Repairing a Family Heirloom, the electronic Canary

[TestSubjectZero]

Hi guys,

I'm looking the repair a old electronic Canary. It's a small circuit power by a 9v battery that makes a bird chirping sound every once in a while.
The issue it has right now is that the chirping "volume" is very low. My guess is that maybe the electrolytic capacitor are underperforming (while I was manipulating it one of them lost some liquid).

I would love some help in better understanding the circuit and some guidance on where to look for issues.
I found online that the circuit is:
 novotill-ElectronicCanary-BirdCircuitBatt.png (3.97 kB. 361x251 - viewed 317 times.)

I've got a background in electric engineering so got basic understanding of electronics but this circuit is beyond my skills  .  I've my own oscilloscopes and multimeter to test stuff.

[drvtech]

If the caps are so old that one has released some electrolyte then you should definitely replace all of them. Fit 25V parts instead of 10V as 1) 10V is too close to the supply voltage 2)the cost difference will be insignificant and 3) the size of modern ones will be plenty small enough to fit.

[TestSubjectZero]

Thanks for the info, will do!

I was also wondering what would be the impact of changing the caps characteristic on the chirping tones?
I guess the capacitance has the biggest impact but how about the voltage rating?

[jdutky]

increasing the rated voltage on the caps should not affect the tones generated (as that is determined by the capacitance), it only increases the safety margin in the circuit (so that the capacitors don't short out and destroy themselves from too high a voltage being applied). In general it is always safe to replace a lower voltage rated capacitor with a higher rated one.

The low volume could also be the result of the resistors having drifted out of spec. Older electronics often used "carbon composition" resistor, which were less expensive than metal film or carbon film resistors, but tend to absorb moisture over time and rise in value. I know that the schematic says to use metal film resistors, but it's a hard learned truth that what's on the schematic, and what is actually soldered onto the board in front of you, can easily be two very different things.

Finally, it could be the transistors, but doing anything more than a rudimentary check of the transistors may require equipment that you don't have. The simple check for the transistors (these are NPN, because the arrow points away from the base) is to use a multimeter in diode test mode, put the red probe on the base, and the black probe on the collector or emitter pin. Measure the diode voltage drop from base to emitter, and from base to collector. Both should be somewhere between 0.5 V and 1.0 V. This doesn't really tell you if the transistors are working as expected, but it does tell you that they aren't blown.

Feel free to skip the transistor check, however, because you say that the device is making chirp sounds, which indicates that the transistors are working. Concentrate on the caps and resistors.

[TestSubjectZero]

I've changed the electrolytic capacitor with no success. I've ordered some resistances and ceramic capacitor to test as well.
What are the chance that it is the transformers or the cone speaker?

I've attached some photos in case it may help.

[Paul Rose]

If it works, but is just too quiet (and you remember that it used to be louder), then it might be the speaker or the transformer associated with the speaker.  Probably not the other inductor.

The schematic had DC resistances for the secondary and primary.  If those check out, the transformer is probably OK.   Checking those with the transformer disconnected is best, but checking those in-circuit, with switch off (or battery disconnected) should be close enough.

[drvtech]

Yerck! That's a lot of corrosion products on the choke (L1). You should try to clean the whole thing up as the gunk is probably conductive. You've got all the data there on the schematic so you can check the DC resistance of the choke and the output transformer with a meter. If you suspect that the speaker is faulty you can always try replacing it temporarily with another e.g. one from your hifi system.

[Whales]

> What are the chance that it is the transformers or the cone speaker?

Woah that's grimy! 

Good chance something is wrong with that transformer/inductor thingy in the pic.  It needs a thorough clean to get rid of unwanted conduction paths and some probing with a DMM to make sure it's not open.  Further tests are a bit more difficult (eg to detect shorted windings) but can be done if you know the original transformer's approx specs.

For the speaker: do a quick DC resistance check with your multimeter and see if it's near the expected value.

[Zero999]

It also appears to be pretty low quality construction, reminiscent of some walkie-talkies I had as a child.

It's unlikely to be the resistors, ceramic capacitors, transformer, or inductor.

It could be the speaker, or either one of the transistors. I think the speaker is more likely, but it's more expensive to replace than the transistors.

Has it been next to any strong magnetic fields? That can demagnetise the speaker. I rememeber using a walkie-talkie to make my own music radio station, when I was a child. I put the walkie-talkie closeof my dad's stereo speaker, for a long time. Afterwards, the walkie-talkie didn't sound anywhere near as loud, as it did before. I believe the big magnet, in the 50W stereo speaker, demagnetised the tiny magnet, in the 0.5W walkie-talkie speaker.

What does the speaker look like? is the cone clean and in good condition, or is it torn, wrinkled and holey?

[TestSubjectZero]

I haven't tried cleaning the transformer but I tried using my in-ear headphones. Sounds like the speaker may be the issue. I almost blew my eardrum.
I've tried measuring the DC resistance of the cone speaker and strangely I can't get a reading; it shows as a open circuit.
 

[Paul Rose]

I've tried measuring the DC resistance of the cone speaker and strangely I can't get a reading; it shows as a open circuit.

That's almost certainly your issue. 

[TestSubjectZero]

Omg I just realized that the sound that I though came from the speaker was coming from the circuit itself. 
One of the component is chirping, I thought it was the speaker.

Thanks so much for everybody's help, you guys are great!

[james_s]

Transformers and inductors can reproduce sound if the windings or core are able to vibrate. It definitely sounds like the speaker is your problem.

[Zero999]

Omg I just realized that the sound that I though came from the speaker was coming from the circuit itself. 
One of the component is chirping, I thought it was the speaker.

Thanks so much for everybody's help, you guys are great!

I'm glad you've found the fault. Just out of curiosity, does the speaker physically look damaged?

When you get it working, please post an update, along with an mp3 file of the sound it makes, if possible.

[TestSubjectZero]

The speaker did not show any signs of damage but under further inspection I found out that one of the small wires running to the solenoid was cut.
I had to remove the center cover to see it.


I've attached a audio file of the circuit working with a slightly smaller speaker then the original one (5cm vs 5.75cm)

[james_s]

I've had small speakers like that fail a few times in a similar way, sometimes you can repair them by soldering the break although replacement speakers are readily available in a range of sizes.

That sounds surprisingly realistic for such a simple analog circuit.

[Zero999]

The speaker did not show any signs of damage but under further inspection I found out that one of the small wires running to the solenoid was cut.
I had to remove the center cover to see it.
(Attachment Link)

I've attached a audio file of the circuit working with a slightly smaller speaker then the original one (5cm vs 5.75cm)

Are you going to try to repair the speaker, will you find one the right size, or just use the smaller one, with a carboard cut-out adaptor, if necessary?

[TestSubjectZero]

I found a paper cone speaker of 5.7cm so I'm waiting for it to arrive and that will be that!

[Zero999]

I found a paper cone speaker of 5.7cm so I'm waiting for it to arrive and that will be that!

Yes, it's probably not worth repairing the old one. By the way, don't put the old one in the bin. Take the magnet off the back, which can be very useful because it's quite strong.