Can a soft-starter affect transformer magnetisation? - diyAudio
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Old 10th December 2009, 07:28 AM   #1
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Default Can a soft-starter affect transformer magnetisation?

Daft question I know, but I'm grasping at straws: I'm rebuilding an Electrcompaniet "Electro25" class-A power amp, and have moved the rather electrically-noisy old C-core tranny into an offboard box, and incorporated a resistor/relay soft-start mech which is switching-in as it should.

However, the tranny is providing only about a tenth of o/p voltage expected... It could very-well just be that the tired old tranny is faulty, but perhaps someone here knows better?

Thankings, Sod
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Old 10th December 2009, 07:33 AM   #2
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Quote:
Originally Posted by earl of sodbury View Post
Daft question I know, but I'm grasping at straws: I'm rebuilding an Electrcompaniet "Electro25" class-A power amp, and have moved the rather electrically-noisy old C-core tranny into an offboard box, and incorporated a resistor/relay soft-start mech which is switching-in as it should.

However, the tranny is providing only about a tenth of o/p voltage expected... It could very-well just be that the tired old tranny is faulty, but perhaps someone here knows better?

Thankings, Sod
Not if it wired correctly.

2 minutes with a cheap DVM will tell you. Check actual voltage on the primary and secondary.
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Old 10th December 2009, 07:43 AM   #3
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Quote:
Originally Posted by cliffforrest View Post
Not if it wired correctly.

2 minutes with a cheap DVM will tell you. Check actual voltage on the primary and secondary.
Thanks Cliff, as I should have said, voltage on the primaries is as it should be, on the secondaries a little more than a tenth of what it should be.

Looks like the last straw just went-under...
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Old 10th December 2009, 10:37 AM   #4
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Hmmm ...

Does it get warm?

And resistance of all the relevant windings?

New circuit or existing amp?

Possible secondaries out of phase?

Grub around looking for any other straws to clutch ....
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Old 10th December 2009, 11:29 AM   #5
AndrewT is offline AndrewT  Scotland
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primaries out of phase? Oh, that would blow the fuse.
What fuse is fitted?
What resistor value is fitted?
What is the primary resistance?
What is the transformer VA?

Is the relay actually shorting out the resistor?
A soft start only needs to be in line for a few cycles of the mains waveform, i.e. >=60ms (3cycles)
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Old 10th December 2009, 03:14 PM   #6
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I'll try and respond to the questions I have the answers to - the unit is at home.

It doesn't get warm.

Resistances: need to measure.

This is from a 30y/o amp.

Secondaries are not connected to the load at present, but are otherwise connected exactly as in the original circuit.

Not sure of fuse value, but it is a very low value fast blow, and has not blown.

I don't know the transformer VA and cannot find any information on it - Electrocompaniet won't release schematics &c.

The soft start is indeed shorting the resistors.

It's a horribly complicated old thing with primaries for about 6 different possible input voltages, and secondaries to provide 3 separate sets of outputs at different voltages, one of which is centre-tapped... It's more than my tiny brain can cope with to envisage quite how it's wound... Gordian knot comes to mind...

Thanks chaps
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Old 10th December 2009, 03:24 PM   #7
AndrewT is offline AndrewT  Scotland
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if you measure the ends of two different secondaries, the meter reads very odd voltages that vary with time and with the model (impedance) of the meter.

You must first identify the secondaries and label them.
I suggest you get an insulated terminal strip and screw a lead end into each terminal. Then hopefully you'll start to make sense of what you have.
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Old 10th December 2009, 04:09 PM   #8
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Thanks Andrew, soon as I have a chance I'll give it a bash. Ta, Paul
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