Help diagnose power transformer issue Electro-Voice AC one crossover

[camshash]

Hi everyone, hope you all are doing well.

I have an Electro-Voice AC one active crossover, which employs relays at each audio output to prevent power-on thumping in speakers. During normal operation I power the crossover on and you can hear the relays switching correctly and the unit works with no issue. Now after about ~10 minutes the relays will click and there is no output, though the unit is still recieving power and input signal. You can see the signal indicator lights still operating, though the outputs are muted.

Now I originally thought the issue might be with one of these relays which are placed just before the outputs , but all the channels would stop working so I figured that wasnt the problem since its rare that all 5 relays would be malfunctioning (Do relays even go bad?)

After cracking the case on the unit a strong smell of burning plastic/components hit me and after searching for the culprit I saw the power tranaformer was smoldered on the corner (see first picture).

I checked all fuses, they are the correct values as well as the input selected for 100-120VAC (Im in USA)

Any idea what the cause of this might be? Im assuming the relays are getting an inconsistent voltage from the PS after the power transformer gets really hot and they mute the outputs. But what would cause the power trans to get this hot?

Any input is appreciated. Thanks

 

[murrayatuptown]

Unpotted toroids that mount with a bolt thru the core cab be shorted if the bolt isn't insulated from chassis. Level 1 foolproofing is to provide a rubber sheet to insulated the bolt.

Level 2 foolproofing is a potted housing like you have. Usually a sign of higher quality & cost.

Large toroids can experience high inrush current...but probably not that little guy.

My guess is it failed...but why? Primary is usually closest to core, with secondary/ies wound over that. That's a low power device...something really ugly must have happened for signs of trauma to make it outside the potting.

I assume that's not hard to procure a replacement of (unless it was a custom), but you could probably find something in Talema's standard product line (but I didn't look at their website).

120 or 120/240 primary and two 22 vac 5 VA (227 mA ac) windings.I think the 80751K looks like their p/n format.
Here's the Talema website:

https://talema.com/products/low-profile-pcb-mount-toroidal-transformers/
Digikey & others distribute Talema.

i suggest removing the transformer, or at least disconnect it from its rectifiers and start looking for shorted rectifier diodes or filter capacitors.

if it's not a secondary short, the insulation may have failed for unknown (to us) reason, causing an internal short...that might explain the unusual visual fault...

murray

 

[murrayatuptown]

80751K doesn't appear on Talema website or Google with Talema in front of the p/n. I would hope if it used to be standard and was discontinued, they'd have data available. I searched for Talema 80000 series and found nothing. So it might have been made for EV.

Since I find no datasheet, I don't know the dimensions or PCB footprint.

They do have other 22 V transformers, but I didn't look for dual winding as I don't know what will fit. They have some open (non-encapsulated) options, but same issue deters digging too far. Being 10 VA total may make it extra small.

09/01 on transformer makes me think Sept 2001 or Jan 2009 if the swap MM/YY. So if it ever had been a standard part they removed from website, Talema may have a suggestion. If they consider it a proprietary build for EV, EV may have some support options or suggestions.

But before you find a replacement, you might like to isolate the transformer secondary from the rectifier & PS as mentioned earlier. I would also investigate the primary windings with an ohmmeter also. It would be wise to have an idea of there is an external (to the transformer) problem that would damage a replacement.

If you do not draw any conclusions, removing the transformer would answer any dimensional requirements if Talema or EV can't/won't provide answers.

I would not do any further powered measurements. The transformer has clearly failed and there is little to no benefit to taking powered measurements in the condition it's in. Maybe I should say YOU should not do them. I think anyone who adds opinions would agree.

Edit: I should add that it is unsafe or could potentially become unsafe. You see it, you smell it. Game over. Start your forensics.

 

[murrayatuptown]

There is a possibility it has shielding as well. Seeking answers from Talema and EV will save you some effort and possibly money if you replace it with a substitute and find you are not satisfied with something EV determined they needed/wanted that a substitute lacked.

 

[NoisyIndividual]

It's possible pin 1 or 13 became dry jointed, and has since started arcing and burning the PCB.
If you took out the PCB you may find you can remove the charred parts of the PCB and hard wire the pin where it supposed to go.

 

[camshash]

Hey yall, thanks for the replies. Very helpful info!

Yes it appears Talema doesnt make the 80751K anymore, but I found a 70045K which is the same spec, just rated for 115V instead of 120V. I removed the old one and put the new one in and the unit is working normally now. However, it does seem the xfrmer is getting quite hot. I left my DMM thermometer on the plastic housing and it read 115F after ~30 mins, but again the unit functions normally and the relays don't mute the outputs. I've left it on and running for hours and all is well, but the xfrmer is probably getting way hotter than it should. I wasn't able to check the temps as the unit was put back together, but it's probably much higher than 115F

The old xfrmer is reading consistent values with the new one and is not shorted. This tells me the root problem lies somewhere else. No caps look bloated or abnormal, and fuses are all the right values. I've read that the only real way to test caps is the remove them from the circuit, same with diodes. Though the diodes in power supply circuit are all reading nominal 0.6V.

 

[NoisyIndividual]

In your first photo, the PCB in that bottom corner looks to be burnt to a crisp.
Is this not any sort of issue?

Maybe you could show a photo of the underside of the PCB where the transformer is soldered?

 

[camshash]

Good point, yes I checked continuity first and it passed, though the trace could be weakened. I just put it back together but give me a minute and I can get a picture.

 

[NoisyIndividual]

I think the PCB may be faulty and is sinking current from the secondary winding.

 

[camshash]

You can see where I added more solder as the trace before wasnt as luminous, but was still passing continuity. Should I try adding a jumper?

 

[NoisyIndividual]

Those 2 pins close together are the opposite ends of one secondary winding?

I think the PCB is arcing inside and shorting them out.

 

[camshash]

Yes that is one side of the secondary, how do I test if it is being shorted?

 

[NoisyIndividual]

Well it has burnt the PCB to a crisp for starters.
And it will be getting hot.

And probably smelling like its burning.

And causing a voltage drop at the regulator.

 

[NoisyIndividual]

You will need to remove the transformer and cut away all the burnt bits of PCB and then hard wire the pins to where they are supposed to go, as I stated in my first post.

 

[sodderboy]

Something not working, stinky smell, not blowing the mains fuse, usually points to partially dried caps.
Check the date codes on the electrolytics. It worked fine for years without a beefier trace.
Relays dropping out would point to what is supplying the voltage. Is the relay voltage correct, with no measured AC component? 12VDC for instance should not be 8.6VDC with 2VAC riding on it.
Mike

 

[Voyager10]

The relays are probably controlled by a circuit which checks whether the supply voltage has stabilised within some tolerance, as switch-on/off thumps are basically what you get if "DC" voltages are changing.

If there's a problem with the power supply such that it falls out of tolerance when it gets hot, chances are that's what's causing the relay symptoms.