>Fellow VG owners,
>
>We have been occasionally blowing the fuse in our source control unit over
>the past year and getting an error message "source filament in limit".  We
>would replace the fuse and were right back in operation.  This week we blew
>the fuse and got a new message "faulty source suppressor".  Now when we
>install a new fuse, it blows almost immediately and we are back to the old
>error message "source filament in limit".  I'd appreciate any
>ideas/experience anyone has regarding this problem. My fear is that I am
>going to have to replace an entire board (which I don't know how to get a
>hold of). Please say it ain't so... 
>
>Regards (and Happy Holidays) to all,
>Margaret
>-------------------------------------------------
>Margaret Lansing


Margaret

Jerry Pulchan replied "Dear Margaret and Howie,
We recently experienced a similar problem (frequently blown fuses) with
our Optima. First, check that the capacitor on the source control board is
OK (as it was in our case). We ultimately traced our problem to a voltage
rectifier. It is located at the back of the source control box, towards
the (left if you are facing the unit). It is easy and inexpensive (few
bucks) to replace the volatge rectifier. You may also want to check that
the connections to the volatge rectifier (look for four red wires) are
good. I suspect that the source control units for a Prism and Optima are
similar. Good luck."

I would agree that the most likely cause of the blowing fuse in all SIRA 9,
SIRA 12, SIRA 24, Prism and Optima instruments is one of the components in
the filament drive circuit.  This consists of the capacitor, bridge
rectifier and 2 transistors found on the high voltage mother board at the
back left hand corner of the unit.  All these instruments use the same
source and HT units.

If the filament really is in limit or short circuited then the electronics
will limil the current drawn to < 4 amps ( for thoria coated filaments).
The fuse that keeps blowing should be 5 amps.

Most of the components mentioned above, apart from the capacitor, are only a
few dollars, so it is proberbly worth replacing all at one go.  Especially
as getting to the components is not all that easy.

As far as the suppessor fault is concerned, on all the older instruments,
this was generated in the HT unit, not the source control and is used in the
collector, so this must be a seperate fault.  Usually caused by a loose
connection between the HT unit and the head amplifier.  This cable is the
one with a BNC (looks a bit like a TV arial plug) at the head amp.  If you
dissconect the suppressor at the head amp, with the source on, you should
get the error message.  Then lay the BNC plug down on the bench so that the
body of the plug is grounded and the error message should go away.  The
suppressor should measure -45v between the centre pin of the BNC and it's body.

Clemente Recio mentioned a different source control unit used on the SIRA 10
and SIRA series 11 instruments.  With the these instruments the source is
controlled from a manual control unit called the "8-80" because it was
originally designed for use in an instrument called the 8-80!  However this
unit is completely different to the units on the other instruments.

Cheers

Peter Stow
                                 ISO-MASS Scientific Inc.
                                319-5940 Macloed Trail S.W.
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