Hacker AL42 Hum - UK Vintage Radio Repair and Restoration Discussion Forum

Dancer1 · 20th Jan 2021, 5:55 pm

Hi all.
I am a newbie who has been lurking on the forum for a bit. Many thanks for all the information and help. I own a Hacker AL42 that I bought as recently serviced. It does have new caps and resistors. However, it has an audible hum that increases with the volume knob. It hums with or without input.

I opened up the back and was looking around the tone control unit. Should there be a ground wire where the arrow is pointing (based on the schematic)?

Many thanks,

Ambrose

kirstyd · 20th Jan 2021, 6:26 pm

On the hacker Gondolier [has the same amp] there is a black wire connected to this point with the other end attached to the metal body of the speaker.

Dancer1 · 20th Jan 2021, 6:42 pm

Many thanks for the quick response Kirstyd. The power lead's earth is attached to the chassis as shown in the picture below. Anyone know whether I should be happy with this and also whether I should connect an earth (the black cable as identified by Kirstyd) from the tag identified above on the tone controls to the chassis? What would be the likely effect on hum?

Many thanks.

Nickthedentist · 20th Jan 2021, 7:13 pm

Hello and welcome, Ambrose.

You could try adding a ground lead from there to the LS chassis, but I'd be surprised if that were the culprit.

I will have a look at mine on Friday if I remember, to see if such a lead is fitted.

Does the hum fade to nothing if the volume control is turned down to zero?

N.

Dancer1 · 20th Jan 2021, 7:56 pm

The hum fades to a point where you have to have your near right next to the speaker to hear it, but it's still there. I will hook up a lead and see what happens. What are the usual culprits for humming? What should I check?

kirstyd · 20th Jan 2021, 7:57 pm

If my memory serves me well I believe there is also a lead coming from one of those connections its other end connecting to the sort of kitchen foil that is stuck to the base of the cabinet. Leaving this off did cause hum.

kirstyd · 20th Jan 2021, 8:22 pm

I have taken pictures of the inside of mine they may help you. I did the refurb last year. You can see the black wire going to the speaker. You can also see the foil bridge held in place by two staples. I replaced the mains lead on mine putting the earth where the old one was connected.

kirstyd · 20th Jan 2021, 8:24 pm

As you can see there is nothing connected to that tag in your first post.

Dancer1 · 20th Jan 2021, 8:31 pm

Thanks Kirstyd. Pictures do help a lot. I shall try a foil bridge to see if that makes a difference.

Richardgr · 21st Jan 2021, 2:10 pm

Since the hum is amplified (increases with volume control), doesn't this imply ripple on the B+ power supply? Your smoothing cap looks to be original, maybe it would be worth updating that.

Another consideration is the heater supply - is that grounded?

Edit: I notice you said it had been serviced, but that smoothing cap and the resistor look remarkably unsullied.

Dancer1 · 24th Jan 2021, 2:45 pm

Based on Richard's observations, I broke out the DMM and traced the issue to C11/C12. I have changed this and the hum is gone. Many thanks to you all for your help. I also checked the rest of the caps and most are new and in range (including the original Millard Mustards). Now I can enjoy the system without the annoying hum.

20th Jan 2021, 5:55 pm	#1
Dancer1 Diode Join Date: Nov 2020 Location: Lincoln, Lincolnshire, UK. Posts: 9	Hacker AL42 Hum Hi all. I am a newbie who has been lurking on the forum for a bit. Many thanks for all the information and help. I own a Hacker AL42 that I bought as recently serviced. It does have new caps and resistors. However, it has an audible hum that increases with the volume knob. It hums with or without input. I opened up the back and was looking around the tone control unit. Should there be a ground wire where the arrow is pointing (based on the schematic)? Many thanks, Ambrose Attached Thumbnails

20th Jan 2021, 6:26 pm	#2
kirstyd Heptode Join Date: Mar 2014 Location: Leeds, West Yorkshire, UK. Posts: 902	Re: Hacker AL42 Hum On the hacker Gondolier [has the same amp] there is a black wire connected to this point with the other end attached to the metal body of the speaker.

20th Jan 2021, 6:42 pm	#3
Dancer1 Diode Join Date: Nov 2020 Location: Lincoln, Lincolnshire, UK. Posts: 9	Re: Hacker AL42 Hum Many thanks for the quick response Kirstyd. The power lead's earth is attached to the chassis as shown in the picture below. Anyone know whether I should be happy with this and also whether I should connect an earth (the black cable as identified by Kirstyd) from the tag identified above on the tone controls to the chassis? What would be the likely effect on hum? Many thanks. Attached Thumbnails

20th Jan 2021, 7:13 pm	#4
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,846	Re: Hacker AL42 Hum Hello and welcome, Ambrose. You could try adding a ground lead from there to the LS chassis, but I'd be surprised if that were the culprit. I will have a look at mine on Friday if I remember, to see if such a lead is fitted. Does the hum fade to nothing if the volume control is turned down to zero? N.

20th Jan 2021, 7:56 pm	#5
Dancer1 Diode Join Date: Nov 2020 Location: Lincoln, Lincolnshire, UK. Posts: 9	Re: Hacker AL42 Hum The hum fades to a point where you have to have your near right next to the speaker to hear it, but it's still there. I will hook up a lead and see what happens. What are the usual culprits for humming? What should I check?

20th Jan 2021, 7:57 pm	#6
kirstyd Heptode Join Date: Mar 2014 Location: Leeds, West Yorkshire, UK. Posts: 902	Re: Hacker AL42 Hum If my memory serves me well I believe there is also a lead coming from one of those connections its other end connecting to the sort of kitchen foil that is stuck to the base of the cabinet. Leaving this off did cause hum.

20th Jan 2021, 8:22 pm	#7
kirstyd Heptode Join Date: Mar 2014 Location: Leeds, West Yorkshire, UK. Posts: 902	Re: Hacker AL42 Hum I have taken pictures of the inside of mine they may help you. I did the refurb last year. You can see the black wire going to the speaker. You can also see the foil bridge held in place by two staples. I replaced the mains lead on mine putting the earth where the old one was connected. Attached Thumbnails

20th Jan 2021, 8:24 pm	#8
kirstyd Heptode Join Date: Mar 2014 Location: Leeds, West Yorkshire, UK. Posts: 902	Re: Hacker AL42 Hum As you can see there is nothing connected to that tag in your first post.

20th Jan 2021, 8:31 pm	#9
Dancer1 Diode Join Date: Nov 2020 Location: Lincoln, Lincolnshire, UK. Posts: 9	Re: Hacker AL42 Hum Thanks Kirstyd. Pictures do help a lot. I shall try a foil bridge to see if that makes a difference.

21st Jan 2021, 2:10 pm	#10
Richardgr Heptode Join Date: Oct 2019 Location: Täby, Sweden Posts: 704	Re: Hacker AL42 Hum Since the hum is amplified (increases with volume control), doesn't this imply ripple on the B+ power supply? Your smoothing cap looks to be original, maybe it would be worth updating that. Another consideration is the heater supply - is that grounded? Edit: I notice you said it had been serviced, but that smoothing cap and the resistor look remarkably unsullied.

24th Jan 2021, 2:45 pm	#11
Dancer1 Diode Join Date: Nov 2020 Location: Lincoln, Lincolnshire, UK. Posts: 9	Re: Hacker AL42 Hum Based on Richard's observations, I broke out the DMM and traced the issue to C11/C12. I have changed this and the hum is gone. Many thanks to you all for your help. I also checked the rest of the caps and most are new and in range (including the original Millard Mustards). Now I can enjoy the system without the annoying hum.