Hacker GP 42 low hum - UK Vintage Radio Repair and Restoration Discussion Forum

gram065 · 28th Apr 2017, 7:32 pm

Now I admit this could be me being anal but I've just being restoring a GP42 that I purchased at a local antiques fair. Mechanically everything has turned out fine and the original Acos 96 has been replaced with a BSR SC11M. I know the golden rule is not to trust a machine of this age before checking the amp but as there were signs of recent work represented by some capacitors and resistors being replaced I gave it a whirl. Everything seems to be working ok except for a very low hum from the speaker which increases slightly when the turntable is switched on by moving the front control lever to "Manual" on the turntable ( a Garrard 2025TC). Now I probably would have accepted this, (the hum level was so low) had it not been that my GP 15 Cavalier, which is also fitted with a BSR SC11M in the arm of a Garrard SP25, has no sign of such a hum. As I say everything is working ok with the exception of the low hum. Any ideas of the cause and also the increase in volume when the turntable is activated? I've put 3 photos along with this of the Cavalier and the Gondolier and pics of the GP42's amp in case anything is obvious.

'LIVEWIRE?' · 28th Apr 2017, 8:10 pm

There are two red electrolytics which appear to be the originals. Without the circuit diagram in front of me, I'm not sure which they are, but it may be worthwhile checking/changing these for the nearest eqivalent modern values.

Edward Huggins · 29th Apr 2017, 11:32 am

Was the original Acos GP96 cartridge faulty then? I'm wondering whether swapping cartridges (and with those fiddly connectors) might have caused this as the BSR SC11M is not an elegant fit to the narrow Garrard tone arm head. Check the deck earthing as well.

gram065 · 29th Apr 2017, 6:14 pm

Thanks for the response Livewire. This afternoon I swapped the turntables between the Cavalier and the Gondolier...no problem with either of them?? Swapped them back and there's no sign of a problem. I wondered if it was an earthing/contact problem on the deck. Whatever, it's not there now but will bear your suggestion in mind. Thanks again for your response.

gram065 · 29th Apr 2017, 10:23 pm

Hi Edward...sorry I must have missed your post earlier. The 96 was still functional but I'd had such good results with the 11 on my Cavalier I decided to swap it out. Yes the connection of the SC11M was fiddly. I opted to adapt the 96's mount. See photos. This held it secure and worked. In swapping the decks to see if I could identify where the problem lay the hum disappeared so I assume there was an earth prob somewhere..now rectified. Thanks for your post

RojDW48 · 30th Apr 2017, 12:46 am

I repaired a couple of GP42s recently and I recall that the cartridge connection tag strip under the deck (which looked absolutely fine!) was nevertheless the source of a mild earth hum. So tighten, re-solder etc if it happens again.

gram065 · 30th Apr 2017, 7:02 am

It was that very tag strip that I suspected. Thanks for your post.

28th Apr 2017, 7:32 pm	#1
gram065 Retired Dormant Member Join Date: Dec 2013 Location: Lewes, East Sussex, UK. Posts: 42	Hacker GP 42 low hum Now I admit this could be me being anal but I've just being restoring a GP42 that I purchased at a local antiques fair. Mechanically everything has turned out fine and the original Acos 96 has been replaced with a BSR SC11M. I know the golden rule is not to trust a machine of this age before checking the amp but as there were signs of recent work represented by some capacitors and resistors being replaced I gave it a whirl. Everything seems to be working ok except for a very low hum from the speaker which increases slightly when the turntable is switched on by moving the front control lever to "Manual" on the turntable ( a Garrard 2025TC). Now I probably would have accepted this, (the hum level was so low) had it not been that my GP 15 Cavalier, which is also fitted with a BSR SC11M in the arm of a Garrard SP25, has no sign of such a hum. As I say everything is working ok with the exception of the low hum. Any ideas of the cause and also the increase in volume when the turntable is activated? I've put 3 photos along with this of the Cavalier and the Gondolier and pics of the GP42's amp in case anything is obvious. Attached Thumbnails

28th Apr 2017, 8:10 pm	#2
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Hacker GP 42 low hum There are two red electrolytics which appear to be the originals. Without the circuit diagram in front of me, I'm not sure which they are, but it may be worthwhile checking/changing these for the nearest eqivalent modern values.

29th Apr 2017, 11:32 am	#3
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,338	Re: Hacker GP 42 low hum Was the original Acos GP96 cartridge faulty then? I'm wondering whether swapping cartridges (and with those fiddly connectors) might have caused this as the BSR SC11M is not an elegant fit to the narrow Garrard tone arm head. Check the deck earthing as well. __________________ Edward.

29th Apr 2017, 6:14 pm	#4
gram065 Retired Dormant Member Join Date: Dec 2013 Location: Lewes, East Sussex, UK. Posts: 42	Re: Hacker GP 42 low hum Thanks for the response Livewire. This afternoon I swapped the turntables between the Cavalier and the Gondolier...no problem with either of them?? Swapped them back and there's no sign of a problem. I wondered if it was an earthing/contact problem on the deck. Whatever, it's not there now but will bear your suggestion in mind. Thanks again for your response.

29th Apr 2017, 10:23 pm	#5
gram065 Retired Dormant Member Join Date: Dec 2013 Location: Lewes, East Sussex, UK. Posts: 42	Re: Hacker GP 42 low hum Hi Edward...sorry I must have missed your post earlier. The 96 was still functional but I'd had such good results with the 11 on my Cavalier I decided to swap it out. Yes the connection of the SC11M was fiddly. I opted to adapt the 96's mount. See photos. This held it secure and worked. In swapping the decks to see if I could identify where the problem lay the hum disappeared so I assume there was an earth prob somewhere..now rectified. Thanks for your post Attached Thumbnails

30th Apr 2017, 12:46 am	#6
RojDW48 Nonode Join Date: Jun 2014 Location: Bristol, UK. Posts: 2,074	Re: Hacker GP 42 low hum I repaired a couple of GP42s recently and I recall that the cartridge connection tag strip under the deck (which looked absolutely fine!) was nevertheless the source of a mild earth hum. So tighten, re-solder etc if it happens again. __________________ '....don't go mistaking Paradise for that home across the road!' (Bob Dylan)

30th Apr 2017, 7:02 am	#7
gram065 Retired Dormant Member Join Date: Dec 2013 Location: Lewes, East Sussex, UK. Posts: 42	Re: Hacker GP 42 low hum It was that very tag strip that I suspected. Thanks for your post.