Help connecting power wires in Amp Head

volkatron

Member
2010-07-25 3:41 pm
Hi, I took my amp apart to change the pots and forgot how to connect the power wires. I've attached a picture of my problem. I'm in Australia so I think the power is 240v. Thanks very much.
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
It's a bit scary if you don't know your own line voltage... you "think" it's 240 :)

The input to the PCB should be straightfoward as T7 and T8.

How many wires have you from the mains tranny ? Do the colours not correspond with the labelling.

Maybe another photo showing more of the wiring.
 

volkatron

Member
2010-07-25 3:41 pm
Wiring

Thanks. Here is another photo of the wiring. By the way I will not attempt this myself, I will get an electrician to do it but I just want to learn.

Also, the colours don't match up to the wiring. My friend (an electrician) has taken this amp apart before and I remember him saying the wires aren't matching up to the same colour, it was just a guessing game for him. Just wondering if anyone knew what it is, as he has moved away. Cheers
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
The transformer has two wires, the brown and blue. It's a single primary winding so no possibility of error here.

I take it you have a meter that measures ohms ?

Still can't fully make out the connections on the socket in the piccy, but it's all common sense.

The brown and blue wires from the tranny connect across the mains... and it doesn't actually matter which way around they go for this unit, although brown is live and blue is neutral in the UK.

The big question is whether the switch is a "single" or "double" pole type. In other words is just one side of the mains switched on and off or both. I think double pole here.

If you want to do this yourself I can tell you what to do. Even better would be a piccy of the underside of the PCB with the switch, but if not a meter is all you need to identify connections.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Put your meter on "low" ohms range such that the display shows 0.00 when you short the test probes together.

I've remarked your picture as I can't be sure of the printing.

Now do this, first having pulled all the leads off that PCB.
Copy and paste below and put either "short" or "open" next to each.


WITH THE MAINS SWITCH OFF

1. Connect one meter lead to Neutral and then read to all the other connections in turn.
A.
B.
C.
D.
E.

2. Transfer the meter lead to "Live" and repeat.
A.
B.
C.
D.
E.
F.

3. Now turn the switch ON and repeat starting again with lead on "Neutral"
A.
B.
C.
D.
E.
F.

4. Now swap lead to "Live" and repeat.
A.
B.
C.
D.
E.
F.
 

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Here it is:

WITH THE MAINS SWITCH OFF

1. Connect one meter lead to Neutral and then read to all the other connections in turn.
A. 000
B. 000
C. 1
D. 1
E. 1

2. Transfer the meter lead to "Live" and repeat.
A.1
B.1
C.1
D.1
E.1
F.1

3. Now turn the switch ON and repeat starting again with lead on "Neutral"
A. 000
B. 000
C. 1
D. 1
E. 1
F. 1

4. Now swap lead to "Live" and repeat.
A. 1
B. 1
C. 1
D. 1
E. 1
F. 1
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Something odd going on as there is no connection from "Live" to any of the other points when the switch is on. Is that fuse OK ? Measure across it on ohms, it should read short circuit (0.00)

Neutral and A and B appear connected... that's as expected and means the switch is a single pole type switching only live. On the first block of readings I missed out F... an ommision, not intentional but assume it reads open (1)

This is how I would imagine it to appear... in black is the connections you have established... in red is what I would expect with the switch feeding one or more of the other connections. So check the fuse first.
A bright torch shone under the PCB might reveal the patterns too.
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
The connections are as I thought then, however normal good practice is to switch the LIVE side of the incoming mains, and for that to go through the fuse FIRST. That's standard safe practice.

You said at the start that your friend said the colours don't match.

I would expect the BLUE wire from the tranny to go to the neutral of the incoming supply and the BROWN to go to the PCB... yours seems the other way around.
That's not a safety problem in itself as the transformer is "non polarised" and normally the wires from a transformer aren't colour coded anyway.

Can you confirm that with the mains switch turned ON that there is continuity from the glass fuse (both ends) through the switch to one of the connectors on the PCB. Your readings up to now indicate NO continuity... so we need to find out why. Where does the other side of the switch go ?

Up to now (from the photos) it looks as though the BROWN wire from the transformer goes to the mains inlet socket directly... is that correct ?

The other terminal from the mains inlet goes to LIVE on the PCB.

So that leaves the BLUE wire from the tranny which needs to connect to the switch to complete the circuit.
And it's the continuity through the switch we haven't established yet.
I would say the BLUE lead connects to the lower right tab in the piccys and that the amp should work then.

Just if your wondering... there's no safety problem in all this... it's a very simple circuit switching one side of the mains only... the only mystery is why you can not read continuity through the switch when it's on.
 
indeed AC in OZ was 240, but is supposed to be now 230 to bring it more inline with the EU etc, many places still have 240 though regardless of what the switches say. brown is normally live here and we generally have a 3 pole 'balanced' power scheme of live/neutral/earth though plenty of more consumer grade audio gear is only 2 pole. its possible the 240 vs 230vac thing had him confused...hey OP?? hehe

for sure take a picture next time, dont worry i'm pretty sure we've all done it, I know I have
 
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Thanks so much for all the help guys, but I decided to get a mate to help me on the weekend, I'm just to new at all this to fully understand it and if I do it wrong, that could be the end of me...

On a totally unrelated question though... I just bought a guitar pedal from America and have to use a different power supply then they gave me. I have read something about a voltage converter.

The one I got with the pedal from America says:

INPUT: 100-240V ~50/60hz 0.4a
OUTPUT: 12V - 1.0A


I have this one that was laying around the house:

Arlec Australia - Product Details

Also, I have this one that says:
INPUT: 100-240V AC 50-60HZ
OUTPUT: 9V DC 1000ma

Can I use either of these? Thanks
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Little PSU's come in two types.

If it works on (says on it) 100-240vac it's what's called a SMPS (Switch mode power supply).
It's electronic and works with no adjustment. They are usually lightweight too.

If it's a conventional transformer it will specify one particular voltage only, say just 120. This type of PSU must only be used at it's stated voltage.

The one in your link says it outputs 12 volts so should be OK... but watch the polarity of the plug you use on the output. The 500 milliamp rating should be ample for a pedal.