7 May
2011
7 May
'11
10:58 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Brent Hilpert
Sent: 07 May 2011 18:02
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Desoldering a chopper transistor
easier
to
I need to desolder a chopper transistor from a
PCB in the PSU of my
PDP11 so
that I can test it and if necessary replace it.
As you are just trying to isolate the device for testing, sometimes it is open the circuits elsewhere to achieve the isolation.
For example, if the
collector goes to an offboard transformer through a connector, then that
lead
is easily isolated; the base is probably fed by a
resistor and lifting one
lead of
the resistor will isolate the base. With 2 of the 3
connections to the
transistor
open, it is isolated for testing purposes.
TO-220 packages (3 leads with tab) can be very difficult to remove in one
piece when - as Pete was mentioning - the holes are to small. Some
manufacturers fail to increase the drill size for the wider TO-220 pins,
they
can be very tight with no room to wiggle the lead to
break the final
solder
bridge. Sometimes slightly twisting the lead might do
it. Sometimes you
have
to rock the device up and out of the holes with
several heating cycles.
It is indeed a TO220 and I do need to test it. After sending my request I
had an idea and took the board to a local jeweller, he was able to remove it
for me by using a very fine drill to drill out the just the pins themselves.
The contacts look to be intact and even if they are not I will be able to
use wires to reconnect the transistor pins.
Thanks
Rob