Roberts R500 high pitched whistle oscillation - UK Vintage Radio Repair and Restoration Discussion Forum

FIXITNOW · 7th Jan 2019, 12:23 pm

Got this Roberts R500 on the repair bench it works fine and is loud enough and tunes in and finds lots of station (MW), but all across the band is this high pitched whistle/ oscillation ,when tuned to a station its still there in the back ground and loud enough to annoy, but does reduce with the volume pot .

I have the trader sheet but am unsure were to start I have not come across this problem before.

The set is as found and had nothing more than the pot and switch cleaned

Any help to point me in the right direction would be welcome
(another radio in same location works fine)

cheers Vince

Nuvistor · 7th Jan 2019, 12:27 pm

Possibly unstable IF amps or mixer stage unless it is interference, any other radios have similar problems?

Check decoupling capacitors and screening cans if it’s instability.

ronbryan · 7th Jan 2019, 1:11 pm

AGC decoupling electrolytic capacitor C11 10uF dried out?

Ron

FIXITNOW · 7th Jan 2019, 1:34 pm

Will check tonight cheers

FIXITNOW · 8th Jan 2019, 8:42 am

good call guys
removed IF unit and checked C11 (10uF) measured 44pF so replaced another of same type cap also in there C13 should be 2uF measured on my Chinese tester 25pF
replacing both has sorted this many thanks

7th Jan 2019, 12:23 pm	#1
FIXITNOW Octode Join Date: Feb 2007 Location: Ely, Cambridgeshire, UK. Posts: 1,976	Roberts R500 high pitched whistle oscillation Got this Roberts R500 on the repair bench it works fine and is loud enough and tunes in and finds lots of station (MW), but all across the band is this high pitched whistle/ oscillation ,when tuned to a station its still there in the back ground and loud enough to annoy, but does reduce with the volume pot . I have the trader sheet but am unsure were to start I have not come across this problem before. The set is as found and had nothing more than the pot and switch cleaned Any help to point me in the right direction would be welcome (another radio in same location works fine) cheers Vince

7th Jan 2019, 12:27 pm	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Roberts R500 high pitched whistle oscillation Possibly unstable IF amps or mixer stage unless it is interference, any other radios have similar problems? Check decoupling capacitors and screening cans if it’s instability. __________________ Frank

7th Jan 2019, 1:11 pm	#3
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Roberts R500 high pitched whistle oscillation AGC decoupling electrolytic capacitor C11 10uF dried out? Ron

7th Jan 2019, 1:34 pm	#4
FIXITNOW Octode Join Date: Feb 2007 Location: Ely, Cambridgeshire, UK. Posts: 1,976	Re: Roberts R500 high pitched whistle oscillation Will check tonight cheers

8th Jan 2019, 8:42 am	#5
FIXITNOW Octode Join Date: Feb 2007 Location: Ely, Cambridgeshire, UK. Posts: 1,976	Re: Roberts R500 high pitched whistle oscillation good call guys removed IF unit and checked C11 (10uF) measured 44pF so replaced another of same type cap also in there C13 should be 2uF measured on my Chinese tester 25pF replacing both has sorted this many thanks