ITT-KB Weekend Automatic - distortion? - UK Vintage Radio Repair and Restoration Discussion Forum

bugbear · 11th Mar 2019, 10:33 am

I just got this neat little radio (destined for the Kitchen shelf).

It's essentially clean, and works well.

But at anything other than quite low volume there's a lot of distortion.

Is this simply a limitation of the product/design or "other?

My personal suspicion, based on limited knowledge, is that the smoothing capacitors in the main PSU aren't supplying a solid voltage.

Diagnoses, or pointers to diagnostic techniques, welcomed.

BugBear

orbanp1 · 11th Mar 2019, 12:52 pm

Hi BugBear,

Chances are the final audio amp in the radio is off.
RM has the schematics:
https://www.radiomuseum.org/r/itt_we...automatic.html

Do check if the common emitter point of the final transistors (CS1909, CS1910) are indeed at half the supply voltage (DC), and continue from there.

Regards, Peter

crackle · 11th Mar 2019, 1:43 pm

Like all ITT-KB radios, distortion is not normally a feature. Don't rule out checking the speaker for rubbing or loose diaphragm inner support.
The ITT Service information is also on the KB Museum website, it includes a printed circuit layout to help you locate the components. http://www.kbmuseum.org.uk/itt_servi...omatic_ITT.pdf
Good luck, they sound good when working correctly.
Mike

buggies · 11th Mar 2019, 7:23 pm

The circuit shows a battery/mains switch - so that should quickly eliminate the power supply.

Graham G3ZVT · 11th Mar 2019, 8:29 pm

First think I'd suspect with distortion at low volume, is an out-of-centre speaker cone

bugbear · 15th Mar 2019, 3:07 pm

Thanks for the practical suggestions; a quick go with a set of batteries, and wiring external speakers via the headphone socket should answer some questions, at least for starters.

BugBear

bugbear · 19th Mar 2019, 10:53 am

Oddly, the radio has simply "settled in" and is now behaving much better.

I note that the RM website doesn't have a "covers off" photo of the innards. Would they like one (I took one for my own purposes) ?

[img]https://farm8.*****************/7808/46695600424_d231ec5037_b.jpg[/img]le_weekend by plybench, on Flickr

BugBear

crackle · 19th Mar 2019, 7:28 pm

I think possibly an electrolytic capacitor has reformed, this can happen sometimes. I have had radios which dont work at all then after a little while they slowly start to work, and often they continue to improve over the next 30 minutes or so.
Thanks for the photo of the insides. https://www.radiomuseum.org/r/itt_we...automatic.html

Mike

11th Mar 2019, 10:33 am	#1
bugbear Triode Join Date: Nov 2018 Location: Diss, Norfolk, UK. Posts: 15	ITT-KB Weekend Automatic - distortion? I just got this neat little radio (destined for the Kitchen shelf). It's essentially clean, and works well. But at anything other than quite low volume there's a lot of distortion. Is this simply a limitation of the product/design or "other? My personal suspicion, based on limited knowledge, is that the smoothing capacitors in the main PSU aren't supplying a solid voltage. Diagnoses, or pointers to diagnostic techniques, welcomed. BugBear

11th Mar 2019, 12:52 pm	#2
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: ITT-KB Weekend Automatic - distortion? Hi BugBear, Chances are the final audio amp in the radio is off. RM has the schematics: https://www.radiomuseum.org/r/itt_we...automatic.html Do check if the common emitter point of the final transistors (CS1909, CS1910) are indeed at half the supply voltage (DC), and continue from there. Regards, Peter

11th Mar 2019, 1:43 pm	#3
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: ITT-KB Weekend Automatic - distortion? Like all ITT-KB radios, distortion is not normally a feature. Don't rule out checking the speaker for rubbing or loose diaphragm inner support. The ITT Service information is also on the KB Museum website, it includes a printed circuit layout to help you locate the components. http://www.kbmuseum.org.uk/itt_servi...omatic_ITT.pdf Good luck, they sound good when working correctly. Mike

11th Mar 2019, 7:23 pm	#4
buggies Heptode Join Date: Jan 2009 Location: West Lothian, UK. Posts: 761	Re: ITT-KB Weekend Automatic - distortion? The circuit shows a battery/mains switch - so that should quickly eliminate the power supply. __________________ George

11th Mar 2019, 8:29 pm	#5
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,711	Re: ITT-KB Weekend Automatic - distortion? First think I'd suspect with distortion at low volume, is an out-of-centre speaker cone __________________ -- Graham. G3ZVT

15th Mar 2019, 3:07 pm	#6
bugbear Triode Join Date: Nov 2018 Location: Diss, Norfolk, UK. Posts: 15	Re: ITT-KB Weekend Automatic - distortion? Thanks for the practical suggestions; a quick go with a set of batteries, and wiring external speakers via the headphone socket should answer some questions, at least for starters. BugBear Last edited by bugbear; 15th Mar 2019 at 3:13 pm.

19th Mar 2019, 10:53 am	#7
bugbear Triode Join Date: Nov 2018 Location: Diss, Norfolk, UK. Posts: 15	Re: ITT-KB Weekend Automatic - distortion? Oddly, the radio has simply "settled in" and is now behaving much better. I note that the RM website doesn't have a "covers off" photo of the innards. Would they like one (I took one for my own purposes) ? [img]https://farm8.*****************/7808/46695600424_d231ec5037_b.jpg[/img]le_weekend by plybench, on Flickr BugBear

19th Mar 2019, 7:28 pm	#8
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: ITT-KB Weekend Automatic - distortion? I think possibly an electrolytic capacitor has reformed, this can happen sometimes. I have had radios which dont work at all then after a little while they slowly start to work, and often they continue to improve over the next 30 minutes or so. Thanks for the photo of the insides. https://www.radiomuseum.org/r/itt_we...automatic.html Mike