V76 input transformers acting weird

[Winston OBoogie]

Thanks, I'd forgotten about that discussion.

So C.J, says the lams used in the V76/V72 input transformers (Cobalt) had higher magnetorestrictive figures than nickel, which is probably the reason for not dipping or potting them.

 

[Michael Tibes]

I've shipped the two defective transformers back and the seller send me two other ones.
Same thing...
What's weird, is that they were from a spare department of a radio station. Why should they keep defective transformers?

@ Bonnie1: you say they generally don't get bad just sitting on the shelf.

Two reasons come to my mind why the radio station might have shelved broken transformers: either a tech guy kept them for possible rewinding or they were actually ok when they were shelved and did break in storage. For example I heard that V74 input transformers like to self destruct due to acid which is released when the foam inside the can deteriorates. I can't verify that, but it explains the lots of broken V74s I've come across. There might be long-term effects nobody could foresee when the units were built. And probably nobody did foresee that anyone in 2021 still might be interested in them.

Finally i higly recommend bypassing the four 2u capacitors in the middle of the primarys. These are installed to avoide eventually asymetrical 48V phantom to pass the pimarys. If these capacitors by accident are charget, a discharge will induct a very high voltages to the fragilde secondaries, proabebly the reason that so many of these BV511 transformers goes dead.

I doubt that these caps were added because of phantom power. The known V76 schematics are dated 1959 and 48V phantom was introduced 1966 if my quick research was correct. Also I remember a video showing arcing in a V72 or V76 transformer due to 48V phantom. The isolation of the wire must be / have become incredibly thin. I have not connected 48V to any of the V family inputs since then.

Is there noone in Europe who can rewind these transformers?

It isn't possible that you're measuring the transformer wrong? Just to be sure...

Michael

 

[Winston OBoogie]

I doubt that these caps were added because of phantom power.

You're 100% correct.
Together with the IPT primary inductance, they form a high pass filter. Independent of the front panel filters on the 80 and 120 versions.
This particular filter can be bypassed by shorting two internal pins with a soldered wire bridge.
Neither the V76M measurement amplifier, nor the V76S proper studio version had this high pass filter.

Nothing whatsoever to do with phantom.