Roberts R250 - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 20th Jan 2021, 10:50 am

How can you manage to connect a PP9 battery the wrong way around & thus fry the audio IC?
The positive & negative terminals are completely different.
Has anyone subbed a LM386 in place of the original IC?
Where would you add a diode to protect the audio stage?

Scotorvm · 20th Jan 2021, 11:27 am

Hi Neil, I think the audio chip fries, not with a wrongly connected battery, but the wrong PSU, I think this model needs a jack plug with +ve on the outside, most 3rd party are centre +ve.

I have a few of the original spec IC's, they work a treat, sometimes need to replace a shot cap close by too. Not sure re the diode.

I can post you an IC if you would like one.

Cheers
Lee

Neil Purling · 22nd Jan 2021, 7:26 pm

Well I wondered how these radios were getting 'cooked' because of the difference in shapes of the two battery terminals. I never considered the mains adapter polarity being the reason. Very sneaky of them to swap plug polarity.
I wonder how come the whole radio doesn't get blown, just the audio stage?

Herald1360 · 22nd Jan 2021, 7:31 pm

Probably there's enough circuit resistance in the supply decoupling etc to stop the earlier stages from frying, unlike the audio power stages.

20th Jan 2021, 10:50 am	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Roberts R250 How can you manage to connect a PP9 battery the wrong way around & thus fry the audio IC? The positive & negative terminals are completely different. Has anyone subbed a LM386 in place of the original IC? Where would you add a diode to protect the audio stage?

20th Jan 2021, 11:27 am	#2
Scotorvm Pentode Join Date: Jul 2020 Location: Falkirk, Stirlingshire, UK. Posts: 200	Re: Roberts R250 Hi Neil, I think the audio chip fries, not with a wrongly connected battery, but the wrong PSU, I think this model needs a jack plug with +ve on the outside, most 3rd party are centre +ve. I have a few of the original spec IC's, they work a treat, sometimes need to replace a shot cap close by too. Not sure re the diode. I can post you an IC if you would like one. Cheers Lee

22nd Jan 2021, 7:26 pm	#3
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: Roberts R250 Well I wondered how these radios were getting 'cooked' because of the difference in shapes of the two battery terminals. I never considered the mains adapter polarity being the reason. Very sneaky of them to swap plug polarity. I wonder how come the whole radio doesn't get blown, just the audio stage?

22nd Jan 2021, 7:31 pm	#4
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Roberts R250 Probably there's enough circuit resistance in the supply decoupling etc to stop the earlier stages from frying, unlike the audio power stages. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O