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Tektronix 492P repair tips


Tektronix 492P repair tips

Faults found on a 1983 model TEK 492P spectrum analyzer and fixing them:
  1. about 10 dB loss of overall gain and thus calibration setting out-of-range when aiming for -20 dB full scale display on cal.
  2. some (10 dB) lack of gain with 100 kHz BW filter setting
  3. intermittent contact inside LO1 coupler
  4. all numerical displays on screen are gibberish
  5. severe lack of analyzer (Y) gain
  6. noisy blower
  7. modifying the input to have DC blocking (note: Mini Circuits BLK-18 limits lowest usable freq. to 10 MHz)
Rule 1: Don't adjust any of the trimmer resistors, unless you really know what you are doing - read the Service Manual.
Rule 2: Don't adjust any of the trimmer resistors.

Useful tools: manuals,  a second spectrum analyzer with tracking generator (or at least signal generator and spectrum analyzer), a microwave bolometer power meter (HP-432), 8 mm wrench, extension card set (~$100)(3 cards and long SMB cables), DMM, ESR meter if the SMPSU has bad electrolytic capacitors. Depending on the faults to fix, you only need some of these tools. If you know the TEK 492 inside out, you need less tools, but still may need RF test gear. You may need spare parts; but buying a second analyzer is cheaper than buying a full set of spare cards and units for the 492P as they typically have price tags around 100€ each and there is a big stack of cards.

Loosen 4 big screws on casing's rear side's blue plastic stands and pull back and away the casing with the handle (the enclosure back side has very sharp edges, don't cut your hands!). If the screws are sealed, ask for a permit or any warranty may expire. If you need to replace the fan or repair SMPSU; at right upper back corner, FIRST: remove the PSU aluminum clamp securing inter-connector and carefully and gently pull open the blue flimsy rectangular multi-pin connector bolted inside the right side frame wall (see photo). Now, proceed by loosening the SMPSU unit securing screws (2+2) at rear side and pull PSU back and outwards - carefully!

Faults #1 and #3. The apparent loss of gain was actually accompanied with very bumpy frequency vs. level response, with dips down to -30 dB (see photo) - typical if having stubs along some RF path. Check the RF input N-connector to mixer signal path by opening mixer's RF- input SMA and sweep the attenuator array + other stuff on the RF path with another TG and analyzer -  got a fairly flat response? Click through the attenuator scale and see the response is still flat and level drop steps are as expected (-10 dB, -20 dB, -40 dB). If not OK, solve which part is damaged and repair it. Use a magnifying glass to see if the mechanical actuators on the RF attenuator have any cracks. If you see cracks, you may try to prevent them from breaking up by using very tiny amounts of Loctite Super Glue, but don't glue any of the moving joints solid.

If RF input feed to mixer was OK, check how power behaves at LO1 SMA output at front panel. If LO1 connector puts out flat power spectra across YTO tuning range, then use a spectrum analyzer (or power meter) covering 2...7 GHz and measure LO1 power level and it's flatness at mixer LO(1) input connector (from the SMA connected to mixer's LO port). Set 492P span at max. - slow or manual sweep. If you only have microwave power meter, such as HP432A, sweep manually through the whole YTO tuning range (whole band) and see if the LO1 injection power remains flat through the tuning range. If the level varies, and the unit has OPT. 3 installed, there are 2 couplers + the Bias-return slug on the LO1 feed from YTO. Find out which component is loosing LO1 signal and fix or replace it. Without OPT. 3 low serial # units; there is only LO1 coupler; check it does not have high loss. High serial # units all have the Bias-return slug.

Problems found: LO1 coupler has bad/intermittent contact inside; the SMA connector' center pins are not soldered to PCB traces. The PCB material does not allow it. Carefully remove the coupler, open it, but note on which side (top or bottom) of the PCB each of the flat SMA center pins go (mark them)  to and how PCB is oriented so you know how to reassemble the stack right. Use some slightly abrasive material and clean the PCB foils and SMA center pins for good contact, reassemble and test.

The faulty Bias-return slug caused a loss of displayed signal level and bumpy response curve; the input-side high-pass pi-filter's coil was cut at GND side and acted as microwave stub draining much of the LO1 power and causing loss of analyzer gain, etc. troubles.  Check with ohm meter the slug's input is close to zero ohms and output is 50 ohms, or feed some RF power of LO1 range (2...6.4 GHz) through it and measure how much it attenuates - it should not! Fix or replace or temporarily bypass it - the analyzer should work now if the LO1 to the mixer is now up at proper level and the mixer is not damaged.

The RF relays that make changeover switching of RF front-end's LPF vs.. preselector (YIG filter) between Band 1 and higher bands, may suffer from poor intermittent contact. If you find which relay is bad, and can open and clean it - fine, otherwise replace it.

#2. 100 kHz BW LC IF filters inside VR1 and VR2 units are de-tuned. You need the extension cards to tune them! Please be very careful when inserting any modules back on the back plane - the connector pins may bend if you force the card in wrong.  The Johanson variable trimmer capacitors might have contact problems. If you open the VR unit's covers, check the narrowest BW IF crystal filters do not have cracks on glass envelopes.

#4. A solder blob on PCB (manufacturing Q.C. issue) near card-edge connector inside the VR1 unit, caused one foil trace of the BCD data bus to be shorted to GND. Similar short may occur anywhere along the bus, so use DMM (or scope) and find it - it totally messes up the CRT numeric displays.

#4. this is from Glenn, K0BO, with thanks! Full Y-scale display can not be reached, but numerical displays are OK on screen; apparent lack of Y-gain. Detector&log amp. input level OK = 0 dBm, 1 kohm R4046 in log.amp unit is open, DC voltages wrong around the Q4035 stage (too positive) and this  amp. stage does not provide enough gain. Replace the 1 k ohm 1 W resistor.
Download the full desciption by Glenn!

#6, Noisy cooling fan. The original fan is run with AC from an internal low-voltage AC generator. Remove the old low-voltage 4-wire AC fan, unplug it and install new quiet PC-type 12 V DC fan of same size (note it blows air in to the analyzer, not out) and feed the new fan with DC from the -17 V DC line (fan's black wire goes to -17 V, red wire goes to GND).

#7. Modification: Add a DC block on the analyzer input. Get a Mini Circuits BLK-18 SMA adapter and install it inside, ahead of attenuator array. You may need SMA adapter and jumper cable to make it fit. This limits lowest frequecy response some, but it looks flat down to 700 kHz  (BLK-18's specs says 10 MHz), so if you need to use 942P way below 1 MHz, find some other DC block that is for lower frequencies and use it instead. The idea is to protect RF attenuator and 1st mixer from accidental DC feed to analyzer RF input.

You may find some de-tuning has occurred on 100 MHz Helical filters in the 100 MHz IF unit. Center the 100 MHz CAL signal carefully and use widest resolution filter (1MHz)  and peak the trimmer caps for maximum Y-gain on CRT screen center. Note the pass-band should remain symmetric and Gaussian in shape. Re-calibrate the analyzer at AMP CAL adj. screw at front panel to compensate for possible overall gain change. The full IF filter calibration includes setting Fc with 1 kHz filter at center, tuning every filter's response to symmetric Gaussian (LC) or flat-top (XTAL) in shape and then adjusting each filter-set's gain in VR units, so they all are all leveled. Don't touch the so-called "20 dB and 10 dB 1st and 10 dB 2nd" trimmers in the VR unit.

If you have microwave power reference to level calibrate band gains, they are adjusted from VR unit too. The Band 1 adjustment can be done with the internal calibrator, but for bands 2 and above, you need some microwave signal source of exact known level, preferably up to 21 GHz. Good RF power sources for lower microwave bands are those unlicensed low-power 2.4 and 5.6 GHz FMTV transmitters which exact output power is first measured with a calibrated microwave RF power meter (such as HP-432).

The other faults generally reported, are related to digital display memory, either H or V memory board. Errors 59 and 60 may intermittently show up sometimes and are related to phase-locking difficulties - if it goes away easily by turning span switch - ignore it, the design was not exactly perfect.

RF-to-DET&LOG AMP chain RF signal reference levels when a -30 dBm signal is fed to analyzer input connector, no preselector and no front-end attenuator is used:

At Ref level setting of -30 dBm and -30 dBm RF input, the CRT display should indicate a peak with full scale on linear scale selected (adjust from front panel slot LIN screw) and on LOG scale selected, (adjust from front panel slot LOG screw). If the numerical displays are shown at proper locations on upper and lower areas of the screen, the deflection unit works OK. If you can not get the peak to go full Y scale, the analyzer lacks gain. The table below list references to verify if the RF sections can't produce the 10 MHz 0 dBm (=F.S.) level to det&log amp unit. Check first the Log amp unit's input level (inter-unit SMB jumper cable). It should be about 0 dBm. If not, proceed backwards up the list towards RF input connector. If the level is correct (0 dBm), then the detector&log amp unit is the most likely fault.
@analyzer RF input connector
(frequency may be 100 MHz, but if upper bands are checked, then 3, 6, 10 or 18 GHz can be used depending on band section - do not forget to peak the preselector at front panel)
-30 dBm fed from ext. signal source, power on the analyzer, set analyzer Fc to carrier frequency, set Ref level setting to -30 dBm
1st LO ouput conn. (front panel) +10 dBm max. (SMA)
2nd converter input -45 dBm 829 MHz or -45 dBm 2072 MHz depending on selected analyzer band (SMA)
2072 to 110 MHz converter's LO input +10 dBm 2162 MHz (SMA)
2182 MHz phase locked 2nd LO output to 829 MHz 2nd converter 0 dBm 2182 MHz
829 MHz 2nd converter 100 MHz input  0 dBm
110 MHz IF amp input -37 dBm when using 2072 MHz 2nd converter or -39 dBm when using 819 MHz 2nd converter (SMB)
110 MHz Helical filter input -23 dBm 110 MHz
3rd converter input -20 dBm 100 MHz
Cal out (front panel) -20 dBm 100 MHz (BNC)
VR1 unit (IF section input) -35 dBm 10 MHz (SMB)
1st filter select input: -19 dBm -19 dBm (internal unit connection)
10 dB gain steps input: -27 dBm -27 dBm (internal unit connection)
10 dB gain steps input: -5 dBm -5 dBm (internal unit connection)
Band Leveling input: -11 dBm -11 dBm (internal unit connection)
2nd filter select: -3 dBm -3 dBm (internal unit connection)
Post VR amp. input: -16 dBm -16 dBm (internal unit connection)
Log amp. input:  0 dBm 10 MHz (SMB)

Based on your previous experiences using contact treatment chemicals (oil and grease), you may apply them on trimmer resistors and connectors, etc. They will not remove oxidation without some abrasion, but they may prevent oxidation (to proceed) for a while by sealing small gaps and covering contact surfaces.

Spare parts and manuals are found both from US and Europe for a price, use Google to find, but be aware the serial number range issue; a manual for low serial numbers (below 2xxxxxx..) is only about 85 % valid when going through the calibration procedure with a unit that has serial # beginning with 4xxxxxx and note: the Service Manual does not have any schematics included. Some TEK docs are found on the web too at some free manual download sites (like KO4BB). Norway Labs in Oregon, USA, manufactures and sells extension card kit for the TEK 49x series, mail to Matt North. Spares are available at "Tektrotom" in Europe. The manufacturer's support has expired.

The above repair tips may perhaps apply also to TEK 494, 495 and 496 models. These equipment are so old you are bound to have some troubles (before long) and need to fix them, whatever promises you get when buying one. When purchasing an analyzer, get some warranty or R.o.R. Don't pay a top-price if the risk is all yours -  25 kg of electronic junk to dispose of, is not what you need. If the price is less than 1/3 of valid price, suspect it is a fraud attempt - never pay with Western Union wire transfer and never deal with sellers that have no name, no address, no bank account. Ask for equipment serial number and find out if it can be valid. Look up from web's forums if the dealer has a good reputation.

Incidentally, if you have found a second-hand non-US-made (Japanese) analyzer, it might as well develop or have problems, but you probably find no documents, no source for resonably priced spare parts if any and the only option may be the official service where the repair costs more, than you paid for the analyzer...very hard to keep such equipment  in working condition.

The new TTi PSA-2701T is handy and small, but lacks dynamic range, see the comparison image above.
The harmonics you see with it, are not there when you use an analyzer with more dynamic range.


Rev. 2010-10-18
© I. Yrjölä, 1999... 2014