HP 141T power supply question

[Glenn]

I have looked at the power supply schematic for the HP 141T with the
intention of improving it, and found something that is very odd (also
found others - maybe another time):

Old one - "neon tube" stabilized (pdf-page 35):
141T Display.pdf

New one - zenerdiode stabilized (pdf-page 10..12):
141T_changes.pdf

(both downloaded from here: http://bama.edebris.com/manuals/hp/141t/ )

The three lower driver and power series regulator transistor base
resistors - to high voltages - bothers me. I would prefer e.g. 100 ohm
to the emitter.

If the output transistors turn off, just for millisecond, the base is
pulled "downward" to the high voltages. The (-)Vbe should never be
greater than 5 volt for most transistors - even for the high power.
Almost every datasheet tells you so.

-

What is your opinion about this? What was the PS designers thinking? I
know that the instrument was the state of the art. Please help me to
understand this PS design.

Glenn

(the question is also asked in news://sci.electronics.design )

 

[[email protected]]

I have looked at the power supply schematic for the HP 141T with the
intention of improving it, and found something that is very odd (also
found others - maybe another time):

Old one - "neon tube" stabilized (pdf-page 35):
141T Display.pdf

New one - zenerdiode stabilized (pdf-page 10..12):
141T_changes.pdf

(both downloaded from here:http://bama.edebris.com/manuals/hp/141t/)

The three lower driver and power series regulator transistor base
resistors - to high voltages - bothers me. I would prefer e.g. 100 ohm
to the emitter.

If the output transistors turn off, just for millisecond, the base is
pulled "downward" to the high voltages. The (-)Vbe should never be
greater than 5 volt for most transistors - even for the high power.
Almost every datasheet tells you so.

-

What is your opinion about this? What was the PS designers thinking? I
know that the instrument was the state of the art. Please help me to
understand this PS design.

Glenn

(the question is also asked in news://sci.electronics.design )

It looks to me like the Q7-8 diff amp could put -20V Vbe across
Darlington pair Q6-3 under fault conditions (R37 CW leg open). Pretty
unlikely failure mode.

Whether or not they could take it depends on which'un trannysauruses
dems was (is). CR13,14, and 18 suggest the answer was "no."

Cheers,
James Arthur

 

[miso]

Either way, that is an instrument that stood the test of time. I was
given one a few years ago. One of two things I use with the old style
IEC plug.

 

[tm]

I have three of those mainframes.
<http://802.11junk.com/jeffl/pics/home/slides/test-equip-mess.html>
Two HP141T storage scope and one HP140T non-storage. I also know of
four more in use in various homes and labs. They are highly reliable
and not really in need of any improvements.

I'm curious. What problem are you trying to solve by making changes
to the power supply?


--
Jeff Liebermann [email protected]
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Hi Jeff,

The old version supply uses a neon bulb as the reference for the 100 volt
supply. This regulator drifts over time as the bulb ages. It gets to a point
that the supply cannot be adjusted to spec and the special bulbs are hard to
find. The problem gets worse as the 100 volt supply is used as a reference
for the other supplies.

The new design changed to a 9 volt low TC zener that makes things much
better.

HTH,
tm

 

[Glenn]

On 28/04/13 17.51, Jeff Liebermann wrote:
....

You really should post your question in some HP scope forum or
newsgroup as it's really a repair problem, not a design problem.

....

Was done at the same time, but I have followed this group some time and
read many posts from many electronics expierienced people. So I thought
this group was as good as the HP scope forum.

It's probably easier to just replace the neon lamp. Neon lamp failure
is a common problem in the really old HP scopes, which usually appears
as display jitter. What HP used were Signalite NE-2 or NE-2H type
lamps, but stabilized with Krypton 85, which has a 10 year half life
and is probably long dead after 50 years. All of the modern NE-2H
lamps are made for illumination, not regulation. I don't think anyone
makes them for regulators these days and certainly not with
radioactive gas inside.
<http://www.febo.com/pipermail/volt-nuts/2010-March/000279.html>
<http://forum.allaboutcircuits.com/newsgroups/viewtopic.php?t=3901>

I also just found a page that made me suspect the problem might be "not
enough radioactivity" in the tube stabilizers, but I was not sure. The
radioactivity ionizes the gas, which is then much easier to start.

Now - I think that the problem of the sometimes blown fuses are because
of the tube stabilizer can not start, and therefore the voltage
regulator send max. voltage out which trips the overvoltage SCRs in the
plug-ins at -12.6V. SCR trip at approx. 14.4V and the unregulated
"-12.6V" is approx. -18V.

It should be possible to find an approximately 70v zener or build a
zener simulator that would suffice as a drop in replacement for the
neon lamp regulator. Looking at the schematic, the +100v adjustment
has enough range to work with any regulator diode between 75v and 85v.
The range can be increased by decreasing R26 (22.1K) or increasing R24
(4.32K). The current through the neon lamp is currently(sic) 3.2 ma,
which may need to be increased slightly by decreasing R21 (5.6K).

I would like to make a new PCB with the zenerdiode circuit, but it
should not look like a rat nest...

The "zener" would be chosen to be a TL431-like IC.

Note that the nearby VR4 is a 69.4v zener. If you can find another
one, it might be usable.

As others has written (might be in the HP forum) these zeners is not
temperature stabilized.

That 69.4v zener is already occupied, it is used to save the transistors
from voltages higher than 70V at start - and at regulator output short
circuit. Then the fuses blows som hectomilliseconds later.

Glenn

 

[Glenn]

I have three of those mainframes.

....

I have four 141T

The reason was to use component from one or two to repair the others.

But I have repair and calibrated all power supplies.

Two of the 8555A "wheels" has been disassemblied, drilled, glued so they
work again. The plastic that holds the contact-metalblade arm had broken.

I have these problem now:

* one scope can not get high enough intensity and can not get in focus,
but it is useable. Tripler blown? I presently do not own a multimeter or
a probe that can measure 6.6 KiloVolts.
* two blows fuses occasionally
* two 8555A has blown diode first mixers (one blown by me - the antenna
cable was not earthed before measurement) - but I have two other 8555A
that now works fine. One has a missing "pointer" belt though. Then it
should be easy to take a module from the other busted ones - NOT! they
use a different socket plug - how can this happen for the same product ;-)
* one power supply uses zener - and the other three use tube
stabilizers. The difference came as a surprise But later the newer
schematic was found.
* one chassis is missing many knobs.

I presume that the storage do not work - or I have not read enough about it.

-

The reason I bought them is that they for the most part use common
components, and I would like to know how they work. The schematics are
fascinating to study.

Two years ago I would never expect to own a single multi GHz spectrum
analyzer. Now I have approx. 3.5.

I also use the preselector in front of 8555A, so signals do not get
aliased into the band of interest.

Glenn

 

[Glenn]

On 28/04/13 17.51, Jeff Liebermann wrote:
....

You really should post your question in some HP scope forum or
newsgroup as it's really a repair problem, not a design problem.

....

Hi Jess

Actually my question was about electronics (re)design

Glenn

 

[Tim Williams]

I also just found a page that made me suspect the problem might be "not
enough radioactivity" in the tube stabilizers, but I was not sure. The
radioactivity ionizes the gas, which is then much easier to start.

Ya know, I wonder if you could take the bulbs down to the local physics
department's "neutron howitzer" and get some neutron activation going.
It's not going to replace Kr85, but there's probably a few elements
present which will absorb with a reasonable cross-section (i.e., it won't
take weeks of exposure to have a useful effect) and half-life (most
neutron activation experiments generate half-lives in the minutes range,
which is great for detecting them, but not so useful for ionization and
stability). It would need to be aged to allow those short-lived isotopes
to decay, of course.

Tim

 

[Dimitrij Klingbeil]

....
It should be possible to find an approximately 70v zener or build a
zener simulator that would suffice as a drop in replacement for the
neon lamp regulator. Looking at the schematic, the +100v adjustment
has enough range to work with any regulator diode between 75v and 85v.
The range can be increased by decreasing R26 (22.1K) or increasing R24
(4.32K). The current through the neon lamp is currently(sic) 3.2 ma,
which may need to be increased slightly by decreasing R21 (5.6K).

....

That range looks almost right for a SG202B (??202?) type gas stabilizer,
its range is nominally 81-86V. These things were made in large enough
quantities in the Soviet Union, back when that existed, so that they're
still easy to get and cheap. Fleabay has some for a buck plus shipping.
Also Pollin (an electronics discount shop from Germany) sells it for 2?.
They sell mostly to Germany and Austria, but can ship internationally.
link: http://www.pollin.de/shop/suchergebnis.html?S_TEXT=SG202B

There is no guarantee that it starts (this lamp has a maximum specified
strike voltage of 135V and most are about 30 years old now), but that
spec is pretty loose to allow for a lot of age-related wear and tear.
Maybe try 2 or 3 in parallel and use the one that turns on as it will
be the lowest-voltage one. If that works, it may replace the original
more or less "as is" without significant modification.

If you decide to put one to use, note the polarity (center pin = anode),
otherwise it will drift out of spec before its rated lifetime.

Dimitrij