Hi Ian,
-
:From: Ian Roberts <ITR@Nanoteq.co.za>
:This is a 13.8 V 100 Watt 70 cm solid state linear
:acquired new over the weekend.
:The device is most reluctant to drop the rx/tx and bias
:relays after the exciter has been switched to RX, and
:continues to produce around 5 W output. Switching
the exciter (ICOM IC490) off usually cuts the amp's
:output and it drops the bias voltage, etc.
-
It's got a problem, maybe more than one.
-
:The user "manual" mentions that for ATV use feedback
:loops consisting of a cap and resistor between the
:collector and base of the push-pull MRF648s should
:be removed as this will suppress the sync performance.
-
This is true. Video Signals are a whole different animal
through amplifiers should one desire true reproduction
of the original drive. Many ATV sites are on the web if you
care to read up on their amplifier designs. I will assume
you don't want to use it for ATV.
-
:This loop is intended to stabilize the amp against low frequency
:oscillations. From this I assume that the amp is potentially unstable.
:The problem can be partially solved by setting the "SSB delay" to
:minimum so that the SSB hang time is shorter, but when the amp is hot
even this does not work all the time.
-
The problem needs to be fixed, not worked around or patched.
-
:It seems that the RF-derived PTT circuit, two diodes charging a cap at
:the base of a Darlington transistor, is too sensitive and is responding
:to noise within the amp's case. However, there are no settings for this
:threshold and in any case the input to this circuit is via series "stray
C".
:I am thinking of placing some "C" across the PTT input to partially
:bypass this input so that the amp requires more RF drive to go TX. I
:hope that the rx/tx relay activates before the bias relay, otherwise
:every rx/tx cycle is hot-switched. However I cannot hear two relays and
:have a suspicion that both functions are incorporated in one relay....
-
: Anyone have some advice for me regarding the TX lockup?
: 73,
: Ian ZS6BTE
-
Sure Ian,
-
For this example, I will assume your amplifier is a solid
design, just out of "normal alignment."
-
Ensure your DC power supply is a low Z source if possible.
Some of the average bench top DC supplies are not so hot
for solid state amplifier operation. It's just one thing to think
about.
-
Get the data sheet for the transistor off the web, read up on it.
-
To realign the Amplifier. Terminate the amplifier into a metered
quality load, add an rf power attenuator in series with a quality
watt meter (Bird 43 or similar on the input between the exciter
and the attenuator. If possible use the mfgr instructions to
align the amplifier.
-
Below is the rf path for this example:
-
Radio-Bird_Meter-Attenuator-Amplifier-Metered_Load, also
you must meter the power supply voltage and current.
-
The method below should be similar to many mfgrs
instructions.
-
If you don't have the mfgr information, set the exciter to just
below the desired/normal drive level plus the rf attenuator
value. Your trying to drive/setup the amplifier tuning "jig" in
an ideal bench top system, using the attenuator to help
ensure stability and a more true 50 ohm exciter/source.
-
Each tuning should be done as quick as possible using the
proper insulated RF type tuning tools.
-
Key the exciter and quickly tune the input circuit for best exciter
reflected power as shown on the bird 43 before the attenuator.
-
Quickly tune the output cap slightly each direction to see how
the power drops off. Your looking to tune the cap in the direction
where the power output and current drawn increase in a relative
direct proportional amount.
-
At some point near the max power output, current drawn by the
circuit will increase with only a very slight increase in power out.
This is the threshold your hope to find. You would like to set the
amplifier output tuning to the point where that current/power
departure threshold is.
-
You bring the output tuning cap up to the point where the power
and current draw start to depart from each other and stop before
the actual departure. The departure is described as the point
where the rf output rise with current starts to increase in value,
much less than the continuing and increasing current draw.
-
If the amplifier is a quality stable design, your problems
should hopefully be gone. It might not hurt to add some
rf bypass caps to the main DC and bias sources. One would
hope that you have a friend with a two-way radio communications
service monitor which includes a spectrum analyzer. Try to
get someone to look at it in operation.
-
Always get a few opinions of any information you receive,
including what I write here. The procedure I describe was
developed using information from standard amplifier tuning
methods which tend to be similar for a lot of mfgrs.
-
Your results may vary. If you have problems with the tr relay
after that, the amplifier for sure needs a lot more work.
-
cheers
skipp
nospam4me@juno.com
-
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