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Old 2nd Apr 2020, 4:39 pm   #1
Mikpowl49
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Join Date: Apr 2020
Location: Pitlochry, Perthshire, UK.
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Default Beolit 600 - Add Bluetooth, ground polarity problem

Hi, a newbie here, so please be kind and forgive my ignorance!

I've just rescued a Beolit 600 from junk status and I'm trying to make it into a more useable device by adding Bluetooth. The idea is to do this by adding internal components and not damaging the existing equipment. I've sourced a miniature Bluetooth board and the plan was to drive this from the switched 7.5V dc supply inside the set and to feed the output to the Aux or Line-in channel on the main board. The problem is that the Beolit is positive ground, but it seems that the ground on the bluetooth output is interconnected with the negative feed i.e. negative ground. This gives a dead short circuit between the positive and negative grounds. Is there any way that I could 'decouple' the feed from the Bluetooth card from the line-in circuit so that the ground polarity is not a factor? or any other suggestion? Thanks, Mike
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Old 2nd Apr 2020, 8:19 pm   #2
Goldieoldie
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Default Re: Beolit 600 - Add Bluetooth, ground polarity problem

Hi
You could use a minature audio transformer ( available on eBay ) or from a scrap radio
That would give you the isolation
Cheers
Pete
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Old 3rd Apr 2020, 9:46 am   #3
Mikpowl49
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Default Re: Beolit 600 - Add Bluetooth, ground polarity problem

Great thanks, Pete. I shall give it a try.
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Old 3rd Apr 2020, 8:21 pm   #4
Goldieoldie
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Default Re: Beolit 600 - Add Bluetooth, ground polarity problem

This ebay number ( type it in the search box ) 120788551407 might be suitable
The same seller sells one with a low impedance .
It depends on where you are interfacing
They are so cheap you could buy both both to experiment!
Cheers
Pete
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Old 3rd Apr 2020, 8:37 pm   #5
tri-comp
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Default Re: Beolit 600 - Add Bluetooth, ground polarity problem

I've mounted Bluetooth adaptors in valveradios with great success.
All the boards I've used required a supply voltage of 5VDC and had a relative low audio-output.
I applied a simple single-transistor amplifier to raise the output voltage to about a Volt RMS.
If you use a coupling capacitor (1uF ~ 4,7uF) in series with the output signal, it makes no difference to the outputsignal if you connect the screen of the signal cable to the Bluetooth adapter positive voltage-terminal instead of connecting it to the module ground (negative voltage terminal).
Just be sure to mount the module-ground (negative-terminal) insulated/floating from the radio-chassis.
In short, the module should work right off the internal supply once you get the supply-voltage to the module in range.
Use could use a low-dropout 5V, 3-terminal voltage-regulator if your Bluetooth module requires a supply voltage of 5VDC.

Rgds,

/Torben
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Old 5th Apr 2020, 8:08 am   #6
Gabe001
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Default Re: Beolit 600 - Add Bluetooth, ground polarity problem

Can I ask you guys how you are muting the radio output when you install a Bluetooth module (if the radio doesn't have a gram input/mute setting)? Thanks
Gabriel
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Old 6th Apr 2020, 5:06 pm   #7
Mikpowl49
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Default Re: Beolit 600 - Add Bluetooth, ground polarity problem

Thanks for your help, guys. I immediately jumped onto ebay and ordered a couple of 600ohm 1:1 line isolating tx and they arrived this morning (1 spare); only a couple of quid including postage. And that's done the trick! A fine old radio and design icon can once more be put to practical use!! Not entirely noise free but very acceptable.

F.y.i. the bluetooth board came from China via ebay and is designed for car fitting. It accepts a range of dc voltage so no need for any extra voltage regulation - it's coupled directly to the switched incoming 7.5V dc supply. About a fiver, amazing. The board is about 50mm by 30 mm so fits neatly inside, attached to one of the cover panels by velcron.

To answer one of the queries re- muting, the Beolit is set up so that an external source can be linked to the amplifier section directly by pressing both FM and MW buttons simultaneously. This also mutes the RF channels. Ideally I'd like to be able to switch power to the B/tooth only when the Aux is enabled (i.e. FM+MW) but so far I haven't seen an easy way to do this.

Thanks again, and I'll look into the plan to raise the line in voltage. Plenty of time with the lock-up going on and fortunately ebay is up and running!!

Mike
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