Replacement Toroidal Transformer

[RishiRishiRishi]

Hi,

I recently purchased an Allen & Heath SD12-2 - it doesn't power up and looks like the toroidal power transformer has been removed.

Could anyone please help recommend a replacement transformer that I can use to get this up and running, in relation to VAC and VA, or even a link to a suitable part from UK? I've attached the schematic, but unfortunately it doesn't supply a part number for the Tx but does have reference voltages - which might help to identify a suitable replacement.

Many thanks!

 

[Rob Flinn]

Did you try asking Allen & heath ?

 

[NoisyIndividual]

https://uk.farnell.com/w/c/transfor...tages=2-x-15v&range=inc-in-stock&sort=P_PRICE
Check the physical sizes of these with the space available where the old one used to fit, that should give you some idea of the VA rating of the original.

 

[RishiRishiRishi]

Did you try asking Allen & heath ?

I did, and their response was: "We also cannot give service support to unauthorised personnel. However, from the circuit diagram, it is clear to an electronics engineer/repair what is needed. If you can find an engineer or repair person who looks at the drawing and knows what to do, go with that. If your engineer is puzzled, please go to someone else."

 

[RishiRishiRishi]

https://uk.farnell.com/w/c/transfor...tages=2-x-15v&range=inc-in-stock&sort=P_PRICE
Check the physical sizes of these with the space available where the old one used to fit, that should give you some idea of the VA rating of the original.

Thank you! So am I right in thinking the size dictates the VA rating of the original, and would their be any major differences between let's say a 15, 30, 50 or 80 VA in this application?

 

[NoisyIndividual]

Yes.
You would not want to fit a transformer with a lower VA rating than the original, you'd be much better off buying the largest one you could get in there.

 

[PermO]

Too small transformer will result in a hot transformer, worst case, it's internal fuse will pop.
Do you have the old transformer ? did you do a beep test ?

Usually transformers don't break without a reason (short circuit at the secondary)

 

[RishiRishiRishi]

Yes.
You would not want to fit a transformer with a lower VA rating than the original, you'd be much better off buying the largest one you could get in there.

Too small transformer will result in a hot transformer, worst case, it's internal fuse will pop.
Do you have the old transformer ? did you do a beep test ?

Usually transformers don't break without a reason (short circuit at the secondary)

Thanks - unfortunately I don't have the original transformer to test, or use as a guide to determine the replacement. The mixer was sold with the transformer removed.

I've done some maths to try to figure out the minimum VA, based on the 15V transformers recommended by NoisyIndividual, and the 500 mA fuse supplied with the unit. What do you think?

15V x 0.5A = 7.5VA, therefore minimum VA should be 7.5VA?

 

[NoisyIndividual]

There is a 1A fuse going to the 2SD234 24v regulator transistor. This would break at around 40VA.

I think I woud base my estimates more towards that ballpark, not 7.5VA.

There will be a few Watts of heat generated in the Zener circuit and the 2SD transistor itself.

Ideally you would use a 24v bench supply and measure the current consumed by the electronics, then the current consumed by the lights at 15v AC, then add a few Watts for the Zener etc.
Then maybe double that or preferably more to choose the transformer.

 

[RishiRishiRishi]

There is a 1A fuse going to the 2SD234 24v regulator transistor. This would break at around 40VA.

I think I woud base my estimates more towards that ballpark, not 7.5VA.

There will be a few Watts of heat generated in the Zener circuit and the 2SD transistor itself.

Ideally you would use a 24v bench supply and measure the current consumed by the electronics, then the current consumed by the lights at 15v AC, then add a few Watts for the Zener etc.
Then maybe double that or preferably more to choose the transformer.

Thanks! I don't have access to a 24V bench supply unfortunately. So since there's a 1A fuse going into the 2SD234 regulator transistor, should I look for something around 50 - 80 VA transistors from the list you sent?

Also I noticed from the data sheets of some of the transistors in the list, some have 2 leads on the primary side, and others have 4 leads - does this matter in this case? In the schematic they have 4 leads, but two are tied together.

 

[NoisyIndividual]

Well, since the Farnell list contains a range of transformers from 15VA to 80VA, all between £20 and £30, I think I would pay the money and still go for the biggest one that fits. Remember the original one seems to be toast, maybe it wasn't really big enough.
And as someone else said, make sure there isn't a fault that killed the original transformer in the first place.

The primaries are either one 240v coil, or 2 x 110v coils you wire in series, as in the schematic, either will do.

 

[Walrus]

A quick fag packet calculation says that a 30 volt ac transformer will give about 41 volts DC on the first smoothing capacitor, and that is without taking into account transformer regulation!
Would not a 12-0-12 transformer be a better choice?

 

[RishiRishiRishi]

A quick fag packet calculation says that a 30 volt ac transformer will give about 41 volts DC on the first smoothing capacitor, and that is without taking into account transformer regulation!
Would not a 12-0-12 transformer be a better choice?

From the gut shot it appears as though there's one mounting hole, where the transformer should be, which makes me think it used to have a toroidal tx (top right on attached pic). I'm not able to find a compete gut shot to confirm this

Well, since the Farnell list contains a range of transformers from 15VA to 80VA, all between £20 and £30, I think I would pay the money and still go for the biggest one that fits. Remember the original one seems to be toast, maybe it wasn't really big enough.
And as someone else said, make sure there isn't a fault that killed the original transformer in the first place.

The primaries are either one 240v coil, or 2 x 110v coils you wire in series, as in the schematic, either will do.

That's a good shout - is there anything in particular you would look out for, to check for a fault that killed the original transformer prior to me fitting a new one?

 

[Graham Nystrom]

Farnell is a good source for Multicomp pro or Triad magnetics toroids. They will have the voltage and VA rating you need and it shouldnt be more than $40 or $50 max. I’d be looking at 50-80 VA range.

 

[RishiRishiRishi]

Farnell is a good source for Multicomp pro or Triad magnetics toroids. They will have the voltage and VA rating you need and it shouldnt be more than $40 or $50 max. I’d be looking at 50-80 VA range.

Thanks! The mixer should be arriving today at which point I'll measure it up and see if I can fit a 50 or 80 VA in there. Will be sure to update once I get started. Is there anything in particular you'd look for to see if there was an issue causing a fault with the previous transformer?

 

[MidnightArrakis]

Thanks! The mixer should be arriving today at which point I'll measure it up and see if I can fit a 50 or 80 VA in there. Will be sure to update once I get started. Is there anything in particular you'd look for to see if there was an issue causing a fault with the previous transformer?

Here in the U.S. there is a company called ANTEK, Inc. that specializes in toroid power transformers. Based upon the criteria I read within this thread, there are these:

They are specially designed to work on all standard 115V or 230V mains voltages at 50Hz or 60Hz. These transformers have heavier gauge wires than the normal requirement to avoid the copper lost during the full power output. And they also have the magnetic shield around the outside to reduce the magnetic leakage.



--------$20.00



------$26.00

>>> MOUNTING HARDWARE <<<


In addition, as I was reviewing your included schematic and with me not even being familiar with this mixer, I noticed that this mixer must be rather old due to how the output XLR connector is wired up. Way back in the 70's, some XLR connectors were wired-up as "Pin-3 High". Meaning, the "HOT" audio-signal was on Pin-3 of the XLR. This resulted in all manner of confusion and problems back then because some audio equipment manufacturers and even the PA sound companies differed on whether they used "Pin-3 HIGH" or "Pin-2 HIGH"!!! I know this because I had personally encountered this issue on more than one occasion. As an example, I was on the sound-crew of a QUEEN concert here in the U.S. sometime during the late-70s and the promoter decided to use two separate concert sound-reinforcement companies for a particular concert. One sound company was the one that QUEEN had brought along with them from Europe and the other one was a large regional sound company from here in the U.S.

After each sound-crew had setup their own systems and performed their own soundchecks, they then ran some audio XLR cables between the two systems so they could both output the same audio from the FOH mixer. So, when they plugged-in the XLR cables connecting the two systems together, guess what audio they got??? >> NOTHING!!! << WHY??? Because the European sound company had their XLR's wired as "Pin-2 HIGH" and the U.S. sound company had their XLR's wired as "Pin-3 HIGH"!!! So, each sound company had "shorted" the other company's audio output to the "Low-Side" of a balanced-signal resulting in ZERO AUDIO!!! The fix??? The U.S. sound company relinquished and rewired the specific cables connecting the two systems together to match the European sound-company's "Pin-2 HIGH" standard and then everything worked just fine!!!

So.....since your ALLEN & HEATH mixer is showing its output XLR connector as being "Pin-3 HOT" and the audio world had finally settled on an international XLR wiring scheme sometime back in the 1980's as being "Pin-2 HOT", I would strongly recommend that you also rewire your output XLR to match this or you'll end up with the exact same situation as I detailed above, but you probably won't understand -- WHY -- your mixer now doesn't have an output when you run a cable from its output XLR connector to some other piece of equipment!!!


Just my 2-cents worth or.....2-pence or 2-quid!!! With today's inflation, it could be 2-Euros now!!!

/

 

[RishiRishiRishi]

Here in the U.S. there is a company called ANTEK, Inc. that specializes in toroid power transformers. Based upon the criteria I read within this thread, there are these:

They are specially designed to work on all standard 115V or 230V mains voltages at 50Hz or 60Hz. These transformers have heavier gauge wires than the normal requirement to avoid the copper lost during the full power output. And they also have the magnetic shield around the outside to reduce the magnetic leakage.

View attachment 121862
View attachment 121863
--------$20.00

View attachment 121864
View attachment 121865
------$26.00

>>> MOUNTING HARDWARE <<<
View attachment 121866

In addition, as I was reviewing your included schematic and with me not even being familiar with this mixer, I noticed that this mixer must be rather old due to how the output XLR connector is wired up. Way back in the 70's, some XLR connectors were wired-up as "Pin-3 High". Meaning, the "HOT" audio-signal was on Pin-3 of the XLR. This resulted in all manner of confusion and problems back then because some audio equipment manufacturers and even the PA sound companies differed on whether they used "Pin-3 HIGH" or "Pin-2 HIGH"!!! I know this because I had personally encountered this issue on more than one occasion. As an example, I was on the sound-crew of a QUEEN concert here in the U.S. sometime during the late-70s and the promoter decided to use two separate concert sound-reinforcement companies for a particular concert. One sound company was the one that QUEEN had brought along with them from Europe and the other one was a large regional sound company from here in the U.S.

After each sound-crew had setup their own systems and performed their own soundchecks, they then ran some audio XLR cables between the two systems so they could both output the same audio from the FOH mixer. So, when they plugged-in the XLR cables connecting the two systems together, guess what audio they got??? >> NOTHING!!! << WHY??? Because the European sound company had their XLR's wired as "Pin-2 HIGH" and the U.S. sound company had their XLR's wired as "Pin-3 HIGH"!!! So, each sound company had "shorted" the other company's audio output to the "Low-Side" of a balanced-signal resulting in ZERO AUDIO!!! The fix??? The U.S. sound company relinquished and rewired the specific cables connecting the two systems together to match the European sound-company's "Pin-2 HIGH" standard and then everything worked just fine!!!

So.....since your ALLEN & HEATH mixer is showing its output XLR connector as being "Pin-3 HOT" and the audio world had finally settled on an international XLR wiring scheme sometime back in the 1980's as being "Pin-2 HOT", I would strongly recommend that you also rewire your output XLR to match this or you'll end up with the exact same situation as I detailed above, but you probably won't understand -- WHY -- your mixer now doesn't have an output when you run a cable from its output XLR connector to some other piece of equipment!!!
View attachment 121871

Just my 2-cents worth or.....2-pence or 2-quid!!! With today's inflation, it could be 2-Euros now!!!

/

Thanks for the info! I also noticed the wiring of the XLR's and was going to switch that over! The mixer is indeed pretty old, from the mid 70s I believe.

I noticed you've recommended 24V transformers, where as NoisyIndividual recommended 15V - looks to me on the schematic part of the secondary transformer wiring goes straight to a VU meter, and is annotated with 15V. I see there is a 24V DC signal further down the schematic, but based on this I thought 15V on the secondary side would be more appropriate. What do you think?

 

[RishiRishiRishi]

So I've measured up the insides of the mixer, and purchased the toroidal transformer - max I can fit in there is a 2x15V, 50VAC. I've drawn up a wiring diagram (attached) if you could take a look at it and check it looks ok, particularly on the secondary side, that would be hugely appreciated!

My plans are to:

Connect the yellow and black wires together, and connect them to the positive pin on the right hand VU meters (both VU meters have 4 pins, positive, negative and two that appear to be wired to the board. The left hand VU meter is completely wired up with the positive pin on LH being wired to the negative of the right hand channel. Only the right hand positive pin is free and appears to be cut).

Connect the orange wire (0V) to the negative/ ground pin of the bridge rectifier.

Connect the red wire (Vsec) to the positive pin on the bridge rectifier.

 

[Walrus]

I still think a 15 volt transformer is to high.
30 Volt, bridge rectified will produce 30 x 1.414=42.42V minus 2 diode Volt drops gives about 41.02 Volts.
The regulation of the VTX-146-050-115 (if that is the one you ordered) is 14%, so maximum off load voltage could be 46.77 Volts.
This is more than the maximum rating of the two smoothing capacitors at 40 Volts!
Even if one allows for copper losses under load, I would think the final unregulated DC voltage will still be too high.

 

[NoisyIndividual]

As is often the case, it would be prudent to replace the 40v Electrolytics here anyway, with caps rated 50v or 63v at 105oC.
And possibly the Zener diode.