Tektronix 49x, 275x schematic

Tektronix 49x/275x-Series Spectrum Analyzer

____________________

John Miles, KE5FX
[e-mail]

March 29, 2009

Service Notes

This public-domain document may be reproduced and distributed freely.
The latest edition is available at http://www.ke5fx.com/49x_notes.pdf

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Errata and helpful notes on calibration/service procedures

Page numbers refer to 494/P service volume 1, 070-4416-00, but in most cases there are
equivalent pages/sections in all other 49x/275x manuals

3. Deflection amplifier (gain and frequency response), p. 5-6

Step 3b: Apply a 500-kHz signal... In later editions (e.g. 494A/P volume 2, 070-5561-00), this was
corrected to 500 Hz.

Step 3b: ... and set Triggering to Int. I've found that at least some models need to be left in FREE
RUN mode to trigger on the specified video waveform.

The 49x/275x deflection amplifier underwent several revisions throughout production, including a
complete redesign in 1988. Regardless of your instrument model, refer to the manual change
information in the 494/A manual (070-5560-00) if your instrument has one of these newer
subassemblies.

Don't forget to disable digital storage before adjusting the X/Y deflection gain. Recalibrate the
digital storage subsystem immediately after any adjustments are made.

5. Frequency Control System Calibration, p. 5-9

Step e: When observing the markers, either turn digital storage off, or follow the procedure in 13.
Digital storage calibration first. I recommend calibrating digital storage immediately after the
deflection amp gain, so you aren't confused in any subsequent steps by storage-registration
errors. This is reflected in later manuals such as the 494A/P's.

Step e-3: ...apply 0.2 us time marks from the time mark generator to the RF input. Any 5-MHz
sinewave source is OK for this, especially if it's strong enough to create some additional harmonic
distortion in the front end. Select 100 kHz RBW to yield narrower markers.

Step f: Max Span Dot Don't bother to change the resistor based on the response at the left end of
the display, as the manual specifies. Change the resistor only if the dot marker is substantially
out of alignment at higher-frequency markers, e.g. at 1.8 GHz in step f-3. (The 1.8 GHz marker
may be too weak to see; neighboring markers are OK for this test as well.)

Figure 5-10: Adjustment and test point locations for calibrating the frequency control
system, p. 5-11

TP1073 is located on the Span Attenuator circuit board, not the 1st LO Driver circuit board as
shown in the 494/P and earlier manuals.

8. Log Amplifier Calibration, p. 5-14

This procedure involves various interacting control settings, ambiguous instructions, and printing
errors. Don't adjust your logamp unless you actually need to. Symptoms such as excessive
amplitude drift will not be corrected by calibration alone.

While Tektronix continually upgraded their instruments to use the latest modules in production,
the same cannot be said of their service manuals. Calibration-procedure change notes were
never issued in many cases, so it's best to follow the instructions in later 49x/275x instrument
manuals whenever they apply to the modules in your instrument. When searching for the correct

schematics and service information for your instrument, date codes on components are often a
better clue than the model nomenclature on the front panel.

For example, you may not need to install your logamp on extenders and remove its covers, as
recommended in step 8b on page 5-14 of the 494/P manual, if your logamp's control adjustments
are easily accessible through the slots in the housing as they were in all later instruments.

The first step in logamp calibration is to identify which of the three possible Log and Video
Amplifier test point and adjustment locations figures (5-17 in 070-4416-00, or 5-13 in 070­5560-00 with two possible control layouts) applies to the module in your instrument. An analyzer
with the older-style logamp uses figure 5-17 in the 494/P manual, while logamps in newer
492A/492B/494/495/497 analyzers and their 275x-series counterparts use the physical layout in
figure 5-13 of the 494A/P manual.

The catch is that at least two different layouts were used for the controls in figure 5-13. At some
point, probably around the time Tek redesigned the board using surface-mount components in
the late 1980s-early 1990s, the trimmer functions were reordered while their physical locations
were left alone. In earlier logamp assemblies, the order is as specified (left-to-right on the
component side):

INPUT REFERENCE LEVEL R1012
LIN MODE BALANCE R1025
OUTPUT REFERENCE LEVEL R1030
LOG GAIN R1037
LOG FIDELITY R1060

In later assemblies, the correct control order is:

LIN MODE BALANCE R1025
LOG GAIN R1037
OUTPUT REFERENCE LEVEL R1030
INPUT REFERENCE LEVEL R1012
LOG FIDELITY R1060

Step h: Return the step attenuator to 0 dB. Display should be full screen (+6 dBm); if not,
readjust the signal generator output for +6 dBm. This confusing step was modified in the later
494A/P manual (step i) but never actually "fixed." The overall goal is to achieve and maintain
fullscreen (reference-level) response with +6 dBm at the input to the logamp. It's not clear what
Tek's intention was when documenting this part of the calibration procedure, but it appears the
intent is for the vertical position control to be adjusted to maintain a fullscreen indication with +6
dBm. If someone has a better interpretation, please email me and help keep this document up to
date!

Note that 492/496 logamp modules used 0 dBm as a reference level rather than +6 dBm.

If your signal generator has a calibrated attenuator (HP 8656/8657/8642/8662 etc.), you obviously
don't need to bother with the step attenuators. Users of HP 8640s and other generators with
vernier level controls will probably find it easier to use the outboard attenuators as specified.

When servicing your logamp module, watch out for corrosion from any surface-mount electrolytic
capacitors that may be present. 494AP-era logamps were among the earlier surface-mount
assemblies produced by Tektronix in the late 1980s. The quality of the surface-mount capacitors
used at the time was terrible, as owners of other Tek instruments such as the early TDS-series
DSOs can often attest. I've had to junk at least one logamp module due to instability and
intermittents caused by this corrosion.

8. Adjust Resolution Bandwidth and Shape Factor, p. 5-14 of 494A/P manual (070-5560-00)

Step i: At the conclusion of this part of the procedure, reconnect J693.

13. Digital Storage Calibration, p. 5-22

The layout shown for the trimpots on the horizontal digital storage board in figure 5-24 is wrong.
The correct order for these controls (from left to right) is:

OUTPUT OFFSET R1039
OUTPUT GAIN R1041
INPUT OFFSET R1046
INPUT GAIN R1048

Unlike the logamp controls above, I don't believe this control layout varies with the board revision.

As noted elsewhere, the digital storage controls should be aligned much earlier in the calibration
process than the 494P manual recommends. Perform this procedure immediately after servicing
or recalibrating the deflection amp.

15. Preselector Driver Calibration, p. 5-24

Follow the procedure 13. Adjust Preselector Driver, p. 5-21 in the 494A/P service manual when
aligning the preselector. It's normal for the 19- and 21-GHz response peaks to be rather broad,
or to occur at the far extent of their respective shaper adjustments.

In most cases it will not be possible to achieve perfect preselector tracking in any one band, much
less across all bands, and in any event, thermal factors will limit repeatability. You can spend
hours tweaking the preselector response if you like, but as soon as you put the cover back on the
analyzer, the effort will be wasted.

Ultimately the response with the PEAKING control centered may be 3-4 dB down at various
frequencies in the different bands, but with correct alignment, it should be possible to peak the
response at any given frequency between 1.7 and 21 GHz without leaving the shaded area. If
your preselector response already meets this criterion, there is little benefit in adjusting the
preselector driver assembly.

If the TM500-series time-mark generator and comb generator specified by Tek are not available,
an HP 33002A SRD comb generator module driven by a +23 dBm, 100 MHz signal source makes
a good substitute. You can also perform the procedure with a CW-capable sweep generator
(e.g., HP 86290A-H08) or synthesizer (HP 8673B or equivalent), but it's more time-consuming.
Absolute amplitude control is not needed for preselector calibration.

Maintenance: Replacing the Crt, p. 6-29

Like the logamp alignment instructions, this is an area of the manual where the engineers and
technical writers at Tektronix never quite found themselves on the same page. Personal injury
or equipment damage can result from following Tektronix's CRT installation procedure.

The key point is simple: before reinstalling the clear plastic implosion shield and metal bezel
frame, you should loosen the four plastic mounting blocks around the perimeter of the new CRT's
face. This will allow the CRT to rest as far back in the instrument as possible while the bezel is
being tightened. The goal is to be able to tighten the bezel's four Allen screws in a cross pattern
without putting any pressure on the CRT face. You should be able to slide the implosion
shield around freely with one hand while you tighten each bezel screw. If the plastic shield
binds up as you tighten one of the bezel screws, the mounting-block screw at that corner is still
too tight.

In particular, don't follow step 6 on page 6-30 of the 494P manual (070-4416-00) ("remove the
bezel and tighten the mounting block screws..."), or any similar instructions that may be in your
edition of the service manual. Instead, once the bezel screws are tight and you've verified that
the plastic shield is still free to slide across the CRT face, tighten the mounting block screws
evenly in a cross pattern to approximately 8 inch-pounds, with the bezel still in place. This
procedure will bind the CRT with a safe amount of pressure, allowing it to undergo normal
handling jolts without overstressing either the CRT face or the plastic implosion shield.

It's not necessary to follow any instructions in the manual regarding removal of the old CRT from
its shield cladding. There's no reason to do this unless your replacement CRT didn't come with
its own shield.

Troubleshooting topics

General notes on power-supply service

Regulation tolerances for the low-voltage supply buses are specified in service volume 1, and are
not usually a problem. However, the +100V and +300V supplies are frequently below their
specified values. If any 66-kHz ripple or sagging is observed, all of the miniature 2.2 uF/200V
axial-lead electrolytic capacitors in the power supply should be replaced. These are inexpensive
high-failure-rate parts, so they should be replaced in any event if the power supply is otherwise
disassembled for service.

I recommend checking the ESR on any replacement high-voltage electrolytics as well. Several
"new" capacitors in this voltage range have proven defective when obtained from surplus
sources.

High ESR in the filter capacitors on the lower-voltage rails (+17V, +5V) has been reported as a
cause of excessive power-supply temperature, so when servicing the power supply, you should
check ESR on all electrolytics as a matter of habit. Check heat-sink fasteners for tightness, and
renew the heat-sink compound while you're at it. Avoid losing track of the mica washers and
aluminum spacers that may fall out when you remove the PCB.

Use 105 degree C-rated electrolytics where possible, e.g., 3.3 uF 350V 105C, Digi-Key
493-2046-ND. These can replace all of the high voltage capacitors on the power supply (6x

2.2uF/200V, 1x 1uF/350V).

part #

According to a Usenet post (sci.electronics.equipment, 2-Sep-01), the large stud-mounted
transistors with Tektronix part number 151-0703-00 cross-reference to the industry-standard part
number 2N6586 (10A/450V, 12.5 MHz fT, TO61 package).