Joe McElvenney wrote:
Hi,

I been beating my brains out for the last few days
trying to run down a problem on a 20-year old Icom-740
transceiver with not a lot of luck. Perhaps someone here in
the group has a trick or two up their sleeve to get me out
of the hole I've been digging myself?

The situation is this -

The rig won't receive but transmits OK and there are no
other fault indications.

The apparent cause is the regulator for the 8V receive
line shutting down due to current overload (i.e. more than
100mA which is the limit of the MB3756). At present it is
running at about 3.3V with around an 80mA load (lower than
100mA but the foldback circuit on the chip is operating).

All other supplies are normal; in fact the other two
supplies out of the MB3756 are on the button and I have
changed anyway it just in case. If I isolate the line and
apply an external 8V the current goes to over 400mA in a
manner that suggests there is a semiconductor junction or
two in the fault path.

As these rigs are a mass of interconnections (the main
board alone has 25 plugs/sockets on it), normal fault-
finding procedure would be to monitor the current and yank
the plugs one by one until the excess load disappears and
then to have at it with a VOM. Unfortunately the system of
on-board links that Icom use makes this a more difficult
task than you would think. Over the last few days I've been
sure I'd found the leak several times but was badly
mistaken on each occasion. Of course, the fault may be an
above ground supply line to supply line leak which wouldn't
show up as an earth leakage path.

The tool I really need is a miniature Hall effect DC
amp-clamp but there doesn't seem to be one available. In
the past I've used a hp current tracer on PCB tracks but
even if I had one now it wouldn't be easy to use here.
However, I do have a bulky hp-1110A (2mV/mA) scope current
probe and a 10526T pulser, so tomorrow I'll try that route
with or without the power on.

Sorry about the windy posting friends but if anyone has
any idea on ways to trace wayward current leaks, please let
me know.

I had a similar problem with a shorted supply rail in an IC-746.

Since it was more accurately a *near* short, and the voltage regulator
was pushing out quite a large current, there were actually several
millivolts all along that rail. By using the DVM on its very lowest
range, and taking the least significant digits seriously, it proved
possible to trace the voltage gradient downward to the short.

The fault is worth mentioning too. It was one of those Icom ribbon
jumpers, that I had inserted slightly off-centre in its slot. The
surprising thing was that the transceiver had worked OK for about a
month... but all that time, the ribbon had been creeping slowly
sideways... until finally it had moved far enough to cause a short.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek