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Fw: TX Problem IC970A 2.4 GHZ module (forward from ICOM group)

W7FG Vintage Manuals has reprints of the UX98 and UX97 schematics. Unfortunately no theory.

The UX98 is a 1 watt unit. Be sure the wattmeter is accurate to 2.4 GHz at low power levels.
There should be a blue "plastic" heat spreader between the module body and the rear heatsink of the radio, likewise about 4 mounting screws drawing the heatsink, blue spreader and module together. I am not sure the duty cycle of the UX98. 2 to five minutes might be a bit severe.

This being said, although there is a forward power sensor, there does not appear to be a reverse power or thermal sensor so the ALC re-action is a bit unusual and may indicate a problem elswhere in the module.

My primary UX98 was a bit sluggish to come up to power first time used after being out of service when I first bought it. I suspect the T/R relay contacts had some tarnish.

My UX97 module had a difficult to trace intermittent which I found was a regulator pass transistor on the casting having a cold solder joint. I found this by visual inspection.

Th UX98 output transistors are FET's which I believe are made by Mitsubishi. There are power control, bias and several jumpers on the board. The manual gives no advice on adjustment. I would consult with the device manufacturer before "tweaking" or damage might occur. You might check the AMSAT-BB about this module and the devices in the PA. I beleive the final devices have "flown" on an AMSAT satellite. So somebody may be familiar with them.

Someone mentioned touching the components and looking for hot spots. A CCD camcorder with the IR filter removed might give you a fast indication of the hot spot. 

Because the unit takes a long time to recover, I suspect the problem area has a lot of thermal mass, so check the regulator transistors as well as the PA.

Please have your friend keep me and the AMSAT-BB posted on his findings as there is little in the way of a support knowledge base for this rare and valuable equipment.

Joe Leikhim K4SAT

Message: 1         
   Date: Wed, 2 Mar 2005 05:51:53 +1300
   From: "NICK.WALLACE" <NICK.WALLACE@xtra.co.nz>
Subject: Fw: TX Problem IC970A 2.4 GHZ module

Subject: TX Problem IC970A 2.4 GHZ module

Hi All,

I am posting this on behalf of Doug VK4OE. We were wondering if anybody else had encountered the same problem which is as follows. I will pass any info received onto him.

Thanks in anticipation.

Nick   ZL1IU

>From Doug.....

Just to save you some typing, I'll describe again what I am
experiencing, which only relates to the 2.4 GHz module in transmit
mode....the other features of this excellent radio are currently working

The problem is one of "fading" transmitter power as something obviously
is heating up in the 2.4 GHz transmit path.  The scenario is that the
radio has been switched off for a time long enough for it to cool to
room temperature internally, then it is powered up and switched to 2.4
GHz transmit mode (continuous, e.g. FM).  The RF output control is set
to maximum.

It transmits the full 1 Watt for 2 to 3 minutes (or 5 minutes if ambient
temperature is cooler) then, over a period of about a minute, the
transmitter power gradually falls away to around 50 mW.  Whilst this is
happening the ALC gradually climbs to a high level.

Momentarily switching to RX or switching the whole radio off briefly
then on again do not lead to a return to 1 W output.  Only powering off
and letting the whole radio cool for an hour or so leads to a return to
1 W output, and being ready for the whole fault sequence to occur again.

It is conjectured that there is something wrong with one or more
component in the RF path between the TX amplifier and the antenna
connector, something which is thermally active.  Has anyone else had a
similar problem and found the solution?

Thank you
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