Earthing a vintage Champ

knuckleberryfinn

A friend of mine has a late '50s Champ which he got super cheap in Australia years ago.  I was round his the other day and had a look in the back of it for the first time - it still had the death cap to ground, and no mains earth!  Also, he was running it through a woefully underpowered stepdown transformer.  

So I said I'd make it safe for him, which I have done.  However, it is making my dim bulb tester glow pretty bright.  It glowed a bit even with the amp off, so I thought it might be down to the step down transformer.  All the voltages were proportionally as expected, so I thought I'd try flipping the switch with no dim bulb - TRANSFORMER BUZZZZZZZZZZ.  I turned it off pretty damn quick.

Anyone know what the issue is?  I am thinking that one of the transformer earths needs to be disconnected, but that's just a hunch.  I can't find a relevant thread anywhere.  Arse.

DJH83004

The first question would be are you sure you have you made it safe? It is a Class1 piece of equipment, so there should be earth continuity of less than 0.1ohm from the earth pin of the BS1363 mains plug to the extraneous metal parts, if you are not sure put a PAT tester on it.

If it just has a 120VAC two core cord from the amp to the transformer with no protective conductor, it will fail a PAT test and is not therefore safe to use.

Actually having re-read your thread, and you have made it safe so I guess you have done the above.

knuckleberryfinn

Yes, I've done that.  But it's now in self-destruct mode.  I've changed nothing except replacing the two core lead with a three core, made sure the live wire goes straight to the fuse, and earthed the chassis.  I've got rid of the death cap too, obviously.  Calling @ICBM @ecc83 @jpfamps...

ICBM

That doesn't sound good. Like you have a leak from live to ground somewhere... if you've removed the death cap it sounds like it might be the transformer. Try swapping the live and neutral connections to it and see what happens.

DJH83004

I hope I'm not stating the obvious, but the vast majority of small UK/US commercial step down transformers are actually auto-transformers not isolation transformers, so the neutral carries through and the live voltage is dropped across the winding. so It is essential that the live and neutral leaving the unit are correct with respect to earth ( the neutral being connected to earth at th sub)

knuckleberryfinn

Not stating the obvious at all, at least not to me. So switching the live and neutral should sort it? I'll check.

IvisonGuitars

Just wear some wellies when you're playing it, safe as houses....

ICBM

knuckleberryfinn said:

Not stating the obvious at all, at least not to me. So switching the live and neutral should sort it? I'll check.

Not necessarily, but it may help identify where the problem is.

Although not good practice, having a stepdown autotransformer connected wrongly shouldn't cause a problem because both live and neutral should be isolated from the earth in the amp.

What I meant was to swap the live and neutral at the *amp* transformer.

Don't try it without your current limiter in line until you've isolated the problem. If it was pulling current even with the amp switched off that isn't a good sign.

Do you have a multimeter? I hope so if you're doing this sort of stuff! If so, first measure the resistance from the transformer winding (either live or neutral) to the chassis. It should be an open circuit.

ecc83

Yes IC I also suspect he has a primary to frame leak. The problem is a DMM might not show it, such faults often need a few tens of volts to break down (but who has a Megger these days?!) .

Taking THE GREATEST CARE he can put a lamp in the amp chassis to good earth path. Even if it does not light, the presence of more than a volt or two across the lamp would indicate a leaking transformer.

Jusfort! Does the amp have a single pole mains switch and if so are you SURE it is in the live side?

Dave.

DJH83004

If it is the 5F1, which it does sound like, L1 (I will call it) goes to a single pole switch on the back of the volume pot and L2 goes to the fuse they then go directly to the primary of the transformer, so to make 100% sure I would disconnect everything and do the tests straight onto the primary winding, remembering it is a 120v winding.

knuckleberryfinn

Thanks for the replies - as ever, I really appreciate the help.  

I've only just had a chance to look at the amp, and it doesn't look good.  I switched the live and neutral wires in the plug as a quick test, and I'm getting exactly the same results in the dim bulb tester: dull glowing bulb when the amp is off, and a bright glow when turned on.  Primary to ground measures open; I haven't tried a lamp to ground yet.  I also tried it with the rectifier out, and it's the same, so definitely a transformer issue.

@ecc83 The switch is a single pole mounted on the back of the vol pot.  Initially I had it wired like this: live > fuse > xformer > switch > neutral.  I swapped the leads so live is now going to the switch, but no change.  I am wondering if the step down transformer is faulty - the amp worked okay according to the owner, though I didn't test it myself.

I'm not really sure what to do next - the lamp I suppose.

EDIT: Is it possible that the massively underrated step down transformer that was being used with the amp previously masked the problem by not supplying enough current to the amp?

ICBM

If it's still drawing current when the amp switch is off you have a leak to earth somewhere. Does disconnecting the earth wire from the amp (not the stepdown transformer) stop it?

Be careful...

knuckleberryfinn

@ICBM Do you mean the mains earth at the chassis, or the secondary earth?  I am assuming you mean the mains earth (hence the danger warning) which I have removed, though with no change: still a dull glow with the amp off, and a bright glow with the amp on.  

However, I did just try unplugging the amp from the step down transformer, and the dim bulb tester glowed dully with nothing except the transformer plugged in at all!  

ICBM

knuckleberryfinn said:

@ICBM Do you mean the mains earth at the chassis, or the secondary earth?  I am assuming you mean the mains earth (hence the danger warning) which I have removed, though with no change: still a dull glow with the amp off, and a bright glow with the amp on.  

However, I did just try unplugging the amp from the step down transformer, and the dim bulb tester glowed dully with nothing except the transformer plugged in at all!  

That sounds like a fault in the stepdown transformer. There is a current draw from an unloaded transformer, but it's so small it shouldn't light the bulb (assuming it's a 100W or at worst a 60).

Do you have another one you can try? Even one of those basic plug-type Maplin ones, which are rated for 45W so just enough for a Champ, I think.

knuckleberryfinn

Ah, I was using a 25W bulb because the Champ is such a low wattage amp (I was working on the idea that the bulb should be roughly double the wattage of the amp - not sure where I read that now).  I've fitted a 100W bulb, which does not glow with the amp off.  But of course, it protects the amp less - the transformer buzz now occurs with the dim bulb tester in place too, and the bulb glows brightly.

EDIT: Is this looking like a power transformer replacement is necessary?

DJH83004

To be 100% honest your friend would be better with a UK voltage mains transformer anyway, they are available from TAD as direct replacements or Trans-Tronic in Chesterfield will rewind the original for you. They have just done a 1958 Tweed Champ 5F1 for me, so externally it looks original, but now has a UK voltage primary windiing. 

RiftAmps

+1 on a Transtronic rewind

knuckleberryfinn

DJH83004;1129877" said:

To be 100% honest your friend would be better with a UK voltage mains transformer anyway, they are available from TAD as direct replacements or Trans-Tronic in Chesterfield will rewind the original for you. They have just done a 1958 Tweed Champ 5F1 for me, so externally it looks original, but now has a UK voltage primary windiing. 

I was thinking the same. I think the step down transformer is still within the 30 day return window, so that'll cover most of the cost hopefully.