[GreenKeys] Re: Dovetrons

[gil at baudot.net]

Hi Doug:

Bummer about your computer. 

It's true that a transformer-input power supply is isolated from a
direct path to the AC line, but the AC safety-ground should always be
connected to any chassis.  With an ungrounded chassis, a fault in the
internal wiring can result in a shock hazard.  I'd like to point out to
everyone that ground is even more important when working with ham
equipment, which is quite often modified (and not always properly).

It is quite likely that the EMI filter in these Tempest Dovetrons uses a
capacitor from each line to chassis-ground as part of the design, which,
as was pointed out previously, will leave an ungrounded chassis right
around half the line voltage, or about 60 volts rms.  This is 60V with
respect to earth ground, which is the AC safety ground (and recall that
the AC neutral wire is strapped to earth ground back at the breaker
box).

I use GFI protection on all lab outlets.  My Dovetron would trip any of
three different GFI outlets I tried.  I did not find any resistive
paths from the chassis to either line, but all it would take to trip a
GFI would be a small imbalance in the line currents.  This imbalance
could result from slightly-different values of these line-to-ground
caps in the filter.  I plan to remove the emi filter when I get a
chance, and try it on a GFI again.

But note that you need to remove the filter, and you can't simply bypass
the filter by jumpering the line across it -- that will take out the
series inductance in the filter, but it will still have line-to-ground
capacitance, and hence still leave the chassis at 60V.

The Dovetron's Audio-Input is 600-ohms, and I'd bet it is
transformer-coupled.  Had they used an isolated balanced connector like
an XLR, instead of the BNC (which has one side grounded to the chassis),
you may not have blown your pc.

gil

> -------- Original Message --------
> Subject: Re: [GreenKeys] Re: Dovetrons
> From: "Doug Alderdice" <ka2wft at arrl.net>
> Date: Sat, February 12, 2005 3:55 pm
> To: "John Lawson" <jpl15 at panix.com>
> Cc: greenkeys at mailman.qth.net
> 
> At 05:45 PM 2/12/2005, John Lawson wrote:
> >   This is a fairly common 'gotcha' when connecting machines from 
> > different eras to one another.  It's truly a bummer that you've had your 
> > mobo scrambled - at least your machine's still basically functional and 
> > the contents of your HD are safe.  I've had much more terrible results...
> 
> Yes.  I am thinking that I am lucky.
> 
> >   There is a line filter, sometimes called by the generic 'Corcom' but in 
> > reality made by various companies, CorCom included. If this has broken 
> > down it would tend to produce this fault, and it *would* be about 60Vac.
> >
> >   Also - is the Loop Supply bleeding to something? Or do you hev the 
> > Tempest version with just TTL outputs, requiring a seperate loop-keyer.
> 
> This is the Tempest unit, one of the "Project Big Bird" batch that a number 
> of us on this list got a couple of years ago.  It was not connected to 
> anything else other than AC in and audio in.
> 
> >  If the problem persists, you can use a small 1:1 audio transformer of 
> > suitable impedance to isolate the two ground systems....  but I'd sure 
> > dig in and find out why the Dovetron is presenting such a solid leak 
> > current in reference to it's internal grounds.
> 
> I am suspecting the EMI filter on the input of the power 
> transformer.  Nothing else on the AC line in goes to ground, and Gil Smith 
> mentioned yesterday that he blew a GFI breaker with his unit.  Sounds like 
> we all better bypass that EMI filter.  I admit to not having a ground on 
> the power cord I wired up for the Dovetron.  I knew it had the transformer 
> for the power supply, so why bother with a ground for a quick test?  Errgh.
> 
> For now I am not doing anything with this Dovetron until I bypass that EMI 
> filter.
> 
> 73.
> 
> Doug, KA2WFT
> 
> 
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