Odd AGC fault in Bush VHF62 - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Breward · 19th Sep 2006, 1:43 pm

Poor AGC action on AM, and poor limiting on FM suggested leaking bypass capacitors on the AGC line on one of these sets, especially as AGC line - to- chassis resistance was low. However, even with the capacitors out and the AGC line isolated from chassis, a resistance of less than 1 Meg could still be measured between the AGC line and chassis. Horrible visions of insulation breakdown in the switching system . . . . but no!

The fault was found to be a leak from grid to rf screen and cathode within the second IF valve (EF89). The insulation between screen grid or anode and the other electrodes was fine Has anyone else come across this type of fault? Is the EF89 particularly prone to 'cold' leaks of this type?

Cheers,
Neil

19th Sep 2006, 1:43 pm	#1
Neil Breward Retired Dormant Member Join Date: Aug 2005 Location: Keyworth nr. Nottingham, UK Posts: 140	Odd AGC fault in Bush VHF62 Poor AGC action on AM, and poor limiting on FM suggested leaking bypass capacitors on the AGC line on one of these sets, especially as AGC line - to- chassis resistance was low. However, even with the capacitors out and the AGC line isolated from chassis, a resistance of less than 1 Meg could still be measured between the AGC line and chassis. Horrible visions of insulation breakdown in the switching system . . . . but no! The fault was found to be a leak from grid to rf screen and cathode within the second IF valve (EF89). The insulation between screen grid or anode and the other electrodes was fine Has anyone else come across this type of fault? Is the EF89 particularly prone to 'cold' leaks of this type? Cheers, Neil