Electronics > Projects, Designs, and Technical Stuff
Replacing 74xxS logic
(1/2) > >>
Jester:
I have an old school Tektronix TM503 timer module that comes in handy once in a while for long duration timing (like an hour or a day). Uses a bunch of 74xxS logic and it runs quite hot. The original "S" parts are pretty power hungry.

What series would be the best to replace these with?

LS will only sink 8mA vs. 20mA for the "S", so this may or may not be a problem depending on the particular circuit (fanout)

ACT have good sink and source capability, improved logic levels and similar propagation delay, and they are also inexpensive.
schmitt trigger:
I you replace *all* of the ICs from 74S to 74ACT, the respective input currents will also be reduced, and therefore the the 74ACT's lowered sink current should suffice.

Having said this.....Consult the datasheet that the reduced input current is proportional to the reduced output drive.

EDIT: I would assume that the original components are all thru hole. Will you obtain all the replacements in thru hole also?

David Hess:
I have done the same thing to a number of my TM500 series instruments.  FACT or ACT has about twice the delay of schottky TTL.  AS (advanced schottky) is a tiny bit faster at half the power and FAST (fast advanced schottky TTL) is a tiny bit slower at 1/4 the power.
SeanB:
Most of the time you can just replace S with LS, though the only times you get issues are things that do odd things, like having RC networks to do delays, or oscillators. If you do all at one there is no problem with fan outs or fan in, as the families all have the same fan out and fan in irrespective of family, provided you stay in the family. ACT will also work, just you might need to buy a good number of DIL to SMT adaptor boards, a few dozen leaded components to use as lead donors, and a few hours of soldering to get them all done.
Jester:

--- Quote from: David Hess on April 09, 2019, 05:22:42 pm ---I have done the same thing to a number of my TM500 series instruments.  FACT or ACT has about twice the delay of schottky TTL.  AS (advanced schottky) is a tiny bit faster at half the power and FAST(fast advanced schottky TTL) is a tiny bit slower at 1/4 the power.

--- End quote ---
David,

Do you recall what series you used?

Mostly socketed DIP packages in my case. I might just swap out what ever is available in DIP and see what happens. Hopefully the temperature drops a bit, the power supply relaxes and everything keeps working for a few more decades;-)
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod