HELP NEEDED! WHAT TRANSFORMER DO I REQUIRE.?

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g9builder

Well-known member
Joined
Dec 17, 2009
Messages
87
Location
Midlands UK
Hi All,

I am trying to repair a powered studio monitor. I have traced the problem to the toroid transformer. It keeps blowing fuses. The pcb is fine ( I have tested it using the transformer from the other speaker and all is good).

So what I am after is a replacement transformer. I have tested the voltage on the known good transformer and I get a reading on the two secondaries of 34v.  

I have attached a copy of the schematic for the power supply section that I have obtained. Can anyone shed any light on exactly what transformwer i need. All the ones i can find appear to be very large that have 35v secondaries and looking at the schematic 35v would seem high anyway!!

I'm rapidly loosing my hair over this and really need this speaker back up and running.

Any help greatly appreciated.

Cheers




 
Are you reading that 35v from unloaded transformer secondaries, wires dangling, not connected to anything?

Anyway,
the above schematic would indicate the transformer you need should be around 2X25V. I'm estimating this from TP6 and TP7 connectors that should have a voltage of 31VDC.

25VAC going through a full wave bridge rectifier and finally to a big cap should be roughly 35VDC at that point (25VAC * 1.4 rectifier and finally smoothing). 35VDC present at that point is very likely close enough. 2X22V would take you even closer but you are probably not going to find one.

But how much of 2X25VAC? More information is needed for the VA rating. How much current draw right after C46 and C47?
 
hi Kingston,

many thanks for the reply.

Yes I was testing the transformer with nothing connected - just the wires dangling.

Re tyhe 31v  - thats why I thought 35 v was too high because it needs rectifying yet.! :)

Regarding current draw ? I don't know and do not know how to find this. All I know is that max power consumption for the speaker is 80w with 20w when idle.

Does this help.?

Many many thanks

 
Unloaded transformers will always show higher VAC than when they are drawing current.

I don't want to guess anything based on mere speaker power consumption wattage.

What I would do is disconnect the wire going from C46 to R47 and TP6, and insert an ammeter in between. that means your multimeter on the ampere measuring setting.

Another similar measuring point would be J11-1 or J11-3. They should both show similar current.

You want to find out the rough max current draw at that point. Then blast pink noise or some really loud music through the speaker and see what the current draw is. Then we can estimate the VA rating for the new transformer.

Is there any writing at all on the old transformer? No hints at all?
 
Hi,

I will test for the current tonight but have tested the voltage from the transformer under load (idle) and the voltage was 27V on both secondaries. So should a 25v transformer be ok?

 
Hi,

Ok tested for current and I got a reading of 0.07 in 10A mode so is this 70MA?

Does this sound about right.?

Played some music through but the reading didn't change.

 
I bet an Avel Lindberg would work well.  Based on the schematic it looks like you need a 22-0-22 secondary at a minimum of 375VA (fuse rating).  They don't show a 22-0-22 at this power rating, but they offer more than what's on their website if you call them.  The dimensions of the broken transformer could probably be used to estimate the original power rating.
 
g9builder said:
Ok tested for current and I got a reading of 0.07 in 10A mode so is this 70MA?

70mA. Sounds about right for a very small speaker. What speaker/amp is this by the way?

You won't need a big transformer. Let's triple that 70mA for safety buffer, about 200mA for each secondary. 50V*0.4A = 20VA.

So you want a 2X25VAC 20-40VA transformer. You will find those easily available.

PS. What is the physical size of your broken transformer. That should give a rough guide if the findings above are way off.

Andre, 375VA rating is way way off based on the measurement above. That thing would also be HUGE.
 
You're right, I missed the fact that the speakers are small... but shouldn't the fuse be rated a bit lower to protect the transformer?  Or is oversized to handle inrush current?  Also I think a 25V secondary may be too high, especially since 22v secondaries are common in the lower VA range.
 
Thanks both,

The speaker is a Tapco S8.

The original transformer is approx 80 mm by 40mm.

I was thinking of going for an 80VA to be on the safe side. What do you think?

Thanks both for all of your help.

 
Andre said:
Or is oversized to handle inrush current?  Also I think a 25V secondary may be too high, especially since 22v secondaries are common in the lower VA range.

That fuse might be just there for some odd dead short accident. Not gonna do much else.

And yes, 2X22VAC would be the optimal secondary voltage based on the schematic. Otherwise might have to drop to 31V with an additional resistor calculated for the correct voltage drop at TP7 and TP6. Whatever is cheapest or most convenient. It will most likely work just fine with even 2X25VAC. Simple monitor amps are tolerant, and the 15V rails are regulated already.

g9builder said:
The original transformer is approx 80 mm by 40mm.

I was thinking of going for an 80VA to be on the safe side. What do you think?

The original is more like 15-30VA based on the size. Toroidal, right? 20VA is safe already, 40VA is very safe. 80VA unnecessary.
 
A toroid transformer's weight is a good cue to guess-timate it's VA rating (as is an EI-transformer's iron core size). get one with the right voltage and similar weight and dimensions.

Jakob E.
 
Contact some toroidal transformer manufacturers.

Some manufacturers are able to strip and reverse-engineer dead transformers and then make new or re-manufacture using the core from the stripped transformer. I have used this approach on a number of failed toroids over the years ranging from small twin-secondary devices to multi-winding console transformers of 1kVA. On all occasions I did not need to supply the transformer spec - simply a request of "This toroid has died, can you create a replacement?".

Regards,

Gareth.
 
Very fine quality, decent prices and helpfull skilled people here: http://www.toroidy.pl/index.php?option=com_content&task=view&id=6&Itemid=13
 
Thanks to all that helped.

Transformer bought and installed and the speaker is back working...

Once again - many thanks

This forum rocks!!! ;D
 

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