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Old 16th Feb 2019, 1:15 am   #1
Krolroger
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Default Transformer replacement Beomaster 4000

Hello All,

I am planning to replace a delaminated transformer in my restored Beomaster 4000 (mid seventies kit) because it buzzes horribly. This is a well known problem on this model and on the 3000 which preceded it.

The existing transformer is on a EI core potted in an airtight box. The replacement will be a single primary toroid rated at 280VA with two secondaries, one of 22 volts @ 4.4 VA and one of 47 volts @ 275 VA.

I have had quotes from Canterbury Windings, Toroidy in Poland and Airlink for a suitably sized replacement, though I've not figured out yet how to mount it. All have been helpful; Airlink cheapest by some margin.

I would welcome some advice about the optional add-ons available with an "audio grade" unit. In this application, for example, is electrostatic screening a good idea and if so how do I terminate it on a double insulated device which isn't earthed? If I attach it to chassis ground will I introduce hum? Or should I leave it disconnected or not have it all?

What is the general view of self resetting thermal cutouts which I can see are a good idea in theory until they fail and render the trafo u/s.

As for mounting, does the washer arrangement top and bottom improve the shielding or should I go for the threaded insert with M6 stud? I haven't a huge amount of headroom.

I'd be grateful for any insights into this process which is a first for me and I would like to get it right.

Pic shows the original layout.

Regards,

Simon
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Old 16th Feb 2019, 10:32 am   #2
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Default Re: Transformer replacement Beomaster 4000

Quote:
Originally Posted by Krolroger View Post
... I would welcome some advice about the optional add-ons available with an "audio grade" unit. In this application, for example, is electrostatic screening a good idea and if so how do I terminate it on a double insulated device which isn't earthed? If I attach it to chassis ground will I introduce hum? Or should I leave it disconnected or not have it all?

What is the general view of self resetting thermal cutouts which I can see are a good idea in theory until they fail and render the trafo u/s.

As for mounting, does the washer arrangement top and bottom improve the shielding or should I go for the threaded insert with M6 stud? I haven't a huge amount of headroom.
Hi Simon,
My thoughts, others may differ, are:

Electrostatic screen, not necessary, but if it comes for pennies / nothing connect it to the 0 volt / chassis ground. Don't leave it floating.

No to thermal cut outs for the reason you stated. Add an external fuse to the 47 volt secondary if it bothers you.

If you have limited room go for the potted insert. The bolt and washer fixing add nothing to any screening, but needs more height. You could 'pot' your own insert fairly easily, but it will likely be just as cheap to buy already done.

Question, does the original transformer mount to the vertical LH side or to a plate underneath we cannot see?

Alan
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Old 17th Feb 2019, 2:35 am   #3
Krolroger
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Default Re: Transformer replacement Beomaster 4000

Hi Alan,

Thanks for your reply.

The original transformer is contained within the airtight box on the left of the pic. The box is screwed onto the rear by means of a bracket, and the left side rail. I will make a new support plate out of sheet aluminium with a neoprene layer, attached to the original fixing points.

The electrostatic screen is a mere £1.33 extra as it happens but I would be happy to omit it. Potted threaded insert is about £3.

The Goss (Grain Orientated Silicon Steel) band round the circumference sounds like a useful addition.

Any thoughts on soft start circuitry to limit inrush? There are any number of cheap Chinese modules available using thermistors/relays but I don't like the board layouts/use of capacitor droppers and wouldn't trust most of the components. Perhaps it's not necessary with a 280VA toroid..

Simon
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Old 17th Feb 2019, 4:41 am   #4
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Default Re: Transformer replacement Beomaster 4000

Toroid manufacturers soo GOSS toroids as an excuse to cut costs by running at high flux density and this makes for an acoustically noisy transformer. I've had to double vacuum impregnate some as an attempt to quiet them. In the end I potted them.

Talk to the transformer firm about low acoustic noise versions. Power rating will be lower for a given core size, price will be higher but the result is more domestically compatible.

David
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Old 17th Feb 2019, 1:58 pm   #5
Krolroger
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Default Re: Transformer replacement Beomaster 4000

David, I'm already maxed out on diameter. Does low flux density mean less inrush current?

Also, should I be specifying a potted (impregnated?) unit?

Thanks,

Simon
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Old 17th Feb 2019, 2:30 pm   #6
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Default Re: Transformer replacement Beomaster 4000

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David, I'm already maxed out on diameter. Does low flux density mean less inrush current?
Also, should I be specifying a potted (impregnated?) unit?
It usually means less inrush.

Many transformers saturate on the DC component if the turn-on transient depending in when in the cycle the switch closes.

sizewise, you already have plenty of watts rating in that size. Lower VA in the same size will be OK

I just wanted to chuck in the warning that toroids can make a lot of noise, so talk to the manufacturer.

David
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Old 19th Feb 2019, 12:55 pm   #7
Krolroger
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Default Re: Transformer replacement Beomaster 4000

David,

The manufacturer says that for a low noise version, I should derate the power rating by 12.5%. That would take the rated power down to 245VA from 280VA

My question is, for a 60w per channel amplifier/tuner, is that going to be sufficient for the transients? Will it result in the transformer running hotter?

The original is rated at 275VA, and I found the following post by a highly regarded restorer of all things B&O very helpful.

http://beolover.blogspot.com/2016/01...surements.html

Incidentally, it's possible to remove the thermal cutout from the circuit if it fails as it's wired in post manufacture.

Thanks,

Simon
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