Vox Power Transformer - Help!

[markdanieluk]

Hiya,

I have an old Vox Valvetronix AD30VT from a good while ago. I haven't used it since one day (10+ years ago) when it went up in a puff of smoke! From what it looks like it's the power transformer, as when I've opened it up the part looks a bit worn and 'not happy'...

I'd love to get it working again, but as it's quite an old amp I've struggled to find the part for it and I don't know enough about electrics to figure out an alternative if there is one. Can anyone advise??

Images attached.

Mark

[maxkracht]

First you need to figure out why the transformer broke. If this is beyond your experience, find a professional. Here's the service manual. https://elektrotanya.com/vox_ad15-30vt. ... nload.html Making some guesses here, but I believe you need around a 45v winding with center tap, somewhere around a 5 or 6v winding and another 6.3v winding for the filament. You might need to get a couple transformers to meet all of these needs. Not sure how much current you need for the 45v winding, the 5 or 6 v winding probably doesnt need much, the 6.3v winding needs at least 300mA for one 12ax7.

[pdf64]

... From what it looks like it's the power transformer, as when I've opened it up the part looks a bit worn and 'not happy'...

I can see an obvious problem with the transformer. Transformers are dipped in varnish, are you sure it's not that coating which is causing you to think it's gone bad?

[Stevem]

I have seen discoloration like that and it does not for certain mean the transformer is bad, as it could just mean that it’s running hotter then it should.

Did the amp blow its fuse like it should if the transformer shorts out?

I am willing to bet that if you find the amps output IC pack you will find it cracked and blown open.
This is likely where the smoke came from.

[Stevem]

I have seen discoloration like that and it does not for certain mean the transformer is bad, as it could just mean that it’s running hotter then it should.

Did the amp blow its fuse like it should if the transformer shorts out?

[R.G.]

Good advice you're getting.

The transformer looks funny, not least because the windings are enclosed in a plastic cover. Probably the windings inside are varnished as well. It might, maybe be bad, but I like the idea that the power amp chip died and caused the smoke.

Instead of just replacing it, the transformer should be tested first before replacing it. There are ways to do that with a multimeter. After the transformer is tested, whether it's bad and needs replacing, or good and doesn't need replaced, the whole amplifier needs brought up slowly and carefully while testing for the output power amp chip being functional.

What is your electronics-technician skill level? Without knowing this, we can't advise you very well. Do you have at least a digital multi meter, soldering iron, and experience working on PCBs? If you don't, take it to a tech.

[pdf64]

... From what it looks like it's the power transformer, as when I've opened it up the part looks a bit worn and 'not happy'...

I can't see an obvious problem with the transformer. ...

[Stevem]

Here’s what the average output IC pac looks like.
This one is bad even though its plastic case is not cracked or literally blown open as they most times do.

These are not easy to change out at all without a desoldering station so I would suggest you take the amp to a tech with the ability’s not only to troubleshoot it, but to get this part changed out if need be.

[maxkracht]

These are not easy to change out at all without a desoldering station

This is an understatement. I've had to work on these after someone without the skill or equipment buggered up the board. Not fun at all... If you must do this, with minimal tools, clip every lead first with flushcutters, then remove the pins one by one. Trying to get the chip out intact isn't going to turn out well unless you know what you are doing.

[markdanieluk]

Hi all,

Thanks for all the advice! I haven't had chance to have a closer look since I posted this, but hopefully I will this weekend! I'll keep you all posted.

Mark

[markdanieluk]

I have seen discoloration like that and it does not for certain mean the transformer is bad, as it could just mean that it’s running hotter then it should.

Did the amp blow its fuse like it should if the transformer shorts out?

I am willing to bet that if you find the amps output IC pack you will find it cracked and blown open.
This is likely where the smoke came from.

This is plausible - I remember at the time I was giving it large with everything turned up (in my young and reckless teenage years!), so I imagine it was getting pretty warm.

I didn't pay much attention to the fuse, but I tried a new one in it when I first came back to it a few months ago, but to no avail. I'll have a look for that output IC pack when I next have a look.

Thanks!