8 Apr
2012
8 Apr
'12
11:13 a.m.
Swapping the motor control board did indeed
resolve the problem. So now I
can look at the faulty board and see if one of the power transistors on
there
has failed.
Thanks
Rob
I have been desoldering the power transistors and testing them with a DMM
using the diode tester. There are eight in all, four test OK as two diodes.
However, the other four are marked 8702 TIP125, have a Texas Instruments logo, and do not test as two diodes. Looking at the
datasheet here
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/4128.pdf it
would seem that it is not they are not simple transistors and so perhaps my
measurements are showing them to be working OK. Here is what I measure with
the diode tester:
1st - b-e 1.95V e-b 0.82V b-c OC c-b 0.66V c-e 0.58V e-c OC
2nd - b-e 1.91V e-b 0.83V b-c OC c-b 0.68V c-e 0.58V e-c OC
3rd - b-e 1.90V e-b 0.81V b-c OC c-b 0.67V c-e 0.59V e-c OC
4th - b-e 1.75V e-b 0.75V b-c OC c-b 0.56V c-e 0.58V e-c OC
It looks like the 4th one is slightly different to the other three, but I
have no idea if this is far enough out of tolerance as to cause the motor
not to turn.
It doens;'t look wildly out to me.
I assuem you don;'t ahve a transistor curve tracer (it's not a common
piece of test gear, most of the time hobyists get them becuase somebody
gives one away....).
You might try a simple swtichign trst using this sort of circuit :
1k
+---\/\/------+------------------------o +ve
| |
\ |
\ (X) 12V, 2W (or so)_
o | bulb
| ----- 12V PSU
| | c |
+----------|b | Device under test
| e |
-----
|
+------------------------o -vve
You can build ti o na solderless breadboard, there's nothing critical.
For the 12V pSU, use a bench supply if you have one, or the 12V output of
a PC PSU 9which you know to be working!). The switch could even be a
couple of wirses to toch together, the lamp should light only when the
switch is closed.
-tony