22 Apr
2009
22 Apr
'09
8:07 p.m.
On Tue, 2009-04-21 at 20:04 +0100, Tony Duell wrote:
Ah yes.. I rememeber those . There were various 'Great Conversions of
Semiconductors' to replace them with a bipolar chopper transistor like a
BU208A.
The PSU is, of course, a switching regulator run
from a mains-frequency
step-down transformer. Strangely there's no crowbar circuit. If that
chopper transistor shorts, the 5V line leaps to about 30V with fatal
effects on all the chips. I don't know why HP cut corners in this way.
Shades of the Sony sets of the late 70s to mid 80s with the GCS power
supplies (Gate Controlled Switch, or Ghastly Catastrophic Semiconductor, depending on who you ask). Oh, except that clapped
full-wave rectified
mains across all the following regulators when (not if) it failed dead
short. Which promptly failed dead short too, with hilarious
consequences.
This reminds me of the Boschert 2-stage PSUs used in some classic
computer equipment (PERQ1s, for example). The basic topology is that you
rectify and smooth mains (giving about 350V DC) and then ring that down
to about 150V using a non-isolated switching regulator. The output of
that feeds a couple of transistors running as a free-running oscillator
driving the main chopper transformer. The output of that is rectified and
smoothed. Feedback is then applied from the main (often +5V) output to
the 150V regulator. Controlling the output of that indirectly controls
the output voltage of the whole PSU of course.
Now for the nasty failuyre mode. The first chopper transistor (in the
150V regulator) goes short-circuit. So the output of that stage leaps to
about 350V. The second oscillator keeps on running, so all the PSU
outputs jump to over twice what they should be. At this point you hope
the crowbar fires, if notm you have a _lot_ of ICs to replace. If the
crowbar does fire, it shorts one of the outputs of the PSU. This causes
the second oscillaotr circuit to work harder and draw more current. This
is detected by the current limit circuit, which removes the drive from
(you guessed it) the 150V regulator chopper. Unfortunately, that is
shorted, so removing the drive doesn't do a darn thing. The 2 transsitors
in the second oscillator then go short-circuit. Now, across the 350V
rectified mains supply you have some shorted transistors, some windings
(both the inductor of the 150V regulator and the chopper transformer
primary), anf the current sense resistor (say 0.15 Ohms). That resisotr
then unrs out, taking the sense transistor with it. And then the chopper
control IC (a 723 IIRC), a few other transsitors and some small passives
'join the choir invisible'. Oh, and a couple of PCB tracks melt.
Yes, I have seen the aftermath of this. I also had to repair it....
These would come in with the customer saying "It
went bang and then the
picture and sound went off. It's probably just a fuse, it won't be too
Oh, the fuse proaly has failed. But as we all know, 'a transistor
protected by a fast-acting fuse will protect the fuse by lowing first'.
And the same applied to GCS devices.
-tony