[NLRS] Killed my amp? Probably

[Chris Elmquist]

Is the input power to the amp (aka, output power from exciter) reducing?

If the SWR seen by the FT897 looking into the amp is not ideal, it may
be cutting back its transmit power.  Your description of watching it
drop seems to fit my experience with automatic power deduction due to
poor SWR seen at the output of these rigs.

This can happen for the usual reasons such as bad jumper coax, bad
connector, loose connector, degrading T/R relay in the amp, etc.

Sometimes the amp input can be tuned for a better match to the exciter
if everything else checks out.  Sometimes you can improve things by
changing the length of the coax jumper between the exciter rig and
the amp.

If you can put the watt meter or SWR meter between the FT897 and the
amp, you can see how it looks but be aware that doing so will change
the system (different cables and lengths) so you might not see what is
really happening when the meter is not inline.

Possibly trying a different exciter rig into the same power amp, might
reveal that the power amp is working OK and the exciter is just unhappy
looking at that amp.

Chris NØJCF

On Thursday (09/03/2015 at 04:12PM -0500), David Palm wrote:
> 
> 
> Unfolding saga -- now I see I'm wrong.  I don't have a 4-110, I have a
> 4-310 which should take up to 30 watts of drive.  So I'm well under that
> limit, but last night I watched the output of this amp fall off as I was
> key down and now it's only putting out 30 watts for 17 watts in.
> 
> I'm also seeing a bit of SWR -- not huge, but noticeable -- when I put the
> amplifier in line.
> 
> Any other ideas on what to check on this dude?  I've got it open on my
> bench.
> 
> 73,
> 
> David  W9HQ
> 
> 
> On Thu, Sep 3, 2015 at 3:51 PM, Barry Malowanchuk <ve4ma at shaw.ca> wrote:
> 
> >
> >
> > Better RF Parts or else you may get a Chinese copy
> >
> > Barry VE4MA
> >
> > ----- Original Message -----
> > From: "Dr. Gerald N. Johnson" <geraldj at netins.net>
> > To: nlrs at mailman.qth.net, "Badger Contesters" <
> > badgercontesters at mailman.qth.net>
> > Sent: Thursday, September 3, 2015 2:00:06 PM
> > Subject: Re: [NLRS] Killed my amp? Probably
> >
> >
> >
> > RF Parts probably also has the part.
> >
> > For the future you need to attach a cable to the amp input with 1.25 dB
> > loss so the 20 watts from the 897 is only 15 at the amp. That's just a
> > few feet of RG-58. ARRL handbook chart shows 12 or 13 dB / 100 feet off
> > the top of the graph though. So ten feet should be pretty close. And the
> > driver probably is bad.
> >
> > 73, Jerry, K0CQ
> >
> > On 9/3/2015 11:58 AM, David Palm wrote:
> > >
> > >
> > > OK, time to tell on myself.  I was checking out my gear last night.  222
> > > MHz ampflier is working fine and playing well with the transverter there.
> > > So good to go.
> > >
> > > On 432 it's a different story.  Here's what you get when you don't use
> > > stuff for a long time and forget when you have.  I connected the rig to
> > the
> > > amplifier, an RF Concepts 4-110.  I had long forgotten that this amp can
> > > only take a max of 15 watts input.  My FT-897 puts out just over that.
> > So,
> > > I was keying the amp and getting about 80 watts out, which is actually
> > > rather low, especially with that much drive.  But then as I watched, the
> > > output just started dropping before my eyes, until I basically am getting
> > > little to no amplification at all.
> > >
> > > I presume that I burned out the driver stage.  The schematic says that
> > this
> > > is an MRF646.
> > >
> > > Those of you who know about such things, does my diagnosis seem correct?
> > > And do you know of any better source to get an MRF646 than this guy, who
> > > has them for $20 apiece:
> > >
> > http://www.ebay.com/itm/Motorola-MRF646-/281687443775?hash=item4195e1493f.
> > > Hopefully this will not be a big problem to swap out and be back in
> > > business.  If you've done such things before and have some good advice,
> > I'd
> > > love to have it.
> > >
> > > Doh!
> > >
> > > 73,
> > >
> > > David  W9HQ

-- 
Chris Elmquist