Bush DAC10 mains hum - UK Vintage Radio Repair and Restoration Discussion Forum

slidertogrid · 15th Feb 2007, 12:25 pm

Hi
I have a Bush DAC10 which had been awaiting attention for a few years,
last week its time came, I reformed the smoother, replaced the wax caps replaced the dial lamps and shunt resistor and the set works well, however after it has run for a while it develops a low level hum, this can only be heard at low volume but it is annoying, I have tried a known good UL41, checked the spare pin in the valveholder is not connected to anything and tried splashing the smoothing with another to no avail.

What have I missed? or do some sets hum more than others?
I had a DAC 90a with similar problems but after disconnecting the unused pin and trying a few UL41s it is silent even on min volume.

Any suggestions gratefully received!

G0PKH - Pete · 15th Feb 2007, 12:37 pm

It could be possible that another valve has a very slight heater cathode leak. Might be worth swapping them one by one to prove.

Had this on a 90a once.

Pete

slidertogrid · 15th Feb 2007, 9:52 pm

Thanks Pete for your reply, I had already tried a rectifier, I have now tried a complete set of valves from a known good DAC90a, no better, I then gave all the valveholders a good clean and scraped the bottom of the valves between
the pins, this has made a little improvement I think, but it still hums a little when warm,
My next line of thought is to try running the valve heaters on DC I am sure Iv seen a thread on this some time back?
Anyone got any ideas on how I could do this?
Thanks, Rich.

Biggles · 16th Feb 2007, 11:52 am

Try replacing the smoothing caps, it could be that they are failing when hot?

Biggles.

paulsherwin · 16th Feb 2007, 12:45 pm

Quote:

Originally Posted by Biggles

Try replacing the smoothing caps, it could be that they are failing when hot?

My thinking exactly. I've never been a big fan of reforming knackered old smoothing caps. You could connect two new caps temporarily and see if it sorts the problem out.

Paul

Biggles · 16th Feb 2007, 1:34 pm

Whoops, I've just re-read your original post and it says you've already tried subbing the smoothers.

Biggles.

Hunts smoothing bomb · 16th Feb 2007, 3:25 pm

Just a stab in the dark but is the screen feed resistor still within spec?

Cheers
Lee

Tim · 16th Feb 2007, 5:37 pm

Locating the source of hum on this set is complicated by the fact it uses half wave rectification,meaning Hum deriving from the HT or heaters/AC supply will both be at 50 Hz. Shorting the control of the UL 41, and UBC41(short out the volume control) in turn may give you a clue. Pick up is most likely to occur in the AF triode/volume control section.
Don't overlook the control itself.
And the mains suppression capacitor.

yestertech · 16th Feb 2007, 7:22 pm

Quote:

Originally Posted by Tim

Locating the source of hum on this set is complicated by the fact it uses half wave rectification,meaning Hum deriving from the HT or heaters/AC supply will both be at 50 Hz. Shorting the control of the UL 41, and UBC41(short out the volume control) in turn may give you a clue. Pick up is most likely to occur in the AF triode/volume control section.
Don't overlook the control itself.
And the mains suppression capacitor.

I agree with Tim here - it's important to note whether the hum is induced into the UBC41 stage or the UL41 itself.
When the fault manifests itself, what happens when you earth :
1. The UL41 control grid ?
2.The UBC41 control grid ?

Being as it's temperature sensitive , logically it should be down to the hottest items ie the valves. Also being temp. sens. many of the tricks about screening and routing of cables will not apply
If valves from a known good set exhibit EXACTLY the same symptoms, then points 1 and 2 above are especially important to determine which components to suspect next.
It could be worth subbing the UL41 valveholder if it looks like neither 1. or 2. above affect the problem ?

Andy

PS Are this set and the known good one fed from the same power arrangements ?

slidertogrid · 16th Feb 2007, 9:29 pm

Hi ,
Thanks everyone for your advice, I tried disconnecting the original smoothing cap and lashing in two replacements, no different, next I metered the grid resistor and it read a bit high, replacing it made a slight difference, I then soldered the screened lead to the body of the volume control as well as its end tag, success ... no hum! its now been on for 2 hours or more and its as humless as can be.

Thanks again everyone for your help! sometimes you get stuck and nothing works and a few pointers in the right direction is all thats needed!

Rich.

15th Feb 2007, 12:25 pm	#1
slidertogrid Octode Join Date: Apr 2005 Location: Peterborough, Cambridgeshire, UK. Posts: 1,898	Bush DAC10 mains hum Hi I have a Bush DAC10 which had been awaiting attention for a few years, last week its time came, I reformed the smoother, replaced the wax caps replaced the dial lamps and shunt resistor and the set works well, however after it has run for a while it develops a low level hum, this can only be heard at low volume but it is annoying, I have tried a known good UL41, checked the spare pin in the valveholder is not connected to anything and tried splashing the smoothing with another to no avail. What have I missed? or do some sets hum more than others? I had a DAC 90a with similar problems but after disconnecting the unused pin and trying a few UL41s it is silent even on min volume. Any suggestions gratefully received! __________________ The rotation of the earth really makes my day... Last edited by Darren-UK; 13th Aug 2007 at 10:48 pm. Reason: Excessive use of emoticons.

15th Feb 2007, 12:37 pm	#2
G0PKH - Pete Retired Dormant Member Join Date: Aug 2003 Location: Stansted Essex Posts: 263	Re: Bush DAC10 mains hum It could be possible that another valve has a very slight heater cathode leak. Might be worth swapping them one by one to prove. Had this on a 90a once. Pete

15th Feb 2007, 9:52 pm	#3
slidertogrid Octode Join Date: Apr 2005 Location: Peterborough, Cambridgeshire, UK. Posts: 1,898	Re: Bush DAC10 mains hum Thanks Pete for your reply, I had already tried a rectifier, I have now tried a complete set of valves from a known good DAC90a, no better, I then gave all the valveholders a good clean and scraped the bottom of the valves between the pins, this has made a little improvement I think, but it still hums a little when warm, My next line of thought is to try running the valve heaters on DC I am sure Iv seen a thread on this some time back? Anyone got any ideas on how I could do this? Thanks, Rich. __________________ The rotation of the earth really makes my day...

16th Feb 2007, 11:52 am	#4
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Bush DAC10 mains hum Try replacing the smoothing caps, it could be that they are failing when hot? Biggles.

16th Feb 2007, 1:34 pm	#6
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Bush DAC10 mains hum Whoops, I've just re-read your original post and it says you've already tried subbing the smoothers. Biggles.

16th Feb 2007, 3:25 pm	#7
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Bush DAC10 mains hum Just a stab in the dark but is the screen feed resistor still within spec? Cheers Lee __________________ Lee

16th Feb 2007, 5:37 pm	#8
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Bush DAC10 mains hum Locating the source of hum on this set is complicated by the fact it uses half wave rectification,meaning Hum deriving from the HT or heaters/AC supply will both be at 50 Hz. Shorting the control of the UL 41, and UBC41(short out the volume control) in turn may give you a clue. Pick up is most likely to occur in the AF triode/volume control section. Don't overlook the control itself. And the mains suppression capacitor. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

16th Feb 2007, 9:29 pm	#10
slidertogrid Octode Join Date: Apr 2005 Location: Peterborough, Cambridgeshire, UK. Posts: 1,898	Re: Bush DAC10 mains hum Hi , Thanks everyone for your advice, I tried disconnecting the original smoothing cap and lashing in two replacements, no different, next I metered the grid resistor and it read a bit high, replacing it made a slight difference, I then soldered the screened lead to the body of the volume control as well as its end tag, success ... no hum! its now been on for 2 hours or more and its as humless as can be. Thanks again everyone for your help! sometimes you get stuck and nothing works and a few pointers in the right direction is all thats needed! Rich. __________________ The rotation of the earth really makes my day... Last edited by Darren-UK; 13th Aug 2007 at 10:46 pm. Reason: Excessive use of emoticons and somewhat rude word removed.