DECstation 220 - Out of Ideas?

Pete Turnbull pete at dunnington.plus.com
Sun Jun 25 10:11:59 CDT 2017


On 25/06/2017 11:46, Rob Jarratt via cctalk wrote:

> I have done a bit more tracing of components and have come across a
> possibly anomaly. The signal that won't go low enough goes to the
> input (pin 2) of a 74LS125 buffer.

As soon as I saw "74LS125" I thought, "Oh, yes, another one".  That's a 
fairly early design tri-state buffer, and in my experience they're not 
very robust.  We used to replace lots of those, though admittedly I'm 
referring to the DIL version.  Worth checking it out.

If you can't desolder all the SMD pins, try this trick: for each side, 
thread a piece of enamelled copper wire under the chip near the pins. 
Tie or wrap one one to some nearby component and hold the other end in 
your hand.  Starting at the end with the wire you're holding, heat each 
pin with your soldering iron, and drag the wire under it, thereby 
breaking its contact with the board.

Failing that, apply a tiny amount of flux to the pins, and heat it up 
with a hot air gun, preferably a temperature-controlled one, and lift it 
off with forceps (or flip it over with a small screwdriver).

-- 
Pete
Pete Turnbull


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