U87, figure 8 is dead

[saint gillis]

I'm checking a Stam Audio U87, the pcb is Dany's one (U1621 V1.2)

Cardioid mode is ok, omni too but the back sounds a bit dull compared to the front I must say.
But 8 figure has a huge level drop.

The 2 backplates have electric continuity, I guess that is not normal right?



Also the back diaphragm has this little scratch that does not look good :

 

[dmp]

Are there separate wires to the two backplates? It might have been modified to use a common backplate, if there's just one wire.
You might have to trace where the capsule wires are connecting to the pcbs. The omni/card/fig8 switch uses R1 (330k) to pull down either the rear diaphragm (omni) or the rear backplate (fig8) to ground. For a single backplate, look at the U67 for switching. The backplate isn't changed and the rear diaphragm voltage is changed for omni / card / fig8.

 

[Khron]

Is this a clone of the "vintage" first generation U87, or the later U87A?

 

[saint gillis]

There are 2 backplate wires (one wire for each backplate)
I have to check but it looks like a first generation U87 clone

 

[Khron]

Photos of the boards (front & back of the mic)?

Even if there are wires coming from each backplate, doesn't mean they couldn't have internally made contact somehow.

 

[saint gillis]

I'll take pics, but I have measured with the backplates wires disconnected from the board

 

[ccaudle]

Even if there are wires coming from each backplate, doesn't mean they couldn't have internally made contact somehow.

Didn't Soliloqueen have problem with one batch of K87 capsules put together with screws which were slightly too long and could contact both backplates? Maybe a similar mistake here.

 

[Khron]

Through-holes were smaller than intended, increasing the chances of accidental contact.

 

[doqmemory]

The 2 backplates have electric continuity, I guess that is not normal right?

It is hard to tell from the picture, but it looks like you might have the connection to the front backplate also touching the rear backplate. You have to be careful here, since the tiniest connection will tie them together.

Has figure 8 worked correctly in the past?

 

[doqmemory]

Is this a clone of the "vintage" first generation U87, or the later U87A?

This is @poctop 's U87i board, no DC converter.

 

[saint gillis]

Yes it's Dany's boards,
Pictures of the boards :


When I first opened the mic I found the backplates connectors touching each others, like that :




Also you can see signs of struggle with the screws.
I've also measured continuity between the 2 backplates with the connectors fully removed..

 

[amplexus]

I know this is perhaps a silly question: Were you checking with headphones on oir in close proximity to speakers? Have you verified the backside is "dull" sounding on a recording played back or with a source that's not your own voice?

 

[saint gillis]

No there's clearly a problem with the 8 fig, the level is at least -30dB, and the signal from the back is almost null.
When I opened the mic I found one of the backplates wire not properly tied , floating and touching the other backplate.

Now I've removed all the wires from the capsule, and I measure a continuity between both backplates.

Is it possible that a short between the 2 backplates has provoked a damage in the capsule? looking at the schematic it seems unlikely..

Is it normal to have a huge volume drop in the 8 fig when the 2 backplates are shorted?

 

[elskardio]

Is it normal to have a huge volume drop in the 8 fig when the 2 backplates are shorted?

You absolutely need isolated backplates to get a figure 8 pattern with this circuit.

Soliloqueen sells a really nice k87 capsule

 

[saint gillis]

Thank you

In this mic, the original Braingasm capsule has been replaced by a Heiserman capsule (not by me), I found one of the backplates wire floating, and the screws fixing the backplates wires a bit damaged like too much force has been applied when fixing them.

What seems the logical explanation for the backplates now becoming in electrical contact within the capsule ?

 

[elskardio]

I’m no expert… but the backplates look damaged above the screws. Maybe this is causing your short…

 

[Khron]

Never seen a toothed washer used on a capsule connection, by the way...

 

[saint gillis]

Anyone knows how the 2 backplates are isolated?

 

[elskardio]

there’s a thin plastic film in between

 

[saint gillis]

Maybe that can be replaced ?