Transformer, earthing and hum

I recently recapped my amp, I still however have a bad hum that is there from the point of switching on, it's not affected by the volume control at all. Initially I thought I may have fried my mains filter caps by connecting them the wrong way, but after replacing them again I still have the issue. I've ruled out fluorescent lighting, and then turned my attention to the mains step down transformer. As I'm in the UK, my model has a transformer in the bottom of the cabinet to step voltage down. Apparently this should deliver 117v, but mine has a 110v one that looks like it's been added by someone much later in life.

The speaker cable for the 2 sub woofers runs directly next to it, could this potentially be the issue? This may be more for the UK guys to answer.

The wiring looks shoddy generally, I notice the psu plug has been cut off and the wires go into connector blocks that have been attached to the transformer. My knowledge here is not great, so I have a few questions on the correct way this should be hooked up. At present the earth supply comes straight off my UK mains cable, is tagged into the transformer and effectively the cabinet and everything else is earthed from this point. This sounds correct as transformers wouldn't have their own earth on the 110v side, or would they?

This also got me wondering about earthing for ballasts, I notice the US style lighting plugs have only two pins, should a second earth wire be run to earth each ballast? I'm in the process of replacing the later added UK voltage ballasts with US ones and the correct tubes. I notice the ballasts have earth tagged to their brackets, but these are painted so not sure what benefit they provide!

I'll post a pic up of the transformer later on, as if appreciate an experienced eye on it. The bottom of the cab is a mess of connector blocks for the badly installed UK lighting and I will be removing this all shortly.

Hello Psychman,

What is jukebox?

With your lighting it sounds as if has suffered from some gepetto influence.
It always amuses me that the lengths some people will go to rather than buying the correct parts.

Regards
Alan

Alan Hood
ami-man
UK

It's a rockola 442, and yeah it has seen all sorts of gepetto measures in its life, I keep finding more!

This is the present transformer, looks like a mess and the PSU plug has been cut to enable it to be run into a connector block



Its also a 110v transformer. I have an original one which states 117v as the correct output and also has the correct PSU socket. I also have a cable with the PSU plug on it so am considering removing the above, and swapping my PSU cable and transformer to an original one like this:

This is the transformer I have:



Any idea where I should put the earth? There doesnt seem to be anywhere on the old transformer for it. Should I effectively make an earth bar inside the cabinet and run all my earths from that?



Apologies for all the questions, I'd rather get things right when dealing with mains electricity!

Hi Adam,

I would remove those side cheeks (remove all four screws) and check the the mains wiring and the 117 output, we sold a couple of Rowe Ami transformers a while ago, so I replaced the wiring before sending them out.

By the looks of yours I would think the cables are brittle underneath those metal side cheeks.

Regards
Alan

Alan Hood
ami-man
UK

Thanks Alan I'll do that. The old transformer I have seems to be an isolating one also, which I imagine is the safer bet too.

Few questions though if you don't mind, what do I do with the earth?

Does the transformer itself need to be earthed? Because I can't see anywhere to tag one on. Maybe I can just make an earth point and earth everything from there? Aside from the cabinet should my lighting ballasts be earthed?

Also it would seem my present transformer outputs anywhere from 105v to 108v at best given line fluctuations. Could this cause issues with my jukebox? In particular I wonder if the hum I have is either being induced in the speaker cable from it, or whether some of the smoothing caps or rectifiers aren't performing as they could on an "out of spec" voltage. I'm not familiar with USA voltages so maybe it's still acceptable amounts.

Swapping the transformer should give my machine closer to 117v, which I've been told is correct

Hi Adam,

When you have checked the wiring under those side cheeks (usually just soldered cables that are insulated) it may be better on the 220/240 volt side to rewire it with a length of 1.5mm 3 core, connect the two cores and bring the earth out through the entry hole or strip off the earth wire sufficently to go under the side cheek and around one of the fixing bolts/screws. You can then run an earth to the 117 volt power supply.

Earths a great thing while the earth is connected, but can be a death trap if the earth on an applience is faulty and then there is a short making everthing that is earthed live. Double insulated goods was a great step forward IMHO.

Regards
Alan

Alan Hood
ami-man
UK

OK, sounds like a plan. By double insulated do you mean earthed AND an isolating transformer?

OK, I installed the old transformer. Internal insulation inside the cheeks was fine with no splits or cracking etc, and as it was attached underneath the waxed paper I decided to keep it and reinforce it with some layers of insulation tape.

I cleaned off some paint and rust from the cheeks, cleaned the rusty screws and bolts with white vinegar, then polished off with deoxit. I earthed to the nut and bolt on there, did a continuity test from my mains earth pin and had perfect continuity to the transformer, and amp and chassis etc.

Works fine, But I am now getting 120v supply instead of the estimated 117v, guess my line is a bit higher. I did notice my gripper motor now runs faster!!

Hum is still there as ever, regardless of transformer positioning, so I think i need to investigate my amp now.

I am happier now my wiring is better and I also have an ISOLATING transformer and NOT an autotransformer, so I feel the job was worth it.

Some observations....apart from both my basket and gripper motors running a little faster than before - a good thing?!

It messed up 1 record pickup - it hasn't done this in 3 months since I got everything adjusted and the basket bent back into shape.

At one point it seems the attract relay stuck and the basket repeatedly spun scanning for records.

Is this the affect of my relays behaving differently under higher voltage?

Before my old transformer was giving me a meager 105v, this newer one is putting out 120v

I am ASSUMING here-If you truly were down to 105 AC input--that's at THE lowest limit that the phono will function--T-8 flour. lamps will not usually light below that voltage--However, now being at 120 VAC, should not really make a big difference ?
The "28VDC", is what might make the difference--AC being higher, may have increased the "nominal" DC voltage-- but it will not cause that "relay problem"-- Ron Rich

My motors were very sluggish at times before. I didn't notice any tube issues, but then some gepetto had changed all my tubes for shorter ones and used UK voltage ballasts and run them straight from the mains supply - perhaps because of the low voltage I now wonder!!

I'm slowly removing these ballasts and putting the correct 110v ones in, along with the correct wattage and size tubes