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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