Topics

Is my Tek 468 beyond repair?


John
 

Bit more progress today!. The replacement CPU arrived and has been installed. I am now getting all of the correct signals on the CPU board as per the service manual and comparable with Tony's "quick once around the processor" report. I have re-installed the two ICs that were removed and the display came to life on the next power up. It initially shows [ . . . .] then [8.8.8.8] then all of the digits flicker with 0's for a while until it settles to [ 0. 0.]. I am getting no display on the CRT at the moment although I do see a trace when I hit the beamfinder button.

So it seems that the computer is now working. Since the "Lamp Test" is ocurring as shown by the [8.8.8.8.], it seems that the ROM Checksum Test and the Lamp Test have passed. Next comes the "RAM Verificaton Test" and I am unclear as to whether this is succeeding or failing. I have so far been unable to find what [ 0. 0.] means although the manual does suggests that a RAM check will return a zero for "No error". It does also say that the display should blank and the scope should start operating which does not appear to be happening.

I will wiggle the chips on the RAM board, but if anyone has seen [ 0. 0.] and can tell me what it meas that would be appreciated.


robeughaas@...
 

The vintageTEK Museum now has replacement 468 EPROMs: https://www.ebay.com/itm/114450705368


--
Bob Haas


Colin Herbert
 

Bit of a setback that they don't ship to the UK. They will ship other things to the UK, so I wonder why not these?
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of robeughaas@gmail.com
Sent: 07 October 2020 18:38
To: TekScopes@groups.io
Subject: Re: [TekScopes] Is my Tek 468 beyond repair?

The vintageTEK Museum now has replacement 468 EPROMs: https://www.ebay.com/itm/114450705368


--
Bob Haas


robeughaas@...
 

Sorry. I will update the listing to allow international shipping.

--
Bob Haas


robeughaas@...
 

The listing already allows international shipping.

--
Bob Haas


Colin Herbert
 

I see that a revision has been made, but the listing still clearly shows that there is no shipping to the UK in two places. I'm not certain that I would buy these, but there might be others in the UK that just might.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of robeughaas@gmail.com
Sent: 07 October 2020 23:23
To: TekScopes@groups.io
Subject: Re: [TekScopes] Is my Tek 468 beyond repair?

The listing already allows international shipping.

--
Bob Haas


Roger Evans
 

Looking at the listing in the UK we see the following:

Shipping to: Worldwide
Excludes: Africa, Asia, Central America and Caribbean, Europe, Middle East, North America, Oceania, Southeast Asia, South America

Without getting my atlas out I think that leaves Australia, New Zealand and Antarctica!

Roger


John
 

I made a more details assessment of the scope this morning. First, I gave the ICs on the ADC/memory board a wiggle just to make sure they were all properly seated and the pins were making contact. After that I powered up the scope and let it warm up a bit.

I had a rather scary moment after about 5 minutes when the storage computer suddenly stopped working and things appeared to be back to square one! However after a few minutes to cool down, the computer fired up again. In normal operation, the internal fan would be blowing cold air across the CPU and ADC boards so I figured that perhaps the processor crashed through overheating so a PC fan was set up to blow cold air onto the boards and it then ran fine for several minutes while I investigated all the scope functions.

A trace is present on both channels in both storage an non-storage modes. Triggering seems a little wobbly and I there is a "ghost trace" at times but it does trigger OK up to 10MHz at least. Neither inverse triggering nor invert mode seem to work. Attenuators are OK but clearly in need of adjustment. The time-base also seems fine for a running race, but I couldn't get anything on X/Y mode, not even a spot in the centre.

In analogue mode the trace seems rather soft and thick even at the best focus, but once storage mode is enagaged, the trace becomes reasonably sharp although it does show some dots at higher frequencies which I presume are the sample points. Hitting the 'Save' button "freezes" the display which presumably means it is displaying a captured snapshot. Cursors appear when the cursor buttons are used although it only seems possible to adjust the dotted line. Pushing the button at the far right seems to reverse the dotted and solid line and the cursor position. From these few observations it does seem that storage computer functions are working except perhaps the display.

Once started up, the display shows ' 0. 0.', i.e the second and fourth digit. Digit 3 seems to faintly light up and change when the cursor is moved, while digit 2 and 4 show mostly '8' although segments do not light up with an even intensity and do change as the control is adjusted. I am begging to suspect that the ' 0. 0.' displayed at startup is a display problem. Looking carefully it seems that after the "Lamp Test" only digit 4 and 2 flash zeros during startup.

In the meantime, I found an eBay seller that sells a 2364 to 2764 adapter boards for vintage computers. They do need pins and sockets to be added, but the final result should be neater. I checked the board traces against the diagram in the PDF about adapters and they seems to match. I intend to build the adapters and use the EEPROMs I have kindly been supplied. The original ROMs are evidently fine for now but I do want to have a plan B. I also would like to make a service ROM as well as a reverse adapter so as to be able to put the Mostek roms into an EEPROM reader and verify their checksum.

Incidentally, I think someone was asking earlier about where to find the various "signatures" in certain steps. There is a list of them in the Service Tests section of the manual (staring page 5-36, details from page 5-45 onwards), however, it appears that both the Service ROM and a "Signature Analyser" are required to read them. I presume that the Signature Analyser is a dedicated bit of Tek kit designed for the purpose? The service ROM routines (functions) are also documented in that section. Some of these routines appear to generate a display on the CRT, others do not.

For now, I will be investigating the display board.


robeughaas@...
 

I have made yet another attempt to straighten out worldwide shipping. I only offer the excuse that ebay has the most stupid system of setting worldwide shipping.

--
Bob Haas


Colin Herbert
 

Yes, it works now. Thanks, Bob. There are no apologies needed, since we can communicate with you readily.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of robeughaas@gmail.com
Sent: 08 October 2020 16:31
To: TekScopes@groups.io
Subject: Re: [TekScopes] Is my Tek 468 beyond repair?

I have made yet another attempt to straighten out worldwide shipping. I only offer the excuse that ebay has the most stupid system of setting worldwide shipping.

--
Bob Haas


John
 

I have traced the problem with the display to one of the driver chips, an 74LS374. A replacement is on its way. It seems an 74LS377 had been used and there are a number of these on the board as well as the odd 74LS374 or two which suggest they may have been factory fitted. The datasheet shows them to be very similar but not quite the same. I am not sure of the reason for the substitution? The diagram calls for a 74LS374 so this is what I have ordered.

I am still a little concerned about the storage computer because sometimes there is a delay starts up. Other times it starts immediately. Occasionally it does not start at all and I have to power down and back up again and it will start. It seems that my perhaps my assumption about overheating may have been incorrect? Someone did say that the storage computer should work without needing to have the fan blowing on it. This CPU certainly runs cooler than the original one did but there is still a fair bit of heat coming off the two boards (CPU and ADC/memory). I would not necessarily say that it is excessive, but certainly considerable and it would make sense to have a fan blow cold air through the enclosure to maintain an airflow and remove the heat.

Once I get the adapters done I can eliminate the possibility of one of the EEPROM's playing up.


Chuck Harris
 

Part numbers on tektronix schematics should not be used
as being representative of what is needed for the circuit.
Use the parts list in the manual that covers your scope's
serial number.

Further, tektronix was very big on making changes to their
scopes after they entered production... sometimes serious
changes. Using a wrong serial number manual on a leading
edge product like the 468 could result in major errors.

If you want to tell if a part was installed by the factory,
go to the part's date code. It is the best representation
of when the part was installed.

-Chuck Harris

John wrote:

I have traced the problem with the display to one of the driver chips, an 74LS374. A replacement is on its way. It seems an 74LS377 had been used and there are a number of these on the board as well as the odd 74LS374 or two which suggest they may have been factory fitted. The datasheet shows them to be very similar but not quite the same. I am not sure of the reason for the substitution? The diagram calls for a 74LS374 so this is what I have ordered.

I am still a little concerned about the storage computer because sometimes there is a delay starts up. Other times it starts immediately. Occasionally it does not start at all and I have to power down and back up again and it will start. It seems that my perhaps my assumption about overheating may have been incorrect? Someone did say that the storage computer should work without needing to have the fan blowing on it. This CPU certainly runs cooler than the original one did but there is still a fair bit of heat coming off the two boards (CPU and ADC/memory). I would not necessarily say that it is excessive, but certainly considerable and it would make sense to have a fan blow cold air through the enclosure to maintain an airflow and remove the heat.

Once I get the adapters done I can eliminate the possibility of one of the EEPROM's playing up.






John
 

An interesting point about the manual. The serial number of this unit starts B0xxxx which, according to the manual means it was manufactured at Tektronix, Inc, Beaverton, Oregon, USA. The manual does cover this serial numbers in the format B000000.

Regarding the date code, I'm not sure whether a date code is indicated? The part is marked as follows:

74LS377N 8020
SA

I am not sure whether the first letter on the second line is 'S' or '5'

According to the datasheets, both the 74LS377 and 74LS374 are octal D-type flip flops and same pinout, but the latter has a slightly higher operating frequency. Given that the application is to drive a 7-seg LED display I didn't expect the speed to be critical here. I received the ICs today and popped one in. The display now works properly. All four zeroes now flash at startup. A number is displayed when the cursor is enabled and it increments/decrements according to the cursor movement.

Unfortunately, there is still a problem with the CPU clock which seems to be intermittent which is almost certainly causing the intermittent start-up and sudden stop problem so it looks like I am going to have to have another look at that PSU/timer board.


Chuck Harris
 

The date code is the 8020. It means the 20th week of 1980.

-Chuck Harris

John wrote:

An interesting point about the manual. The serial number of this unit starts B0xxxx which, according to the manual means it was manufactured at Tektronix, Inc, Beaverton, Oregon, USA. The manual does cover this serial numbers in the format B000000.

Regarding the date code, I'm not sure whether a date code is indicated? The part is marked as follows:

74LS377N 8020
SA

I am not sure whether the first letter on the second line is 'S' or '5'


satbeginner
 

8020 would indicate manufacturing in 1980, week 20.

Leo


John
 

Thank you. Learned something today then!

According to one source the scope was produced from 1980 to 1985, so that would indicate that this unit is a fairly early example. The serial number is in the low 21000s appears to correlate with that. That might also explain the absence of a couple of features/buttons on the display board.

Maybe I could ask something further? Do capacitors have a similar date format? Would, for example 9914MC (assuming no other codes present except capacitance, voltage and temp rating) indicate 1999, week 14?


John
 

The crystal has been replaced and the clock seems stable after a 1hr soak so this is looking good.

I ran through the storage functions today and the good news is that all storage functions are operating as described but I do have a couple of questions regarding the cursors:

Firstly in VOLTS mode, when ONE trace is showing the display correctly shows 0.304V for the 300mV cal trace. However when TWO traces are displayed, the display shows 1.52 and the μS LED is lit?

Secondly in TIME mode I get the dots as expected and placing them across a full cycle of the 1kHz cal signal I get a reading of 1056mS with the time/Div set to .2mS. One cycle of a 1kHz wave should take 1mS so I'm not sure how this correlates?

I next intend to run through the analogue functions, but would appreciate some insight on the above readings first. In any case I am relieved that the storage computer now appears to be fixed. Can I say thank you to everyone who provided information and comparison data.


John
 

With further investigation I have concluded that TIME cursors look like they are measuring pretty spot on. With a 1kHz signal it, in fact, measures 1.056mS which is correct. At 10MHz the measurement was 0.097μS which is just shy of 0.1μS which is also about right. The issue with the VOLTS measurement when there are two traces present is still a problem.

Storage does run into Nyquist at about 15MHz though and the display then becomes inaccurate. Storage captures simple waveforms but seems not to be able to cope very well with complex forms such as AM modulation even at 100kHz. I guess given the relatively small memory buffer by today's standards this is perhaps not surprising.


John
 

I have a question about the options that this scope has and X-Y mode. On the back it lists:

Opt 4
Mod (stamped on chassis between the two option labels)
Opt A2

What is option A2? I can't find a reference to this in the manuals?

The front panel seems to indicate that the X-Y and the first 3 tim/Div's are in a "STORAGE ONLY" zone. Does this mean that X-Y mode is only available when storage is turned on? In any case analog mode produces a dot and storage mode a horizontal line but both are positioned to the extreme left and off the CRT screen. Both can be made at least partially visible only by rotating the position control to the extreme right or using the beamfinder but neither can be positioned anywhere near the centre of the CRT display.

I note from the options section in the Service manual that there exists an "Analog X-Y" option. From my reading of the description this would seem to apply to X/Y plotting via GPIB rather than X-Y display on the CRT but I am not sure.

Clearly there is a voltage being applied to one of the horizontal plates and I am in the process of tracing this back to source. On the 465, X-Y mode places a dot at the centre of the CRT as might be expected in analog X-Y mode but this 468 is behaving differently and it would be helpful to know whether it should behave the same - at least in analog mode - or whether it requires an option to be installed or something?


John
 

Just a bit of an update on the X-Y mode troubleshooting so far.

While a trace is running there is a sweep angle on both plates as might be expected. In X-Y mode there is 100VDC on one plate, zero volts on the other.

With U564 (4x transistor DIL package, horizontal amp section) removed, the spot is centered.

With pin 2 of U564 grounded the spot is centered.

With Q760 and Q772 removed, the spot is now hard right instead of left. Beamfinder is once again required to locate it. Approximately 0.6V is present on U564-p2/Q760-c/Q772c.

With Q760 and Q772 present U564-p2 is at around 180mV. It seems to me that it needs to be 0V but I am not sure how to diagnose the imbalance.