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Old 3rd Jun 2016, 10:38 am   #1
Dangerous's Avatar
Join Date: Jun 2016
Location: Mansfield, Nottinghamshire, UK.
Posts: 2
Unhappy Help for Marconi 2383

Hi, Can anyone help me out here?
I have a Marconi 2383 in bits at present and I have traced a fault to the 1st 4.4GHz amp in the receiver module. There is no gate bias at all and still no drop across the drain resistors. The other identical modules in the unit (AL1) show a bias of about -1.4 v and drain voltage of about 8V. This is all regulated by a clever circuit where the drain current is monitored by an op amp and long tailed pair. Rising drain current biases the stage on, falling drain current turns it off which appears to concur with my findings. In addition the other two AL1 amplifiers show about 15 Ohms to ground, whilst this unit shows open-circuit.
It has been intermittent for years on calibration, but always started eventually. Then last week the Cal trace was well down and finally vanished. There is no 502.6MHz IF output and the tracking generator does not work. This seems to point to the lack of 4.4GHz drive which is common to both sections.
Does anyone have a spare receiver unit they could sell me, even if it is not working, or maybe the module itself, which looks fun to get out, but worth a try at replacing.
The FET claims to be HMF0300, but is buried under a glued on plastic cover. Not sure if the device is a die only thing or SMD. It must be good to work at 4.4GHz, especially in 1986 when the unit was built!
I also need a rigid coax lead, N type (50 Ohms) to SMA for the tracking generator to front panel connection which is missing.
I have (almost) full manuals for the 2380, 2382 and 2383, and can provide copies on CD if anyone else needs them.
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