31 Mar
2004
31 Mar
'04
5:18 a.m.
Does anyone have any suggestions about debugging a TU58?
This is a dual TU58 from a vax 730. The symptom is that when I try to
boot from it the tape rewinds a bunch of times and then reads off the
end of the tape.
I removed it and poked all around with a scope and the schematics. The
motor control is fine as well as the tachometer. The read channel shows
no activity, even when I put a tape in and move the tape. The micro
seems happy.
I connected it to "tu58ctl" and I can talk to it and send/receive
commands. But when I try to position the tape I get a "motor stopped"
error.
I'm begining to wonder if the 3 TU58 carts I have have been bulk erased.
I have no "known goods" and I've never read these tapes. I made the
assumption they were good but that might be a mistake.
(wish I had those mythical proms which can write carts :-)
I put a scope on the signal from the read heads and I see nothing when
the tape spins by. I find that very odd. I would think I'd see pulses
even if the blocks where all zeros... So nothing makes me think the
tapes have been bulk erased.
Is there any way I can test the read channel without having a good tape?
(I assume not, but I know very little about tape drives, perhaps there
is some 'technique').
I know the tapes are useless if they have be bulk erased - but will the
symptom be no read channel activity? (I assume so).
blah. if I've wasted all this time on bulk erased tapes %$#%@#@!#$
-brad
31 Mar
31 Mar
6:03 a.m.
On Wed, 31 Mar 2004, Brad Parker wrote:
Is there any way I can test the read channel without having a good tape?
Using a known non-magnetized small screwdriver (or a freshly-degaussed
one), observe the head output with the scope, and move the tip of the
screwdriver blade very close to the head face. If you want, you can wrap
a bit of cellophane tape (1 thihkness max) around the tip to keep from
scratching anything. You should see some disturbances on the scope, if
you are actually watching the raw analog sigs from the head... or you may
'fool' the electronics into thinking that it's read a bit... anyway, this
can give you an indication that the head is good... beware using an
ohmeter on magnetic heads, unless you have a degausser for same and use it
afterwards - the DC voltage from the ohmeter will magnetize the head, and
this is a Very Bad Thing for subsequent tapes.
Cheers
John
10:12 a.m.
can give you an indication that the head is good...
beware using an
ohmeter on magnetic heads, unless you have a degausser for same and use it
afterwards - the DC voltage from the ohmeter will magnetize the head, and
this is a Very Bad Thing for subsequent tapes.
Is this still a problem with modern digital ohmmeters? It certainly was a
problem with analogue ones. I once went on a merry dance finding why a
Sony reel-to-reel VTR was giving low head output, it turned out I'd
magnetised the heads while checking them for continuity. Not enough to
damage tapes, but enough to upset their magnetic characteristics...
-tony
10:09 a.m.
Does anyone have any suggestions about debugging a TU58?
This is a dual TU58 from a vax 730. The symptom is that when I try to
boot from it the tape rewinds a bunch of times and then reads off the
end of the tape.
I removed it and poked all around with a scope and the schematics. The
motor control is fine as well as the tachometer. The read channel shows
no activity, even when I put a tape in and move the tape. The micro
seems happy.
I asusme both drives behave identically. That would suggest that the
heads were both OK (it's unlikely they've both failed at the same time).
I connected it to "tu58ctl" and I can talk to it and send/receive
commands. But when I try to position the tape I get a "motor stopped"
error.
I'm begining to wonder if the 3 TU58 carts I have have been bulk erased.
I have no "known goods" and I've never read these tapes. I made the
assumption they were good but that might be a mistake.
That, alas, is the most likely explanation, I think...
(wish I had those mythical proms which can write carts :-)
I put a scope on the signal from the read heads and I see nothing when
the tape spins by. I find that very odd. I would think I'd see pulses
even if the blocks where all zeros... So nothing makes me think the
tapes have been bulk erased.
Do you have any other tapes of the same form factor (say an HP85, or an
HP9825, tape)? If so, try those and look at the head signal with a 'scope
(it'll be in the millivolt range, of course). If you get something with
those tapes, but not with the TU58s, it's a fair bet they've been bulk
erased.
Is there any way I can test the read channel without having a good tape?
Do you have a tape head demagnetiser? If so, use it to _carefully_ inject
a 50/60Hz field into the head. Look at the output of the head with a
'scope. You might be able to see the signal at the output of the
amplifier stages too, it depends on what, if any, filtering is present.
-tony
7571
days inactive
7571
days old
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ard＠p850ug1.demon.co.uk -
brad＠heeltoe.com -
jpl15＠panix.com