2 years ago, I attempted to troubleshoot and repair an intensity modulation issue on my 564, but ended up giving up after a few unsuccessful attempts. I had replaced a few power supply caps, and all voltages & ripple were within spec. I thought maybe it could be an issue with the CRT? Last month, a spare CRT popped up 30 miles away for $40, so I grabbed it. I made the swap, and the issue remained.

I have since changed out the remainder of the electrolytic caps in all ‘low voltage’ supplies (all but the -12.2V – I don’t have a good way to mount new caps, and the ripple is really non-existent). I swapped the vertical and horizontal plugins, and never saw the issue, but I think this is a red herring, as when I put just the vertical plugin in the horizontal slot and feed it a 60Hz ramp, I do see the issue. I did this 2 years ago, and confirmed it today. The other thing I did to eliminate my suspicion of the horizontal plugin was to pull the transistor that drives the intensifying signal, Q294. When I pulled it, the issue diminished a little bit, but was still present.

Other things that didn’t work:

Disconnecting the intensifying pulse input – had to leave R838 & R839 connected, otherwise the beam intensity was unstable.

Maybe there was some heater to cathode leakage in the HV regulator tube? I swapped it out with a 12AU7 (similar pin out, close enough performance), the same issue occurred.

The ripple of on the primary of the HV transformer seems negligible, and in line with what I’d expect on a simple unregulated supply. Allowable ripple isn’t listed on the manual like it is for the other rails.

May be on to something: I’m seeing at least 5V of ripple on the grid of V800.

I thought the ripple on the 300V supply was in spec, but I just noticed a filter cap I missed on the other side of the regulator, that’s tucked away in the back of the scope. No change after swapping out that cap, but I think we’re getting closer. When I see problem, I’m also seeing ripple measurements jumping around. When the trace is solid, the ripple is rock steady at less than 5mV.

Things to suspect next:

• V674, the 6AU6 error amplifier
  Nope.
• V677, the 6AS7 series pass tube
  Damn, this was looking like the culprit at first, but the issue returned not after not too long. Swapped the old tube back in and issue eventually returned.
• C670 blocking cap – worth a try?
  Nope.

Worth noting that we can still see the ripple on the 300V rail even if we pull the plug-in.

Turning our attention back to the HV section, looking at the input of the HV regulator error amp, before R816. It’s bang-on at -106V. The problem has been intermittent, and hasn’t happened for a few minutes since I gave that area of the HV supply a blast with cold air (compressed air upside down). Yep, OK, it’s back and the ripple is evident here as well – still not clear if we’re getting closer to the cause, or are just seeing the effect on the +300V rail. OK, this kind of sucks, but I’m getting to the point in the program where we just start swapping out caps. Also waiting on a 6CZ5 to swap out V800

• C841 – .047u – no change
• C849 – .001u – no change
• C801 – 0.2u ? just a disc cap
• C807 – .001u – says it needs to be 600V, but there’s only ~500V across it. Gonna risk a 600V for now.

SOLVED. It was V634. The 6DJ8 in the -150V supply. The supply that has NOTHING TO DO with any of the HV Circuits, and which exhibited NO RIPPLE. F*&k, this was a hard-earned win.

Side note – I nuked Q294 while I was troubleshooting – it runs the intensifying pulse, and I was yanking it and stuffing it back in live. I put in some basic bitch NPN transistor and it works fine.

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.

Replacements:

• -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 –

Continued here, 2 years later…

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.

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.

  • Tek 7854 Doesn’t Come On... : 2014 –  this subject seems to be broken across a few different threads.  David Hess talks about designing a replacement board, but I can’t find any evidence of it.

  • 7854 ROM, Again: 2014, 16 messages – talks about the Pentti Haka ROM process.
  • Tek 7854 Info, Repair, Restore: early 2016, 109 messages – no new information, besides the mention of a 1.03 ROM version.
  • 7854 RAM card upgrade only, any interest?: 2016 – 2017, 128 messages – There’s a lot of discussion on the memory map.  Nathan Johnson was working on a prototype of a replacement board, but the trail goes cold.
  • 7854 Fixed, now for some newb questions: late 2018, 18 messages – Author mentions he had to forgo the GPIO board to get the new ROMs to fit.
  • 7854 firmware: 2013, David Hess talks about firmware versions (info below)
  • 7854 ROMs: The Whole Story: This is the thread I started in April 2020.
• 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

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 TMS9900.  Here’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.

WRONG – Reading 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.