[Premium-Rx] RA3701 Sub-Octave Filter issue

[Ian Ropper]

Hi guys,

 

I posted this question a few months ago, but I am posting it again, having
done some extensive investigation/debugging.

 

I have a couple of ST86496 Sub-Octave Filter modules for the Racal RA3701.
My problem is that try as I may, I cannot get S-O modules to work within any
of the three RA3701s that I currently have.

 

Essentially both modules, when enable, introduce about 50dB of attenuation
at 1.5MHz, deceasing to about 30dB at 30MHz.

However, both modules pass BITE ... so what is going on?

 

I have ascertained that something is causing the module to mute when
enabled.

The S-O module will respond to an RX MUTE by shorting Pin 14 of SK1 to 0V at
the rear of Processor. When this is done, bith TP2 and TP4 on the S-O module
go high. This energises RL28 and RL27/RL33 respectively.

RL28 disconnects the input of the attenuator section from the from the
aerial and connects it to the internal noise generator, while RL27 and RL33
essentially select the internal attenuator, even if it is set ot 0dB.

However LK1 on the S-O module can be removed to prevent the module
responding to this particular MUTE command. I have removed the link to no
avail.

 

There is however a second module MUTE signal and this is the culprit. This
comes via bit-5 on the Module Data Bus and goes (eventually) to the same
OR-Gate that the above mentioned MUTE goes to. When the S-O module is
enabled via the front panel, a logic-1 is latched through to the output of
ML4 pin 14 and the module is muted as above.

 

I may be clutching at straws here, but initially I thought this might be a
firmware problem coupled with a legacy from the real-time continuous BITE
monitoring since the inputs to ML4 are not static. Note: the Module Data Bus
is common to all modules. However, the two S-O Modules that I have came with
two of the RA3701s that I have ... so I have to conclude that the firmware
in the Processors is compatible.

 

Is there anyone out there who recognises my problem and knows how to resolve
it?

 

Thanks in advance,

 

Ian, GM0UHC

www.chavfreezone.me.uk