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.
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 loop is intended to stabilise 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.
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 lock-up?
73,
Ian ZS6BTE
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