7 Nov
2010
7 Nov
'10
12:04 p.m.
On 06/11/10 20:03, Tony Duell wrote:
I suspect, and suspected at the time, that you areperfectly correct :-)
Before we get another flamewar, I will point out that just about every
seller I've dealt with at radio rallies has been extremely honest. If
something doesn't work, they say so. If it's missing important parts,
they say that too.
I feel that the enthusiast-type radio amateurs are more likely to be
totally honest than the traders (the former don't want to cheat a fellow
ham), but to be fair I've never had what I consider a bad deal from any
of them.
Aha...This remionds me ao a character I saw at a
couple of radio rallies
many yearsa ago. He had a pile of working hard drives. And another pile
of ''untensted' hard drives. Hmmm...
Read as: "I've tested all of these, but I'm selling the broken ones as
'untested'." Then you get the fiercely honest ones -- I approached
a seller at a
radio rally, and asked about some VRLA batteries he was selling.
"Ah, they're only a couple of months old -- brand new. Serial number's
here, and manufacture date here. Charged them up half-way the other day,
hang on, I've got a multimeter here somewhere.... Oh, and I'll wire a
car headlamp over one if you'd like to see it running under load..."
I've met plenty of sellers who are happy to let you remove covers and
have a look inside. And I rememebr a chap with a box of assorted valves
who provided a multimeter so you could at least check if the heater was
OK.
I wonder if theprevious owner plugged them into a
PC power supply and
then discovered that either they didn't work, or somebody told him he'd
probably done some daamge, so you ended up with them
Indeed. I've got a lead on a pair of double-sided drives from a PCW9512.
They're basically the same drive design, but without the write protect
sense pin (optical sensing!), a white front panel and a slightly
different mounting cage.
Fingers crossed...
Indeed.
On the to-do list (it's a bit late to be getting
out the DMM at this
time of day!)
You put the DMM away? That suprises me ;-) I'm seeing around 2V on all three pins of the head
-- 'top', 'bottom'
and the centre tap. If I switch to AC coupling, 200mV/div, I see what
looks like a very noisy sine wave on the centre tap, and then the coils
Trhis is for the selected head? That can't be right!~~~~~~~~~~~~~~~~~~~~
have the same noisy sine, plus an alternating pulse.
If I subtract the
CT from either coil, I can see the magnetic pulses. These look like the
0-180deg (positive going) part of a sinewave, then a gap, then the
180-360deg (negative going) part.
-- In read mode --
Not a sausage at 200mV. Dropping to 50mV/div, I can see a heck of a lot
of noise, but nothing that looks remotely like the 125kHz data I fed it.
There's nothing interesting on TP1, TP2 and TP3 either. With drive stopped:
TP1 -- 258.45kHz sine, ~200mV amplitude
TP2 -- same sine wave, 180 degrees out of phase
TP3 -- ground
Adding the two channels together gets me a flat line with a bit of
noise. Subtracting (per the instructions in the service manual) gets me
a 400mV pk-pk sine...
It's as if the ASIC were oscillating internally...
It may well do this. There's probablhy an AGC circuit that cranks up the
gain if there's no signal from the head.
I asusme the head windings are continuous, and that the heads are
actually touching the disk...
But that head CT signal sounds totally wrong.
-tony