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Tektronix T922R Service Notes

The Tektronix T922R oscilloscope was discontinued in the middle 1980s; Tektronix ended long term support in 1995.  Tektronix no longer stocks parts unique to the T900 Series scopes.  Nevertheless, the T922R remains in daily use in wireline logging trucks all over the world.  This page contains some random notes on servicing these old workhorses.

Old Tektronix manuals are truly works of art.  Reading the manual and studying the schematics is an education in and of itself.  The T921/T922/T922R manual is a combined operating and service document.  As a courtesy we offer the T921/T922/T922R Manual for download, a 14.3 megabyte pdf file with a higher resolution version available on CD from the publisher (see the bookmark for details).  If you need to troubleshoot a T922R, start with the flow chart on pages 5-4 and 5-5 of the manual.

If the oscilloscope is completely dead, of course first check the fuses.  T922Rs have been known to blow fuses when turned on in cold weather.  F722, a 1/16 amp slow blow fuse in the 100 volt power supply circuit, has a habit of blowing if the line voltage goes high (as in generator runaway). The power indicator pilot lamp on the T922R flashes if the line power varies more than 10% above or below the value for which the internal power selection switches are set.  T922Rs below serial number B011950 have a neon power indicator pilot lamp, and those with serial number B011950 and up have an LED power indicator.  Some of the T900 Series did not work well with 50 Hz line power; the power transformer core saturates and radiates 50 Hz noise into other parts of the scope circuitry.  A mu-metal shield for the transformer cures the problem.  The extent to which this problem specifically occurs in the T922R is not well documented.

As with all electronic equipment, component failures are increasing as the T900 Series oscilloscopes age.  The filter capacitors on the power supply board are now prone to failure; look at C742 and C743, both 2,200f at 16 volt electrolytics, and C722, a 100f at 160 volt electrolytic.  In plastic case units only, VR784, a 33 volt zener diode, can fail and cause loss of the CRT display.  VR4367, a 4.3 volt, 0.4 watt zener diode on the vertical board, causes a no trigger condition when it fails, a common problem.  If R458 gets very hot or burns out, check C458 for a short, check T460 for a shorted primary, and/or check Q458 for proper operation.

The .022f, 400 volt vertical input coupling capacitors (C4102 and C4202) sometimes go open or short out.  If your T922R works when connected to a panel equipped with its own scope output coupling capacitor (SIE bond panels, for instance), but misbehaves when connected directly to the line, suspect a shorted C4102.  The vertical input dual JFET transistors (Q4122 and Q4222) fail occasionally.  They can be replaced with an ECG/NTE 461.  These two problems with the vertical input are often caused by exceeding the rated input voltage, but also look for a shorted CR4123 or CR4223 as a cause of the dual JFET failure.  U4134 and U4234, CA3086 / LM3086 devices, are prone to failure, causing the trace to go full high or full low with location only possible using the "Beam Find" button.

Problems have been reported with the various interconnect cables.  Check all internal wiring for bad connections at the various connectors.  The scale illumination bulbs, DS816 and DS818 can be replaced with a 1764 lamp, but the 2187 is rated at 7,000 hours as opposed to the 1764's 4,000 hours with only about a 10% decrease in light output (if you can stand a 40% reduction in light output, the 2185 is rated at 50,000 hours).  Shorts or opens are sometimes seen in the high voltage tripler assembly (U460).  These are potted assemblies not intended to be repaired at the component level.  At present there is no known source of replacement tripler assemblies, except from scrapped instruments.  Stan Griffiths, scope guru, has indicated he is looking for a solution to this problem; visit Stan's Scope Site, and the Tektronix Resource Site which he co-created.

There were two CRTs used in T922R scopes:  from serial number B010100 to B011371 a 154-0279-00 CRT was installed, and in serial number B011372 and above a 154-0804-00 CRT was used.  The 154-804-00 CRT in the later production T922R scopes (it was never used in the plastic case version T921 or T922) has a "halo reducing mesh" which was probably incorporated due to complaints about sharpness, and likely because of the VDL application.  The 154-0279-00 CRT does not have the "halo reducing mesh" feature.  Either CRT can be installed in any T922R scope, but if the 154-0804-00 is installed in an early model T922R, or in a plastic case scope, be sure to take CRT pin 12 to ground.

The T922R cam switches frequently give problems.  It is not uncommon to find dead positions in one of the "VOLTS/DIV" switches located on the vertical attenuator boards, or in the "SEC/DIV" switch located on the horizontal board.  A very dirty vertical attenuator cam switch can cause an erratic display in all positions.  Reassembly of cam switches is a nightmare, so cleaning is usually attempted first.  Only a residue free cleaner should be used, and one that will not attack the plastic parts in the switch.  Do not lubricate the switch contacts (no WD-40 or the like).  Tektronix recommends plain old isopropyl alcohol.  We have had good luck with Caig Laboratories' DeoxIT cleaner.

Check out Cleaning Secrets Revealed for some tips on how to make your scope look like new.  The plastic polishing technique outlined near the bottom of said page is useful for removing scratches from the CRT implosion shield and/or the face plate (lens or filter).  Done properly, even a badly scratched face plate can be made to look like new.

Below is a cross reference list of some of the T922R semiconductors.

T922R Tektronix to Generic Semiconductor Cross Reference

   TEK PART NO  GENERIC  TEK SCHEMATIC DEVICE LABELS
   151-0126-00  2N2484   Q446
   151-0127-00  2N2369   Q134 Q136 Q144 Q146
   151-0188-00  2N3906   Q166 Q446 Q454 Q2108 Q2124 Q2134 Q2136 Q2174 Q2176
                          Q2326 Q2332
   151-0190-05  2N3904   Q160 Q162 Q416 Q424 Q810 Q2246 Q2274 Q2314 Q2354
                          Q2422 Q2432
   151-0192-03  MPS6521  Q2314 (Also TPS6521)
   151-0199-00  2N3640   Q112 Q122 Q4174 Q4176 Q4274 Q4276 Q4302 Q4303 Q4310
                          Q4311 Q4312 Q4313 Q4322 Q4324 Q4336 Q4346
   151-0216-02  MPS6523  Q2176
   151-0223-03  2N5769   Q2412 Q4372
   151-0224-00  2N3904   Q792
   151-0301-00  2N2907A  Q772 Q774
   151-0302-00  2N2222A  Q752 Q754
   151-0347-00  2N5551   Q426 Q722 Q726 Q734 Q796 Q818 Q2334 Q2344
   151-0350-00  2N5401   Q434
   151-0358-00  D44R4    Q458
   151-0423-00  TIP50    Q458
   151-0434-01  2N4261   Q4322 Q4324
   151-0478-00  TIP31A   Q756 Q776
   151-0497-00  TIP47    Q736 Q812 Q816
   151-1042-00  2N5454   Q2104 (Q2106) Q2114 (Q2116) Q2242 (Q2244)
   151-1090-00  D/2N5397 Q4122A,B Q4222A,B (Use ECG/NTE461)
   156-0197-03  LM3086   U2126 U4134 U4234
   156-0405-02  N9602N/F U2224
  Note:  A suffix other than "00" indicates a selected transistor.

If you need more information or are working on other Tek equipment, the Tektronix Resource Site has an amazing Tektronix to Generic Transistor Type Cross Reference.
Sphere also has a semiconductor cross reference, and other useful stuff.
Much of the above was inspired by Paul Knight, our logging equipment repair guru.

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Manual background used with the kind permission of the Tektronix Resource Site.
03-05-01
Last 10-20-10