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Old 12th Jul 2020, 10:32 am   #1
Gabe001
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Default Cossor 464 bluetooth

Dear colleagues (or radio buddies acc to my wife),

I've just returned a cossor 464 to working condition. It didn't need much; 2 out of 3 electrolytic capacitors reformed and the wax capacitors were changed. It works great as an AM radio

The problem is that it does not have a gram/Pu socket. I was wondering about how to introduce Bluetooth in the minimally invasive way, so that I can listen to the music I like. I know that BT is a dirty word and I would never do this at the expense of a working radio, so bear with me.

My thought is a seperately powered BT module with integrated amplifier connected directly to the speaker via 2 wander plugs in the ext LS socket. Hence, to use the radio as a BT speaker, I would apply 12 v to the BT module via a DC jack. To use the radio as an AM radio I would apply power via the mains. This means that the two are completely isolated electrically up to the point of the speaker.

Could you very kindly review the addition below on the circuit diagram in red and let me know if it makes sense?

These is no HT across the ext LS connections, but in case of a short the capacitors and the double pole switch should offer protection to modern electronics down the line.

What I'm not sure of is whether keeping the UNPOWERED Bluetooth module connected to the speaker via the wander plugs in the ext LS socket will have any ill effect on the output transformer/vintage components when the cossor is in use as an AM radio.

I hope I haven't made any glaring errors, my background isn't engineering/electronics.

Thanks
Gabriel
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Old 12th Jul 2020, 11:04 am   #2
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Default Re: Cossor 464 bluetooth

I doubt that would work..... the BT module would need a low impedance loudspeaker level output. With an added audio amplifier it might but I would be more concerned for the fate of that amplifier with radio audio stuffed up it than any ill effects the radio might suffer.

Doing it "conventionally" with a BT receiver installed in the radio can be achieved with no more than a few easily undone soldered connections to get the audio in at the volume control.
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Old 12th Jul 2020, 4:24 pm   #3
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Default Re: Cossor 464 bluetooth

Thanks Chris. Does the radio audio need to be disconnected at the volume control in favour of the Bluetooth audio? I'd rather have them both fully functional.
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Old 12th Jul 2020, 5:08 pm   #4
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Default Re: Cossor 464 bluetooth

In the past I have used bluetooth modules designed for car radios.

These often have a built in usb and FM radio as well.

You can use the aux function so that the radio works as normal.

They come with little remote controls.

The down side is they seem to be variable in facilities and connectivity.

But they are very cheap so worth a play.

The ones I last used no longer available but this one looks very similar

https://rover.ebay.com/rover/0/0/0?m...2F292383393124

There are loads similar in different styles and price

Cheers

Mike T
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Old 12th Jul 2020, 5:30 pm   #5
Gabe001
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Default Re: Cossor 464 bluetooth

Hi Mike yes I've seen those. My main question is whether you can safely use an amplified signal from one of these to drive a non energised radio speaker via the ext LS socket.

Chris thinks (above) that if I then turn the radio on, to use it as a radio not a Bluetooth speaker, the Bluetooth amplifier won't like being force fed the radio signal

Gabriel
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Old 12th Jul 2020, 6:52 pm   #6
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Default Re: Cossor 464 bluetooth

The one I linked is line out not for driving a speaker directly.

The output of the module would connect to the volume control. The input to the module would connect to the wite removed from the top of the volume control as suggested by Chris.

Cheers

Mikr T
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Old 12th Jul 2020, 7:00 pm   #7
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Default Re: Cossor 464 bluetooth

Thanks mike
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Old 12th Jul 2020, 7:55 pm   #8
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Default Re: Cossor 464 bluetooth

The KRC-86B Bluetooth module will automatically switch over between Bluetooth and an external input. The issue with this module is that it is fitted with 100k resistors on the inputs and outputs, but these can removed for more valve friendly loadings.

In outline you would connect the module's output to the radio's volume control and take the input to the module from the wire that formally went to the volume control.

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Old 13th Jul 2020, 8:47 am   #9
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Default Re: Cossor 464 bluetooth

Quote:
Originally Posted by wd40addict View Post
The KRC-86B Bluetooth module will automatically switch over between Bluetooth and an external input. The issue with this module is that it is fitted with 100k resistors on the inputs and outputs, but these can removed for more valve friendly loadings.

In outline you would connect the module's output to the radio's volume control and take the input to the module from the wire that formally went to the volume control.
Thanks. There is a good post on wiring this up on this forum - someone put one in a Hacker and kindly shared the circuit diagram. I'll have no problem replicating something like that

I note other people have mentioned that the module sounds low. I suppose this is due to the resistor issue you mentioned. I'll purchase a few and have a play with them.

I've never removed anything from a printed circuit board before. Is it a simple matter of unsoldering the piece and bridging the gap? It seems a bit daunting considering the tiny size of these components

Gabriel
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Old 13th Jul 2020, 9:52 am   #10
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Default Re: Cossor 464 bluetooth

Yes and no....

If the resistor is in series with the signal path and you want to get rid of the signal loss it causes then yes. If it's in parallel with the signal path and is loading down a high impedance signal then no, just remove it.

Depending on the circuit(s) involved there may be other considerations as well but this one sounds straightforward enough.
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Old 13th Jul 2020, 10:40 am   #11
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Default Re: Cossor 464 bluetooth

They are in parallel with the signal path and must be removed and the pads left empty. I'll try and put some pictures up later.
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Old 13th Jul 2020, 11:44 am   #12
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Default Re: Cossor 464 bluetooth

Quote:
Originally Posted by wd40addict View Post
They are in parallel with the signal path and must be removed and the pads left empty. I'll try and put some pictures up later.

Thanks yes that would help a lot, I can remove components it's the bridges I was worried about

One final thing, can I power the module from the mains AC using the transformer (220v -> 5v) in the attached picture, rather than off the radio transformer? It's cheap and cheerful with excellent reviews and can handle up to 1 amp, which is more than enough

Gabriel
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Old 13th Jul 2020, 12:15 pm   #13
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Default Re: Cossor 464 bluetooth

Cheap and cheerful, but is it safe and will it interfere with the radio?

Since the module only needs 5v DC at a maximum of about 30mA I'd look at getting it from the radio.
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Old 13th Jul 2020, 5:49 pm   #14
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Default Re: Cossor 464 bluetooth

Attached picture shows 4 off 100k resistors removed (circled in red).

Note in this Ferranti battery set installation I got better results using the very low output impedance of the Bluetooth setting to simply overdrive the input to the volume control. Resistors still needed removing to ensure when Bluetooth off there was no extra loading on the radio signal.
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Old 13th Jul 2020, 6:28 pm   #15
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Default Re: Cossor 464 bluetooth

Thanks you very much WD, much appreciated. I've saved this for reference and will place the module on order shortly and have a play.
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Old 23rd Jul 2020, 5:36 pm   #16
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Default Re: Cossor 464 bluetooth

Diode, regulator, capacitors and module have arrived. Just waiting on the prototype board.

Having another look at your picture.
Are you using just one of the input channels? Also am I correct in my interpretation that you've summed the left and right output in the set you're working on?

Thanks
Gabriel
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Old 23rd Jul 2020, 6:06 pm   #17
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Default Re: Cossor 464 bluetooth

It might be easier to obtain a BLUETOOTH speaker and place it BEHIND the COSSOR.
For added realism, take the back off.
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Old 23rd Jul 2020, 6:10 pm   #18
Gabe001
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Default Re: Cossor 464 bluetooth

Quote:
Originally Posted by Brigham View Post
It might be easier to obtain a BLUETOOTH speaker and place it BEHIND the COSSOR.
For added realism, take the back off.
Maybe, but there's no fun in that!
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Old 23rd Jul 2020, 8:03 pm   #19
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Default Re: Cossor 464 bluetooth

In that setup I am not using the input connections at all. The output (L&R summed together) is connected to the volume control through a capacitor. How it works is as follows: when there is no Bluetooth connection the module's Analogue switch IC is switched to route input to output. Since I have removed the 100k resistors the output connection is now very high impedance and the radio functions as normal. The capacitor and a 3M3 resistor to ground stop the radio's AGC DC voltage from being clamped by the limited voltage range of the IC.

When Bluetooth is connected the Analogue switch changes over and the output impedance in this state is a few hundred ohms easily overdriving the radio audio. It works very well.

I did initially try routing the radio's audio from input to output, but with this particular radio this gave instability and poor performance.
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Old 23rd Jul 2020, 8:16 pm   #20
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Default Re: Cossor 464 bluetooth

Here's a different module I've fitted into my Blaupunkt Santos. The Santos has a pick-up input so switching or resistor removal not required. The challenge here is that the radio is a live chassis type. The heaters are all run at 6v3 with one side connected to chassis. I modified an eBay 5v regulator kit to half wave rectification and used it to feed the KRC68. See attached pictures.

Module is mounted inside set for safety as mains reversal could make the pickup sockets live!
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