Meat Plow wrote:

On Mon, 06 Sep 2010 14:13:08 -0700, whit3rd wrote:

On Sep 6, 6:57Â*am, Richard Rasker wrote:

I've been asked to fix a special piece of lab equipment, a
Hewlett-Packard 214B Pulse Generator, dating back from 1980 or
thereabouts.

The problem is relatively straightforward: a chain of components in the
-155V supply has blown:
* PCB fuse F602, value unknown
* Q601 and Q602 (MJ15003)

Since the fuse (F602) has literally exploded, there's no way to
determine its original current rating.

Look for labels, or silk-screened indications of the fuse type and
rating...
you never know.
It might be possible to analyze the exploded bits, of course. Collect
them all!

If it helps, MJ15003 is rated for 20A, max. The safe-operating-area
limit is a polygon on log(I)/log(V) paper with nodes at
[ 20A, 10V], [5A, 50V], [0.5A, 140V].


I've repaired some old SW radio gear where it was impossible to get
technical data. That never stopped me though. I understood the circuits,
mixers, IF, Hartley osc etc.. and applied what I knew typical values
existed for the circuit to work. Similar to what you suggest. Same with
more recent mosfet power amp from the 80's. Sound Code Systems or SCS.
Couldn't find at the time (too early for the primitive internet search
engines) any service errata. But I knew the parameters of the devices
and what they should spec and effected a repair. The amp is still in
service here. If the OP has a couple blown MJ's and a toasted fuse that
shouldn't require scientific intervention.

I'm legally responsible for whatever goes wrong (further damage or injury)
if I put in the wrong parts, most notably fuses. Furthermore, this thing is
used in a university laboratory by quite a number of people, so I really
need to get it right. So in this particular case, guesswork is out of the
question (but if I had to guess, I'd say that the fuse should be something
between 1 and 4 amps, and the tubes look quite similar to PL519's).

Richard Rasker
--
http://www.linetec.nl