[AMPS] TX lock-up on Mirage D-1010-N

[Ian Roberts]

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




--
FAQ on WWW:               http://www.contesting.com/FAQ/amps
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com