP shows that these are wire-ended types inserted into plastic bases with their wires pointing straight down so they pass through the holes in the base. Then, the wires are folded up along the outsides of the base.

It may be possible to release the bulb from the plastic base by bending both wires so they point straight down, then push on the wire ends to eject the bulb from the base.

[SLE_71R]

Wow, thank you all very much for your detailed replies!

Thankfully, the bulbs aren't glued to their bases, so they can indeed be removed without issue. I'm trying to sort a few other vintage sets out, and I (foolishly) assumed that the vast majority of bulbs could be freed from their bases, and found myself in the predicament mentioned. I bought some spare bulbs, only to learn they are glued in, and thus can't be used... if anyone has some short W1 bulbs spare, please let me know! But I digress.

Yes, it looks like the board has seen some heat from those diodes. I haven't looked up close at them, but will there be enough information on each diode to discern what specs the replacements should be? I'm struggling to see any useful information on them from the photos I've taken, but will give them a proper look during daylight.

I can't imagine much airflow is happening inside this casing - there are virtually no holes. Would it be worth drilling some to allow the heat to escape in the region of these diodes?

I will try to find a cap that is 16V - nice tip! Thanks again to you all for taking the time to reply.

[Herald1360]

Are the devices switching the bulbs actually transistors? Small triacs also feature in this sort of light string controller.

[SiriusHardware]

That's a thought, maybe the lamp strings are running on AC controlled by triacs. The screen printing (Q4-Q7) would normally be used to identify transistors, though. I could expect some other designation if they were SCRs or Triacs.

[Glowing Bits!]

If the OP is still looking for W1 bulbs, I should have 80 that was taken out of a scrap multifunction set, the controller died sadly.
As for the output, the triac devices usually switch 24v AC, unless LED's are used then it involves a rectified supply.

[SiriusHardware]

I had originally thought that maybe the diodes had run too hot because of a prior fault on the lamp strings, but that could only be the case if the lamps were running on DC from the output of the bridge.

If not, you have to wonder what it is in the control circuit which has been drawing so much power through the bridge - unless the capacitor has failed low-resistance of course.

[The Philpott]

- If it can be made to operate as intended....it may not need extra cooling. As the controller only sees 24v you could run it with the cover off once the diodes and capacitor are changed and see how hot things get.
Dave

[SLE_71R]

Looks like the flux was never cleaned properly when the board was soldered?

I can only make out three numbers on the diodes - 004. I can't tell if there are more numbers/letters on the underside, which is hard to see. Guess I will find out when desoldering them?

I was surprised to see the caps in this are all 105c rated. Regarding replacement, should much attention be paid to finding one with or over a particular endurance rating? (i.e. worthwhile going for something over 5000h, or...?)

I do have a Seek thermal camera, so could indeed get an idea of how hot it runs once up and running again

[Cobaltblue]

Please post the pictures in the thread.

Cheers

Mike T

[SLE_71R]

Originally Posted by Cobaltblue

Please post the pictures in the thread.

Cheers

Mike T

Hello Mike, apologies. I thought it would be nice to present some high resolution photos, as attachments seem to get scaled down once uploaded. No worries though - is my reply still accessible? I haven't got a copy of what I typed out.

Please omit the links and approve if possible, or proceed in whatever way is appropriate. I will then post the photos as attachments.