Re: Great Design

alchazz wrote:
On Sat, 25 Apr 2009 12:20:55 -0700, Tim Wescott wrote:

alchazz wrote:
I was amazed the other day. At my town's recycling center, I found two
variable power supplies made by HP, model 721A. I cleaned them up,
plugged them one at a time into a fused extension and turned them on.
Voila, they both worked!

The date code on the PCB is 12-15-59. These beauts are ~50 years old
and work like a charm. I tested them throughout their operating range,
0-30VDC, 0-225MA.

The pass transistor is a 2N375, with a P(max) of 45W and an Ic(max) of
3A. I wonder if it is germanium? I also wonder what the predicted MTBF
was of these guys? I know the current ones are targeted to 3yrs.
How you run a bit of power semiconductor has a huge effect on the MTBF
(more properly the MTTF) -- derate it so that it runs cooler, and it'll
last a lot longer.

The transistor is rated at 3A max. In this application it was current limited to 225mA. The heatsink is the entire chassis. What is more amazing is that the large filter capacitor has not failed. It may not be effective. I'll have to check the ripple later to see what's what. And that one is easily replaced.

Last year I needed another audio signal generator and retrieved a HP 200AB from my garage, (SN 1651). It was one of the first HP instruments, introduced in 1952 to replace the older 200A and 200B. It had been unused for over fifty years. I was worried about the electrolytic capacitors, and debated about starting it on a Variac to "re-form" them if needed. Running the rectifier at low filament voltage might cause more damage. Instead, I just plugged it into 120 V AC, after replacing the line cord, on which the rubber had deteriorated. (LA smog!?)

It worked perfectly; even the calibration was within a couple of cycles at the 60Hz where I checked it. There's no 60Hz hum in the output, so the filter caps most be OK!

They don't make 'em like they used to!

--
Virg Wall, P.E.
.

Relevant Pages

  • Re: Great Design
    ... variable power supplies made by HP, ... Voila, they both worked! ... I also wonder what the predicted MTBF ... The transistor is rated at 3A max. ...
    (sci.electronics.design)
  • Re: Great Design
    ... variable power supplies made by HP, ... I also wonder what the predicted MTBF was of these guys? ... Wescott Design Services ... "Applied Control Theory for Embedded Systems" was written for you. ...
    (sci.electronics.design)