Bush AC 41 loud noise - UK Vintage Radio Repair and Restoration Discussion Forum

Pepelephew · 14th Apr 2014, 8:06 pm

Hi All,
I spent some time restoring this Bush AC 41 earlier this year and was very happy with the final result. However about a week ago the sound faded and died on all wavelengths.`i switched it off and had a quick look to see if anything obvious was wrong, (nothing noticed). On switching it back on again I was greeted with very loud hum/noise. This happens on all wavelengths but not on Gram.The noise increases/decreases with the volume control. Nothing whatsoever can be tuned in.
There is no response from any wavelength except this incessant noise.
Any pointers please

Thanks
Bill

Nickthedentist · 14th Apr 2014, 8:17 pm

Sounds daft, Bill, but have you tried another set in the same spot in case it's interference from a SMPS etc.?

And have the waxies all been changed? A failed decoupler could be causing motorboating, I suppose,

Nick

Sideband · 14th Apr 2014, 9:50 pm

Possibly a cathode/heater leak/short in one of the valves pre volume control. The clue has to be with the gram position where there is no hum. Only the UBC41 and UL41 are used in that position so that leaves the UF41 or UCH42. I'd try either of those first before diving in deeper.

Pepelephew · 14th Apr 2014, 10:09 pm

Hi lads
Yes this set was sitting beside anther one . No problems with the second set only the ac41
Brought it to a different room.......same problem.
The valves used in this set are ECH 42, EBF 40, EL 41 and EZ 40
All are lighting up ok but I have not checked any voltages
All waxes were replaced and out of tolerance resistors replaced when I worked on this set earlier.

Bill

jonnybear · 14th Apr 2014, 11:04 pm

This Radio is a bit of a strange beast, as it uses the triode section of the ECH42
as a preamp for the Gram when not being used as the radio oscillator, I cannot see it as a heater cathode short as the ECH42 & the EBF40 cathodes are direct to chassis,I am wondering if you have either a detached C5 causing instability or worst case a arching wave change switch.
John

Top Cap · 14th Apr 2014, 11:22 pm

In the GRAM position it appears that the EBF40 is not in circuit, could the fault be here?
Have you tried pulling the ECH42 in a RADIO position to see if the noise goes?
If it doesn't then try pulling the EBF40 where perhaps the problem may be?
Not serviced this chassis before so some speculation on my part, it would be nice if you could trace it to a faulty stage by some method of isolation like this.

Sideband · 15th Apr 2014, 10:30 am

OK sorry I thought this set used the U range valves in series from a transformer. Same idea though and Top Cap is describing a way of eliminating the valves quickly (easier with parallel heaters)!

Possibly a short in one of the valves and worth pulling the ECH or EBF out quickly just to see if the fault changes.

Pepelephew · 15th Apr 2014, 3:27 pm

Sorry I misled you with the valve line up. This set uses EBF 80 and it's the only valve that I don't have a spare of.
Perhaps a better description of the noise is like a very loud Brrrrrrrrrrr. I replaced the other valves one by one with no change.I am waiting on a replacement EBF 80 which will take a few days. I am away from the set for a few days so I will report back when I get the valve,hopefully that will sort it
Thanks
Bill

Sideband · 15th Apr 2014, 6:20 pm

What happens if you just pull the EBF80 out? OK to do this with the set on but you can remove it, switch on and see if the hum is still there.

Pepelephew · 16th Apr 2014, 7:58 am

I am away for a few days but will follow your advice when I return
Bill

Silicon · 16th Apr 2014, 11:15 am

Capacitor C3 (0.05uF) is the screen decoupling capacitor for V1 and V2.
Has it been changed?

jonnybear · 16th Apr 2014, 1:54 pm

I stand corrected Silicon ( eyes ain't what they used to be) it is C3 .05uf, not C5 as I stated in earlier thread.
John

Pepelephew · 16th Apr 2014, 4:20 pm

I had changed old wax caps when I first refurbished this set in January
I don't gave the service sheet with me to be absolutely sure but I will check this when I return
Bill

Pepelephew · 23rd Apr 2014, 8:12 pm

Hi Lads,
back after a few days hols....
C3 cap was changed recently, removed ECH42 and noise stopped but started again when valve re inserted.
Same for EBF 80. Removed valve and noise stopped ,fitted a replacement valve and BRRRRR noise started again. A bit stumped, this noise overrides everything on all bands except gram.

jonnybear · 24th Apr 2014, 5:01 pm

Sounds like instability in RF/IF section, check the following R2, 680k, C15,.05uf
R8 1.5m ohm,R10,680k, r12 330k.
other than that what voltages have you got on both ECH42 & EBF80. also check for dry joints and poor earthing points, especially around where you have changed components.
John

Pepelephew · 24th Apr 2014, 8:10 pm

Thanks John,
I will follow up on that and report back
Bill

Pepelephew · 29th Apr 2014, 8:45 pm

Hi Lads,
following John's suggestions I removed the tag strip where I had replaced some components, c15,c3,r8.....was rechecking all solder joints when I found one wire was slightly loose, when I heated the solder it came out without me touching it.
Some of the components I had fitted earlier in Feb. were now reading out of tolerance probably because of the loose wire?...
I replaced them and refitted the tag strip, reconnected everything and surprise surprise success. Set now working nicely again . Thanks to all for your help

Bill

14th Apr 2014, 8:06 pm	#1
Pepelephew Pentode Join Date: Nov 2011 Location: Prosperous, Co. Kildare, Ireland. Posts: 176	Bush AC 41 loud noise Hi All, I spent some time restoring this Bush AC 41 earlier this year and was very happy with the final result. However about a week ago the sound faded and died on all wavelengths.`i switched it off and had a quick look to see if anything obvious was wrong, (nothing noticed). On switching it back on again I was greeted with very loud hum/noise. This happens on all wavelengths but not on Gram.The noise increases/decreases with the volume control. Nothing whatsoever can be tuned in. There is no response from any wavelength except this incessant noise. Any pointers please Thanks Bill Attached Thumbnails

14th Apr 2014, 8:17 pm	#2
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,846	Re: Bush AC 41 loud noise Sounds daft, Bill, but have you tried another set in the same spot in case it's interference from a SMPS etc.? And have the waxies all been changed? A failed decoupler could be causing motorboating, I suppose, Nick

14th Apr 2014, 9:50 pm	#3
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,572	Re: Bush AC 41 loud noise Possibly a cathode/heater leak/short in one of the valves pre volume control. The clue has to be with the gram position where there is no hum. Only the UBC41 and UL41 are used in that position so that leaves the UF41 or UCH42. I'd try either of those first before diving in deeper. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

14th Apr 2014, 10:09 pm	#4
Pepelephew Pentode Join Date: Nov 2011 Location: Prosperous, Co. Kildare, Ireland. Posts: 176	Re: Bush AC 41 loud noise Hi lads Yes this set was sitting beside anther one . No problems with the second set only the ac41 Brought it to a different room.......same problem. The valves used in this set are ECH 42, EBF 40, EL 41 and EZ 40 All are lighting up ok but I have not checked any voltages All waxes were replaced and out of tolerance resistors replaced when I worked on this set earlier. Bill

14th Apr 2014, 11:04 pm	#5
jonnybear Octode Join Date: Nov 2010 Location: Cullompton, Devon, UK. Posts: 1,435	Re: Bush AC 41 loud noise This Radio is a bit of a strange beast, as it uses the triode section of the ECH42 as a preamp for the Gram when not being used as the radio oscillator, I cannot see it as a heater cathode short as the ECH42 & the EBF40 cathodes are direct to chassis,I am wondering if you have either a detached C5 causing instability or worst case a arching wave change switch. John

14th Apr 2014, 11:22 pm	#6
Top Cap Octode Join Date: Mar 2006 Location: Watford, Hertfordshire, UK. Posts: 1,270	Re: Bush AC 41 loud noise In the GRAM position it appears that the EBF40 is not in circuit, could the fault be here? Have you tried pulling the ECH42 in a RADIO position to see if the noise goes? If it doesn't then try pulling the EBF40 where perhaps the problem may be? Not serviced this chassis before so some speculation on my part, it would be nice if you could trace it to a faulty stage by some method of isolation like this. __________________ Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

15th Apr 2014, 10:30 am	#7
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,572	Re: Bush AC 41 loud noise OK sorry I thought this set used the U range valves in series from a transformer. Same idea though and Top Cap is describing a way of eliminating the valves quickly (easier with parallel heaters)! Possibly a short in one of the valves and worth pulling the ECH or EBF out quickly just to see if the fault changes. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

15th Apr 2014, 3:27 pm	#8
Pepelephew Pentode Join Date: Nov 2011 Location: Prosperous, Co. Kildare, Ireland. Posts: 176	Re: Bush AC 41 loud noise Sorry I misled you with the valve line up. This set uses EBF 80 and it's the only valve that I don't have a spare of. Perhaps a better description of the noise is like a very loud Brrrrrrrrrrr. I replaced the other valves one by one with no change.I am waiting on a replacement EBF 80 which will take a few days. I am away from the set for a few days so I will report back when I get the valve,hopefully that will sort it Thanks Bill

15th Apr 2014, 6:20 pm	#9
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,572	Re: Bush AC 41 loud noise What happens if you just pull the EBF80 out? OK to do this with the set on but you can remove it, switch on and see if the hum is still there. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

16th Apr 2014, 7:58 am	#10
Pepelephew Pentode Join Date: Nov 2011 Location: Prosperous, Co. Kildare, Ireland. Posts: 176	Re: Bush AC 41 loud noise I am away for a few days but will follow your advice when I return Bill

16th Apr 2014, 11:15 am	#11
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,163	Re: Bush AC 41 loud noise Capacitor C3 (0.05uF) is the screen decoupling capacitor for V1 and V2. Has it been changed?

16th Apr 2014, 1:54 pm	#12
jonnybear Octode Join Date: Nov 2010 Location: Cullompton, Devon, UK. Posts: 1,435	Re: Bush AC 41 loud noise I stand corrected Silicon ( eyes ain't what they used to be) it is C3 .05uf, not C5 as I stated in earlier thread. John

16th Apr 2014, 4:20 pm	#13
Pepelephew Pentode Join Date: Nov 2011 Location: Prosperous, Co. Kildare, Ireland. Posts: 176	Re: Bush AC 41 loud noise I had changed old wax caps when I first refurbished this set in January I don't gave the service sheet with me to be absolutely sure but I will check this when I return Bill

23rd Apr 2014, 8:12 pm	#14
Pepelephew Pentode Join Date: Nov 2011 Location: Prosperous, Co. Kildare, Ireland. Posts: 176	Re: Bush AC 41 loud noise Hi Lads, back after a few days hols.... C3 cap was changed recently, removed ECH42 and noise stopped but started again when valve re inserted. Same for EBF 80. Removed valve and noise stopped ,fitted a replacement valve and BRRRRR noise started again. A bit stumped, this noise overrides everything on all bands except gram.

24th Apr 2014, 5:01 pm	#15
jonnybear Octode Join Date: Nov 2010 Location: Cullompton, Devon, UK. Posts: 1,435	Re: Bush AC 41 loud noise Sounds like instability in RF/IF section, check the following R2, 680k, C15,.05uf R8 1.5m ohm,R10,680k, r12 330k. other than that what voltages have you got on both ECH42 & EBF80. also check for dry joints and poor earthing points, especially around where you have changed components. John

24th Apr 2014, 8:10 pm	#16
Pepelephew Pentode Join Date: Nov 2011 Location: Prosperous, Co. Kildare, Ireland. Posts: 176	Re: Bush AC 41 loud noise Thanks John, I will follow up on that and report back Bill

29th Apr 2014, 8:45 pm	#17
Pepelephew Pentode Join Date: Nov 2011 Location: Prosperous, Co. Kildare, Ireland. Posts: 176	Re: Bush AC 41 loud noise Hi Lads, following John's suggestions I removed the tag strip where I had replaced some components, c15,c3,r8.....was rechecking all solder joints when I found one wire was slightly loose, when I heated the solder it came out without me touching it. Some of the components I had fitted earlier in Feb. were now reading out of tolerance probably because of the loose wire?... I replaced them and refitted the tag strip, reconnected everything and surprise surprise success. Set now working nicely again . Thanks to all for your help Bill