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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 (over the past 10 years):
  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)
  8. loud bang and heavy smoke from PSU
  9. replacing bursted aluminium electrolytic capacitors from Preselector driver, LO1 YTO, and other plug-in units
  10. finding reason for lack of Preselector tracking on bands 4 and 5 (ones using x3 harmonic mixing)
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 aluminium 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's back side hole 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) and the SMB angle connector coax close to it. Now, proceed by loosening the SMPSU unit securing screws (2+2) at rear side and a few small countersunked screws on the bottom edge and pry the PSU backwards - gently!

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 riser card to tune them! Please be very careful when inserting any modules back inside 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 1 and 2 unit's covers, check the 100 Hz B.W. I.F. crystal filters do not have cracks on glass envelopes. A 300 Hz drift on a no longer hermetically sealed crystal causes distorted band-pass shape and excessive loss of apparent gain on that I.F., that can be corrected by replacing the crystal with a good one, or pulling up the drifted crystal resonace frequency with a series capacitor. After done, adjust 100 Hz I.F. gain on VR 2 for same level as it is on 100 kHz. Another lesson learned while replacing the 100 kHz B.W. Johanson trimmer tuning capacitors on both VR 1 and 2: don't lift the PCB's up from the milled enclosure by pulling from the HC-18/U- type 1 or 10 kHz B.W. crystal filters - the holder enclosure has laterally very little room inside and slight metal cover compression on the sides can cause input or output pin to give intermittent varying short to ground, causing huge loss of gain for the I.F. in question.

#4. A blob of solder on PCB (manufacturing Q.C. issue) near card-edge connector inside the VR1 unit, caused one foil trace of the Intrument Bus (ADRS & DATA) data 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. If the bus fails, all lights on front panel light up at start and buttons and swithces do not work. The bus's ADRS and DATA lines can be cheked with oscilloscope on the void card slot's pins. C.F. tuning knob may attempt to communicate with
Processor when rotated, this helps to check the bus's health, as there may not be traffic on the bus when idle, all lines are at high state (+5 V).  A short or zap of voltage on the bus's data or adress line can damage the line-driver TTL IC on the Processor card, a 74LS244N or 74LS245N.
While working with this, replace the DIP switch on the ROM card. The old one will no longer close reliably, found one of the 8 swithces no longer made contact after it was cycled.

#5. 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 - PCB is next to the fan. Remove the old low-voltage 4-wire AC fan, unplug it and install new quiet PC-type 12 V DC fan of same size, power rating of about 3 W and air flow (note: it blows air in to the analyzer, not out) and feed the new fan with DC from the -17 V DC line with some series resistor to drop voltage closer to 12 V (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.

#8.  Those brownish Rifa RC filter capacitors on the PSU AC side tend to blow up eventually. The one (0.1 uF  22 ohms 250 Vac) fiberglass-taped on the AC line input filter, exploded with big smoke. There are two other similar snubber capacitors on the PCB. Dismantle the PSU PCB and
replace the at least the 3 snubbers. Also those 0.01 uF 125 Vac brown Rifa capacitors seem to have cracks on their casing, so replace them too. Check if the 230 V gas discarge tubes are OK, not cracked or corroded - replace if necessary.
Measure ESRs from all PSU aluminium electrolytic capacitors - I found none expired and no need to replace any. With magnifying glass, look for eroded solder connections on the PCB: the row of three TO-220 regulators' leads may need resoldering.
Clean dust and dirt from the PCB. Add some thermal conductive paste on the heat sink thermal bridge edges, reassemble carefully with all the right length spacers (shorter in the middle thermal bridge, longer on the regulator/PCB heat bridge gap (not necessary to remove regulator top side screws!)). Nylon nut goes on the stand-off near the gas discharge tubes and clamps go in their right places. Check no wire (AC?!) underside the PCB is left pinched.

#9.  Preselector driver card had one 510 uF 25 V aluminium electrolytic capacitor aged and had corroded it's negative lead fully cut from the can. There are some 15 PCS of aluminium electrolytics just in plug-in units. Majority of the elyts are visible on PCBs. Most digital plug-in units have none, some RF units have tantals only. Other enclosed units also have aluminium electrolytic Vcc by-passes that should be checked or replaced. When inserting back the plug-in modules, pay attention the card edge connector is well aligned with back plane pins, or there will be damage and more things to repair.  Out of 33 pcs of aluminium electrolyte capacitors replaced, just 5 were defective: all 510 uF 25 V axial Spragues found on pug-in units and 2 pcs of 820 uF 6 V axial Spragues (corroded leads) on LO1 (YTO's) end-plate PCB. The PCBs are very easy to solder, holes are large and component density is low. During this work I discovered on two plug-in units and one RF- unit solder splashes and blobs. Inspect the PCBs with magnifying glass for splashed solder - if the card is easy to remove, check the underside too. The YTO LO1 820 uF 6 V elyts and the 1 uF BP (subst. with polyester 1 uF 50 V capacitor), can be removed and new ones soldered back on with out removing the round driver PCB from the end of the YTO, just heat the pad and gently pull cap's lead out, remove solder from holes and with short enough leads, place on new capacitors' leads in the holes and resolder from top side only. Same applies to other units. Use 105 °C aluminium electrolytic capacitors preferably.

A shopping list for all TEK 492P aluminium electrolytic capacitors (and 2 replacement type polymers), unusual uF values on shopping list are changed to E10- series next higher value and 6 V and 10 V voltages to 16 V and some to 25 V.
Higher voltage capacitors will do, but will have slightly higher ESR value. 

The unit under work was a "SN B0425xx " version.
 I have replaced 33 pcs of electrolytics (of which of none were on PSU).

desoldered       & ->  shopping list  (does NOT include any PSU unit capacitors)

1 uF 50 V BP vertical 2 pcs  -> 1 uF 50 V vertical MKT polyester  2 pcs
6.8 uF 35 V axial  2 pcs
8 uF 200 V axial  1 pcs
10 uF 20 V axial 2 pcs    -> 10 uF 25 V 2 pcs

10 uF 10 V vertical  1 pcs...repl. below
10 uF 25 V vertical  2 pcs   -> 10 uF 25 V vertical 3 pcs

10 uF 50 V axial   1 pcs
10 uF 100 V vertical  1 pcs
15 uF  20 V axial  1 pcs -> 15 uF 25 V axial 1 pcs
22 uF  10 V vertical  2 pcs  ->  22 uF 25 V vertical  2 pcs

47 uF  35 V axial  1 pcs.....repl. below
50 uF  25 V axial  6 pcs   -> 47 uF 35 V axial  7 pcs

100 uF 20 V axial  2 pcs...repl. below
100 uF 25 V axial  2 pcs   ->  100 uF  25 V  axial  4 pcs

100 uF  10 V vertical  1 pcs...repl. below
100 uF  25 V vertical  1 pcs  -> 100 uF  25 V vertical 2 pcs

510 uF  25 V axial  3 pcs  -> 680 uF  25 V  3 pcs
820 uF  6 V axial  2 pcs  -> 820 uF  16 V  axial  2 pcs


List of aluminium electrolytic capacitors per unit:

1st LO Interface
A16  1 uF 50 V  bipolar vertical aluminium electrolyte
A16  820 uF 16 V axial aluminium electrolyte
A16  820 uF 16 V
axial aluminium electrolyte

2182 MHz Phase Locked 2nd LO
A22  6.8 uF 35 V Tantal?
A22  6.8 uF 35 V Tantal?
A22  6.8 uF 35 V Tantal?
A22  6.8 uF 35 V Tantal?
A22  6.8 uF 35 V Tantal?
A22  6.8 uF 35 V Tantal?

A22  6.8 uF 35 V Tantal?
 A22  47 uF 25 V Tantal?
 A22  22 uF 16 V Tantal? 

25 MHz PLL, Harmonic mixer, 829 MHz 2nd Converter, 829 MHz Diplexer, I.F. Switcher, 829 MHz Amplifier
A23  15 uF 25 V
Tantal?
 A23  15 uF 25 V
Tantal?
A23  15 uF 25 V
Tantal?
A23  15 uF 25 V
Tantal?
A23  15 uF 25 V
Tantal?
A23  15 uF 25 V
Tantal?
A23  15 uF 25 V Tantal?
A23  15 uF 25 V Tantal?

Phase Gate Input
A4  100 uF 25 V axial aluminium electrolyte
A4  100 uF 25 V
axial aluminium electrolyte

Phase Gate unit
A4  none alu. el.  (never open it)

PSU:
A30  total:  24 pcs aluminium electrolytic capacitors

110 MHz I.F. Amplifier
A32  none alu.el.

110 MHz Mixer/Oscillator
A34  none alu. el.

Front-panel
A38  none alu. el.

Video Processor
A40  47 uF 25 V
axial aluminium electrolytic
A40  10 uF 35 V
axial aluminium electrolytic
A40  10 uF 25 V
axial aluminium electrolytic

Preselector Driver
A42  560 uF 25 V
axial aluminium electrolytic
A42  100 uF 16
V vertical aluminium electrolytic
A42  47 uF 25 V axial aluminium electrolytic

 
1st LO Driver
A44  10 uF 25 V
vertical aluminium electrolytic
A44  10 uF 50 V
vertical aluminium electrolytic
A44  10 uF 35 V
axial aluminium electrolytic
A44  100 uF 63 V axial electrolytic capacitor
A44  22 uF 50 V
vertical aluminium electrolytic
A44  47 uF 25 V
axial aluminium electrolytic
A44  100 uF 25 V
axial aluminium electrolytic
A44  100 uF 25 V
axial aluminium electrolytic
A44  1 uF 25 V bipolar vertical aluminium electrolytic


Center Frequency Control
A46
22 uF 50 V axial aluminium electrolytic
A46
22 uF 50 V axial aluminium electrolytic

Span Attenuator
A48  47 uF 25 V
axial aluminium electrolytic
A48  47 uF 25 V
axial aluminium electrolytic

Synthetizer
A50A1  560 uF 25 V axial aluminium electrolyte

Strobe Driver
A50A2 
none alumn. el.

Offset Mixer
A50A3  560 uF 25 V axial aluminium electrolyte

Error Amplifier
A50A4  4.7 uF 35 V vertical electrolytic capacitor
A50A4  560 uF 25 V axial electrolytic capacitor

Controlled Oscillator
A50A5  none alumn. el.

Phase Lock Control
A51  none alum. el.

Memory
A54  none alum. el.

GPIB
A56 
none alum. el.

Processor
A58 
none alum. el.

Digital Storage Horizontal
A60
  none alum. el.

Digital Storage Vertical
A61  none alu. el. (note: plastic washer on one PCB mountig screw near Q2035!)

Log Amplifier
A62  none alum. el.

Deflection Amplifier
A64  none alum. el.

CRT Readout
A66  none alum. el.

VR#2, Band Leveling, 2nd Filter Select, Post VR amplifier
A68A1  none alum. el.

Z-axis
A70  none alum. el.

Sweep
A72  100 uF 16 V vertical electrolyti capacitor

A72  10 uF 100 V vertical electrolytic capacitor
A72  100 uF 25 V axial electrolytic capacitor

High Voltage
A74  8.2 uF 200 V
axial aluminium electrolyte
 
1st LO connector
C107 22 uF 25 V vertical aluminium electrolyte


#10.  The Preselector driver card had one of the card edge connector's outer row contact's latch broken (unit forced-in misaligned?). This damage allows the outer row socket contact to get pushed back when the card is in the slot - you can not see it. Thus, there may be intermittent, or no contact. In this case there was a slot cracked on the card edge connector's green plastics where the contact's latch hooks should be locking on to. This contact carries data from the backplane data bus to change the Preselector tuning voltage gain (path: 74LS377 to LM339N to a J-FET) in to LM308 op. amplifier, changing the Preselector tracking to "mode" x3 to allow 3rd harmonic Preselectror tuning tracking. The edge connector's latching damage was repaired with polyethylen welding gun. A similar broken edge connector was also found on a memory board and was fixed the same way.

The preselector calibration process did not go as the manual says, even the 1st adjustment with 30 uV meter failed to adjust any lower than about 200 uV. Related or not, at some step of the procedure, one or two of the trimmers went to end-stopper when peaking signal and
the front panel PEAKING knob ended up CCW on one upper band. Somehow, by not following the manual, I managed to get the PEAKING knob closer to mid postion on all bands and fair tracking for the preselector, but it took a lot of time and was frustrating.


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). Nowadays there are also synthetized small USB connectable signal generators that cost 1000 - 2000€ and go from upper HF up to 12 GHz or 19 GHz and have stable output power at +10 dBm level.

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.I found replacing the Phase Gate module with a newer one  -02 version, made the Error 59 no longer to appear at all, phase lock was always instant. Switich off the phase lock did sometimes flick on the Error 60, but it went away immediately.

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 (with some 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.

Manuals (op. and maint.) are freely downladable from the Interned, also from Tektronix web site. Spare parts 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. Norway Labs in Oregon, USA, manufactures and sells extension card kit for the TEK 49x series, mail to Matt North. The manufacturer's support has expired. Good repair information is available with Google search, such as at KE5FX.com site.

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, what ever promises you get when buying. Ask for some warranty or R.o.R. Don't pay a top-price if the risk is all yours -  25 kg of electronic junk, 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 seller 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 can not find documents, or 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. 2020-02-05
© I. Yrjölä, 1999... 2020