On Mon, 4 Nov 2019, Jack Reynolds wrote:
Don't know about older ones but 492BP/494AP are pretty reliable, quite easy
to fix, very rugged and pleasure to work with/on. The "on" part is here
because I do fix, refurbish, and recalibrate those relatively often. They
suffer from age (older heavily used instruments require almost total recap,
especially their power supplies where almost all capacitors except 2 big
bulk ones for filtering rectified mains voltage are probably bad by now but
otherwise that hold pretty good. As of capacitors it is almost all
electrolytics and wet tantalums. Tantalums are especially bad -- they are
filled with sulfuric acid that etches through their silver cases and
destroys everything around. Most of them are bad by now. There are many of
electrolytics and wet tantalums inside RF shielded compartments so proper
refurbishing requires almost complete disassembly to get into some of those
and other tight spaces. Replacing wet tantalums might be tricky at times as
they have very high capacity density and it might be difficult to find a
replacement that would've fit into some of RF enclosures.
There are also SMD aluminums inside those enclosures that are ALL bad and
the boards inside are contaminated if not straight damaged yet. Those MUST
be replaced and boards restored which sometimes require partial board
disassembly to get to damaged areas as well as multiple SMD components
replacement that were damaged by leaked electrolyte. BTW, _some_ of
instruments have older black plastic incapsulated aluminums that look like
SMD tantalums instead of regular vertical cylindrical ones. Those are
surprisingly robust -- I have never seen a single failed one.
Yet another capacitor plague, frequently overlooked is those trimcaps with
film dielectric. That film is quite often deteriorates to the point it
breaks to pieces that makes instrument to lose calibration and impossible to
calibrate. Situation is especially bad in 494AP 10Hz filter installed on top
of CRT -- it runs very hot by design and it is almost 100% chance all such
caps inside have their film dielectric turned to dust and shreds by now.
They all MUST be replaced.
Finally one more thing that is quite important is internal fan. Original
instruments had very fancy 3-phase thermally controlled fans driven by a
separate board. Those are difficult to replace if it is the actual fan that
failed (check the control board first) -- they had very low electrical noise
that affects instrument sensitivity / noise floor and thermal regulation is
_NOT_ just a convenience feature to reduce fan's acoustical noise, it is
necessary for proper thermal regulation as many parts are _DELIBERATELY_ run
hot, some of them _HEATED_ and all specs are only met if everything is at
the proper temperature. Don't even think about putting a cheap chinese
computer fan inside. If original fan failed the only way to fix it is to use
a higher voltage good quality fan with added electrical filtering. It would
require removal (or modifying) the control board and it will take for the
instruments longer to heat up.
And sure enough the full re-adjustment will be required after refurbishment.
Other than that they are pretty rugged and reliable, good instruments. One
weak spot is the low barrier Shottky diodes in the first mixer that are
easily damaged by input overload or DC voltage applied but that is common
for all of such instruments.
BTW, all those 275x instruments are _EXACTLY_ same as their 49xAP siblings
with a different front panel. Inside them there is an actual _FULL_ 49x
instrument, fully assembled, just with outer case and front panel removed.
The entire e.g. 2756P is just a bunch of aluminum angles holding a 494AP
sans front panel, a different front panel and outer cover. The 494AP inside
is _ABSOLUTELY IDENTICAL_ to a regular 494AP, with exact same firmware and
even all identity switches settings are same. It is the front panel (that
has its own processor) that tells the firmware if it is 2756P or 494AP.
2756P front panel is more convenient as it has separate numeric keys, not
shared with others but that comes at a cost of significantly bigger
footprint and turning into a bench-only instrument while 494AP is
specifically designed to survive long trips in a truck and constant field
abuse.
I would appreciate some general thoughts on Tektronix stand alone Spectrum
Analyzers. Other than the plugins for 7000 and 5000 series scopes I never
see them discussed here. The H-P group seems to have a love/hate
relationship with the ones from H-p. There are always a few being worked
on. How serviceable are the ones from Tek? Are they so reliable so as not
to need repair often or are they impossible to repair? Any thoughts would
be appreciated.
Thanks,
Jack Reynolds
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