21 Apr
2009
21 Apr
'09
8:28 p.m.
On Tue, 2009-04-21 at 20:04 +0100, Tony Duell wrote:
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.
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
expensive will it?" - and all the semiconductors inside are now black
sand...
Gordon