Tektronix 564 Revisited

I recently picked up a somewhat rare 3A5 auto-ranging plugin for the 560 series, and decided to fire up my 564 to test it out. Since it had been over a year since I powered up my 564, I tried it first in it’s current config, a 3A72 Dual Trace plugin, and a 3B3 Timebase. Two problems were readily apparent:

  1. The varying intensity – I strongly suspect this has to do with AC ripple, since it holds steady when line triggered
  2. The trace is about 2 CM short.

Here’s a table showing measured supply rail voltages & ripple. All voltages are within spec, but the ripple on the -100 V, 125 V & 300 V lines are out of spec.

RailDC VoltageRipple
-100 V-100 V120 mV
125 V125.5 V40 mV
300 V301 V120 mV
-12.2 V-12.21 V2 mV
355 V357 V
420 V435 V1.5 V
475 V493 V280 mV
-3,300 V-3,290 V8 V


  • -100 V: C640, a combo 340uF & 10uF 200 V Electrolytic
  • +125 V: C642, a combo 340uF & 10uF 200 V Electrolytic
  • +300 V: C644, a combo 340uF & 10uF 200 V Electrolytic – this is only a 200V cap since it’s low side is off the 125V supply.
  • +475 V: C646 2x 40uF

So that’s a rebuild of 3 cans. C642 & C644 should take care of the ripple on the +420 V unregulated rail, which feeds the high voltage supply.

Using a fluke 6kV probe into my 7854 scope, I was able to measure about 50 V of high frequency ripple on the 3,300 V supply, however I’m not 100% certain that measurement was accurate. I could pick up about that much just by having the probe within a few inches of the HV supply, so I’m not sure how much of that was inductive pickup through the probe, or even the cable of the probe? I’d love someone to set me straight on that.

Following some recommendations on this old post on the tekscopes group, I gave the ceramic binding posts a thorough cleaning with IPA & cotton swabs. So far, I have not seen the intensity modulation problem return that didn’t seem to eliminate the problem, as it returned the next day, albeit not immediately, but after 30 or so minutes of operation. I did discover that with the cover reattached over the HV transformer & diodes, I was no longer able to detect the HF ripple when measuring high voltage. The AC ripple still disappears as soon as it’s measured.

Replacing C342 & C344 greatly reduced the ripple in the 125 V & 300 V supplies, but did nothing for the ripple on the 420 V unregulated, and hasn’t fixed this intensity modulation issue (I was pretty sure it wouldn’t though). I have confirmed that the intensity modulation is at 60Hz, and isn’t related to any errant signals from the horizontal plug-in by running the vertical plug-in in the horizontal plug-in’s slot, and feeding it a 60Hz ramp. The issue reoccurred.

As for the sweep length on the 3B3 – For now I was able to get the trace to the correct length using the sweep length adjustments pots, however, I had to crank them both (the normal and delayed sweep) fairly far clockwise. Once adjusted to the correct length, I do get sweep waveforms as documented in the manual, so I don’t suspect that I’m compensating for a problem further downstream.

Beam Intensity

There are, I believe 3 distinct different methods to modify the beam’s intensity:

  1. J21-14: Intensifying Pulse – changing the intensity of the beam by altering the ground reference of the control grid supply
  2. J21-13: Unblanking Pulse – deflection blanking
  3. J11-24: Chopped Blanking Pulse –

Tektronix 7854: Continued Woes


Well shit – it seems as though something else went wrong with this scope while I was slogging though the ROM problem.  It’s not the display board.  We’re not even getting to the point where that’s being checked.  It seems to quickly go into this vegetative loop pretty much immediately.  I put back the old (bad) ROMs to at least see if I could get the ROM error pattern again, but nothing – just all lights lit.

Did some probing on the chip-select lines on the ROM card, and it’s clear that it’s going through some quick little loop, but it’s not always the same loop on each restart.  I’m hoping to find a way to reset the CPU w/o cycling power.  I think I can short TP1200-1 to ground, but waiting for someone to confirm so I don’t invariably nuke something else.

I confirmed that all power supply rails are spot-on.


Tektronix 7854 – back on the bench

I bought this unit a few years ago when I happened on a cache of Tektronix gear being sold by the sons of a deceased collector in NYC. I remembered that I powered it up enough to discover that it suffered from the infamous ‘ROM Rot’ issue that plagues these machines.  The machine is seen in it’s vegetative state below – This specific pattern of illuminated mode switches means the machine can’t initialize itself, because it lacks the instructions to do so.

Dude where’s my socket?

Unfortunately, I’d since removed the IEC socket – I vaguely remember it was cracked. Replacing that would be the first order.

Getting to the inside to attach the connector is a bit of an exercise in contortion, but it’s manageable.  The biggest annoyance in working on these supplies is that even after disconnecting all of the connections, it’s still tethered to the mainframe by the cord to the power switch (I’ll bet there’s a way around this, but it’d be at the switch end).

It’s usually hard to plug things in incorrectly due to how they’re keyed, but I take photos just in case.

The other annoyance is reconnecting the line to the HV power supply.  It passes through a hole in the shield of the supply, and once you disconnect it, you can’t plug it back in with the shield in place.  And, there isn’t enough slack to remove the shield, so you have to:

  • disconnect the cable from the other end, up in the high voltage section (careful!)
  • run it through the hole in the shield
  • plug it in to the power supply
  • reattach the shield
  • re-route and reconnect the cable back up into the HV supply.

With the power supply reassembled, we’re back to square 1 – no ROMs.

Tektronix 7854 – ROM Rot

This is an attempt to capture all of the knowlege that exists regarding the ROM issues on the Tektronix 7854 Oscilloscope.  My machine, serial # B042029 suffered from this problem, hence my interest. If somehow you’re reading this and have any corrections or questions, please chime in.

The Case of the Rotting ROMs

One of the more common problems with the Tektronix 7854 is Rom rot.  Over time, the Mostek mask ROMs start losing bits.  It’s a well known issue that has been discussed a number of times.  Why am I bringing it up one more time?  Because I don’t think all of the information has ever been compiled into one place.

There are a number of resources out there:

  • The Vintage Tek Museum documents a method of using modern replacement EPROMs with a socket adapter board.
  • This Zipfile that the Tek museum’s page references.  This contains a text file by Pentti Haka from 1999 that so far is the best explanation I’ve found of what’s going on under the hood (Patched, 16k ROMs)
  • This document from 2011 by Goran Krusell that describes much of what Pentti Haka originally outlined.
  • Dave’s repair blog has some notes on the subject, that I think ended up in the Tek Museum’s page
  • This Zipfile from David DiGiacomo (Patched, 8k ROMs)
  • There are a number of threads the TekScopes group.  Some of these go back 7+ years, have dead links, and dead ends, but there’s still lots of good info.
  • And there’s this thread I’ve got going on the Old Tek Scopes Facebook group.

What’s on the board:

Above is a photo of the original ROM board.  It contains:

  1. 16k words of ‘base code’ starting at address 0000, spanning across (4) 8k byte mask ROMs from Mostek (upper left)
  2. 2k words of patch code 0x8000, spanning (2) 2k byte EPROMs (upper right)
  3. A Field Programmable Logic Array (FPLA) in the lower left

The FPLA (an early FPGA) watches all 15 address lines and outputs 6 address lines and a flag.  When an address is called that has a newer segment of code on the EPROMs, the FPLA decodes the incoming address, puts an altered address on the board’s bus, and sets a flag that switches the chip selects from the main ROM to the patch EPROMs. The patch itself is only a few words, enough to jump to another part of the EPROM where a new segment of code resides.  The EPROMs also contains ‘normal’ code that is directly addressed.

A note on versions:

Early versions had an external battery backup option, as evidenced by a pair of banana jacks on the rear, and separate RAM & ROM boards.  I believe there to be 2 or 3 versions of the firmware for the original board, denoted by the last two numbers of the part#s. The versions of the EPROMs must match the FPLAs.

David Hess believes there were:
2 Mask ROM versions
3 FPLA versions
3 Patch EPROM versions

Dan on the Teksopes group reports his version 1.03 has the following:

  • U100 160-0408-01
  • U110 160-0409-01
  • U120 160-0445-02 (Signetics 3-state FPLA)
  • U200 160-0410-01
  • U210 160-0411-01
  • U400 160-0466-02 (8x2k EEPROM)
  • U410 160-0467-02 (8x2k EEPROM)

On my version, all chips have a -00 suffix.

Dan and I have confirmed that the contents of the -00 and -01 mask ROMs are identical.  Thus the difference in versions is only in the FPLA & EPROMs, and the ROM part numbers were only incremented due to a change in supplier, from Mostek to Motorolla.

Holger, who is working on a replacement board (more below), mentions a version 2.0, which I believe is what’s on the later, combo RAM/ROM boards.

I still don’t know for certain how the suffix on the part numbers map to firmware versions.

Late versions (Serial B100000 and above*) had a built-in battery backup on a combined RAM/ROM/Backup board, which occupied the larger RAM slot, forward of the ROM slot.

* That’s according to the change info, but Bob – N3XKB reports his SN#B100078 unit still has the old configuration.

Raymond on the TekScopes board: explains the difference in behavior in a working unit:

With the original version with external battery backup, recognisable by the two banana sockets (red and black) on the rear panel, you’ll see all lights coming on for a few seconds, then most will switch off, you’ll hear a beep (if that’s enabled with the switch on the rear panel) and you’ll see the text “self test complete” on the CRT, if your Readout intensity setting is sufficiently high.
If you then press the “stored” key (slightly to the left above the blue key), you’ll be able to see the horizontal stored trace (a flat line if nothing stored) and you can see the readout text of the stored mode.
With the newer version (integrated ROM/RAM, internal backup), you’ll either see and hear the same or you won’t see anything, apart from the power light and the graticule illumination if that’s on. It depends on the position of the switch on the rear panel that allows you to start up with self test of from backup power.

Late version combined RAM/ROM card

What are our options?

Fortunately, ROM images are available, and we have a few options in how to make use of them.

relevant EPROMs



Pin count



8k x 8


25 V


8k x 8


12.5 V

20 - 50ns

2k x 8


25 V


16k x 8


12.5 V


None of the above EPROMs are in current production, some are easier to acquire than others, and only the 27128A is programmable by inexpensive USB programmers such as the TL866.  Suffixes are important, as they denote the access speed.  Several people have confirmed that 350nS EPROMs are necessary and have reported intermittent issues in using 400nS chips.

Option 1: Programming pin-compatible EPROMs with original images

This approach uses the original images, and relies on a working FPLA, and matching patch EPROMs.  You can use either (4) MCM68766C35 or CY7C64 EPROMs, and the -00  images on the tek Wiki. (also zipped here)

This is a good option for initial troubleshooting, but as it relies on a working FPLA, may still be prone to future issues.  The stability of the FPLAs is somewhat unknown, though there have been reports of them being a source of trouble.

Option 2: Programming pin-compatible EPROMs with patched images

Most solutions that have been described elsewhere involve the use of patched images, which already include the patch terms.  This obviates the need for the FPLA (in fact it must be removed), but still requires the patch EPROMs.

You can again use either (4) MCM68766C35 or CY7C64 EPROMs to replace the mask ROMs, and if you have anything other than -02 version EPROMs, you must replace those as well with (2) 2716-1 EPROMs.
This zipfile from David DiGiacomo contains the images required.

Option 3: Programming 28 pin EPROMs with combined images

This is the solution documented on the Vintage Tek Museum’s page, and requires (2) 27128A 16k EPROMs, which are also out of production, but easier to acquire and program. Using a pair of 16k EPROMs covers the address space of 4 original 8k ROMs.  These require adapter sockets, which may or may not interfere with the ability to retain the GPIB board, depending on who you ask. As with the above option, if you have anything other than -02 version EPROMs, you must replace those as well with (2) 2716-1 EPROMs.

You must also rewire A13 & Chip Select lines now that the address space is handled by 2 chips instead of 4.  From the Tek Museum’s page:

Wire A13 by connecting both 27128 pin 26 to U425 pin 1.

Wire new CS. Start by disconnecting the existing CS from U200 and U210 pin 20. We did this by simply not connecting pin 22 in the EPROM adapter (which connects to pin 20 on the PCB) and then flying a new CS wire directly to pin 22 of the EPROM:

Option 1: Add an unused gate by connecting both 27128 U200 and U210 pin 22 to U225 pin 11. Connect U320 pin 13 to U225 pins 12 and 13

Option 2: (We have not implemented this so consider it unverified) Cut and lift U420 pins 9 and 13. They will float high or you can connect these pins to +5V.

This zipfile from Pentti Haka contains the images required.



Option 4: Build a new board

Holger Lübben is working on a new version of the combined ROM Diagnostic board.  He’s still testing it and will release it when it’s done.

Notes on the Memory Map

The CPU is the Texas Instruments TMS9900Here’s another good overview of the chip and it’s pin-out.

Paraphrasing from a few notes across the Tekscopes group:

  • The total space is 64K words of 16 bits.
  • The LSB of addressing is not used (no byte addressing), so the address bus is physically 15 bits.
  • All peripheral devices seem to use dedicated select lines, DMA and interrupt communication with the processor, so they do not occupy space in the memory map.
  • Address references printed on the boards are byte level
    • The 16 bit addresses run from 0000 to FFFF (128K bytes or 64K words).
    • RAM addresses start at A000 and run to DFFF (16K bytes or 8K words).
  • Since the LSB is not used, it is 32K words of 16 bits or 64KBytes of total address space.
  • The main ROM is 32KBytes so it extends to 7FFF. On the old ROM board,
    the patch ROM address space starts at 8000.

Here’s a puzzle: Why did Tek provide two unused memory chip sites on the new board? There is only space in the address map for another 8K bytes or 4K words between E000 and FFFF.

If I understand correctly, with the removal of the patch ROM at 8000, Tek may have changed the RAM starting address from A000 (old RAM card) to 8000 (new combined memory card). That would mean the firmware was changed, and the new ROMs may be able to address RAM from 8000 to FFFF, which is double the old amount. They have only installed RAM for 8000 to BFFF, and the unused sites on the new memory card would be for C000 to FFFF.


Diagnostics boards & Calibration Fixtures

Eggie Siert Writes:

Hi to All,

Nice that Brian put some energy in contacting the buyer of the 7854 with the 067-0961-00 Diagnostic Memory Board.

In Post 62147 I copied a list of Fixtures to service a 7854 (see below):

” In the past I made some overview of 7854 dedicated Calibration Fixtures (7854 Service Manual: Maintenance Section 3-7/8/9):


  • 067-0892-00: Tektronix Microlab 1 Mainframe (provides power for service package and in conjunction with the RS232 Compatible Terminal control over the 067-0961-XX Diagnostic Memory Board)
  • 067-0911-00: Diagnostic Test Interface (serves as a interface between the Microlab 1 Mainframe and the digital portion of the 7854)
  • 067-0912-00: Analog Test Board (used to isolate the analog circuitry from the digital portion) Plugs into A29-Display Board
  • 067-0913-00: Extender Board 44-Pin (used with the A30-GPIB and A31-ROM Boards)
  • 067-0914-00: Extender Board 80-Pin (used with the A27-MPU and A28-RAM Boards)
  • 067-0915-00: Extender Board 124-Pin (used with the A26-Control Logic and A29-Display Boards)
  • 067-0961-XX: Diagnostic Memory Board (contains a portion of the service package firmware, as well as 7854 specific troubleshooting stimuli and diagnostics)
  • 070-2972-XX: Signature Tables (is a complete and cumulative (historical) document of the firmware in the digital portion of the 7854)
  • RS232 Compatible Terminal (In conjunction with the Microlab 1 provides control over the 067-0961-00, the A2-Test Processor Board and the 067-0911-00 Diagnostic Test Interface)”

I never saw on eBay or else the 067-0911-00 Fixture which is really needed to take advantage of the 067-0961-00 Diagnostic Memory Board. The 067-0911–00 and up Manual is on TekWiki and you must have access to both big Volumes of the “Test Procedures for 7854 Diagnostic Troubleshooting using the 067-0911-00 and up Diagnostic Test Interface”

 Successful Failure I’m a dumbass






It was this chart that led me to the ROMs in the first place, and unfortunately, it’s also this chart that confirms that I have a bad display board.  After reading out all of the ROMs, comparing them against the copies on the wiki, I was able to identify that U210 was bad.  After a few false starts, I burned a replacement MCM68766C35, verified it, and re-installed the board.  The new pattern of indicator status lights confirms that:
1) I succeeded in rectifying the problem on the ROM board (YEAH!)
2) The display board is also faulty.

WRONGReading Is Fundamental.  I have ALL lights on, which is not listed in the self test.  Which means It’s getting hung up in booting, and I’m likely not done with the RAM card.

Tektronix TM-503 repair

The third bay of my recently acquired TM-503 mainframe ate two plug-ins, releasing their magic smoke.  The it didn’t do this to the module that came originally installed in the 3rd bay, and here’s why:

The mainframe has two power transistors for each bay that are wired to the edge connectors.  They’re meant to be used by the plugin’s power supply.  The DM-502 that was originally in the 3rd bay didn’t actually use these transistors, but the DC504 and FG501 did.  The photo below shows 3 of these transistors.  The one on the far right is for the center bay, and it has a mica disc insulating it from the chassis.  The two on the left did not have those mica insulators.  Some knucklehead that replaced the left-most transistor (center one removed by me), forgot to re-install the mica disc, and I found one floating around the inside when I opened it up.  This meant that one of the pins was shorting to ground, and he was only lucky not to get bit because of the plug-in he was using.

You’ll also see a 5v regulator bodged onto the unit, it’s output has been snipped.


Here’s the damage caused by the error:

A blown trace on the DC504IMG_5776.JPG

And one of two blown resistors on the FG501.  I’ll have to order replacements, they’re .27Ω, of some reasonable wattage.  IMG_5777.JPG

The trace repaired:

And the mica replaced:

DC504 is back to life. It’s not the greatest counter, but it claims it’ll go up to 80Mhz.

Tektronix 514: When are you from?

Here’s some of my notes about trying to understand a little bit more about the 514, in particular, mine (S/N 948)

The usual questions are:

  • When were they made and for how long?
  • how many different variants were there, and which were visibly different vs which ones were just internal revisions.
  • When is mine from, and which are the correct schematics?  Usually the schematics will indicate a range of serial numbers for which a particular schematic sheet is valid for.

Some of the things I observed in the manuals:

  • the schematics in the BAMA manual for this manual aren’t just raw scans, they’ve been re-composited.  It’s evident elsewhere in the manual as well.  Generally whoever put together the PDF manual did a nice job, but it appears they cropped out the serial number info when assembling the PDF schematic pages.  d’oh.
  • There’s a few pictures in the BAMA manual:
                          SERIES “A”
      With the “serial” field on the bottom left of the unit.  Mine like this, except without the ‘SERIES “A”’ text. (no S/N shown next to “Serial” callout)
      with the serial directly under that. (S/N #5181)
  • Comparing the BAMA schematics to the paper one I bought, I can confirm that the BAMA manual schematics are for S/N 3150 – 3408.
  • There’s a note in my manual that says:
    “Major circuit changes occurred in the Type 514AD oscilloscope at S/N 3409. Numbers in parenthesis indicate the earlier values in the Type 514D oscilloscope.”
    So the ‘A’ version is anything after 3409.  OK great.
  • But, looking at the versions of the vertical amplifier schematic, there is:
    1. “Type 514 / 514D Oscilloscope” S/N 101 – 3149.
    2. “Type 514 Series A Cathode Ray Oscilloscope”, S/N 3150-3408 (same one from BAMA) dated March ’53,
    3. “Type 514 D Cathode Ray Oscilloscope” S/N 3409+, dated March ’56, which has notes about changes in S/N 3650+
  • So it seems like there’s the 514, the 514 series A, and the 514A, all available with or without the ‘D’ for delay line.  The ‘Series A’ seems like it might be the precursor to the plain-old ‘A’ suffix, common of later models, but there’s some inconsistencies in naming conventions across the versions of schematics, so it’s difficult to be certain.  These are the earliest years of the company, so it’s not surprising to see evolution in the documentation.

Observations looking through the catalogs:

  • October ’50 514D is listed for $950.
  • August ’51 514D is listed for $950.  no ‘D’ in silkscreen.
  • March ’52: 514D is listed for $950.  Underside photo shows PS caps identical to mine, but ‘D’ looks like a part of the silkscreen.
  • March ‘53: catalog says there was a 514-D, but there’s no page for it.
  • August ’54: there was -AD only
  • August ’55: gone.

The punchline:

  • Made from 1950 – 1956
  • Mine is from somewhere between ’50 & ’52.  Had to guess I’d say ’51.
  • There were a few different milestones:
    1. S/N 101 – 3149
    2. S/N 3150 – 3408
    3. S/N 3409 – at least 5181
  • some minor changes within the first few hundred are called out in the schematics.
  • They weren’t advertised after ’53, but available until at least ’56.
  • BAMA schematics are incomplete (I’ll upload mine, promise)
  • I wonder if there were examples where the delay line was added post-sale, and the ‘D’ was hand-stamped in, like mine?

Canned air, your troubleshooting friend

After being on for about an hour, the intensity dropped to zero again.  After that I’d fire it up, and it would come on but quickly dim, and the intensity control wouldn’t effect the intensity, but move the beam slightly.

I traced the problem to the -15v supply, and finally the capacitor C143 (which I’d already replaced!).  I zeroed in on the problem by cooling areas using canned air with the bottle tilted to get a blast of frost.   The -15v supply would dip down to about 10v shortly after power up, but would rocket back up to -15v when I hit it with the frost, and I’d see the beam change.  A handy trick! With a fresh 10µF in it’s place, and it seems to be working for now.  I did need to hit the CRT bias trimpot to get the beam to extinguish with the intensity control fully counterclockwise.

Tektronix 620 XY monitor repair

This is a wholly unremarkable early 80’s XY CRT monitor.  It’s like, the least interesting thing in my collection, but I’m taking a workshop in vector graphics next weekend, so I figured I’d pull this out of the ‘upstate home for wayward oscilloscopes’ and see if it worked.


It did not.  I got a full intensity spot on the center of the screen, with no ability to move it around or adjust it’s brightness.  I bought the manual from Artek Manuals and off I went.

I actually spent way too much time on this repair; learning how the power supply worked, re-drawing schematics, and making detailed measurements.  I’ll spare you all of that.  It was a bunch of tantalum capacitors.  In gear of this era, it’s a very common mode of failure.  They usually fail shorted, so they’re easy to spot with a resistance measurement in situ.  A shorted capacitor can mean bad news, and tantalums have a reputation for failing violently, spewing their gooey capacitance all over the innards of your gear.  Fortunately, this is Tektronix, so the power supply has a few different layers of protection to minimize the collateral damage caused by single part failure.  The problem manifested as a low voltage coming from the 15v supply.  Under no load, this would float around 19v, but with a modest load, of 100Ω or so, it would happily regulate. The amplifier board had some shorted tantalum capacitors (C397, C398, C401, C402), which brought the load way down to a few ohms.  Fortunately, there’s a clever little current regulator in power supply that drops the voltage if there’s a near-short like this.

Replacing these got the 15v supply working, but uncovered/caused some issues in components elsewhere, notably caps in the -20V & -70V unregulated supplies.  I also inadvertently roached one of the capacitors while testing it, and a probe slip took out a diode.  It was… not my proudest moment.  There’s also a few fuses that saved my stupidity from making even more of a mess.

Here’s some shots of the re-worked area:

With C142 pulledIMG_5267.JPG


Replaced C142, CR142, CR143, C143.  IMG_5268.JPG


I brought it up on the variac and was rewarded with a controllable trace that responded well to inputs, waiting for some vector graphic goodness.  I smell a clock project in my future.

I will say, I’m used to working on the giant old tube gear, which is simply a joy to service.  This on the other hand, was not Tektronix’s brightest moment in industrial design.  Getting this thing apart felt needlessly belabored.  Just getting the boards disconnected from eachother was a giant hassle because of how one of the cables was run and how a shield just overlapped the board enough to get in it’s way.

That’s the power supply on the top, the amplifier board on the bottom, and the connector in the center of the image.  There’s a line voltage cable pinched between the amp board, and a transformer shield, which I had to loosen to get the two apart. To get the amp board out, you had to remove the whole back, which of course the CRT was attached to.

Also, no pilot light?  This was by far my least favorite piece of tek gear to work on thus far.  I’m just glad I got it working and off my bench.

Here’s the compulsory Lissajous pattern.  There’s some DC offset, but I think that’s the Waveforms audio oscillator.