Old radio + Bluetooth amp switch overheat

#1

Hiya

I’m trying something new with my vintage radios.
This time instead of just replacing the old radio output with a Bluetooth amp and using that as the source instead, I want to add a switch so I can switch between the old radio and a Bluetooth amp chip.

I’ve set up power through the switch. The first time I melted a set of batteries and this time when I attached the positive one of the chips on the Bluetooth board gets very hot when I turn the switch to that out put.

I’ve tried two of these boards to make sure it’s not the board that’s the problem. But both have the same problem. The same chip heats up.

At the moment I’m just switching the positive.
Negatives are both connected to negative terminal on battery. Maybe they are shorting there?

Maybe I need to get a triple switch that I can control the positive run, the negative run, and the audio.
Currently just doing double switch for positive and audio. Haven’t got as far as connecting audio to it yet.

Any ideas??

IMG_6944.JPG3024×4032 2.18 MB

(in the pic there are two chips. only the left one is part of the circuit. the positive cable is diconnected. when i touch it to the dc+ on the board it rapidly heats up one of the little chips on the board)

#2

Bit tricky to see in the pic but what voltage do the Bluetooth modules accept and is that what you’re giving them ? Undervolt or overvoltage would cause that issue.

Also , where exactly is your audio wired to ? Is it possible you are shorting the outputs of the demodulator and the Bluetooth module together ? I.e. if both are grounded - if one is sending a high and the other is off, youre effectively shorting power to ground. Perhaps a simple mixer would help there , just a little unity gain opamp circuit would be fine.

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#3

You could use a 4PDT switch instead of the one you have in the picture http://www.maplin.co.uk/p/toggle-switch-4pdt-fh08j

With that you can switch and isolate both power sources and speaker outputs at the same time. I’d avoid driving the audio outputs of either amplifier with the other, it might put an excessive load on the amplifier being driven and a possible ground loop. Naxxfish’s suggestion avoids this issue.

You might still have to use a mixer to down-mix from stereo unless your bluetooth module can do that itself (check the datasheet).

I’ve made a diagram, I hope it’s not too much of a mess.

Diagram done really quickly.PNG1805×879 354 KB

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#4

nice one @Edd!!
i was in Maplins scratching my head yesterday and took a punt on that exact 4pdt.
probably try it out tonight if i get a chance.
fingers crossed!
and thanks for the diagram! that’s super clear now.
btw the module is mono.

@naxxfish the module is 9V and the batteries are 1.5V x 4 = 6V

#5

When you say 9V - is that 9V max ? If it’s regulated , 6V might not be enough if it regulating down to 5V , and it’s a cheap regulator circuit (quite possible …). Try adding another cell to get it up to 7.5V and see if that helps? Or test from a bench supply - that way you can limit the current draw and prevent it from completely melting !

Definitely isolating the two with a 4pdt switch will help, though. You’ll probably get a pretty loud click when you switch between the two though - it’s probably fine, I’m probably just over cautious from my broadcast engineering background :slight_smile: fixing that would be hard, you’d need a soft start circuit somewhere to avoid it.

Good luck!

#6

When you say 9V - is that 9V max ? If it’s regulated , 6V might not be enough if it regulating down to 5V , and it’s a cheap regulator circuit (quite possible …). Try adding another cell to get it up to 7.5V and see if that helps? Or test from a bench supply - that way you can limit the current draw and prevent it from completely melting !

Definitely isolating the two with a 4pdt switch will help, though. You’ll probably get a pretty loud click when you switch between the two though - it’s probably fine, I’m probably just over cautious from my broadcast engineering background :slight_smile: fixing that would be hard, you’d need a soft start circuit somewhere to avoid it.

Good luck!

#7

Also, here’s my logic as to why it could be undervolt:

Bluetooth board is powered - begins bootup sequence
Gets as far as turning on the radio section, or maybe only as far as charging some decoupling caps.
Draws too much regulated current than the regulator can supply with its input voltage - the regulators output voltage drops , turns the module off.
Module turns off, so current draw goes away. Voltage rises again.
Module turns back on, starts the cycle over again.

So what you’d get is many high current spikes that normally wouldn’t be an issue when they’re short duration and only happen at power on - but when you repeat them over and over again in quick succession, they eventually make the regulator get really hot and fail.

In that scenario, the module wouldn’t work at all.

#8

Mystery solved after a lot of trail and error and @courty s help.

The manufacturer of the old radio had put a black cable on the positive terminal and a red cable on the negative terminal of the battery. So i’ld connected the dc on the module the wrong way round.

A black and red herring!!!

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Electronics Night 2nd March 2017
#9

The fiends!

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