Roberts R200 Aux In Modification Help - UK Vintage Radio Repair and Restoration Discussion Forum

Gundog48 · 26th Oct 2018, 12:11 am

I have myself a Roberts R200 which I'm doing some work on. I hope this isn't considered sacrilege here, but the LW band doesn't work, so I've installed a Blue Tooth receiver that runs in its place. It all runs off the internal battery, then goes via an amp to the speaker. Works pretty well, and looks normal from the outside.

I thought it would be nice to use the original amp as opposed to an additional one, and have been looking at where I could connect the 3.5mm (from the Bluetooth receiver) output onto the device. I'm a general electronics hobbyist, but have no background in audio, so I'm not sure how best to go about this. I've looked at perhaps linking it into the volume control knob, I'm not sure.

Could anyone point me in the right direction, or suggest some reading that'll let me figure it out for myself?

Many Thanks!

'LIVEWIRE?' · 26th Oct 2018, 8:42 am

The usual way to add 'aux' inputs to any radio is to disconnect the detector diode from the volume control, and wire the jack socket in it's place. A switch or switched socket can be used to allow the choice of either radio or external signal source. There are detailed instructions on how to do this in the 'sticky' thread at the top of this section.

https://www.vintage-radio.net/forum/...ad.php?t=87095

Although this headed 'adding 'aux-in' to car radios' the same method can be used in portable sets. Bear in mind, though, that the output of the BT receiver is (almost certainly) stereo , whilst the R200 is obviously mono, so series resistors should be fitted, as explained in those notes.

Gundog48 · 29th Oct 2018, 11:26 pm

I've read a lot of the material there and am having limited luck. I've done as the general guidance seems to suggest, and I can connect to it and play music through it successfully, which is all good. But my god is it noisy, and when I say noisy, I mean noisy. Seems to fall in the general territory of a ground loop, but has these 'rising and falling' tones. It is also sensitive to things such as the proximity to my lamp and the tuning position, even with the line from the detector diode to the volume control totally disconnected.

I'm wondering if this has something to do with the positive ground on this unit- I've never dealt with this before. My wiring has a 5V buck converter coming from the input leads from the battery, this then goes through a DC-DC isolating converter and into my bluetooth module. The left and right audio outputs are combined via a 10K resistor on each line into the volume control potentiometer, where the input from the detector diode would go. The ground is connected to the top pin of the potentiometer, which goes to ground (which is +9V in this case.) I'm a bit lost as to what do do now. It's frustrating being so close, yet still being really unusable!

Herald1360 · 30th Oct 2018, 10:37 am

Sounds like the problem is those switching PSUs and the noise they will generate, particularly with them inside the set! If you've ever seen the screening lengths that valve car radio makers went to in order that the built in vibrator power pack didn't upset reception, you'll get the idea.

You might get away with building your own switched capacitor voltage inverter to run the module. That way you could keep a common ground between the module and the set. Audio dc isolation need only be a suitable capacitor in each signal line.

If there's room in the set for 4 AA cells, you could power the module from them (stick a silicon diode in series with the +6V to get near enough 5V) and again maintain a common ground.

26th Oct 2018, 12:11 am	#1
Gundog48 Retired Dormant Member Join Date: Oct 2018 Location: Marden, Kent, UK. Posts: 2	Roberts R200 Aux In Modification Help I have myself a Roberts R200 which I'm doing some work on. I hope this isn't considered sacrilege here, but the LW band doesn't work, so I've installed a Blue Tooth receiver that runs in its place. It all runs off the internal battery, then goes via an amp to the speaker. Works pretty well, and looks normal from the outside. I thought it would be nice to use the original amp as opposed to an additional one, and have been looking at where I could connect the 3.5mm (from the Bluetooth receiver) output onto the device. I'm a general electronics hobbyist, but have no background in audio, so I'm not sure how best to go about this. I've looked at perhaps linking it into the volume control knob, I'm not sure. Could anyone point me in the right direction, or suggest some reading that'll let me figure it out for myself? Many Thanks!

26th Oct 2018, 8:42 am	#2
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Roberts R200 Aux In Modification Help The usual way to add 'aux' inputs to any radio is to disconnect the detector diode from the volume control, and wire the jack socket in it's place. A switch or switched socket can be used to allow the choice of either radio or external signal source. There are detailed instructions on how to do this in the 'sticky' thread at the top of this section. https://www.vintage-radio.net/forum/...ad.php?t=87095 Although this headed 'adding 'aux-in' to car radios' the same method can be used in portable sets. Bear in mind, though, that the output of the BT receiver is (almost certainly) stereo , whilst the R200 is obviously mono, so series resistors should be fitted, as explained in those notes. Last edited by Station X; 26th Oct 2018 at 8:45 am. Reason: Link added.

29th Oct 2018, 11:26 pm	#3
Gundog48 Retired Dormant Member Join Date: Oct 2018 Location: Marden, Kent, UK. Posts: 2	Re: Roberts R200 Aux In Modification Help I've read a lot of the material there and am having limited luck. I've done as the general guidance seems to suggest, and I can connect to it and play music through it successfully, which is all good. But my god is it noisy, and when I say noisy, I mean noisy. Seems to fall in the general territory of a ground loop, but has these 'rising and falling' tones. It is also sensitive to things such as the proximity to my lamp and the tuning position, even with the line from the detector diode to the volume control totally disconnected. I'm wondering if this has something to do with the positive ground on this unit- I've never dealt with this before. My wiring has a 5V buck converter coming from the input leads from the battery, this then goes through a DC-DC isolating converter and into my bluetooth module. The left and right audio outputs are combined via a 10K resistor on each line into the volume control potentiometer, where the input from the detector diode would go. The ground is connected to the top pin of the potentiometer, which goes to ground (which is +9V in this case.) I'm a bit lost as to what do do now. It's frustrating being so close, yet still being really unusable!

30th Oct 2018, 10:37 am	#4
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Roberts R200 Aux In Modification Help Sounds like the problem is those switching PSUs and the noise they will generate, particularly with them inside the set! If you've ever seen the screening lengths that valve car radio makers went to in order that the built in vibrator power pack didn't upset reception, you'll get the idea. You might get away with building your own switched capacitor voltage inverter to run the module. That way you could keep a common ground between the module and the set. Audio dc isolation need only be a suitable capacitor in each signal line. If there's room in the set for 4 AA cells, you could power the module from them (stick a silicon diode in series with the +6V to get near enough 5V) and again maintain a common ground. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O