Christmas Ornament repair - advice on resistors please

[Welsh_Mike]

Hi,

May I have some help please identifying some resistors. We have a Christmas ornament with some sentimental value where some of the LED’s (red and yellow) and a little motor that revolves a miniature christmas tree have stopped working, whilst the other LEDs (blue and green) work fine.

Pic 1 is the ornament (with working blue and green LEDs).
Pic 2 is the rating of the transformer powering it - it can optionally work off three AA batteries but we never do that. (But I did test it on batteries today and it made no difference to the fault).
Pic 3 is the tiny board with four resistors which is the heart of the device.
Pic 4 is the hideous soldering at the back of the board.

Numbering the resistors from the left in pic 3 -

the transformed power comes in through the top of the large resistor to the left #1, the base of which is connected to the bottoms of the other three resistors and to the feed from the empty battery compartment.

The tiny one, #2, feeds the motor that has stopped working.
#3 feeds the red and yellow LEDs that have stopped working.
And #4, to the right, feeds the blue and green LEDs that still work.

All connections appear to be sound. If I apply power to the system the #2 resistor gets very hot, very quickly.

I am pretty competent with a soldering iron (1st ever lesson in metalwork at my Birmingham technical grammar school 62 years ago.) My soldering iron use these days generally is in other hobby work, not electronics. I would like help to know what (and where, as Maplins have disappeared from Cardiff) to buy so I can replace that too cramped little board.

Many thanks for any assistance you can offer.

Mike

 

[Lightium]

What is your question? Do you need help with resistor color codes?

 

[Welsh_Mike]

Sorry Lightium, I am often not very clear about what I want - I get too verbose. Yes, I do need some help with the meaning or identification of the resistor codes please.

My readings of the codes are -
#1 - Red, Red, Silver, Gold = 0.22Ω (0.209Ω-0.231Ω) presumably working
#2 - Red, Black, Black, Gold = 20Ω (19Ω-21Ω) presumably failed as motor not working.
#3 - Brown, Brown, Red, Silver = 1100Ω (990Ω-1210Ω) presumably failed as red/yellow lights don't work.
#4 - Blue, Red, Gold, Gold = 6.2Ω (5.89Ω - 6.51Ω) presumably working as blue/green lights work.

I would expect to replace all four resistors. But I would like to have a second opinion on my interpretation and calculation of the codes. What particularly worries me is the #3 calculation of 1100 Ω which seems way out of line with the other resistors, particularly when it appears to be doing exactly the same job of driving the same munber of LEDs as #4, just different colours.

Thanks for your response.

 

[Lightium]

Your interpretation is good. Have you written a schematic to follow so you can reassemble the circuit on a larger board?

 

[Les Jones]

I agree with your interpritaion of the resistor values. I don't think R2 is faulty. I think there is a short on the wiring between it's output side and the motor and the LEDs that it powers. Unsolder one of the wires to the motor at the motor end. See if the LEDs that were not working now light. Do not leave it on for long as without the normal load provided by the motor these LEDs will be passing more current than they would normally. If the LEDs light then the short is in the motor. Have you measured the value of the resitors with a meter to see if they are about the same as their marked values ?

Les.

 

[Welsh_Mike]

Your interpretation is good. Have you written a schematic to follow so you can reassemble the circuit on a larger board?

Thanks, I have made a drawing I can follow to ensure I can rewire it all (rather more neatly) in the same way. My drawing will do for me but would not by any means pass muster as a proper circuit diagram with correct symbols and conventions.

I agree with your interpritaion of the resistor values. I don't think R2 is faulty. I think there is a short on the wiring between it's output side and the motor and the LEDs that it powers. Unsolder one of the wires to the motor at the motor end. See if the LEDs that were not working now light. Do not leave it on for long as without the normal load provided by the motor these LEDs will be passing more current than they would normally. If the LEDs light then the short is in the motor. Have you measured the value of the resitors with a meter to see if they are about the same as their marked values ?

Les.

Great call Les. I measured the resistors -

#1 - between the incoming transformed power and all the other resisors
measured at 2.2Ω (markings suggested 0.22Ω) Perhaps colours are really Red, Red, Gold, Gold

#2 - to the failed motor
measured at 5.9Ω (markings suggested 20Ω) I can make no sense of the measurement compared to any reasonable interpretation of the colour markings.

#3 - to the failed red/yellow lights
measured at 5.9Ω (markings suggested 1100Ω) I can make no sense of the measurement compared to any reasonable interpretation of the colour markings.

#4 - to the working blue/green lights
6.3Ω (markings suggested 6.2Ω) which is a pretty good match for the colour markings.

The variance from what I was expecting was such that I repeated the measurements several times - the were consistent.

So then as you suggested, I disconnected the link to the motor, and as you expected, all the lights illuminated. So it looks like I have a shorted out motor.

The motor (photo attached) says "TECHNI MOTOR RT-300PA-09555-22.5 3.0V". I have googled that but failed to identify any motors with the same part code, though many look very similar.

Thank you both for the very helpful guidance.

 

[Nigel Goodwin]

Thanks, I have made a drawing I can follow to ensure I can rewire it all (rather more neatly) in the same way. My drawing will do for me but would not by any means pass muster as a proper circuit diagram with correct symbols and conventions.


Great call Les. I measured the resistors -

#1 - between the incoming transformed power and all the other resisors
measured at 2.2Ω (markings suggested 0.22Ω) Perhaps colours are really Red, Red, Gold, Gold

#2 - to the failed motor
measured at 5.9Ω (markings suggested 20Ω) I can make no sense of the measurement compared to any reasonable interpretation of the colour markings.

#3 - to the failed red/yellow lights
measured at 5.9Ω (markings suggested 1100Ω) I can make no sense of the measurement compared to any reasonable interpretation of the colour markings.

#4 - to the working blue/green lights
6.3Ω (markings suggested 6.2Ω) which is a pretty good match for the colour markings.

The variance from what I was expecting was such that I repeated the measurements several times - the were consistent.

So then as you suggested, I disconnected the link to the motor, and as you expected, all the lights illuminated. So it looks like I have a shorted out motor.

The motor (photo attached) says "TECHNI MOTOR RT-300PA-09555-22.5 3.0V". I have googled that but failed to identify any motors with the same part code, though many look very similar.

Thank you both for the very helpful guidance.

It's EXTREMELY rare for resistors to go low value, and of those rare cases where it happens it was down to a particular type of resistor. Much more likely is that it's reading through something else - disconnect one end of the resistor to check them.

 

[Les Jones]

As you have not traced out a wiring diagram of the device I am Just making wild guesses as to how it is wired.
I am guessing that one end of the 4 resistors is connected to a common point which is also connected to either the battery positive or negative. (Possibly via the on/off switch.)
The fact that you get a resistance reading of 5.9 ohms between the ends of resistors 2 and 3 suggests the they are in parallel. There may be a short between them or thy may be intended to be connected in parallel.
I assume that you measured the resistance of the resistors without disconnecting one end of them when doing the measuremant. Due to other paths through the circuit you could get a lower reading than than the marked value. If I was working on the device my first step would be to trace out the wiring diagram (Schematic.) The fact that disconnecting the wire to the motor suggests that that it is wired in parallel with the LEDs that were not working. With that wire still disconnected re measure the resistor values. The motor may not be faulty. Test the motor using a 3 volt power supply and measure how much current it is taking.

Les.

 

[Welsh_Mike]

Right, I disconnected all four resistors from the common terminal that they were all wired to and remeasured their resistance.

Measurements were -
#1 - 2.5Ω
#2 - 20Ω
#3 - 8.3Ω
#4 - 6.5Ω

You were correct, it was not totally clear in the mess under the board but I'm pretty confident resistors #2 and #3 were wired together at both ends, in parallel - the common end obviously by intent, but the other end looked I think to be an accident of clumsy soldering. I separated them for purposes of measurement.

I put 3 volts across the motor terminals and it sprang into life. Measuring the current was way out of my comfort zone (I had to read the multimeter manual several times - I have never measured current in my life) and I could have done with an extra couple of hands but the multimeter was moving constantly between values of 3.9 Amps to 4.1 Amps.

OK Les, put me in the box marked confused. I am not sure what we have proved, and I hope you can give me some knowledge. The only thing I can think of doing is tomorrow I should try to put it all back together with soldering that doesn't accidentally join resistors #2 and #3 in parallel. Attached is my sketch of the wiring to help me put it back together.

Thanks
Mike

 

[Les Jones]

The accidental short between resistors 2 and 3 explains why the motor was stopping the red and yellow LEDs from working. What is the total battery voltage ? ( Or the type and number of cells.) Try removing the motor so that is free to rotate, Re connnect the wire to the motor and see if it rotates. Measure the voltage across the motor terminals.
What is the history of this device as it does not look like that board with the resistors on is original ?

Les.

 

[Welsh_Mike]

Hi Les,

As I mentioned in the OP, there are three AA batteries in series in the optional battery compartment that we have never used until this investigation, so 4.5V . The ornament was bought by my wife in a "christmas section" of a local garden centre at least 17 years ago, and it might have been 20+ years. It has worked absolutely fine every christmas until now. And even this year it worked for three days OK until the day it didn't.

It had never been taken apart before a few days ago. The whole installation of the electrical connections looks very iffy, the plastic unit that is primarily the battery compartment which then has the motor assembly screwed to the top of it has been crudely sawn to fit the opening in the base of the ornament and it was fixed in place by two screws at one end and at the other end where it had been sawn short, it was glued with a thick application from a hot glue gun. I guess that was done 20 years or so ago in a small factory in China.

The motor is rotating fine when it is connected to two AA batteries (to match the 3V rating printed on it). I measured the current across the terminals, it was just fiddly because trying to trap the very fine wires in place against the battery terminals at the same time as holding the probes of the multimeter onto the same terminals was dextrously demanding for 73 year old arthritic hands.

I am going to attempt to solder it all back together again this afternoon, without connecting resistors 2 & 3 in parallel, and see whether we have a joyous result. I will post the outcome.

Many thanks for all the interest and guidance.

Mike

 

[Welsh_Mike]

Well the soldering went well and it all looks a bit neater. For all of about 5 minutes I thought we had cracked it because when I switched on I had all lights and the revolving christmas tree working. I switched it off prepared to reassemble everything back into the housing of the ornament. Switched it back on and only had lights.

Now the lights all work and the motor to rotate the Christmas tree only works if I give it a manual turn to encourage it to start - once it has started it carries on absolutely fine. But switch off and back on again and it doesn't turn until I take it out of its cradle turn it manually and then it carries on.

The other worrying thing is that the resistor that feeds it is extremely hot, far too hot to touch.

 

[Les Jones]

I think we all read the colours of R3 in the wrong sequence. I think the first band is gray (8), The scond red (2) The third gold ( x 0.1) I'm not sure what colour the fourth band is but that is only the tolerence. So it is 8.2 ohms.
I think the gear system may be the problem. I suggest removing the gears from the shaft, cleaning them then lubricating them with low viscosity oil. The motor may require re lubricating with low viscosity oil.
Measure the voltage across the motor when it is stalled and then when it has been assisted in starting. As the motor is driving a worm gear there will be axial force on the shaft either pushing it back into the motor or pulling it out. (depending on the direction of rotation.) This may make the motor tighter to turn when driving the gears.
If the motor is the problem then you might find one on ebay.

Les.

 

[Lightium]

How much space is there between the bottom of the tree and house?

 

[Welsh_Mike]

Hi Les,

I agree with your rethinking of R3.

If I power the device with transformed mains power, the lights all come on, and the motor does not spin, even if it is not connected to its gears and therefore has no load. The current shows nil. If I then manually encourage the motor to spin it carries on fine, the current shows 0.17Amps. But if I then introduce the gear load it stops.

If I disconnect the mains power and instead put in 3 AA batteries, exactly the same things happen. Similar current reading of 0.17 Amps. But if I then introduce the gear load it fails.

If I disconnect the motor from its normal connections and instead wire it direct to 2 AA batteries, it spins up to full speed straight away and successfully drives the load of gears and christmas tree. For this reason, I am dubious about the issue being one of lubrication. I wonder if R2 is denying the motor enough power. Or whether I just try and source a replacement motor as a next step.


Hi Lightium, there is plenty of space for the tree. If you look at the picture I posted yesterday - motor assembly 2 - the tree sticks in the top of the black pillar/spindle on the right hand side of the picture. When it is all assembled that spindle is proud of the building structure by about 10mm.

Do have a happy new year everybody.

 

[Les Jones]

I think there is a poor connection between the brushes and some segments on the the comnutator. I don't think it is possible to dismantle that type of motor to check the brushes and comutator. As a temporary fix you could try reducing the value of R2 (The 20 ohm resistor.) I have had a quick look on ebay for a suitable motor but did not find that motor.
Try searching for 3 volt DC miniature motor . (on Ebay, AliExpress and google.)
You might find a motor with a built in gearbox that would work without the existing gear train. I have seen similar motors in CD and DVD drives so if you any old ones you could see if the contain similar motors.

Les