Bush VHF61/62 - why so deaf on AM - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Keith · 4th Mar 2020, 7:27 pm

Just a final post to let you know the outcome of investgations on this receiver. I finally found the source of the severe crackling on MW/LW which, I believe, was giving a false impression of poor AM performance (possibly by also impacting the AGC voltage).
The problem was internal arcing in the LW wafer where HT is being switched (in this case S12 bypassing R9 to increase the LO voltage on long wave). One of the HT switch contacts was adjacent to an earthy contact on the other side of the wafer. Cleaning the area with IPA had no effect. A 250V megger between the isolated contact and chassis confirmed the problem.

Unbelievably, there was spare set of contacts on the same wafer. Moving the function to these contacts solved the problem (for now, at least).
Thanks for all your replies.

Techman · 4th Mar 2020, 8:44 pm

Well done Keith, there's nearly always a way round these things.

4th Mar 2020, 7:27 pm	#21
Keith Heptode Join Date: Jan 2003 Location: Tewkesbury, Gloucestershire, UK. Posts: 693	Re: Bush VHF61/62 - why so deaf on AM Just a final post to let you know the outcome of investgations on this receiver. I finally found the source of the severe crackling on MW/LW which, I believe, was giving a false impression of poor AM performance (possibly by also impacting the AGC voltage). The problem was internal arcing in the LW wafer where HT is being switched (in this case S12 bypassing R9 to increase the LO voltage on long wave). One of the HT switch contacts was adjacent to an earthy contact on the other side of the wafer. Cleaning the area with IPA had no effect. A 250V megger between the isolated contact and chassis confirmed the problem. Unbelievably, there was spare set of contacts on the same wafer. Moving the function to these contacts solved the problem (for now, at least). Thanks for all your replies. __________________ Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm

4th Mar 2020, 8:44 pm	#22
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Re: Bush VHF61/62 - why so deaf on AM Well done Keith, there's nearly always a way round these things.