I, too, am one that cleverly managed to cook the '240 chip: Too many
clip leads in too small a space allowed the +15 to get into the lock
indicator pin while I was evaluating the unit. It, too, worked OK after
doing that.
Rather than order another 74ACT240 or even just an HC/T240, I rummaged
around and found a 74LS240 and replaced it, making good use of my hot
air rework tool: Removing the old chip and installing the new one was
far easier than it would have been had it been an un-socketed DIP!
I suppose I should be slightly concerned about mixing logic families,
but I can't seem to get myself too concerned about it: The lock
indicator and 1pps outputs work fine and I doubt that I'd really care
about a few extra nanoseconds here and there and the 'LS doesn't really
hog too much current compared to the entire unit, anyway...
(FWIW, I can see the 1PPS on my ancient '465B scope, but having dim room
lighting and turning up the trace intensity to full brightness helps:
It *really* is hard to see, but it's 1uS wide, just as it should be.)
* * *
On random things, on one of my units it had the section of the board
populated with the +5V switching regulator - but it was not powered up:
A bit of sleuthing around with an ohmmeter and with the help of the data
sheet for the regulator chip revealed which two jumpers needed to be
installed and I was in business. If someone else has the "10 MHz only"
5680A with the un-powered 5 volt switcher on board and wants to know
which jumpers to install, I could make that information available if it
isn't to be found elsewhere.
The other "identical" '5680A unit had that portion of the board
unpopulated, but a few minutes with a soldering iron, 7805, a pair of
0.1uF monolithics and some wire-wrap wire brought that one online as
well with the tab of the 7805 soldered to the ground plane in the area
where the parts weren't and with a bit of heat sink compound on both the
body of the regulator and the same section of the insulating sheet
allows at least some of the heat to be transferred to the bottom panel.
Since then, I've modified another in the same way and also had good results.
I was initially concerned about dumping a couple more watts of heat into
the box, but attached to any semblance of a heat sink, it's no big deal
- but I suppose that the extra 100-ish mA above what a switcher might
pull is a bit annoying... (When warmed up, the steady-state current at
15-18 volts stays constant at 800mA for each of the units with the
in-built 7805.)
73,
Clint
KA7OEI
EB4APL<eb4...@cembreros.jazztel.es> said:
John,
You are not alone in this trip. Another member of this list tells me
that he connected the power to the lock indicator pin when he received
his unit and he was eager to test it. His 74ACT240 also got cooked but
the rest of the unit continued working normally.
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
