[Amps] al-1500 parasitic?? or some other problem?

Tue, 14 May 2002 17:06:40 -0700

Dave,

When you said "with and without" the ICE bandpass filter,
did you take it completely out of circuit for the "without"
portion or just switch it to the bypass mode. The LEDS on
the ICE front panel display (I am speaking of the ICE419
with which I am familiar) are not decoupled very well from
the RF path. On one of the units I had, the 7 MHz LED
went leaky and turn into a noise diode when reversed biased.
Turns out that ICE uses a single pull-up resistor common to
all 6 LEDS. When the unit is activated, this reverse biases
the 5 un-lit LEDs for the unused bands with about 10 volts DC.
The 7 MHz LED in one of my units had a high reverse current
and made a very nice broadband noise source. I discovered
the problem during the 2002 ARRL Phone Contest when
I noticed that my broadband noise floor on 10 and 15 meters
was significantly lower with the ICE filter turned OFF!

Anyway, the fact that RF coming from the DC control circuits
was able to couple out to the RF ports of the filter suggests that
some of the RF power passing thru the filter would be able to
leak back onto the diodes thru the control wiring network. I have
seen the LEDs on the ICE 419 blink in unison with the RF
envelope in some cases. I don't remember exactly which
band/antenna combinations would do this, but it always made me
nervous having that much RF across non-linear device like an
LED when operating in a multi-transmitter environment. After I
discovered the noise generating diode, I rewired the diode bias
network in the ICE so that each LED has its own pull-up resistor
and set of bypass capacitors. The LEDs don't blink anymore, and
the un-lit diodes in the inactive positions are no longer reversed bias
(no more broadband noise).

A couple of other commenters suggested bad connection (another
source of diode junctions). This is also a strong possibility. With
all the different rig/amp combinations causing the same trouble,
it is starting to smell like a problem with the wiring (e.g. bad
connection). Sounds like it would be worth borrowing a dummy
load at this point.

Good Luck,

Mike, W4EF.............................

----- Original Message -----
From: "David Robbins" <k1ttt@arrl.net>
To: "'Amps'" <amps@contesting.com>
Sent: Tuesday, May 14, 2002 3:30 PM
Subject: RE: [Amps] al-1500 parasitic?? or some other problem?

> At this point nothing is a stupid question, I am definitely confused.
>
> I am using other radios in the shack to measure it.  It was first
> noticed for the bad clicks on 15m.  The 15m station has an ft-1000mp
> with antennas on a separate tower about 300' away from the 20m tower.
> To eliminate the rx as a problem I also hear it on a ts-680 that is
> connected to a 6m antenna on yet another tower.
>
> Add another twist or two...
>
> There is also a sub-harmonic being generated at exactly 1/2 the tx
> frequency.  It comes and goes exactly in sync with the 1.5x 'harmonic'.
>
> Now the real kicker!  I swapped the al-1500 out for an hf-2500, and the
> 'harmonics' are still there.  And they respond to the tune control on
> the hf-2500 exactly the same as on the al-1500.... so now I am down to
> the 'some other problem'... still related to amps, but apparently not
> specifically the al-1500.  This makes the problem much more important to
> find and fix as the last resort plan was to move the al-1500 to 10m, but
> obviously this is not going to fix the problem now.
>
>
>
> David Robbins K1TTT
> e-mail: mailto:k1ttt@arrl.net
> web: http://www.k1ttt.net
> AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
>
>
> > -----Original Message-----
> > From: Steve Katz [mailto:stevek@jmr.com]
> > Sent: Tuesday, May 14, 2002 21:42
> > To: 'David Robbins'; 'Amps'
> > Subject: RE: [Amps] al-1500 parasitic?? or some other problem?
> >
> > Okay, I'll ask the seemingly stupid but logical question: What are you
> > using
> > to measure this spur?  Have you tried using something else?  (That is,
> if
> > you're using a receiver, have you tried a different receiver?  A
> spectrum
> > analyzer?  Anything different from what you originally used?)
> >
> > WB2WIK/6
> >
> >
> > > Ok, thanks for all the testing suggestions and comments.  Below I
> will
> > > try to answer all the questions and suggestions so far.  I also find
> it
> > > hard to believe it is produced by the amp, but am running out of
> other
> > > causes.
> > >
> > > I have tracked the spur from 21000 to 21450 (tx from 14000 to 14300)
> it
> > > is always exactly 1.5 times the tx frequency, the frequency of it is
> not
> > > affected by the amp tuning controls.  The onset point does vary with
> > > load, near 14000 the swr is about 2:1 and the spur doesn't start
> till
> > > about 1kw output, at 14300 the swr is 1.2:1 and the spur is there at
> > > about 200w output from the amp but is much more sensitive to the amp
> > > tune control.
> > >
> > > I have generated it with and without the input bandpass filter and
> the
> > > output stubs in all possible combinations with 2 different radios.
> > >
> > > I unfortunately don't have an adequate dummy load right now, but
> even at
> > > almost 1:1 swr at the best frequency for the stack the spur is still
> > > there as loud and predictable as ever.  (note the stack is 300' from
> the
> > > shack, with the shack off the side of the beams, and there is no
> other
> > > sign of rf feedback in any equipment, all grounds have been checked)
> > >
> > > I have swapped the ft-1000mp for a ts-870, there was no change in
> the
> > > spur at all.
> > >
> > > The amp does not have a qsk board.
> > >
> > > There is nothing in the rx antenna jack on the ft-1000mp or the
> ts-870.
> > >
> > > I can't believe it is a design flaw as the amp functioned normally
> for
> > > several years with no problems.  And this is so repeatable now that
> I
> > > can't believe we didn't notice it or that someone else hasn't also
> seen
> > > it in multi station contest operations.
> > >
> > >
> > > David Robbins K1TTT
> > > e-mail: mailto:k1ttt@arrl.net
> > > web: http://www.k1ttt.net
> > > AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> > >
> > >
> > > _______________________________________________
> > > Amps mailing list
> > > Amps@contesting.com
> > > http://lists.contesting.com/mailman/listinfo/amps
>
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