"Jim Yanik" . wrote in message
.. .
"BFoelsch" wrote in
:
I recently gave away a 465 that was giving me fits. I don't know what
happened to it, but it had squirrelly problems all over. It was
shipped to Tek for repair in the early 1990's and Tek said it was
unrepairable. Of course I knew better.......
It was used extensively in field service, and was probably vibrated to
death, but it had the damndest problems I ever saw. The ones that
really killed me (other than the vertical attenuator switches) were an
oscillation in the Ch 1 vertical, inability to get a smooth leading
edge on a fast rise (fall was OK) and an intermittently failing
intensified sweep. I spent about 100 hours on that thing and never did
get it to work right. I don't know if that had multi-layer boards or
not, but it acted like some inaccessible connections were
intermittent. I checked and changed component after component, and
there would always be one more trouble. That was my weekend for a few
months; I would start and go througn the WHOLE calibration procedure
(including the stuff that everybody skips, like swinging the line to
check the power supply at high and low line conditions) to see how far
I could get this time. Undoubtedly I would stop at some point because
the adjustment didn't have enough range or some such. I'd fix that and
next week a different set of stuff wouldn't work. The problems were
never expensive stuff, just resistors and capacitors.
That was the ONLY Tek scope that ever stumped me. I even managed to
keep a 647 running, and those NEVER worked.
Yes indeed. The perversity of the inanimate.
So I gave away the 465 and bought a TDS2012. Took a little getting
used to, but I've never looked back. Still have the old reliable 547
and a whole slew of plug-ins in the corner, but they are going to need
a new home before too long.
Trying to support a TEK product without the Tek selected transistors and
other specialized components is extremely difficult.
One of the first things I did when getting a unit that someone else had
tried to repair was to find and remove all the non-Tek xstrs,and replace
with the proper TEK parts.They were often causes of oscillations and bad
HF
responses.Some scopes may have had ferrite beads used in some places lost
or not installed with the new transistors.One other common problem was a
black silver oxide growing on the tiny HF trimmer caps,especially on the
bottom of them,acting as an insulator and making the cap open and
ineffective.
IMO,the HF cam contacts used in the attenuators should be failing and no
longer repairable due to the plastic part that holds the gold contact to
the spring metal degrading and coming apart,or loss of spring tension.
That's a problem with using plastic parts,they outgass and eventually
degrade and lose strength.Some of the atten intermittents can be due to
the
outgassing making a film on the contacts and pads.
Yes, this unit could indeed have had some non-Tek semiconductors in it. I
got the HF ocsillation to stop by putting a few gimmick capacitors, less
that 1 pF, at the output of the Ch 1 vertical amp. Didn't affect the
frequency response any, but the vertical output amp had a few extra time
constants visible on a leading edge that none of the adjustments would
control.
It was a real learning experience, but the patient died. One thing I DID
learn was that gold-plated sockets don't mean a thing if they mate with
tinned leads. When I first got that scope almost nothing worked, and major
recovery was effected by removing and replacing all the socketed
connectiions, which in that scope included virtually all of the
semiconductors. I also suspected that that scope was not a "complete"
instrument; seems to me that the versions and serial numbers didn't really
agree, as if someone tried to make one good scope out of parts from other
ones that were of varying vintages.
Anyway, I decided that I am out of the scope-fixing business. It's kind of
like changing the engine in a car, the first couple dozen are fun, and after
that its just plain work.
--
Jim Yanik
jyanik-at-kua.net
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