Bush DAC 90a loud hum - UK Vintage Radio Repair and Restoration Discussion Forum

John Earland · 20th May 2020, 12:46 pm

Hi everyone,
I hope you can help. I have a a lovely Bush Radi0 - DAC 90A and it gives a lovely glow and rich sound. However, there is a loud hum which gets drowned out if I turn the volume up but is constantly there.

I have replaced the Caps so I am guessing its at least one of the valves? Without replacing them all, would anyone know how I could test to see if they are working properly? They all glow but I've been told that doesn't matter. I only have a multimeter in my arsenal - awaiting delivery of an oscilloscope.

I may need to replace more caps so any help would be greatly appreciated.

Best wishes,
John

pip5678 · 20th May 2020, 12:53 pm

UL 41 valve is often a suspect, some info in this thread below (and others) re pin 4 modification which may help and is easy to try.
https://www.vintage-radio.net/forum/...d.php?t=150751

cathoderay57 · 20th May 2020, 1:34 pm

Hi John, which caps have you actually replaced? Be careful with the live chassis and using the multimeter measure the dc voltage on pin 6 (grid 1) of the UL41. Cheers, Jerry

John Earland · 20th May 2020, 1:56 pm

Thanks everyone - I will check those out. Really helpful. I've replaced 5x 0.05uf and 2x0.01uf and also the 'X' cap? I saw it somewhere that this one goes so replaced it. Also the smoothing capacitors 32-16 (350V). I think that's all. I did this a long time ago and always meant to try to get rid of the hum but only now have I found some time!

Best wishes,
John

John Earland · 20th May 2020, 1:57 pm

Quote:

Originally Posted by cathoderay57

Hi John, which caps have you actually replaced? Be careful with the live chassis and using the multimeter measure the dc voltage on pin 6 (grid 1) of the UL41. Cheers, Jerry

Hi - what should I be getting on the pin? J

cathoderay57 · 20th May 2020, 3:09 pm

Hi John, measured with respect to chassis you should be getting approx zero volts - however, the UL41 suspect faults alluded to in Post #2 can include leakage across the electrodes internally to the valve that can lead to positive volts on the grid. If you have changed C18, the 0.01uF grid coupling capacitor, it should not be leaking therefore any positive volts on the grid can indicate a duff UL41 and possibly a cause of hum. Also worth checking the routing of the wiring and capacitors attached to the volume control. If audio leads or coupling capacitors are close to mains wires this can result in 50Hz hum pickup. Just carefully moving them can reduce the hum level.

John Earland · 20th May 2020, 3:44 pm

That's great advice. Thank you. I'll double check.
Best wishes, John

kalee20 · 20th May 2020, 5:35 pm

Maybe easier - short grid (Pin 6) of the UL41 to chassis. Everything should go quiet.

Next, short grid (Pin 3) of UBC41 to chassis. Everything again should go quiet.

If the UL41 short doesn't make it quiet, fault is in the UL41. If the UL41 short makes it quiet and so does the UBC41 short, fault may be in grid circuit of UBC41, or leakage to the diode anodes in UBC41.

That's all assuming that the HT line is ripple-free - I see you have changed the HT electrolytics. But make sure the common negative connection is OK.

cathoderay57 · 20th May 2020, 8:55 pm

Quote:

Originally Posted by John Earland

awaiting delivery of an oscilloscope.

Not wishing to teach granny to suck eggs but be careful when you get the scope if you're intending to use it on the DAC90A - simply because the screen on the scope's signal input cable is most likely connected to the scope's chassis and therefore almost certainly earthed, whereas the DAC90A chassis will be at mains neutral or live potential depending on how you have wired the mains lead! Cheers, Jerry

cathoderay57 · 21st May 2020, 8:06 am

As a follow-up to Post #9 you can use the scope safely providing that you power up the DAC90A via a 230v mains isolating transformer.

John Earland · 21st May 2020, 8:10 am

Yes! When I got the radio I didn't twig the fact that it was a metal chassis-I am always fearful of metal and electricity. So the first time I plugged it in I was just about the move it when I thought-I’m just going to check and it was indeed live!

John Earland · 21st May 2020, 8:12 am

Quote:

Originally Posted by kalee20

Maybe easier - short grid (Pin 6) of the UL41 to chassis. Everything should go quiet.

Next, short grid (Pin 3) of UBC41 to chassis. Everything again should go quiet.

If the UL41 short doesn't make it quiet, fault is in the UL41. If the UL41 short makes it quiet and so does the UBC41 short, fault may be in grid circuit of UBC41, or leakage to the diode anodes in UBC41.

That's all assuming that the HT line is ripple-free - I see you have changed the HT electrolytics. But make sure the common negative connection is OK.

When you say ‘short’ do you mean place a wire from the pin the chassis?

cathoderay57 · 21st May 2020, 8:50 am

Hi John, yes, that's what kalee20 meant. If you are careful and keep one hand in your pocket and use the other to hold the insulated handle of a screwdriver you should be able to use the screwdriver blade to temporarily short circuit the pin to chassis and listen to see if hum disappears. You don't need to solder a wire on.

kalee20 · 21st May 2020, 10:12 am

Exactly as cathoderay57 says! (thanks!)

cathoderay57 · 21st May 2020, 6:13 pm

John, one last thought - if you are unsure of the UL41 pin configuration make sure to look it up. If you short the wrong pin to chassis you could rupture one or more of the valve heaters! Cheers, Jerry

John Earland · 22nd May 2020, 2:28 pm

Quote:

Originally Posted by cathoderay57

John, one last thought - if you are unsure of the UL41 pin configuration make sure to look it up. If you short the wrong pin to chassis you could rupture one or more of the valve heaters! Cheers, Jerry

Hi, that's great - thank you. I will look that up because I dont know the numbering! All new to me this bit. Many thanks.John

cathoderay57 · 22nd May 2020, 5:02 pm

OK. Pin numbering is viewed from underneath as shown here http://www.r-type.org/static/baseb8a.htm

John Earland · 24th May 2020, 9:01 am

Thanks everyone for all your help. It was indeed the UL41-I replaced it and hey presto! The hum sound has vanished! Many thanks for all your advice and guidance.

60 oldjohn · 24th May 2020, 11:16 am

Pleased you got it sorted. Some valve holders may if your are lucky have the pin numbers printed next to the solder tags if you look closely. The Ever Ready battery set on my bench at the moment has them marked. But I needed a magnifying glass to see them.

John.

Sideband · 24th May 2020, 7:03 pm

UL 41's are rare and expensive. You might be able to fix the old one by burning off the residue inside the glass around the pins. You need a high voltage to do it (around 1,500V at several milliamps). Join the two heater pins together (1 & 8) and then apply the voltage across any two pins (except 1 & 8 of course). The idea is to 'arc' the internal deposits away that are causing the leakage. I have seen a device made up from an old camera flash unit which seemed to do the job quite well.

20th May 2020, 12:46 pm	#1
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	Bush DAC 90a loud hum Hi everyone, I hope you can help. I have a a lovely Bush Radi0 - DAC 90A and it gives a lovely glow and rich sound. However, there is a loud hum which gets drowned out if I turn the volume up but is constantly there. I have replaced the Caps so I am guessing its at least one of the valves? Without replacing them all, would anyone know how I could test to see if they are working properly? They all glow but I've been told that doesn't matter. I only have a multimeter in my arsenal - awaiting delivery of an oscilloscope. I may need to replace more caps so any help would be greatly appreciated. Best wishes, John

20th May 2020, 1:34 pm	#3
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Bush DAC 90a loud hum Hi John, which caps have you actually replaced? Be careful with the live chassis and using the multimeter measure the dc voltage on pin 6 (grid 1) of the UL41. Cheers, Jerry

20th May 2020, 1:56 pm	#4
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	Re: Bush DAC 90a loud hum Thanks everyone - I will check those out. Really helpful. I've replaced 5x 0.05uf and 2x0.01uf and also the 'X' cap? I saw it somewhere that this one goes so replaced it. Also the smoothing capacitors 32-16 (350V). I think that's all. I did this a long time ago and always meant to try to get rid of the hum but only now have I found some time! Best wishes, John

20th May 2020, 3:09 pm	#6
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Bush DAC 90a loud hum Hi John, measured with respect to chassis you should be getting approx zero volts - however, the UL41 suspect faults alluded to in Post #2 can include leakage across the electrodes internally to the valve that can lead to positive volts on the grid. If you have changed C18, the 0.01uF grid coupling capacitor, it should not be leaking therefore any positive volts on the grid can indicate a duff UL41 and possibly a cause of hum. Also worth checking the routing of the wiring and capacitors attached to the volume control. If audio leads or coupling capacitors are close to mains wires this can result in 50Hz hum pickup. Just carefully moving them can reduce the hum level. Last edited by cathoderay57; 20th May 2020 at 3:14 pm.

20th May 2020, 3:44 pm	#7
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	Re: Bush DAC 90a loud hum That's great advice. Thank you. I'll double check. Best wishes, John

20th May 2020, 12:53 pm	#2
pip5678 Pentode Join Date: Jan 2019 Location: Manchester, UK. Posts: 163	Re: Bush DAC 90a loud hum UL 41 valve is often a suspect, some info in this thread below (and others) re pin 4 modification which may help and is easy to try. https://www.vintage-radio.net/forum/...d.php?t=150751

20th May 2020, 5:35 pm	#8
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Bush DAC 90a loud hum Maybe easier - short grid (Pin 6) of the UL41 to chassis. Everything should go quiet. Next, short grid (Pin 3) of UBC41 to chassis. Everything again should go quiet. If the UL41 short doesn't make it quiet, fault is in the UL41. If the UL41 short makes it quiet and so does the UBC41 short, fault may be in grid circuit of UBC41, or leakage to the diode anodes in UBC41. That's all assuming that the HT line is ripple-free - I see you have changed the HT electrolytics. But make sure the common negative connection is OK.

21st May 2020, 8:06 am	#10
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Bush DAC 90a loud hum As a follow-up to Post #9 you can use the scope safely providing that you power up the DAC90A via a 230v mains isolating transformer.

21st May 2020, 8:10 am	#11
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	Re: Bush DAC 90a loud hum Yes! When I got the radio I didn't twig the fact that it was a metal chassis-I am always fearful of metal and electricity. So the first time I plugged it in I was just about the move it when I thought-I’m just going to check and it was indeed live!

21st May 2020, 8:50 am	#13
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Bush DAC 90a loud hum Hi John, yes, that's what kalee20 meant. If you are careful and keep one hand in your pocket and use the other to hold the insulated handle of a screwdriver you should be able to use the screwdriver blade to temporarily short circuit the pin to chassis and listen to see if hum disappears. You don't need to solder a wire on.

21st May 2020, 10:12 am	#14
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Bush DAC 90a loud hum Exactly as cathoderay57 says! (thanks!)

21st May 2020, 6:13 pm	#15
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Bush DAC 90a loud hum John, one last thought - if you are unsure of the UL41 pin configuration make sure to look it up. If you short the wrong pin to chassis you could rupture one or more of the valve heaters! Cheers, Jerry

22nd May 2020, 5:02 pm	#17
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Bush DAC 90a loud hum OK. Pin numbering is viewed from underneath as shown here http://www.r-type.org/static/baseb8a.htm

24th May 2020, 9:01 am	#18
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	Re: Bush DAC 90a loud hum Thanks everyone for all your help. It was indeed the UL41-I replaced it and hey presto! The hum sound has vanished! Many thanks for all your advice and guidance.

24th May 2020, 11:16 am	#19
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 3,987	Re: Bush DAC 90a loud hum Pleased you got it sorted. Some valve holders may if your are lucky have the pin numbers printed next to the solder tags if you look closely. The Ever Ready battery set on my bench at the moment has them marked. But I needed a magnifying glass to see them. John. __________________ My favourite text message "I'll be there in five minutes, if not read again"

24th May 2020, 7:03 pm	#20
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,571	Re: Bush DAC 90a loud hum UL 41's are rare and expensive. You might be able to fix the old one by burning off the residue inside the glass around the pins. You need a high voltage to do it (around 1,500V at several milliamps). Join the two heater pins together (1 & 8) and then apply the voltage across any two pins (except 1 & 8 of course). The idea is to 'arc' the internal deposits away that are causing the leakage. I have seen a device made up from an old camera flash unit which seemed to do the job quite well. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....