Great Design

[alchazz]

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.

Nice.

Al

 

[Jean-Christophe]

The pass transistor is a 2N375, with a P(max) of 45W
and an Ic(max) of 3A.
I wonder if it is germanium?

Yes it is : PNP (Ge)

I also wonder what the predicted MTBF was of these guys?
I know the current ones are targeted to 3yrs.

3 year MTBF doesn't mean that it *will* fail in 3 years !

Nice story, anyway.

 

[alchazz]

Yes it is : PNP (Ge)


3 year MTBF doesn't mean that it *will* fail in 3 years !

Nice story, anyway.

That's right. It could be much less than 3 years. ;-)

Al

 

[Jean-Christophe]

That's right. It could be much less than 3 years.  ;-)

Could be.

I guess these units were stored for a long time before
someone found them and decided to threw them away ?

I don't believe they would function continuously at 45W for 50 years.

 

[Rich Grise]

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.

Nice.

225 mA? (I know you didn't mean 225,000 amps!) Maybe they were
insufficient for the previous owner's needs, or maybe he died.
(I once got a spectacular deal on a one-owner car whose owner had
presumably died. It was about 15 years old, and almost cherry.)

Cheers!
Rich

 

[alchazz]

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.

Al

 

[Nobody]

I don't believe they would function continuously at 45W for 50 years.

A lot of modern PC PSUs wouldn't last 50 weeks sitting on the shelf before
the capacitors start leaking.

 

[Nutz]

225 mA? (I know you didn't mean 225,000 amps!) Maybe they were
insufficient for the previous owner's needs, or maybe he died.
(I once got a spectacular deal on a one-owner car whose owner had
presumably died. It was about 15 years old, and almost cherry.)

Cheers!
Rich

..

I hope it didn't smell strongly of industrial cleaners /
deodorizers................

 

[Rich Grise]

I hope it didn't smell strongly of industrial cleaners /
deodorizers................

I didn't say the guy died INSIDE of it; it had been parked in his driveway
for some time; every little nook and cranny on the outside was littered
with tree debris, notably the heater/defroster air intake and the gap
between the body and bumpers.

But it had that old-guy green plastic thing across the top of the
windshield, and several VFW-type stickers in the back window. And the
brake pads were so worn that one of the front disks had a huge gouge, and
the guy had parked it on a hill, using only the parking gear pawl, like
he'd never even used the parking brake.

I drove it from CA to MN one time, with 20-50 oil, in the winter. The
next morning, it wouldn't even crank - it was like tar in the crankcase.
So, I filled it with 10-40 and drove it for some years until it threw
the timing chain. I sold it to some salvage guy for less than the price
of its new tires. (I had finally got the front end tie rods, A-arm
bushings, Pitman arm, and idler arm all fixed so it had stopped eating
tires.) )-;

Thanks
Rich