Help! TU81 Plus tape drive problems.

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue Aug 17 16:48:10 2004

>
> (I sent this from the wrong email address last night...oops!)
>
> So, I was hoping to find some sort of manual (any kind) for my TU-81+
> via google, but have failed.
>
> So, my TU-81+ doesn't want to power up. I've found the "hidden" power
> switch under the cover that I've flipped back and forth a couple of
> times, I've made sure the 110/220V jumper was set properly (110V), and
> tried different power cords, all to no avail. (I've bypassed the power
> controller in the rack and directly hooked the drive to an outlet.)
>
> The power supply "sounds" like it's getting some power when I plug it
> in, as I hear a small arc, presumably charging up some capacitors.
> However, the fan doesn't start up, and none of the front panel lights.
>
> Does anyone have any suggestions of where to go next? (Tony? :) I
> could just start pulling it apart and going through it, but I prefer to
> have some direction before I do that.

Since you've taken my name in vaio, I'd better reply :-). However, I've
never been inside a TU81+ (or a TU81 for that matter), so this is going
to be somewhat general.

I am going to assume ti's a switch-mode PSU and that it's somewhat
separate from the rest of the machine. In which case I'd start by
indentifying the output connections (it should be fairly easy to find at
least the ground and +5V lines, maybe +/-12V as well). I'd then check
those with a voltemter with the thing powered up -- with SMPSUs if one
output is there and correct, generally the others are OK too, so you
don't need to check all of them, at least not at this stage.

Assuming the PSU is dead, I'd then disconnect the outputs and put dummy
loads (car bulbs, for example) on the main outputs. If it works then,
you've got some short or other overlaod in the rest of the machine. I
think this is unlikely, since that normally causes the PSU to 'tweet' as
it starts up and shuts down repeatedly.

OK, now we know there's a real PSU problem. Pull the plug, and check the
voltage across the mains smoothinc capacitors (these will be large-ish
electrolytics, around 200V rating). If the chopper isn't running but
there are no other problems, it's not unheard-of for these to remain
charged, and they _will_ throw you across the room, even with the thing
unplugged. However, if they are charged, then it means that the mains
input circuit is working.

Most likely the input circuit, in 110V mode, is a voltage doubler using
the 2 caps in series and 2 diodes from a bridge rectifier (either 4
discrete diodes or a bridge rectifier module). You should be able to
indentify the +ve and -ve ouputs of this circuit. Clip a voltmeter across
these points (don't hand-hold the probes, you are working with rectified
live mains which _will_ kill you) and power up. You're looking for about
350V DC here.

If that's working, unplug again, and let things discharge. Now identify
the chopper transistor (a power transistor or mosfet on the mains side of
the PSU) and test it. Maybe it's open (unlikely).

Then look for a resistor of a few hundred k ohms (or a couple in series)
from the +ve side of the output of the doubler to some point in the
choppper control circuit. Check it. THis is the startup resistor and
often causes the problem you're having. If it's open, replace it with a
resistor that will stand a few hundred volts (many small resistors won't,
take care).

Then come back here and let us (me?) know what you've found.

-tony
Received on Tue Aug 17 2004 - 16:48:10 BST