[AMPS] amplifier problems
Richard W. Ehrhorn
w4eto@rmi.net
Fri, 15 Oct 1999 11:41:04 -0600
Funny I don't remember any Jamie Jones at ETO... John Cunliffe, yes. Problems with step-start relays were and are very few.
Principal cause of burned step-start resistors: owner fires up amp with cover off and forgets to disable the HV crowbar - BTW, a dangerous thing for any but experienced and careful techs to do in any case! Next most common (but actually quite uncommon) causes, a failure (short) in HV supply or associated circuits) OR failure of the ~24V supply that powers the step-start relay.
I would never (except in dire emergency such as middle of contest or DXpedition and nothing else in parts store!) replace a step-start resistor with a smaller unit. Going from original 10 ohm/13w to 20W not a bad idea... but make sure no wiring or other components are close enough to the resistor(s) to be damaged if the original cause of R failure hasn't been fixed or occurs again.
73, Dick W0ID
-----Original Message-----
From: Jamie Jones [SMTP:jones@cunliffegroup.com]
Sent: Wednesday, October 13, 1999 2:21 PM
To: Gilmer, Mike; 'AMPS'
Subject: Re: [AMPS] amplifier problems
A lot of time the step start resistor in this units blow because a too large
fuse was in place; it should be a 1 1/4 amp slow blow ceramic fuse.
It will also blow if the relay contacts are bad ( open frame crummy little
things -- bad design to use it in the first place had to change a lot of
them during my time working for ETO ( when Alpha Power was still ETO ). It
also fails with a too large fuse AND the crow bar down with open case (HV
short) and also of course one of the 8874 shorted ( have seen a few of them
:-) ). A 20W resistor will work fine in this application I installed a few
of those.
Jamie
----- Original Message -----
From: Gilmer, Mike <mgilmer@gnlp.com>
To: 'AMPS' <amps@contesting.com>
Sent: Wednesday, October 13, 1999 12:36 PM
Subject: RE: [AMPS] amplifier problems
>
> On Wednesday, October 13, 1999 12:05 PM, measures
> [SMTP:measures@vcnet.com] wrote:
> > >
> > >On Wednesday, October 13, 1999 10:20 AM, measures
> > >[SMTP:measures@vcnet.com] wrote:
> > >> >
> > >> >Hello all...have an old alpha 374 the resister across the step
> down
> > >> >relay, a 10 ohm 13 watt wire would burned up....question...can I
> > >replace
> > >> >this with a 10 ohm 20 watt, what I have available or should I
> replace
> > >it
> > >> >with an exact same wattage??? thanks all Tim WB8OWX
> > >> >
> > >> I have seen ww resistors of the same physical size rated from 10w
> to13w.
> > >
> > >Your point?
> >
> > The ratings game is just that. 13w rated and 10w rated vitreous
> coated
> > ww resistors are 2 inches long and 0.4 inches in diameter.
> > >
> > >> A 10w, 10-ohm resistor should probably work ok in this
> application.
> > >
>
> It sure gets crowded on a head of a pin.
>
> He has a 20W unit, apparently in hand. Why would you feel it necessary
> to tell him a 10W would work and not answer his specific question and
> tell him if a 20W is OK?
>
> >
> > >
> > >> However, there is a chance that the resistor was not defective in
> the
> > >> first place, and there is a circuit fault which caused the failure.
>
> > >>
> > >
> > >True enough, but since a 10W "should probably" maybe it "might not".
> >
> > My guess is that a 7-watt rated unit would work in this (intermittent)
>
> > application.
> >
>
> Maybe even smaller. In this application it probably depends more on its
> construction than its power rating per se. I agree the real problem is
> why it "broke" in the first place. If being underrated caused the
> problem then bigger will be better. If not, then it won't help much -
> just cook longer probably.
>
> I'd treat it much like a blown fuse: replace it once and see what
> happens - if it blows again, troubleshoot. Unlike a fuse, it probably
> won't "blow" right away even if there's trouble. So there'll be time to
> see (smell?!) if the rest of the circuit is operating properly.
>
>
> Ciao
> Mike
> N2MG
>
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