Baffled by extra transformer wire

[WaveyDipole]

I am baffled by the somewhat by the transformer that I have which belongs to an amplifier. The markings on the transformer are: TH3056, also marked H33075

On the circuit diagram, the primary inputs are labelled in 3 groups as follows:

||┌--- 10v
||└--- 0v
||
||┌--- 120v
||├--- 110v
||└--- 0v
||
||┌--- 110v
||│
||└--- 0v

With the switching arrangement this gives and input of 110v, 120v, 130v, 220v, 230v and 240v. What I don't get is that there are 7 wires in all on the diagram, but the transformer has 8 wires on the primary side. Two were loose, one was a black wire with a soldered end which I figured had detatched itself from somewhere. I got continuity to two other points which suggested that it belonged to the winding with the 3 wires and if my surmise was correct appeared to connect to the neutral.

Then there was also a dirty whitish wire which looked cut but not insulated and had just a tiny amount of wire protruding. My MM can't get any resistance/continuity reading from this across to any of the other wires. I'm a little baffled as to what this last wire is for? Should it be connected somewhere and do I have an open primary? But in that case, where does it connect on the circuit? I only appear to need 7 conductors? I read around 65v on this to neutral (with a DVM) when the transformer was live. It certainly does not connect to the ironworks but appears to be running into the windings but what purpose would it serve?

I connected the black where I thought it ought to go and tried powering the transformer through a 40w bulb at first and then directly. With a direct connection and set to 240v, I got secondary voltages of 22-0-22 and 2v on the second pair. They should be 26-0-26 and 6v.

[Ed_Dinning]

Hi this could be an electrostatic screen, wound between the pri and the o/p winding. It is normally earthed.

Ed

[Lucien Nunes]

My MM can't get any resistance/continuity reading from this across to any of the other wires

This suggests to me it could be an interwinding electrostatic screen. If you can see in, you might be able to identify it as lying between primary and secondary, and may be able to confirm this by capacitance measurements unless someone knows a better way?

Oops, post crossed with Ed's

[trobbins]

With respect to the single wire that is likely the ES, I agree that if capacitance is highest for the nearest mains side and secondary side windings then that is a good indicator. If the PT 6V winding was for a heater then that would likely have the lowest capacitance reading.

A megger/insulation resistance test to the other windings would also provide some confidence.

When the transformer is energised, the ES will show an AC voltage on a high resistance DMM due to the capacitance.

[Skywave]

From what you have written, you have two wires which are puzzling you.
The first one you refer to sounds very much like an electrostatic screen. I have found that such a screen is often connected to the transformer core: a resistance check may confirm that.
As for the other mystery wire - the "dirty whitish one" - at present, I have no suggestions: sorry.
If you can send in a few 'photos, they might well help us all.

Al.

[Herald1360]

That white wire isn't the only baffling thing about that primary arrangement. Normally, the low voltage settings would have two primaries in parallel, but there doesn't seem to be a valid arrangement for 130V that way. Do you know how the switching actually configures the windings?

[vidjoman]

As you say "belongs to an amplifier" - have you not looked at the diagrams for the amplifier. It could show you not only the connections but the colours.

[WaveyDipole]

Thanks for the responses. I have a photocopied diagram which is spread across several pages and is not too clear. Apparently it is difficult to find a copy online and I still haven't found one. However I have scanned the page with the PSU and attach an image of the PSU section. It is not very clear unfortunately and I have 'drawn in' some of the lines to the voltage selector switch. The main fuse and switch could just be made out underneath this and connecting to pin 7.

I must admit that I didn't check the black wire to the transformer core, but since there was continuity to one of the windings I surmised it was related to that particular winding. However I did check for continuity between the 'dirty white' wire and the transformer core and it doesn't connect to this or any of the windings. It doesn't seem to connect anywhere. I will also take a reading with the insulation tester and check capacitances to confirm.

On the diagram, it is possible to make out a dotted line in between the core and primary winding. On the scan the bottom end of this has disappeared, but on the page this line terminates with a curved line with an E under it which I just assumed was connecting the core to earth. A little googling seems to suggest that this may indeed indicate an electrostatic screen?

An electrostatic screen would make sense in an audio application and this particular wire appears to be the outermost wire on the primary winding but it does not appear to be placed on the outer edge. If the secondary is wound over the primary, then it could conceivably connect a screen placed between the two windings. Presumably it would be connected to electrical earth?

The 6v line (I am pretty sure that is what it reads) is connected a set of 3 lamps in parrallel. I'm a bit surprised to get only 2v here. I have the switch set to 240v so if my mains is 230v I might expect a slightly lower output voltage but this seems much too low although I am reading it without any load attached.

Herald, ther diagram does not give sufficient indication of how the switching is configured and the switch itself is studded together so I can't dismantle it to have a look at how the conductors are arranged inside. The line between 4 and 4 is a wire link. Unfortunately there is no indication of which colours go where either. One could get 130v using the 120v and 10v windings in series. To get 240v, all 3 windings would be required in series. I don't think I've seen an arrangement quite like this before.

[vidjoman]

What make and model amplifier is it? Someone may have a better copy of the diagram.

[dominicbeesley]

The diagram is poor but there is the hint of a junction on the secondary ground side that would look like a likely connection for a screen

[WaveyDipole]

The amp is a Laney TLA-200. Its a bit unusual since Laney primarily make guitar amps, but this appears to be a HiFi amp that was also integrated with a tuner and optionally a sound system with a turntable. There is a reference to a Thomas Laney Audio (formerly Hi-Sound) under '1974 HFYB range of receivers' on this page:

http://select45rpm.com/pages/hifi/a-...receivers.html

Its also mentioned here:

http://revamphifi.com/receiverarchive/

Not sure what 'HFYB' stands for. I couldn't find anything under Hi-Sound either. If anyone has a digital copy of the diagram or knows where one can be obtained from then this would indeed be greatly appreciated.

[Skywave]

Originally Posted by WaveyDipole

http://select45rpm.com/pages/hifi/a-...receivers.html
Not sure what 'HFYB' stands for.

Hi-Fi Year Book?

Al.

[yesnaby]

Hello,

Don't know if this helps at all but here is a different Laney amplifier with a similar input selector where the white wire is indeed the screen.

Michael

[WaveyDipole]

yesnaby, thanks for that. I was able to locate a higher resolution copy of that diagram. As you say, a different amp, but the primary windings on the transformer have the same layout. The switching layout is almost identical just with connections to pins 5 and 7 being reversed which possibly wouldn't matter. This diagram has the colours and mine has the voltages so it will be interesting to see whether the colours correspond and whether my black was actually dark brown!

In any case, you seem to have confirmed that the white wire goes to the screen so thanks to you and everyone else for clearing that up!

[TonyDuell]

Just as an aside, I suspect that for 130V mains you link the 10V and 120V windings in series _and_ parallel the 110V winding across the 110V section of that. Effectively using the 130V winding as an autotransformer to also run the 110V winding.

I've see a similar thing 'in reverse' for 100V mains where a transformer has a pair of 115V primaries, one tapped at 100V. You parallel them and apply 100V to the tap.

[Herald1360]

I sort of wondered about that connection. I wonder if Ed Dinning could comment on the wire gauge implications?

[WaveyDipole]

I just checked with an LDR and white wire to any other wire reads as capacitance with the capacitance progressively increasing as you move up the sequence of wires on both primary and secondary sides till you presumably reach the ones closest to the screen.

I just had a close call with it as I inadvertently powered it up while still set to 110v. Not sure how or why, because yesterday I definitely had it set to 240v and I don't remember changing it. Anyway nothing much happened except that I got no power and wondered why only to find that the 3amp fuse in the plug had blown which led me to discover my error. I fitted another fuse and then measured the secondary voltages, this time using the AVO, and found that they are exactly as stated, 26v and 6v.

After that incident I also checked and still get inductance readings on all windings so I think everything is still OK but I would have been absolutely gutted had I ruined a perfectly good transformer. I am hoping that no serious damage has been done.

[WaveyDipole]

Sorry, in the above meant LCR (not LDR).

PS, my cheap far east LCR gave me erratic readings when checking the windings and the transformer was configured for 240v. Is this anything to worry about or is it just down to the cheap device reaching its limits (about 20H)? Other readings seemed to be consistent.

[TrevorG3VLF]

One of my digital meters has a tizzy fit when measuring resistances of inductances presumably because it does this with intermittent measurements. Shorting of a different winding may make resistance measurements possible.

Use an AVO.

[Herald1360]

Originally Posted by TrevorG3VLF

Use an AVO.

And try not to be connected across the TX winding when the AVO probe is removed

Had a play with a sim of the TX- the only interesting bit about the primary wire is that the floating 10V winding need twice the current rating of the individual 110/120V windings.