Random DEC Questions

Chris Elmquist chrise at pobox.com
Thu Dec 4 16:57:47 CST 2014

On Thursday (12/04/2014 at 02:34PM -0800), Ian McLaughlin wrote:
> > On Dec 4, 2014, at 2:27 PM, Robert Jarratt <robert.jarratt at ntlworld.com> wrote:
> > I opened up the VT320 tonight. The PSU is set for 240V, which is good. However, I noticed some corrosion around the flyback transformer. I have posted a picture here: http://1drv.ms/1zqDhx5
> > 
> > This looks like corrosion of some kind of retaining clip perhaps, rather than anything functional. Am I right, or should I be concerned?
> The brown between the ferrite core and the white coil housing is normal glue used in the flyback manufacture.
> However, the rust on the metal clip holding the ferrite core together is definitely unusual - I've never seen this before.  Obviously a sign of moisture at some point in its history.  Is there any other signs of moisture?  The clip itself is purely mechanical (holding the two parts of the ferrite core together) and serves no electrical purpose.  As long as it's still structurally sound, it's probably ok.

I have actually seen a lot of this, particularly on VT220 I have restored.
By the looks of yours, I'd say the clip is toast.

It does hold the core together so if it comes apart and there's later
vibration of significance, the core might come apart and then you don't
have a flyback anymore.

I would try to clean up the particles that remain as they are conductive
and if they migrate into other parts of the circuit or if the remains
of the clip come off and end up somewhere else in the circuit, you'll
have a lot more to repair.

I have repaired several flybacks with this problem by removing them from
the board, removing the rusted clip and all the gunk and then threading
a #12 or #14 insulated copper wire through the core and around the
ferrite and then twisting it back on itself to cinch it up.

You might also be able to accomplish similar without removing the flyback
from the board if you can get your cinch wire underneath the bottom of
the core, between it and the PCB and then wrap it over the top.  I think
the core splits right about where most of your clip has disappeared--
about half way up the side of the flyback in your photo.

I haven't it done it this way myself but you might also be able to save
it with some epoxy or even hot melt glue if you can encapsulate both
halves of the ferrite that way.

Good luck.

Chris Elmquist

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