Hello!

I recently got Denon PMA520 vintige stereo amp. for free, but it has a dead power transformer. I want to fix it, but i don't know what kind of tranformer i need. All i know that originaly it has torodial transformer with 37,2 V in second winding.
Can I use laminated steel core transformer?
Can I use transformer with 40V in second winding?

I attaced schematics and some photos of the amp.

I have only basic knowledge about eletronics and realy need your help to sort this out.

Thanks!
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I told the moderators you were having difficulty getting your attachments to show up. Somebody will probably be along to help you with that.

 

It seems that you have not uploaded the attachments successfully in the server, which is weird, since you have received attachment ID codes. Can you repeat the process one more time?

Edit the message, use the "Go Advanced" button and re-upload your pictures.

 

Realy could not find "Edit post" button, so I just added attachmens once more:

Rest of the pbc and components looks undamaged (no burnt components).

View attachment denon_pma520.pdf

 

If you are in N.A. there is Antek that makes Toroidal transformers.
He also lists on ebay as Jonango.
The reason for toroidal in audio equipment is the magnetic field is contained within the core, so hum problems are kept to a minimum.
The diameter and height will indicate the Va required.
Max.

 

You probably couldn't edit your post because your post count is under 10 and you are thus classified as a New Member. I always forget class permissions.

Sorry for the confusion.

 

I can tell you this: You're looking for a transformer that is probably rated as 75 volts, center tapped, or 37.5-0-37.5 give or take a volt or two. The power rating will be less than 460 watts which I derive from 230 VAC input and a 2 amp fuse. The power rating might be quite a lot less, like 200 watts.

The manufacturer went to considerable expense to make this a toroid AND shield it with a metal can. Trying to use an open frame transformer is begging for trouble.

 

I recently got Denon PMA520 vintige stereo amp. for free, but it has a dead power transformer. .................
................I have only basic knowledge about eletronics and realy need your help to sort this out.

How do you know the transformer is dead?

 

I can tell you this: You're looking for a transformer that is probably rated as 75 volts, center tapped, or 37.5-0-37.5 give or take a volt or two. The power rating will be less than 460 watts which I derive from 230 VAC input and a 2 amp fuse. The power rating might be quite a lot less, like 200 watts.

The manufacturer went to considerable expense to make this a toroid AND shield it with a metal can. Trying to use an open frame transformer is begging for trouble.

Ok, it's the help what I was looking for!
So it's ok to use 40,0-0-40,0 V torodial transformer?

How do you know the transformer is dead?

1) Transformer did not show any voltage in secondary winding when I connected transformer to 220V.
2) Transformer's primary winding didn't show any resistence reading when i connected it to digital multimeter.
3)Transformers primary winding does not conduct eletricity.

Is there any other way to check transformer?

 

If you are in N.A. there is Antek that makes Toroidal transformers.
He also lists on ebay as Jonango.
The reason for toroidal in audio equipment is the magnetic field is contained within the core, so hum problems are kept to a minimum.
The diameter and height will indicate the Va required.
Max.

I'm from Latvia so shipping would be expensive...

And I realy can't get toroid out of it's metal case because it is filled with polyurethane putty, to get out toroid I have to destroy either toroid or metal case.

 

Obviously you've checked for a blown fuse in series with the primary winding?

 

It looks as though you have removed the transformer for testing, so it also looks as though you need a new one.

These people are specialists

http://www.ilpelectronics.com/images/pics/transformers_mains.pdf

 

Obviously you've checked for a blown fuse in series with the primary winding?

Fuse was the first thing that I checked, and the fuse is ok. But how it comes, that there is dead toroid with intact fuse?

It looks as though you have removed the transformer for testing, so it also looks as though you need a new one.

These people are specialists

http://www.ilpelectronics.com/images/pics/transformers_mains.pdf

Yup I have removed it.

I have found toroid in my local electronics shop. 40-0-40 V 5 A in total for 220V. Those excess ~2,5 V should not do any harm....am I right?

 

Those excess ~2,5 V should not do any harm....am I right?

That depends how close other components are to their voltage/power ratings.

 

That depends how close other components are to their voltage/power ratings.

Is there any way to tell it by looking in schematics? Rectifier and caps will hold 40V ....I think.

 

I have found toroid in my local electronics shop. 40-0-40 V 5 A in total for 220V. Those excess ~2,5 V should not do any harm....am I right?

The simple way to deal with a slightly higher output from the transformer is to add one or more power rectifier diodes in series with the feed from the main rectifier bridge to each rail.

This will lower the rail voltages to normal levels without huge power disipation or complicated circuitry.

Is this enough information or do you need more detail?

 

Fuse was the first thing that I checked, and the fuse is ok. But how it comes, that there is dead toroid with intact fuse?

Sometimes there is a thermal fuse inside potted transformers, which might have gone open.

 

The simple way to deal with a slightly higher output from the transformer is to add one or more power rectifier diodes in series with the feed from the main rectifier bridge to each rail.

This will lower the rail voltages to normal levels without huge power disipation or complicated circuitry.

Is this enough information or do you need more detail?

So all I must do is to put one or more power rectifier diodes after the original power rectifier diode bridge?

I have attached png. file to show what i mean and photo with
s4vb bridge rectifier.

 

I'm from Latvia so shipping would be expensive...

And I realy can't get toroid out of it's metal case because it is filled with polyurethane putty, to get out toroid I have to destroy either toroid or metal case.

If you are absolutely sure it is the transformer, I would tackle it as you have nothing to lose.
If it is hard epoxy it usually chips away.
Extremely rare for a transformer to go open.
Max.

 

We are all concerned to help here, which is why we keep questioning that the transformer itself has failed.

Denon are famous for failed thermal fuses.

https://www.google.co.uk/search?hl=...0...1ac.1.34.heirloom-hp..9.9.968.fB6XnBRc0Y4