API 2503 Burnt?

[dawsonaudio]

Here's a photo of one of my output transformers.  I've got a rack of 8 312 cards I made some time ago.  This particular channel isn't passing audio really.  What audio is there is pretty low in volume and crackly.  Brand new op-amp installed.  I noticed the discoloration on the 2503 transformer.  Is this normal coloration?  The other's don't have this dark color to them.

Thanks for any help here.

Nate

 

[gridcurrent]

no help,
but was told by api "experts" that 2503s never blow.
this after I had sent a few fried ones to the Doc (rip) to assess.
liking the idea of a coupling capacitor in a 312.

 

[abbey road d enfer]

Here's a photo of one of my output transformers.  I've got a rack of 8 312 cards I made some time ago.  This particular channel isn't passing audio really.  What audio is there is pretty low in volume and crackly.  Brand new op-amp installed.  I noticed the discoloration on the 2503 transformer.  Is this normal coloration?  The other's don't have this dark color to them.

Thanks for any help here.

Nate

It is definitely fried. can you measure the DC resistance of each winding? That would help determine the cause.

 

[dawsonaudio]

The output XLR (secondary) has 2 ohms between pin2 and pin3.  All other output xlr's have around 13.5 ohms between pin2 and pin3.  Should I measure the primary as well?  Hope I'm measuring the DC resistance correctly.

 

[pucho812]

Are you sure the one card is getting the correct voltage?

Have you probed it with a scope and test signal?

 

[dawsonaudio]

Wish I had a scope to probe.

I'll check voltages with unit powered up.  I've only checked the power supply +/- which is holding at 16vdc with it powering the rack of 312 cards.  I'll check around.

Thanks for the help here.

 

[dawsonaudio]

I pulled the card, rechecked all solder connections and everything looks good.  The voltage at each of the 2520's is around 15.4 vdc +/-.  At the power supply input it is 16.0 vdc +/-.  I'm still getting 2.0 ohms across pin 2 and 3 of output.  The other seven channels have around 13.5 ohms across pin 2 and 3. 

Looks like the transformer is a little darker than the others, but looks can be deceptive.  I should say that I haven't really used this last channel in the box in a while so it could have tanked a while ago. 

Any other ideas before I swap the transformer with another channels working one to see if that is the problem?

Thanks again.

 

[Walrus]

2 Ohms is definitely a short circuit, compared to 13.5.
Desolder the tranny and check again, if still 2 Ohms then it's new one time!

 

[dawsonaudio]

Yeah, still 2 ohms when transformer removed.  I'll order another one.  Thanks for the help.

 

[john12ax7]

It would be useful to pinpoint the method of failure so that the new transformer doesn't suffer the same fate.

 

[dawsonaudio]

What would be the most obvious place to look for the output transformer failure?  Never had an audio transformer go toast before.  I have had a/c transformers burnt up before though.

 

[JohnRoberts]

What would be the most obvious place to look for the output transformer failure?  Never had an audio transformer go toast before.  I have had a/c transformers burnt up before though.

perhaps look for DC. A shorted device in a driver circuit could dump PS rail into the iron.

JR

 

[Walrus]

Shorted secondary could mean the volts probably came from whatever you plugged the pre into.
It's also possible that over heating of the primary side from a shorted 2520 caused the secondary winding insulation to melt and short.

 

[abbey road d enfer]

I would bet for the output having been connected to something that had either DC or very large AC.

 

[Whoops]

I would bet for the output having been connected to something that had either DC or very large AC.

I have the same impression also.
If it is just the secondary that fried it was bad usage and at least those transformers are not expensive and can be easily replaced.

You should measure the DC resistance on the primary side also and let us know...