11 Feb
2011
11 Feb
'11
12:53 a.m.
It could be the chip, certainly, I doubt it's the
windings. IIRC the
widings are connected in a 'star' ('Wye') configuration to the 3
outputs,
see the HP drive schemcatics I pointed you to for details. If I cam
right, you could check the resistance between the 3 motor driver ouptus
(6,7,9) with the drive power remoced, they should all appaer to be
shorted to each other.
Tony I've looked through past emails and can't seem to find those schematics. Can
you point me to them again?
The motor driver output pins are not QUITE shorted. There is about 25 ohms resistance
showing which flickers between 20 and 30 as I turn the wheel? Normal?
However, it's also ppossibloe that one of the hall
sensors has failed.
Theseare little 4 terminal parts that detelct the position of the motor
and cause the chip to drive the rigth coils. IIRC, they are connected to
pins 15,16,1,2,3,4[1] of the chip. With the motor supposed to be stopped,
connect a voltbeeter between grounf and each of those pins (if I have
rememebred them right, checkl the data sheet) in turn and slowly turn the
spidnle. You should see a samll change in voltage for each pin, it shoud
'flick' up and down as you turn the spindle. If one or two are not doing
this, suspect the hall device.
I've found them. On this TA7259P they are pins 1-4 and 13, 14. The voltages do show
a variation as the spindle turns. But it's very small. About 0.03 volts and that
variation is consistant over all 6 of them. The voltages differ. Pins 1 and 2 are 6.5,
pins 3 and 4 are 4.3 and pins 13 and 14 are 8.4v
[1] Be careful with the pin mubmbering if you are
working from one of my
HP drive scheramtics. I said the chip has 14 pins and 2 tabs. Toshiba say
th chip has 16 pins, including the tabs. So the pin numbers are
different.
The chip I have has 14 pins.
I notce that one of the screews was badly corroded. Is
there any damage
to the PCB tracks in that area? An open connection could account for your
problems.
The screw is rusty but the PCB tracks are actually in very good condition. There are no
shorts.
One question for you. Where does the white wire from
thr motor connector
go? Is it soldered to some point o nthe bototm of the PCB, if so, where?
No not soldered to the board. The white wire joins the other wires from the two motor
board connectors and goes into an 8-pin socket which plugs into the top of the bottom
circuit board. The pins for this can be seen standing upright on the mid-bottom left on
the board:
http://www.classic-computers.org.nz/temp/lisa2-10-bottom-of-drive-circuit-b…
The white wire goes to pin 8 which is the pin nearest the bottom (i.e south) of the
picture.
Well, what do you think Tony? I've found a place in HongKong I can get these TA 7259P
ICs for $20 (incl shipping). The IC or the hall sensors? Are there any other tests I can
do?
I appreciate the detailed help you're giving me here.
Terry
