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 514 – Back at it

Well, after 4 years and two moves, I’m finally back at this old beast. I’ve chronicled my previous work here, and where I left off was getting a sharp, swept trace, if I disconnected the +225V line from the upper deck where the Vertical circuits are. There are some leaky caps up there, and with them in the circuit, they pull a bunch of other things low, including the HV section (I don’t remember why, but it made sense at the time).

This is now a SEVENTY YEAR OLD machine, and given that every Black Beauty & Bumble Bee capacitor I’ve come across so far has been the cause of trouble, I think the best course of action is simply to replace all of them.

After changing / checking / reforming every cap on the upper deck, I still was having problems with the +225V supply, so I turned my attention back to the regulator. It turned out that the series pass tube wasn’t working: All of the current was flowing through the bypass resistors, so the voltage level was based only on the current. I confirmed this by pulling the Series pass tube (6AS7), and noted no change in the behavior. Working back from the voltage trim-pot, I discovered that the 12AX7 comparator wasn’t working properly. The socket needed some cleaning, and then the whole thing locked right up at 225V.

So, it “works”!
Here it is triggering at 10MHz


  • Sweep linearity: going to change out the bumblebee caps in the timing circuit to see if that helps.
  • Sweep magnifier all sorts of weird.
  • Interplay between vertical position & vertical attenuation controls: need to look into this more – The ‘Vertical Attenuation’ control affects the vertical position of the trace. Not sure if this is supposed to happen or not, but I’d think not.
  • Change in intensity yields change in trace width: It seems to affect the horizontal trace width only, and happens in a counter-intuitive way; the trace gets wider when the image gets brighter.
  • Calibrator has an overshooting leading edge.

Sweep Linearity:

note the difference in slope between the beginning of the sweep waveform, and the end of it. Notice the difference in the slope between the start and the end of the sweep in the traces below. The worst of it was on the 100uS per division setting, and a new cap helped.

Sweep Magnifier:

the sweep wave form when sweep magnifier is turned on and swept through

C124, a .1uF bumblebee was the culprit. Replacing that, and restuffing C126, an electrolytic, solved the problem.

The problem turned out to be, what pretty much every other problem in this scope was – a bad bumblebee capacitor. I’m just replacing all of them as I work across the scope; I replaced this one in passing, and the problem was gone on the next power up.

AC Socket
I’ve never seen one quite like this before. It’s recessed, as are many of the later tektronix sockets, but there’s no ground pins, so the blades are centered.
– Screw spacing = 1 1/2″
– Chassis hole diameter = 1 1/8″
– Socket inner diameter = 1″

According to some folks on the tekscopes group, these were not grounded, but rather accepted common extension cords of the time – searching used General Electric extension cords on eBay yields some good hits.

This picture from the 1952 catalog confirms their shape, and that the socket was not grounded. A member on the Tekscopes group says the manual instructs the user to ground the instrument via the front panel banana jack, but I couldn’t find reference to that.

If I wanted to replace it with a grounded alternative, I’ve got a few options:

  • an IEC cord – Would require some filing of the frame hole to fit, and may require some bodging of the steel case as well.
  • a powercon connector. may fit unaltered, at an angle, but might interfere with the back case, or require it to be modified.
  • midget twist-lock – 2 prong will fit, mostly unaltered, but the 3 pin would require opening the hole in the chassis, and re-drilling the mounting holes.

Here’s the drawing of the 3 pole midget twist-lock. I think this is the best compromise, as it doesn’t involve any adapter plates or square holes – just enlarging some already round holes by 1/8″. Incidentally, note the original date on that drawing. I appreciate that there’s a 55 year old drawing on a manufacturer’s site that’s still a current reference.

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: restoration update

A bag of parts from Newark arrived and I went to work replacing capacitors in the power supply.  First, I finished re-stuffing one of the cans:
IMG_5582.JPG IMG_5585.JPG IMG_5583.JPG IMG_5584.JPG

I’m really happy with how these came out.

Than went to work replacing the old bumblebee caps – here’s a few shots before I soldered and clipped the leads.  C102 is the re-stuffed can freshly installed.



I was conflicted about just ripping out old components and replacing them with new ones, but (and I’m paraphrasing his words from a conversation I had with Kurt): Tektronix used the best components they had available to them at the time.  If they had these newer, more reliable components, they would have used them instead.  You shouldn’t feel bad as long as the restoration is done in the spirit of the original design and keeps up with their level of craftsmanship.  I’m happy with my work so far and think I’m on the right track.

I decided to power this up on the variac.  It was almost midnight, so I didn’t bother with photos or videos, but:

I started to hear the whine of the high voltage power supply around 60 VAC on the input (frequency is around 4kHZ, wonder if this is right?).  Eventually I saw a fuzzy green dot on the screen around 80VAC, that quickly drifted off.  If I turn the intensity control all the way up, I see the familiar glow of on the screen you get when the beam is deflected way off the face.  All other controls are unresponsive.

The +1,500V supply is around 1,000V, the -1,500V supply is around -800V.  I would like to get those missing 1,200V back…  The -140V supply seems to regulate albeit somewhat loosely.  The 225V, not so much, and the 225V adjust control doesn’t do anything.  I’m suspecting I have to replace the reminder of the .01μF bumblebee caps in the supply.  Maybe I should just rename this hobby “replacing old capacitors”.

It’s a start!

514 Status update

A gaggle of capacitors is en route from Newark / Element14, so until they arrive, the 514 is off the bench.  I got enough to replace all bad caps in the power supplies, including the untested HV cans, just in case.  ‘High Voltage Problems’ is a common utterance when speaking about these old beasts, so I want to be prepared to replace anything.  I’ve been staring at the HV power supply schematic for the last week, and I think I’m ready to tackle it.  I got a enough 630v film capacitors in .1, .01 & .047μF to deal with bad bumblebee caps as necessary, as well as 22μF 450v electrolitics to restuff some cans with.

The plan is to replace all the bad caps I’ve identified, replace the rest of the missing tubes (which I have), then slowly bring it up on the variac.  I’m not expecting a trace, but I would like to see a spot, which would indicate that the power supplies are somewhat working.  after that:

  1. Check of all power supplies.
  2. Check the vertical signal path
  3. Get the sweep generator working
  4. Get it to trigger
  5. A thorough cleaning & make a new graticule