Bush DAC90a hum and alignment - UK Vintage Radio Repair and Restoration Discussion Forum

Markjames · 8th Aug 2005, 10:38 pm

I recently bought the above on ebay in working condition but when I looked inside it had been got at. Components had been replaced with disregard for values and safety and the wiring to the bulbs had been messed about with.

I've replaced the capacitors again with correct values etc. and sorted out the wiring but I'm left with 2 problems.

There is a noticeable hum which is not affected by the volume control and it looks as if someone has been playing with the aerial trimmers and oscillator coils as TC4 was nearly hanging out.

I wondered if the UL41 could be causing the hum? smoothing caps etc. have been replaced?

If someone lives in the Bedford area who knows more about these things than I do, I'd be happy to pay them to sort it out, as I don't have the equipment to check the alignment, other than that, any advice would be appreciated.

Markjames · 9th Aug 2005, 9:21 am

Regarding my thread above, I've put the aerial trimmer back in, moved the radio to a different position and rotated it and can now pick up a lot more stations this morning, so sensitivity is probably okay.

Still got the hum though, not very loud, but noticeable at low volume, could it be normal I wonder?

If not, anything you think I should try would be helpful, but I don't have any spare valves to try, just a few resitors and caps.

Also, dropper smells hot, but this is getting better, I assume this is just dust etc. burning off (originally there was whiffs of smoke but they've all gone.

Sound quality and volume good.

Aerodyne · 9th Aug 2005, 5:14 pm

Hi Mark.

The UL41 output valve is notorious for heater-cathode leakage which can manifest itself as a hum regardless of V/C setting, but it is often indecisive when the valve is checked on a valve tester - the best test is replacement, but these are becoming scarce and therefore expensive.
I hope you don't mind me mentioning this, but you do sound as though you are not very experienced. If so, remember that the DAC90A has a live chassis: handle with care.
-Tony

Darren-UK · 9th Aug 2005, 5:40 pm

Hi Mark,

AC/DC radios have a background hum by default and this should only be audible at zero or minimum volume. You should decide if your hum is normal in respect of this before investigating further. You should also ensure the hum is not being generated by some external source such as a pc situated in close proximity.

Regarding the UL41, as has already been stated this valve is notorious and can indeed be a source of hum. However, a failing UL41 will often show up in two other ways; 1. poor volume or 2. normal operation after switch-on ( say up to 15 mins from cold ) and then poor volume, sometimes with a bit of distortion.

Markjames · 11th Aug 2005, 8:57 am

I didn't know that a hum at low and zero volume was normal for AC/DC sets, the hum goes away as I increase the volume so I guess it's normal. Thanks very much for your help.

8th Aug 2005, 10:38 pm	#1
Markjames Retired Dormant Member Join Date: Jul 2005 Location: Bedford Posts: 10	Bush DAC90a hum and alignment I recently bought the above on ebay in working condition but when I looked inside it had been got at. Components had been replaced with disregard for values and safety and the wiring to the bulbs had been messed about with. I've replaced the capacitors again with correct values etc. and sorted out the wiring but I'm left with 2 problems. There is a noticeable hum which is not affected by the volume control and it looks as if someone has been playing with the aerial trimmers and oscillator coils as TC4 was nearly hanging out. I wondered if the UL41 could be causing the hum? smoothing caps etc. have been replaced? If someone lives in the Bedford area who knows more about these things than I do, I'd be happy to pay them to sort it out, as I don't have the equipment to check the alignment, other than that, any advice would be appreciated.

9th Aug 2005, 9:21 am	#2
Markjames Retired Dormant Member Join Date: Jul 2005 Location: Bedford Posts: 10	Re: Bush DAC90a hum and alignment Regarding my thread above, I've put the aerial trimmer back in, moved the radio to a different position and rotated it and can now pick up a lot more stations this morning, so sensitivity is probably okay. Still got the hum though, not very loud, but noticeable at low volume, could it be normal I wonder? If not, anything you think I should try would be helpful, but I don't have any spare valves to try, just a few resitors and caps. Also, dropper smells hot, but this is getting better, I assume this is just dust etc. burning off (originally there was whiffs of smoke but they've all gone. Sound quality and volume good.

9th Aug 2005, 5:14 pm	#3
Aerodyne Octode Join Date: Jun 2004 Location: Hampton Vale, Peterborough, UK. Posts: 1,698	Re: Bush DAC90a hum and alignment Hi Mark. The UL41 output valve is notorious for heater-cathode leakage which can manifest itself as a hum regardless of V/C setting, but it is often indecisive when the valve is checked on a valve tester - the best test is replacement, but these are becoming scarce and therefore expensive. I hope you don't mind me mentioning this, but you do sound as though you are not very experienced. If so, remember that the DAC90A has a live chassis: handle with care. -Tony

9th Aug 2005, 5:40 pm	#4
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	Re: Bush DAC90a hum and alignment Hi Mark, AC/DC radios have a background hum by default and this should only be audible at zero or minimum volume. You should decide if your hum is normal in respect of this before investigating further. You should also ensure the hum is not being generated by some external source such as a pc situated in close proximity. Regarding the UL41, as has already been stated this valve is notorious and can indeed be a source of hum. However, a failing UL41 will often show up in two other ways; 1. poor volume or 2. normal operation after switch-on ( say up to 15 mins from cold ) and then poor volume, sometimes with a bit of distortion.

11th Aug 2005, 8:57 am	#5
Markjames Retired Dormant Member Join Date: Jul 2005 Location: Bedford Posts: 10	Re: Bush DAC90a hum and alignment I didn't know that a hum at low and zero volume was normal for AC/DC sets, the hum goes away as I increase the volume so I guess it's normal. Thanks very much for your help.