7. While observing the
current meter, slowly turn bias pot #1
clockwise to assure that the current is
dropping. Continue turning the pot
clockwise until you reach the cut-off
point where the total current does not
drop further.
8. Record the total idling current
so you can establish where that device was
idling from the factory and then slowly
turn bias pot #1 counter-clockwise until
the idling current is increased by 250 ma.
9. Repeat the adjustment procedure
in numerical sequence for bias pots 2, 3,
and 4. As noted in step 8 above, record
the idling current when you reach cut-off
for each device to establish how each
MRF-150 was set from the factory.
When you have completed the adjustments,
power down the amplifier, remove the clip
leads, install the fuse, and put the covers
back on. When the amplifier is re-assembled
slide it back into its normal operating
position and you are good to go. My
adjustment notes below show how my amp was
set up from the factory. The imbalance
between devices 1 and 2 which form one
push-pull pair probably explains why it was
performing less than optimally. After
running the amplifier for several months I
repeated the adjustment procedure and found
that the idling current for each device was
still within a few milliamps of my first
setting. After correcting the idling current
issue I found that my ANAN with OpenHPSDR
pre-distortion linearization engaged had no
issues correcting the ALS-600 on any band.
Making sure that your ALS-600 amplifier is
idling correctly will help keep you from
ending up on this
page!
------------------------------------------------W1AEX
notes during bias
adjustment--------------------------------------------
Total idling current before
adjustment: 610 ma
With device #1 at cut-off:
535 ma (610 ma - 535 ma = 075 ma idling
current for device 1)
With device #2 at cut-off:
415 ma (610 ma - 415 ma = 195 ma idling
current for device 2)
With device #3 at cut-off:
465 ma (610 ma - 465 ma = 145 ma idling
current for device 3)
With device #4 at cut-off:
435 ma (610 ma - 435 ma = 175 ma idling
current for device 4)
Total idling current after adjustment:
1020 ma (Each device @250 ma x 4
devices = 1000 ma + 20 ma for residual
current draw by devices other than the
MRF-150 MOSFETs = 1020 ma)
If you total up the mathematically derived
factory idling current for all 4 devices you
will see that it comes up to 590 ma. So...
what about the other 20 ma? I found that if
I set all the devices to cut-off there was a
residual current draw of 20 ma. At any rate,
you can certainly do the bias procedure by
zeroing all the devices and then going back
and increasing the idling current by 250 ma
for each device (be sure to observe the
numerical sequence in the photograph above)
but the upward idling current drift problem
is more of an issue that way. I found it
easier to get more precise results by keying
the amp, letting it sit at idle for several
minutes, and then zeroing one device at a
time and then setting the idling current for
that device and repeating this for each of
the remaining devices. Either way will work
fine as long as you are careful.
A two-tone test showed the amplifier
delivering better than -34 dB third order
IMD which is quite respectable. With the
Pure Signal pre-distortion linearization
protocol engaged the amp reached -45 dB or
better on all bands without any difficulty
at full output.
NOTE: During the summer of 2017, as part of
my annual amplifier clean-up, I put the
ALS-600 back on the bench to check the bias
settings. The previous settings were holding
very well but I decided to set each device
to idle at 200 ma for better cooling. There
has been a lot of discussion about what the
optimal resting current is for this
amplifier and the truth is that the range
for acceptable IMD performance is pretty
generous. If you are going to use the
amplifier for RTTY or AM linear the 200 ma
setting is a good compromise as it will
still produce a very clean signal on SSB and
thermal stability will be greater when
running continuous duty modes. On hot summer
days, with the amplifier set to idle at 300
ma or even 250 ma, the thermal protection
switch would sometimes flip the amplifier
into "standby" if I transmitted longer than
a couple of minutes. With an idling current
of 200 ma per device the thermal switch
never actuated during the continuous duty
modes so it looks like it's right in the
sweet spot. Following the adjustment, I
checked the amp's linearity by using the
AmpView utility in the OpenHPSDR mRX PS
software and found that the amplifier was
still correcting perfectly on all bands.
Total idling current after the June 2017
adjustment: 820 ma (Each device
@200 ma x 4 devices = 800 ma + 20 ma for
residual current draw by devices other than
the MRF-150 MOSFETs = 820 ma)
Random
failure of the TX/RX open frame
relay to transition correctly
In the summer of 2014, after a year of
operation, my ALS-600 began to randomly fail
to properly transition from RX to TX as the
open frame T/R relay would not reliably
close. As you would expect, this resulted in
a very high SWR for the exciter as it looked
into the open connection it was presented
with. Mashing the PTT function of the
exciter would clear the problem but it would
randomly show up again after a day or two
and as time went by it began to happen more
frequently. Browsing the forums where
Ameritron amplifiers are discussed revealed
quite a few similar complaints other owners
have had with the open frame relay. This
kind of surprised me because the AL-80B that
I owned for years used the same open frame
relay and it never gave me any problems at
all.
At any rate, I grabbed the ALS-600 and put
it on my work bench to see what was going
on. After pulling the cover off the RF deck
and powering it up on the bench I cycled it
between RX and TX while watching the relay.
Sure enough, after about 10 successful
transitions the contact pivot arm got hung
up and did not fully close. Cycling it a few
times would clear the problem each time it
happened. After watching this behavior
through a few cycles I determined that it
was a simple mechanical problem with the
contact pivot arm. As you can see in the
photo below, there is a spring at the top of
the relay that keeps the RX contacts tightly
engaged until the coil is energized and the
magnetic pull overcomes the spring tension
to pull the pivot arm inward so that the TX
contacts are firmly engaged.
Closer inspection of the relay pivot point
the next time it failed to transition
revealed that one side of the pivot arm was
not pushed completely onto its pivot tab,
causing the pivot arm to be slightly
misaligned. The second picture below shows
where the pivot tabs are located on the
frame of the relay. I grabbed the pivot arm
and carefully slid it off the pivot tabs and
then slid it back on. I noticed that one of
the pivot tabs seemed to fit much more
tightly than the other and so I slid the
pivot arm on and off the tabs several times,
which seemed to free up the movement nicely.
I suspect there might have been a little
burr or other irregularity on one of the
tabs that prevented the pivot arm from
successfully moving through its full range.
The failure probably started to appear when
the pivot arm eventually walked a few
millimeters away from its fully seated
position and got hung up on that rough spot.
At any rate, the relay has worked flawlessly
since the summer of 2014 so I consider the
problem to be fully resolved. If you run
into TX/RX transition problems with your
open-frame relay and want to try this "fix"
just be careful to avoid stretching the
return spring excessively as you slide the
pivot arm on and off the tabs.