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Old 15th October 2006, 08:03 PM   #1
blurk99 is offline blurk99  United Kingdom
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Question Marshall Transformer Questions

Evening all...

I've got a Marshall AS50 that has gone wrong... No sound atall, not even pops or thumps when unplugging instruments etc.

I've given it a visual inspection over the boards inside and there's nothing that appears 'blown' etc.

So i looked at the transformer

Primary - 4 Wires - Blue (Neutral), Brown (Live) Black and White (both no connection, they terminate on the PSU board but they don't connect to anything, not even eachother)

Secondary - 3 wires - 1 Black and 2 Red

Continuity good across all 3 secondary wires

On the primary wires i've got something weird going on.

Black - White = 1.9ohms res.
Blue - White = 33.2ohms
Blue - Black = 35.0ohms
Brown - infinite resistance to all other primary wires.

I think i've shown the transformer is shorted internally, breaking the primary winding and shorting into the 'other' (Black-White) winding - what do you all think? also does anyone have a clue what the Black-White winding is for? they are not connected to anything else within the amp - i've looked under their connections and they just terminate on the PCB.

cheers

james
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Old 28th October 2006, 07:48 PM   #2
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Perhaps they are wrappings for the 110V ??
It seems that the primary is interrupted.
If the Trasfo is interrupted do not esitate replace with a new spare-parts
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Old 15th November 2006, 10:56 PM   #3
Albertb is offline Albertb  United Kingdom
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The extra 2 could be for alternative voltages on the primary. It was not unusual at one time to have a primary winding tapped for 200V, 220V, 240V as suited your local conditions.

Looks to me as though you have a starting point of the Blue or neutral, then the main turns of the coil giving 33.2ohms leading to the tap for connection to your mains live at White, then a few more turns giving 35ohms to the Black, and finally the Brown which we can say should have been around 36.8ohms. The coil seems to be broken in the few turns between the black and brown and is not shorting to anything - (yet).

It may even be possible to test it, (or even to use it temporarily), with the mains across the Blue and Black by swapping the black and brown leads on the board.
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Old 15th November 2006, 11:10 PM   #4
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Just as you use brown=hot, blue=neutral over there, we use black=hot, white=neutral over here. So, it think the transformer has a set of 120VAC taps, plus a set of 240VAC taps. It sounds like it was set up so the same transformer is used for both services, and just soldered differently for the 2 services. 1.9 ohms is very OK for a 120V primary. So, to me, it sounds like your transformer is open on the primary side, in between the brown tap and all the rest.
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