Troubleshooting Hum, Please Check My Work

TrevorK

Member
2006-07-06 3:32 am
I'm relatively new to analog audio. I've been lurking here for a bit now, and trying to learn all I can about troubleshooting hum. I would like to outline what I've found and get some feedback on whether I'm on the right track or if there is anything else I should consider.

I have a Philips 312 turntable and a Harman/Kardon 430 receiver. I'm getting some 60hz hum and I suspect it's from the cartridge picking up stray fields from the transformer in the turntable. Its definitely 60Hz; I've scoped it. Here's how I have come to this conclusion:
  • flipping the 2 prong plugs for the turntable and receiver has no effect.
  • Aux input on the receiver is dead quiet.
  • Phono input with turntable disconnected has some white noise at 74%+ volume but no hum.
  • Phono input with turntable hooked up, but powered off has a little more white noise, but no hum.
  • Hum is present when the turntable is powered on and motor is off.
  • Disconnecting the ground wire between the TT and the receiver makes the hum worse.
  • When the motor is powered on the hum volume is the same as far as I can tell. 33 and 45 rpm, no discernible difference.
  • Motor on or off, the hum gets louder as I swing the arm closer to the spindle. The transformer is on the back left side of the turntable, so swinging the arm brings the cartridge closer to the transformer.
  • The hum gets even louder if I remove the metal platter and swing the arm toward the spindle.

I know this TT was purchased new in 1978. Do transformers tend to age and degrade in such a way as to produce more EMI or is it likely that this stray field has always been there... or maybe now there is something about my cartridge that is just particularly sensitive to it (grado prestige green)? I am considering the idea of replacing the transformer; maybe with a toroidal unit. I suspect a new transformer, especially one this small, would cost less than a new cartridge which should still has quite a bit of life left in it.

I'd appreciate any thoughts or suggestions. Thanks.
 

U230421

Disabled Account
2017-02-07 1:54 pm
The hum is more possible induced in cables from the cartridge to the pre-amplifier.
Transformers hardly age, a toroid has less stray but positioning a 'regular' can improve things considerable.
I have to confess as a dutch citizen that I've never considered aquiring a Philips turntable. Moving the arm over the turntable area yielding in audiable hum differences points to akward if not bad or even loose grounding of neccessary things.
It can also just be a design thing, inaudiable with common equipment in that time.


Have a look inside for odd/loose things, or even better, post a photo of the inner cavities so advise may arise from these forumgrounds!
 

TrevorK

Member
2006-07-06 3:32 am
Thanks for the feedback. Photos attached.

I did attempt to check for loose connections inside the case. There are 1/8in wide spade terminals connecting the fine wires from the tonearm to the RCA cables that exit the enclosure. I removed each, buffed the terminals and reconnected them thinking that any corrosion probably wouldn't be helping me here.
The external wire for the chassis ground connection connects in this area as well via a ring terminal and star washer. I removed and reconnected that as well.
I get continuity between ground and all of the motor control board grounds and the chassis. The motor casing appears to be intentionally isolated as it is mounted on rubber bushings with no clear signs of an intentional ground connection. I get no continuity between the chassis ground and the platter, but its unclear to me if that is intentional or not. The metal tonearm is also floating I believe.
Outside of this and looking for broken wires I am not sure what else to look for inside.

The turntable and the receiver were purchased together by my father. I kinda like the idea of keeping them running together, and making repairs or updates to quiet them down as best as I can. In this case, they are the common equipment of the time.

But maybe it is not supposed to be any quieter. My hope was to get the hum down below the level of the white noise, so I could maybe attempt a phono preamp project at some point to see if I could build something better than the internal phono stage. But as they say, you can hope in one hand...
 

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TrevorK

Member
2006-07-06 3:32 am
Just a further clarification, the red wire with the ring terminal hanging out in space is a ground wire. It connects to the external ground wire and a foil ground plane on the bottom cover. I removed the bottom cover for the photos so I didn't have to hold it up. The transformer is also mounted to the plastic bottom cover near the white 4 pin connector on the lower left. In this photo, the front of the turntable is up.
 

rayma

Member
2011-04-29 8:37 pm
Do you get audio in both channels from the turntable? If not, there is a bad connection somewhere in that channel.
That tt should not have audible hum, is it in both channels?. Some Grados are sensitive to hum though.
 
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If the transformer produces DC voltages after rectification, replacing it with an external wall transformer may help. That helped hum in my RA-88a RIAA mixer quite a lot.
When Peavey or dynaco put a transformer in a line level or MM level device, they 1. wrapped it in steel and 2. put a steel cage around it to separate it from the low level parts. You didn't show a picture to show whether the transformer is wrapped in steel, but this chassis obviously has no separate grounded steel enclosure for the transformer.
I too, enjoy turning base consumer gear, like my RA-88a mixer, into good performing mid-fi or hi-fi products. Shopping for "the best" is for connosieurs of a higher income class than I inhabit. Spent $25 on the $15 used mixer and achieved a really stellar base noise & hum level.
I am fortunate that my BIC940 turntable, $79 in 1979, doesn't hum, wow, flutter, or pick up thuds from my feet when I walk around, or tear up highs off LP's. My mother's RCA player did the last, to the trash with that one.
 
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warrjon

Member
2011-02-04 10:10 am
Vic
Is the arm ground in good condition.

I had the same problem but only with my Linear arm, my Technics EPA100 was fine. But the linear arm has a carbon fibre wand so no shielding for the wiring. Grounding the TT chassis to the phono ground eliminated the buzz. My point is hum an be difficult to trace. The fact it get worse as the arm moves across the platter indicates the hum is induced most likely into the wiring.
 

TrevorK

Member
2006-07-06 3:32 am
I delicately unhooked the grado cartridge without unbolting it from its mount, and plugged in the ortofan. It's definitely quieter. You can still make out a trace of hum but its now down in the white noise. I think this may be some flavor of a "grado hum" issue, whether the entry point is the cartridge or the wires I am unsure. Obviously I did not play any music through the ortofan, but since the head of this arm is plastic, and the closest grounded metal part is the tube portion of the arm (this is actually grounded, contrary to what I indicated earlier. More on this in a moment) I think this was a reasonable test. I can make out the hum through music at higher volumes with the grado. I suspect a new cartridge could make the hum go away.

Some google searches for "grado hum" and "philips 312" produce recommendations from folks for the combination. So either some cartridges are more sensitive than others, or some 312's are nosier than others, or some people have a different tolerance or expectation for hum maybe? Or the grado just exposes a grounding problem that other cartridges are less sensitive to.

If the transformer produces DC voltages after rectification...

No snark intended, but isn't that basically what a rectifier is supposed to do? Unless you are suggesting there may be a DC offset in the secondary winding of the transformer that the rectifier somehow creates? I could try to measure that.

I initially thought the metal tube of the arm was floating, but I think I was just not pressing my probe to the metal with enough force to make a connection through the anodizing.

Being new to this stuff, I have no sense for how common this may be, but this philips uses a little mounting plate that the cartridge bolts to, and that slides in and out of the tonearm head. There is someone on ebay who sells a remake of this plate. It's the best set of photos I could find.
Pats Audio Cartridge Holder - Philips 212 312 Turntable Plus Ortofon Super OM 10 | eBay

You can see the 4 pins for the signal and ground wires. In the head there is a 5th connector in the center that is a chassis ground. My original mounting plate and the photos of remakes I could find seem to not use this for some reason. Maybe just having the connector on the head side reduces crosstalk between channels? Anyway, the metal parts of the arm and this terminal in the head all show continuity to the chassis.

So here is what I am thinking now. I'll probably find another cartridge for the short term, but maybe get another mounting plate and keep the grado for another day. The wooden portion of the plinth has a little bow in the front that makes it not quite fit right. It's just a box sandwiched in there between the plastic base and the metal portion so it would be easy to replace and I have some black walnut in the basement that could be pretty cool for this. As a part of that project, I could mount RCA jacks and a connector for an AC-AC wallwart transformer... which takes a swipe at the transformer issue and the dinky signal cables as well, which would allow me to play with the grado again while also allowing me to backpedal to the original setup if something about this goes pear-shaped.

Also, I am attaching a pic of the transformer. Its mounted to the plastic base. No grounds. I suppose I could try chassis grounding its metal frame, but I got to wonder why that's not already grounded. I suspect that could create a safety issue if one of the windings fails and shorts to it somehow but I am not sure. This turntable has a 2 prong plug.I am leaning toward leaving that alone.

This feedback has been very helpful. Thank you to everyone who has chimed in so far.
 

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Bonsai

Member
Paid Member
2003-07-25 10:44 pm
Europe
www.hifisonix.com
You could try rotating the transformer - the fields from EI types are very large and very much linked to the geometry of the core - so the orientation has a big effect.

Another option is to replace the transformer with a toroid. The Ametherm [sp?] types from Digikey feature very low external fields and judging by the pic, that one is probably a 5VA component, so about $15 in the USA.
 
Certainly worth going down the external AC to AC wallwart route.
Well, I suggested AC to DC wall wart if that is what the transformer produces, which would get the AC 15' away, not under the cartridge. Those 32 v copier/fax/printer power supplies are about $2 at charity resale shops, rectifies & regulates down to +-14 pretty easily. I'm using an 18 v race car DC supply regulated down to +-7.5v in my RIAA mixer. Plenty of headroom for my sources, MM phono, CD player line level, FM radio earphone out, turned down to not clip.