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Is my Tek 468 beyond repair?


John
 

I recently acquired a Tektronix 468. When powered up (with digital mode turned off) it will occasionally display a good trace but usually there is no horizontal sweep. The time-base is working but the scope is behaving a bit oddly. For example, CH1 shows up OK but CH2 shows on both CH1 and CH2!. All power rails are spot on voltage. Having checked through all the signals on test points within the Horizontal Amp, Sweep Logic and Trigger circuits, most signals are present, but one or two are missing. There is no horizontal signal to the input horizontal amp. Various signals are present as illustrated in the service manual in the Sweep display logic and trigger section, but with some notable exceptions. For example, there is no pulse feeding the 74LS02 from the 74LS74 which I suspect is the reason for no or intermittent output. There is no start A Swp pulse at TP49 although all other waveforms in the trigger sections are present. There is a possibility that there may be a problem with the sweep logic, possibly the 74LS74 IC on the main board, but I suspect this may not be the full story because the the digital section does not work at all.

On power-up, the 468 is meant to start with a power-on self-test. The first test is the ROM checksum followed by the Lamp Test and then the RAM Verification Test. The manual indicates that if the ROM checksum fails then it will "neither respond to the front-panel switches nor will it operate". It also says that "Front Panel indication of a ROM failure is that the Lamp test is never started".

Well none of the switches on the storage module respond regardless of whether of whether storage mode is turned on or off, however the main oscilloscope function switches do seem to be working. Nothing appears on the 7-seg LED display except 3 dots. The Lamp Test where all the segments of the display light up does not happen which would seem to indicate that the ROM test has failed.

My question is whether the oscilloscope should still operate in non-storage mode? Or is the failur of the storage module the reason why the scope is behaving strangely? I also have read that a ROM can succumb to something called "ROM rot". I don't know much about this, but can the ROM be re-programmed or replaced? Is the ROM code available anywhere?


tgerbic
 

John,

I have a 468 as well but have not used it in a very very long time.

Unless a near impossible part to find part has burned out it should be repairable.

Since this is a processor controlled scope, you need to make sure the processor part of the scope is working to properly troubleshoot. The ROMs could, over time, lose their programming and need to be replaced. A couple of bits here or there could stop the scope from working. This is what people may refer to as "ROM rot". ROM code is available and the ROMs were originally MK36000s, but can be programmed in a 27C64, since finding and programming MK36xxxs ROMs can be a challenge for most people. I seem to remember that you may need to make some wiring change to use the 27C64 chips or use an adapter, if it fits, in place of the original ROMs.

If you have an EPROM reader/programmer that can read the old 36xxx ROMs, the checksums and part numbers should be:

MK36693J-5
160-0459-01
Checksum C2AC

MK36694J-5
160-0759-01
Checksum EF01

If the checksums don't match, the ROM is probably bad. If you are going to replace one, it is best to replace them both.

Hope this helps.
Tony


John
 

Thank you for the part numbers and checksums for the ROMs. Unfortunately I do not have an EEPROM programmer so am unable to verify the checksums or program new ones. I would need a bit of help determining what programmer would be required to read these MOSTEK ROMs? Alternatively, perhaps someone could advise what programmer to get to program 27C4s? I might be willing to invest but then I would also need the code files, which, despite a bit of Googling I have not yet found anywhere. My Googling has turned up the possibility that either Motorola MCM68764 or Cypress CY7C264-55WE are pin compatible, but again, I have no idea what programmer would be compatible with those and still need the files. It seems that the feathertech archive is no longer available.

I did also come across this resource which suggests they may sell them:

https://vintagetek.org/468-mostek-mkb36000-rom-repairs/

I have sent them an e-mail asking whether the service is still available and at what cost.

In the meantime, if anyone has the images, a copy would be greatly appreciated.


Steve Dench
 

I believe the ROM images are on Tekwiki, I think that is where I got them
when I needed them. You will most likely have trouble finding an original
24 pin Eprom, but I used (as i understand most do) make up an adaptor board
to use a standard 28 pin Eprom like 27C64 etc.

On Thu, 17 Sep 2020, 4:44 pm John, <subs@...> wrote:

Thank you for the part numbers and checksums for the ROMs. Unfortunately I
do not have an EEPROM programmer so am unable to verify the checksums or
program new ones. I would need a bit of help determining what programmer
would be required to read these MOSTEK ROMs? Alternatively, perhaps someone
could advise what programmer to get to program 27C4s? I might be willing to
invest but then I would also need the code files, which, despite a bit of
Googling I have not yet found anywhere. My Googling has turned up the
possibility that either Motorola MCM68764 or Cypress CY7C264-55WE are pin
compatible, but again, I have no idea what programmer would be compatible
with those and still need the files. It seems that the feathertech archive
is no longer available.

I did also come across this resource which suggests they may sell them:

https://vintagetek.org/468-mostek-mkb36000-rom-repairs/

I have sent them an e-mail asking whether the service is still available
and at what cost.

In the meantime, if anyone has the images, a copy would be greatly
appreciated.






John
 

Thank you for the pointer to the TekWiki resource. I have found and downloaded the two ROM files. I do have an AVR programmer and a ZIF socket board that can take up to a 40pin DIL chip but I'm not sure whether this can be used to read the MOSTEK ROM or write the 27C64:

https://www.ebay.co.uk/itm/AVR-Minimum-System-Board-ATMEGA16-ATmega32-USB-ISP-USBasp-Programmer-For-ATMEL/202959381564

I am happy to invest in a suitable programmer provided it is versatile enough to read/write a range of vintage EEPROPM chips.


Miguel Work
 

No, this programmer is for atmel AVR ony

Where are you located?

https://www.youtube.com/watch?v=Gdmsni8cGvY

If can program eeproms for you



-----Mensaje original-----
De: TekScopes@groups.io [mailto:TekScopes@groups.io] En nombre de John
Enviado el: jueves, 17 de septiembre de 2020 10:31
Para: TekScopes@groups.io
Asunto: Re: [TekScopes] Is my Tek 468 beyond repair?

Thank you for the pointer to the TekWiki resource. I have found and downloaded the two ROM files. I do have an AVR programmer and a ZIF socket board that can take up to a 40pin DIL chip but I'm not sure whether this can be used to read the MOSTEK ROM or write the 27C64:

https://www.ebay.co.uk/itm/AVR-Minimum-System-Board-ATMEGA16-ATmega32-USB-ISP-USBasp-Programmer-For-ATMEL/202959381564

I am happy to invest in a suitable programmer provided it is versatile enough to read/write a range of vintage EEPROPM chips.


Roger Evans
 

Page 328 of the Tek 468 Service Manual at KO4BB .com (linked to from Tek Wiki) shows how to isolate the analogue and digital portions of a 468 to verify that the analogue section works correctly 'stand alone'.

Page 330 lists a series of basic tests on the processor board unfortunately it is not specific about which of these rely on the ROMs being correct. You would need another scope to do the testing and it would be useful to know how far down the list you can go before you stop seeing the expected signals. Signals like the 5MHz and 2.5MHz clock, ALE and some activity on RD and WR would indicate that the processor is at least operating even if it doesn't have the correct op codes in the ROMs.

Regards,

Roger


John
 

Miguel, thanks. This is what I suspected. I think unless anyone has a better suggestion I might buy one of these anyway but with all the cables, adapters and bits:

https://www.ebay.co.uk/itm/TL866II-Plus-Minipro-Auto-Programmer-Kit-10-Adapters-BIOS-Flash-Motherboard-USB/363091405640

I don't have a parallel port on my PC so the Wilhelm is out and the Martin Everhards 2700 is rather too specialized.

I am in Leicestershire in the UK BTW. It seems that to read the existing ROMs I will need to build an MKB36000 to 27C64 adapter and then reverse to fit the 27C64 into the MOSTEK slots. From what I have read it seems very likely that the ROMs will be "rotted" by now but it would be interesting I think to check them anyway.


John
 

Roger, thanks for the reference to KO4BB.com site and references to the manual pages. I will check that out and report back.


Roger Evans
 

John,

Since you are in the UK, I can offer to program a couple of 28C64s for you, please e-mail me (rdot gdot evans at talk21 dotcom). There are ','s between my initials. If you would find having a device programmer is useful then please don't let me stop you buying one. Beware the 28C64s are EEPROMs and really need the WEn pin 27 tied to VCC to prevent inadvertant writes in circuit.

Regards,

Roger


John
 

Roger, that's a kind offer which I may take you up on. I will e-mail to ask particulars. However, first, here are the results of the test procedure on page 330:

All voltages are within normal parameters.

Tests 1 - 6 were successful. All timing pulses were present and PRDONE was present on U472-16 (140).

Test 7 thru 11 had no signal present, so no signal on ALE, RD or WR or wavefore on TP477 or U255-5.

I'm not sure what 12 and 13 are about and need to look them up, but U255-9 is "stuck" high.

Since the above tests were not completed satisfactorily, I didn't do on to the Trap Test Procedure or the ROM Checksum. I have yet to go through the process on P228. but I don't understand what Service/Options Switch X10000000 means? I am presuming this refers to the row of DIP switches on the top board but am I right in thinking its simply the configuration of the individual switches from, left to right? So first switch ON and all the remaining ones OFF?

I have noted your point about pin 27 on the 28C64 EEPROM.


John
 

BTW, I should also mention that all the pins on the two ICs that I removed were very tarnished. I tried cleaning (very carefully since they seemed rather fragile) with both IPA and Servisol switch cleaner but it made very little difference. I would have added this to the previous post but can't see a way of editing it?


Roger Evans
 

John,

I don't have any direct experience of the Tek 468 or the 8085 processor that it uses but I am fairly familiar with some of the other 8 bit micros, so this is gleaned from the 8085 manual:

If there is no ALE signal (it should pulse once for each byte to/from memory) then the processor is not stuck in a loop. There are three possibilities 1) it has executed a HALT instruction, in which case pins 29 and 33 on the processor will be at logic zero; 2) one of the interrupt sources is permanently active. The 8085 is the opposite of what I am used to, RST5.5, RST6.5, RST7.5, INTR and TRAP are all active high. If any of these inputs are stuck at logic high they will prevent the program continuing (without reading the 8085 manual more carefully I am not sure if this condition can stop the CPU trying to fetch the next instruction, in which case ALE should be switching) 3) the processor is stuck in Reset, in which case pin 36 will be stuck low, eg due to C174 being short circuit or the Run/Reset jumper being in the wrong position (there is also a jumper on TRAP that needs to be in place).

If there are any 8085 experts around then please correct any errors in the above.

Tarnishing of pins could well be the cause of errors and also if you search this site you will find references to a particular type of IC socket (I think made by TI) that is generally a poor design. Do you have any Deoxit?

Regards,

Roger


John
 

Thank you. I have downloaded some information on the 8085 to review, but in the meantime you have given me some things to look at. I should perhaps have been more specific when I said signals are absent. In fact, ALE is stuck HIGH but definitely not pulsing.

I did find a marking on the circuit diagram that says "LO REMOVES INTERRUPTS" on pin 13 of U255 (74LS74). Pin 9 of same is connected to INTR pin on the 8085.Grounding pin 13 of U255 brings INTR LOW and ALE LOW, but still nothing happens. I performed a reset with pin 13 held LOW but it made no difference. I am not sure how one can tell whether data is being read, but I was intending to research that aspect.

RESET_IN is high unless I move the jumper at which point it goes low while grounded. I will check the status of READY, SID, HOLD, TRAP and the various RST pins to see whether that reveals anything. I see RST 5.5, 6.5 and 7.5 on the diagram.

I will do some further investigations and report back.


John
 

Status of control pins on 8085:

READY H (5V)
SID L
HOLD L
RST6.5 L
RST5.5 H (4V)
RST7.5 500kHz pulse as indicated in manual
INTR H (4V)
RESET_OUT L (1V)
RST_IN H


Daveolla
 

Hi John, there is a zip document on Bluefeather site but someone posted maybe yesterday that it was down, with all the info and bin files to programme.
I also did a compilation of all this with a conversion socket and it was on Bluefeather and maybe on Ko4bb site. I can send you both if need be. On this site in the files section there should be a 12KB file 468.zip and is just the bin files. A more complete file with pinouts and data sheets is Tek468.zip 1890KB, not sure where it is but probably Ko4bb anyway.

Below I will post some of the discussions on Tekscope that I had copied to text files and now reside in my Tek 468 folder. Losts of words of wisdom (also check our archive of previous posts) from Tekscopes member Reed Dickinson our 468 guru.

Dave

-----------------------------------------------------------------------------------------------
http://www.ko4bb.com/doku2015/doku.php?id=test_equipment:tek_468_repair_tips
July 2017

468 Repair Tips - www.ko4bb.com
From the TekScopes mailing list:

I just finished a 468 that had a bad rom.

Just for the record, there is an error in the troubleshooting section relating to the testing of just the analog section of the scope. If the processor fails, you will have a hung up trace off screen on the left side of the scope.

The troubleshooting section states to disconnect the p3008 connector on the A12 trigger board, pull the p483 connector cable and jumper the pins. Then, on the A14 vertical mode switch board move the p307 connector to the p206 test mode port.

This places the scope in analog mode and displays ch1 & 2 chopped.

The only problem is that when you disconnect the p3008 connector, you kill the +5vs source that is used on A14 - p206. The +5vs comes from the power supply on the timing board and powers all the digital logic.

The work around is to use a jumper to the +5 on the interface A15 board (test point) to pin 3, 4, or 5 on the p307 cable to force it to 5 volts, then it will work. This applies +5 to the ch 1 select, ch2 select, and /xy select, and a ground to the /chop select.

I can't tell you how much time it took to find that out. Hope this helps someone in the future.

The bad rom was one of the gold top chips.

Regards, Tom
----------------------------------------------------------------------------------------------------------------------------

Subject: [TekScopes] Appraisal Tek 468
Date: Sat, 7 Apr 2007

Hi Gang:

I have found that the weakest links of a 468 are the ROM's, the 8 segment switch on the top board and the microprocessor power supplies on the rear assembly. I was able to design a circuit to read the ROM's and can now program a 27C64 with a 28 to 24 pin adapter to put these great oscilloscopes back on the air. I also wrote and programmed a ROM that reads the value of the 8 section switch and displays the value in octal on the display, most helpful for locating a problem with the data bus or a stuck level on a tri-state device. The Volume I manual has tables of check sums that will help you diagnose ills and these are indeed useful albeit a bit tiring to work with. I have also read and programmed a 27C64 for the 067-0989-XX test ROM. It uses the same 28 to 24 pin adapter as the other ROM's. If anyone needs an operational ROM or the test ROM drop me a note off line and we can discuss your problem. If you find that your 468 does not display the marching numbers on the 7 segment display during boot-up then check the 8 segment switch on the upper board. It appears to be affected by the flux used in the wave soldering operation and frequently needs replacement. I usually install a 16 pin socket in that position to accept the switch. The microprocessor power supply is a royal PITA to work on. I can offer no quick suggestions here, just use normal troubleshooting techniques. When reassembling do NOT put long screws in the two center holes of the rear bezel as they will crack the PC board in the rear and then you really will have a problem. If that happens you can send the board to me and I can repair it as I made a point-to-point wiring list. Overall the 468 is a truly magnificent instrument. It encompasses all the very best features of the 465B along with the ability to instantly change from an analog to a digital instrument. The 10 MHz digital bandpass does leave something to be desired but is satisfactory for many applications.

Reed Dickinson
reed714@...

-----------------------------------------------------------------------------------------
Read Me text from Tek468.zip

The Tektronix Scope, For The Most Part Is A Reliable Instrument. Its Main Weakness Is The Two Firmware ROMS (U565, And U575) Tek # 160-0459-01 And # 160-0759-01. These ICs , Of Course Are Custom Devices Made By Mostek In The Late 70s-Early 80s. As They Are No Longer Aavailable (It Seems From Anywhere), I Have Found The Latest Firmware Version (So I Am Told) . The Files Listed (2764.bmp, And 2764a.bmp) Are The Pinouts Of The Common 2764 EPROM From Intel, Which I Used For My Replacement ROMs. The File (MK36XXX) Is The Pinout For The Obsolete Tek Roms (U565, U575). The File (Fig. 1.bmp) Is The Hand Drawing Of The Adaptor To Convert A Programmed 2764 To The Mostek 36xxx Pinout. The File (OMK36693) Is The "Bin" Of The New "MK36693", Which Is "Burned" To A 2764. The Same For File (OMK36694). Most EPROM Programmers Should Do The Job. Good Luck On Your Textronix 468 Restoration!!!!


tgerbic
 

Thought I would pull out my 468 and see if I could find anything useful to post. Well no trace on power up :( However it passes self test. Had to replace the 1200uf cap in the power supply to bring it back to life. First repair on this scope since new. Will probably replace all the big PS filter caps in the future in this and my 465B scopes, since I bought them from Tek in the 80s and they use the same parts. Cheap insurance, maybe... The 465B has never been touched. Guess I should go fire that one up too. They have been sitting for a decade.

Self test runs with 8's and dots first then a bunch of zeros flashing around on the display, then it goes blank when the test passes.

I did a quick once around the processor to provide some general signals for a running unit.
1 5Mhz Squarewave
2 5Mhz Squarewave, half size from 5V down to ~2.5V
3 Low, short high pulse on power up
4, 5, 6 Low
7 500Hz digital signal with 2ms positive pulse
8, 9 High
10 100Hz digital signal with 10ms positive pulse
11 High, occasional negative pulse
12 to 19,k and 21 to 33 Address/data/control constant traffic
20 Gnd
34, 35, 36 High
37 132Hz squarewave
38, 39 High
40 +5V VCC

Regards
Tony


Roger Evans
 

John,

I was initially surprised that Reset out was low but that is correct since 8080 peripheral devices have an active high reset pin!

A couple of tests for next time you are looking at the scope:
Check that RST7.5 is 50kHz not 500kHz. 500kHz would indicate that U440 is dead and would probably saturate the processor with interrupts.
Check that pin 14 of U472 is also high so that we are not looking at a failed U472
Measure S0 and S1 (pins 29 and 33), that might show that the CPU had executed a HALT instruction and had at least done something after Reset.

The GPIB interrupt stuck high does need tracking down.

Regards,

Roger


Roger Evans
 

Just correcting my misreading of the schematic for the RST7.5 input. The _input_ to U440 is 50kHz so the RST7.5 frequency should be 500Hz as measured by Tony.

Roger


John
 

Reed, thank you for the note about the switch on top of the board. I will check it out. Power supplies are OK and all voltages present. I read in the service manual about the "service ROM" and wondered whether that would still be available. I will bear you in mind should I need it.

Tom, thank you for the description of the error in the troubleshooting of the analog section. Will be carrying out that procedure soon so will bear that in mind.

Daveolla, thank you for the description of the files. I contacted Bruce and he has granted me access to the server. The files are now hosted on ftps and you have to e-mail Bruce for the access details. I did find the 468.zip file but not the bigger Tek468.zip. I did find a PDF of the adapter pinout in the files section on the TekScopes group.


A couple of tests for next time you are looking at the scope:
Check that RST7.5 is 50kHz not 500kHz. 500kHz would indicate that U440 is dead and would probably saturate the processor with interrupts.
Sorry, my mistake. Its at 500Hz which is as per point 5 in the test procedure, not kHz.

Check that pin 14 of U472 is also high so that we are not looking at a failed U472
Yes, pin 14 (gate output) is high, hence RST7.5 is high. Pin 13 (gate input) is at about 1.5V. Grounding either makes no difference. G1 (P1) and G2 (P19) are low.

Measure S0 and S1 (pins 29 and 33), that might show that the CPU had executed a HALT instruction and had at least done something after Reset.
S0 and S1 are both LOW which would seem to indicate a halted state. I tried monitoring both during reset/power up thinking that there should be some activity, particularly on S1 as the CPU boots and attempts to read the ROM but nothing happened - it just stayed low.