On Wed, 27 Jul 2011, JIMMIE wrote:

On Jul 27, 7:21*am, Scott wrote:
On 7-26-2011 20:34, JIMMIE wrote:





30 year old piece of equipment operating pretty much flawlessly for
that entire period. VCC and VEE on the 1448 is +/- 15 volts and the
chip is overheating..... even with inputs and outputs disconnected the
chip still overheats(130 F). I noticed that the manufacturer's sets
the MAX *power supply voltages at +/- 15 and understand this was
probably designed on the hairy edge of failure but like I said its
been working for 30 years and replacement chips work no better, what
gives. Power supply can not be adjusted and there are other chips on
the board that require the +- 15. d BTW cooling the chip makes it
work. operating the chip from an external +- 12V power supply makes it
work but modifying the circuit may not be an option. I saw where there
are some CMOS versions of the 1488, I was wondering if they would
handle the +- 15 volts better.

TIA
Jimmie

Can you cut the circuit trace of the voltages (Vcc, Vee) feeding the IC
and strap in a pair of diodes (in series) such as IN914 types in each
supply line? *Each diode drops about .7V, so you would end up with about
+/- 13.5V at the IC. *If you want even lower voltage at the IC, add in
another diode or two in each supply line. *Ensure proper polarity...in
the Vcc supply lines, the diode cathodes would go toward the IC and the
Vee diodes would have the anodes toward the IC. *Just a thought. *Or, if
you know how much current the device draws through each supply (Vcc and
Vee), you could place a single zener diode in series with the supply
voltages to the IC and the polarities would be opposite of using 1N914
diodes. *Just find a zener of about 2.5-3V that is rated at about twice
the current the IC draws from each supply.

N0EDV- Hide quoted text -

- Show quoted text -

Thanks Scott, that was my first thought but I am not allowed to
modify the board... I was hoping the CMOS version of the old chip may
work at the elevated voltage level. Im thinking the one that has
worked all these years was a cherry picked mil spec device, its PN is
751448

So?

It's a line driver, not ttl. It would start with "75" from some
manufacturers.

If it's worked all this time, then look at it from a different angle. The
IC has gone bad in such a fashion that it works at lower voltages.
Replace the IC, and likely it will go back to working. The heat issue may
be that it's bad, not because it's receiving too much voltage.

After thirty years, don't suddenly think the issue is too much voltage.

If the IC was working and doens't now, then I wouldn't trust it even at
lower voltages. But if you're fussing over supply voltage, then reveal
what else uses the +-15v lines, chances are good they aren't fussy either
(whereas the voltages to the line driver define the RS-232 levels and
someone may have deliberately chosen to go for the maximum RS-232 voltages
rather than lower voltages that may not be so obviously high and low.

Michael VE2BVW