Power Supply capacitor keeps blowing

I am building a mic preamp and have now blown this electrolytic cap twice.

It is in the phantom power circuit.

As far as I am aware, these blow from being installed backwards, or over voltage. 

When it blew, I was measuring 48vdc from the output, but it may have been negative (things happened fast when it exploded!).

I am using a hammond 369ax (I will attach the diagram for this).  The transformer has the main 250vac out, with a ct and a 50vac tap.

The main b+ line is using a bridge diode, and I have the ct and the 50v wired to the 50vac in - should I be using the ct as 0 like this?

The first time it blew I was measuring 150vdc and determined the regulator was bad.  I replaced this, so now the only thing I can think of is that I wired the ct to the wrong side of the bridge rectifier.  Will this flip the polarity out of the diode?

It is C13

According to the transformer datasheet, yes, you'll want all grounds to connect to the red-yellow center tap on the "big" secondary.

Those 50V AC should be at no-load, if i'm reading the specs correctly ("N.L.V." = No Load Voltage?), so... even rectified, that should come out to about 70V DC.

If you have another cap to spare, remove the regulator and the diode (D1 in your schematic), and see what voltage you can measure across it.

To be on the safe side, i'd use the "largest" primaries (in order to get the lowest voltages on the secondary), at least for initial tests.

This is what I’m confused about.

The transformer has the normal dual secondary connected to a bridge rectifier.  Bridge rectifiers don’t use a center tap, right?

I got this thing as an incomplete project, and that power transformer fried - worried about this happening again!

The first time I tried this, I connected the 50v tap by itself, so a single wire, to the ac 50v in.  The other side references ground, so I thought it was ok, but the cap blew and I measured very high voltage 150vdc.

With the ct floating, the other taps work, measure correctly and the amp works (although I’m also working out an output transformer wiring issue.  I can scope clean output coming from the end of the circuit.

I just heard from the builder that this won’t work with this transformer due to how the tap is implemented.

I would still like to better understand the issue with ct and bridge rectifiers

> using a hammond 369ax

You didn't buy that beast for a little phantom. What ELSE is it powering? HOW is it connected?

You "can" get +175V and +70V on this winding.

If you are thinking to get +350V, then you can't be using the CT and 50VAC taps to get a grounded 70V.

For 350VDC:

The first plan below *may* work. Put a 1k 1/4W in series with the 50V tap for smoke checking. Note that this gives more raw voltage than intended, may stress the regulator and caps.

The second plan will work, and is simpler, but throws near 2 Watts of waste heat.

This is a pair of my Classic Solo mic pre's that someone has partially built in a box  and sold to the OP.. The Classic Solo really needs a separate 50VAC winding for the phantom power which feeds a bridge rectifier and then a TL783 regulator. I think initially the OP connected the HT bridge across the 250VAC winding and then the phantom bridge between the centre tap and the 50VAC tap which resulted in smoke.

Bottom line is this transformer is not really right for this application but there may be ways round it. I have suggested running a 70V zener off the HT and bypassing the phantom bridge but I am sure there are other solutions.

Cheers

Ian

ruffrecords said:

This is a pair of my Classic Solo mic pre's that someone has partially built in a box  and sold to the OP.. The Classic Solo really needs a separate 50VAC winding for the phantom power which feeds a bridge rectifier and then a TL783 regulator. I think initially the OP connected the HT bridge across the 250VAC winding and then the phantom bridge between the centre tap and the 50VAC tap which resulted in smoke.

Bottom line is this transformer is not really right for this application but there may be ways round it. I have suggested running a 70V zener off the HT and bypassing the phantom bridge but I am sure there are other solutions.

Cheers

Ian

Click to expand...

Thanks Ian, I wasn't wanting to drag you in - you did your part!  Great PCBs and I'm very excited to use these in a session, I bet they'll sound excellent!

I fried the initial PT due to a wiring mishap.  They had used a cutoff from one of the primary wires as a ground tied to a transformer bolt under the transformer, so when I grabbed "blue" to wire the primary I grabbed the grounded blue.  I also grounded the CT, and then connected just the single 50v tap to the phantom circuit plus side.  The fuse blew, but it also smoked.

I have not had this confirmed - was hoping someone could school me here.  I am under the impression that you cannot use a CT with a bridge diode rectifier?  I usually build tube guitar amps with tube rectifiers or 1n4007 strings, so this is learning territory for me.

I guess I am confused how the multi taps work in a transformer like this. 

> I am under the impression that you cannot use a CT with a bridge diode rectifier?

Nearly every stereo transistor power amplifier uses a CT and bridge.

But not grounded the way you are trying to do it.

So should I ground the CT on this?  Right now it is heatshrinked in a bundle with the 50v tap.

The preamp is working, sounding great and quiet, except for the 50v.  I am thinking it will be easiest and most efficient from a heat perspective to just buy a 50v transformer and be done with it.

rockinrob86 said:

So should I ground the CT on this?  Right now it is heatshrinked in a bundle with the 50v tap.

The preamp is working, sounding great and quiet, except for the 50v.  I am thinking it will be easiest and most efficient from a heat perspective to just buy a 50v transformer and be done with it.

Click to expand...

Like I said in my eamil to you, the phantom supply in this case really needs a separate winding so buying a 50VAC transformer is the easiest solution. Transformers with 50V secondaries are uncommon but ones with 2 x 2V and 2x25V are readily available. You don't need a big one for phantom power. a 5VA or 10VA type will be plenty. Something like this will do:

https://uk.farnell.com/block/fl6-24/transformer-6va-2-x-24v/dp/1131553

Cheers

ian

Run a 12VAC:120VAC transformer *backward* from a 6VAC tap. That gives 60VAC. Around 84V raw DC.

AC couple the phantom supply by using capacitors between the transformer and the rectifiers?

In a pinch, and with a chubby smoothing capacitor, you miiiight even manage to get away with a half-wave rectifier as well, as the simplest(?) solution.

Scrolling back a bit, that's actually exactly what PRR suggested, in his first reply
https://groupdiy.com/index.php?topic=72658.msg920201#msg920201