Thanks for the help. What can I use for a signal? I
don't have a signal generator. I tried testing it with a 3v 800ma wall wart
using the EXT INPUT and didn't get a dot or anything.
I tried using a 1.5v battery on the lower left input probe and, while holding
the test button down I could see the dot jump up and down 1.5 square up each
time I contacted the battery terminal. I can't get a dot to appear without
holding the test button, so I feel like the knobs and switches probably aren't
adjusted correctly.
It worked the same way with the lower left INPUT when I tried the wall wart.
The dot jumped up 3 squares, but only when holding the test beam button.
Do you think I'm doing the test correctly, or are there other switches and
knobs I need to try different settings with?
Thank again!
Andrew
--------------------------------------------
On Sun, 10/8/17, 'Paul
Bicknell' paul@...
[hp_agilent_equipment] <hp_agilent_equipment@...>
wrote:
Subject: RE: [hp_agilent_equipment] HP AN/USM 281A Oscilliscope needs repair (San Antonio TX)
To: hp_agilent_equipment@...
Date: Sunday, October 8, 2017, 1:05 PM
?
Hi re
fixing 180 from memory
Before
removing the transistors a simple
test
Have
you tested all the power supply rails
on the pcb for the main frame for this test I would remove
the plugging just in
case they are causing any problems ?
?
Display
switch on right hand side 3rd
one down turn it clock wise this puts the X amplifier into
external with the intensity
turned up you should have a spot
turning
the pot fully clock wise gives
maximum sensitivity ?( right hand side 2rd one
down magnifier
should be set to 1 )
now
connect a signal into the external ?BNC
input below ?I just held a length of wire and put into the
bnc socket ?you
should see a horizontal line if not it is probably the drive
transistors
if
you have movement connect the BNC input
below ?to the calibration terminal you should see 2 dots
one on left one
on right if one hasten moved then it is that pare of
transistors that has a
fault
?
this
was my first HP scope to repair ?Paul
B
From:
hp_agilent_equipment@...
[mailto: hp_agilent_equipment@...
]
Sent: 08
October 2017 18:02
To:
hp_agilent_equipment@...
Subject: RE:
[hp_agilent_equipment] HP AN/USM 281A Oscilliscope needs
repair ( San Antonio TX )
?
?
I have a fluke
digital multimeter, a weller soldering
station, and not much else. I'm slowly building up what
I can when I have a
specific need or find a good deal.
I bought a set of probes for it off ebay
http://www.ebay.com/itm/P6100-2PCS-Oscilloscope-Probe-Kit-100-MHz-Clip-Test-Probe-For-Tektronix-HP-Rigol/322409718031?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649
There's no rush for me to fix it, so I can take the next
10 years if need be. Should
I de-solder the transistors for testing?
I heard that with very high voltages, the thickness of the
gloves are what's
important. That's why lineman's gloves and spark
plug boots are so thick.
I have a thick pair of rubber gloves that I'll probably
be wearing for safety
after leaving it unplugged for a while.
I also need to make a high resistance capacitor discharge
tool. I believe I
have some high capacity resistors I've salvaged from
some equipment.
Thanks for the tips. Any further help is appreciated.
Andrew
--------------------------------------------
On Sun, 10/8/17, Reginald Beardsley pulaskite@...
[hp_agilent_equipment]
< hp_agilent_equipment@...
>
wrote:
Subject: RE: [hp_agilent_equipment] HP AN/USM 281A
Oscilliscope needs repair ( San
Antonio TX )
To: hp_agilent_equipment@...
Date: Sunday, October 8, 2017, 11:30 AM
?
What test gear do you have? That will help people
advise you. Did it come with a probe?
The manual suggests approaches appropriate to a
commercial
environment where labor is expensive and parts are
readily
obtained. In your case labor is actually what you're
after. Except you'll call it "experience"
after you're done. Being able to say you have
repaired
a 100 MHz analog scope is non-trivial. That's a good
scope, though old age will make it a bit cranky. It will
serve you well even when you get newer gear.
You can make a limited test of the transistors by
measuring
the forward resistance of the B-E and B-C junctions with
an
ohmmeter. The B-C resistance will be smaller than the B-E
resistance because the larger area of the collector
trumps
the higher doping level of the emitter. Use the highest
resistance ranges. You may encounter problems with DMMs
that don't supply sufficient voltage to turn on the
transistor. I learned this trick using an analog VOM. It
doesn't work with the Harbor Freight cheapy DMMs.
If you've got one of the Harbor Freight DMMs that
they
give away, measure hFE with it. That should suffice for
testing the two transistors, But beware of the crappy
transistor sockets, You should probably solder some leads
with grabbers or alligator clips. I just tried two red
ones. One worked, the other didn't, but both were
intermittent. An old yellow HF DMM had a better socket
and
worked just fine (once I plugged it in correctly ;-)
Connect to the probe compensation signal for testing.
Make
sure you know where the high voltages are and stay away
from
them. I've heard it's rather painful at best and
lethal at worst. If you do need to work near it, keep
your
left hand in your pocket and wear rubber sole shoes. HF
nitrile gloves might not be a bad idea, though I
don't
know how good they'd be. Not quite sure what is
recommended for southpaws. Be aware that the HV
capacitors
and the CRT can hold a charge for quite a while. Before
you
EVER touch anything in the HV section, ground it with a
screwdriver first with the unit unplugged.
Whenever you get tired or confused, stop. It took me over
a
week to fix the horizontal section in my Dumont
working a
couple of hours each evening.
Most important....
Have Fun!
Reg
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