It has been 2 decades since I last had a 141T, so I don't
recall too many of the details, but one thing I thought I
would mention: If the power transistor in mention has a
mica/silicone insulator between its case, and a heatsink,
don't assume that the insulator is good, or that the plastic
bushings are good...
I ran into a silly problem on a EIP counter's power supply,
where the manufacturer plated through a hole that was used
to pass a zip-tie. The plating was making intermittent
contact between a power supply rail on one side, and
ground on the other side. I removed the plated through
hole's plating, and all was well.
I studied, and studied the schematic, and couldn't see how
the circuits shown could behave the way they were, because
the circuits shown on the schematic didn't represent the
flaw caused by the erroneous plating.
So, don't assume that the schematics are even capable of
showing the failed circuit. Verify it.
And, any ohm meter can read really close to zero, just
short the leads, and note what it says. From then on,
subtract that value from whatever low ohm value you
read.
-Chuck Harris.
On Wed, 4 Sep 2024 22:04:37 -0400 "Bill Berzinskas"
<bberzinskas@...> wrote:
Ugh, I put it down for a few days to work on an old Zenith my wife
got for my birthday. Today, I put a fresh pass trans in, and fired
it up. I got -2.5v for a few seconds and then it looks like it took
out the transistor. Replaced
and same story.
Mike - with F4 removed, I measure ~50k ohm at the terminal you
mention, so that rules out C9? R49 does look ok.. my DMM only
measures tenths, so I get a floating .2 to .3 ohm. I guess I typo¡¯d
the decimal or mentally transcribed wrong..
Dave - I ran through each part you mention. R44 was a little suspect
in that the schematic says 20k but it measured 18k, still in range
but I¡¯d have expected it to go UP. In a fit of rage, I replaced C8,
C9 , R44, R46 and still blows pass trans. Raw voltage in is
around 22v last I checked.
At this point, I¡¯m wondering if there is something under the board
fighting me. It didn¡¯t look like I was the first person in here,
and I¡¯ve been working from the top side only. Is it worth pulling
the board to look at? I could go eBay a bunch of old exotic
parts, I could try to shotgun some moremodern parts in.. I do
feel like I¡¯m loosing the battle on this rail though. Anyone in
North Carolina? :-)
On Wed, Sep 4, 2024 at 9:45?PM VK5HL via groups.io <vk5hl=
[email protected]> wrote:
Hello All
I have a hp 141t when it is powered up there is no trace but when I
turn off the power the trace briefly appears in top right corner.
Is it the blanking circuit or do I need to do an trace alignment or
something else. I need some guidance
Has anyone encountered this problem before and let me know what you
did.
73's Leroy de VK5HL
-----Original Message-----
From: [email protected] <
[email protected]> On Behalf Of Chuck Harris
Sent: Tuesday, 3 September 2024 1:41 PM
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] HP 141T Power Supply
Help Needed
To amplify your comment, many times older linear supplies, when
they are brute force recapped, with the best, long life, low esr
caps you can find, will become beacons of chaotic oscillation.
I put a low ESR ceramic capacitor in the feedback loop of a unity
gain OPAMP circuit the other day, and was amazed that it went
totally unstable. It was an older LM301 variety, where the response
characteristics of the opamp are a blank slate until the designer
adds in the stability network. The designer's efforts were wiped
out by my ceramic capacitor replacement for his 1uf back-to-back
emulation of a non-polar electrolytic.
Sometimes the modern best replacement is not suitable for use in an
older instrument.
-Chuck Harris
On Mon, 2 Sep 2024 22:40:20 +0100 "Dave_G0WBX via groups.io"
<g8kbvdave@...> wrote:
Hi.
It's just a generic term (damping is another) to slow down the
response of a loop, to prevent it going unstable.
Not an uncommon issue in some linear PSU's like those -12.6V or
-100V regulator topologies that are not "conventionally" laid out.
Unless you 'scope the thing, you often don't know it's gone
unstable, other than some passive parts sometimes cook for no
obvious reason. (That R49 for example! See Mike's comment in
another mail.)
It is also not uncommon, when modern transistors are used to
replace older failed parts (that are not available any more) and
the new device has "a lot" more gain at HF than the originals,
that regulator or other control loops suddenly become "unruly"
under some operating conditions.
Sometimes a low value "Base Stopper" resistor (a few Ohms, or a
ferrite bead) in series with the new device's Base connection can
restore sanity!
Take care.
Dave B.
