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SERVICE INFORMATION FROM HEWLETT-PACKARD
JANUARY-FEBRUARY 1983
Improving Bandwidth and
Risetime in 174x
Oscilloscopes
Doug Moloney, Hewlett-Packard
Low bandwidth and slow risetime
problems i n your 174X series of
oscilloscopes may be caused by cor-
rosion of the delay line's braided
shield. What HP has discovered is
that under extreme operating condi-
r '
tions (wide ambient temperature
variations) the braid becomes cor-
roded from gasses emitted by the
outer PVC (polyvinyl chloride) jack-
et of the delay line.
This PVC outgassing is very similar
to the vinyl dashboard and seats in a
car emitting gasses to create the film
on the inside of the car windows.
( I .
Regular BNC cables with PVC jack-
ets are susceptible to the same prob-
I lem. However, due to the constant scope is operated in a constantly con- times while watching the CRT
I
I flexing in normal handling, the in- trolled environment, it may take for high-frequency response
I dividual strands of the braided years for the contamination to be- improvement.
shield rub against each other to keep come objectionable.
the contact between them clean. 4. If response improves, keep
squeezing the cable. You can ex-
Since the delay line is rarely moved Identifying a faulty delay line is pect to see improvements of 1 to 3
once it is installed in the scope, the easy. nanoseconds or 10 to 60 MHz. If
PVC contamination may continue to no improvement is observed, per-
build until the braid's conductivity 1. Remove the scope's bottom cover. form the pulse response adjust-
decreases to the point where high- ments and repeat step 3.
frequency attenuation becomes sig- 2. Input a fast risetime pulse or 100
nificant. This process can occur in a MHz sine wave and note t h e 5 . Perform the pulse response ad-
relatively short time if the scope is risetime or amplitude. justments and high-frequency
f subjected to repeated and radical performance tests in the manual.
changes i n ambient temperature 3. Locate the delay line (accessible
(extreme heating and cooling of the from the rear of the A1 preamp If your oscilloscope meets its specifi-
delay line). On the other hand, if the assembly) and squeeze it several cations, no further action is required
Part No. 5952-0113 QHewlett-Packard 1983
WWW.HPARCHIVE.COM
and the delay line reliability will be Or, if desired, a new delay line can service notes from the address on the
as good as a new one. However, if be installed that is no longer suscep- service note order form. The service
your oscilloscope does not meet its tible to the PVC contaminants. The notes are:
specifications, additional trou- new delay line part numbers are: p7
1740A-20A
bleshooting should be performed to
verify the true problem before re- 01740-61633 (for all 174X except 1741A-13A
1743) 1742A-5A
placing the delay line.
01743-61613 (1743 only) 1743A-6A
1744A-7A
For more information on individual 1745~-1
oscilloscopes, order the following 1746~-1
The previous issue of Bench Briefs
More On Printed Circuit Board showed a picture of a dendrite
formed between two circuit traces
ework, Repair, and Cleaning and indicated that the cause was ex-
cessive use of circuit cooler. It's true
Dendrite that using the circuit cooler helped
cause t h e dendrite, but it was
A dendrite, as shown in the photo, is a resistive short between these two mainly the moisture condensed out
solder nodes. of the atmosphere t h a t gave t h e
dendrite its start. The water causes
a bridge across the circuit traces,
and the difference in voltage poten-
tial forms a n electrolyte corrosion
cell. Since corrosion is electroplating
in reverse and gold is a noble metal,
it becomes a cathode and the copper
trace under the gold becomes the 3
anode. The copper anode is depleted
and the metal follows the water path
to the gold.
You can create your own dendrite
phenomena by placing DI water be-
tween two traces on a n old circuit
board and applying a voltage poten-
tial of 30 volts. Watch the dendrite
grow under a microscope. In some
tests here at HP it grew across the
gap in less than 60 seconds.
Videotapes on P C Board Printed Circuit Board Contact Ordering:
Reliability-25 min. Contact your nearest HP sales or
Reliability HP part 90661R* service office and specify the tapes
The last issue of Bench Briefs dis-
Analysis of Multilayer Ceramic you want, or contact Hewlett-
cussed three new videotapes avail-
Capacitors-27 min. Packard Video Products directly at:
able from Hewlett-Packard concern-
HP part 90662R* Hewlett-Packard
ing printed circuit board reliability.
Unfortunately, the ordering infor- * = Tape Formats: Video Products
mation was incomplete. 1819 Page Mill Road
A = VHS-SP
Palo Alto, California 94304
The complete tape part numbers are: B = Beta 1
(415) 857-2381
D = %" Umatic
For more information on many other -\
Example: HP videotapes and videotape prod-
Printed Circuit Board Reliability-
36 min. 90660RB orders the 36-minute tape ucts, ask for the videotape catalog
HP part 90660R* in Beta 1 format. HP part number 5952-0157.
2 BENCH BRIEFS JANUARY-FEBRUARY 1983
WWW.HPARCHIVE.COM
;onformal Prevent contamination of the
dielectric surface by field soil (oil
coatings and the methoc
moval. The above IPC refer ___-I
Coatings vapor or fingerprints), which can
cause electrical leakage in humid
a good job of describing all the vari-
ous coatings, and reference I
environments. 700B does an equally good job
Inhibit t h e growth of fungus, scribing how to remove them.
thereby improving the electrical
characteristics. Even nonnutrient I can, however, describe the 1 e c U l 1 1 -
What They Are
surfaces (the fiberglass board) can mended handling procedure for some
A conformal coating can be a thick of Hewlett-Packard's PPC (Protec-
support fungus growth when con-
or thin layer of protective material tive Polymer Coating) coated boards
taminated with field soils such as
that is applied to one or both sides of used in several of our DVM products
a n assembly. The thin coating oil vapor.
(e.g., 3468A/3478A DMMs). The
closely follows the contours of the Suppress electrical flashover be-
tween conductors a t high PPC provides a high-impedance bar-
board and components; it "conforms" rier to leakage between nodes and
to the shape of the assembly, and altitudes.
circuit traces. The PPC is applied be-
ideally produces a film of consistent fore t h e board is loaded and
thickness over the entire surface. It A secondary function of the confor-
mal coating is to help support the wavesoldered and all solder points
can be so thin as to be unnoticed, or are left open. The PPC is the solder
so thick that you can't even see the components so that the entire mass
of the component is not carried by mask.
unit (i.e., a "potted" assembly).
the solder joints.
If you are confronted with another
Assemblies are frequently given a manufacturer's board where the en-
conformal coating to assist them in A lot more information on conformal
tire solder-side is covered by a thin
functioning under harsh environ- coatings is contained in the follow-
ing publication: conformal coating, the repair proce-
mental conditions. When correctly dure would be similar.
chosen and carefully applied, con-
formal coatings will help to protect IPC-CM-770B
Guidelines for Printed Board 1. Remove the PPC film by scraping
a n assembly from the following with an x-acto knife. Be careful to
hazards: Component Mounting
remove only the film.
-
0 Humidity -obtained from-
2. Make the repair or alteration fol-
0 Dust and dirt lowing the accepted procedures at
The Institute for Interconnecting
0 Airborne contaminants-e.g., and Packaging Electronic Circuits your company. The volume 22,
smoke, chemical vapors number 4 issue of Bench Briefs
Conducting particles-e.g., metal contained some information on
chips, filings this subject.
Accidental short circuit by
dropped tools, fasteners, etc.
0 Abrasion damage Removing the Conformal Cleaning the Repaired Area
0 Vibration and shock (to a certain Coating Clean the repaired area according to
extent) Occasionally, you are going to be the following steps. The flux
0 Tampering or modification faced with repairing a board that removers we use contain bipolar sol-
has a conformal coating. You may vents. That means they contain two
Generally speaking, the primary have to either replace a component kinds of solvent, one to dissolve the
function of the conformal coating on or actually modify or repair part of rosin and one t o dissolve the ac-
printed circuit boards is to protect the circuit board. To do this means tivators in the rosin. Once you dis-
the electrical characteristics of the you are going to have to determine solve the rosin and activators and
board. And high-impedance boards which removal method (there are spread them all over the board, you
are the ones most susceptible to several) should be used for a specific then have to remove the rosin, the
leakage in almost any environment. coating. activators, and the flux remover.
Therefore, conformal coatings on The key to proper flux removal is to
' printed circuit boards are used espe-
cially to:
It is beyond the scope of this article
to try to describe all the different
use large quantities of solvent for
thorough cleaning and rinsing.
JANUARY-FEBRUARY 1983 BENCH BRIEFS 3
WWW.HPARCHIVE.COM
1. When applying the flux remover, fingers-they leave oily finger- board to remove all traces of mois-
hold the board with the edge con- prints, which cause corrosion. We do ture. Allow one hour at 7VC (158◦ Jabse Service Manual Search 2026 ◦ Jabse Pravopis ◦ onTap.bg ◦ Other service manual resources online : Fixya ◦ eServiceinfo