Resetting a Tektronix 4051

[stepleton]

I'm debugging a Tektronix 4051 with a hardware fault somewhere.

It would be convenient to be able to reset the machine without power-cycling it. What would be the best way to do this: pulling RESTART-0 to 0V?

(For now: address and data lines are behaving normally, but something appears to be holding \IRQ lines in assert...)

 

[daver2]

Yes, grounding RESTART-0 to 0V should do the trick.

The RESTART-0 signal is generated from OPAMP U541B in the power supply - in series with a resistor of 820 Ohms. The maximum current that can flow is therefore 5/820 = 6 mA - so no damage should occur...

EDIT: Just relooking at the power supply schematics. The 820 Ohm resistor is a current limiting resistor for the 4.7V Zener diode, so it is possible for a higher current to flow - depending upon what supply voltage U541B is fed from. Guessing at 15V, this could cause a maximum current flow of 15/820 = 18 mA. Still fairly small... If you are concerned, you could modify either the positive or negative input to the OPAMP - These have very high-valued resistors associated with the inputs, so you could use a low-valued resistor as your 'short'.

If something is dragging the /IRQ line low it will probably be one of the PIAs...

Try U265, U461 or U565 (all MC6820).

Don't forget that add-on packs (e.g. U5 in the communications pack) can also generate interrupts.

Dave

 

[stepleton]

Thanks Dave,

I did some very light reverse engineering on my 4051 MaxiROM pack, which has a place for a reset button but nothing fitted there. I find that this button indeed simply shorts RESTART-0 to ground. If VintageTEK are OK with doing it that way, then it's good enough for me.

Turning to the PIAs, it may be useful to describe my setup a bit more. I have two 4051s here, one that "works" and one that doesn't. (The working one still has a fault: a collapsed X axis when displaying graphics but not text. This likely implicates the X-axis vector delay filter described on page 6-22 of vol. 1 of the Service Manual. I'll attend to this later as it would seem likely to me to be an easy fix, and I don't really have the benchtop space to open more than one 4051 comfortably.)

I've opened the broken 4051 into the "service configuration" shown on page 3-27 of Service Manual vol. 1, but with two differences. I've unplugged the RAM expansion and set it aside, and I've also disabled the tape drive by disconnecting the J2 ribbon cable (as well as tape drive power connector J82; debatably J87 should also be unplugged, but it's not). This is done owing to a tape drive fault that causes it to run the transport motor incessantly whenever it has power. If the computer cannot operate without a well-behaved tape drive plugged into it, then this might be the problem. Otherwise, it's an issue to sort out later.

In common with the page 3-27 illustration, the backpack is not fitted and therefore doesn't factor into troubleshooting here.

The CPU board reveals a set of five XC6820s from 7501 and an XC6800B from 7520. Early chips (XC not MC)? In any case, the sockets are all the (reportedly unreliable) TI side-wipe type. To try finding an obvious 6820 fault, I attempted to run the machine with either U265, U461, or U565 removed (one at a time); in each of the three cases, \IRQ remained asserted. With the keyboard PIA removed, all of the indicator lamps remained illuminated: perhaps not surprising as the keyboard PIA controls those. Swapping the positions of these three PIAs did not change observed behaviour.

Although it's somewhat inconvenient to swap parts between my two 4051s, exchanging the keyboards is relatively easy. So I tried this, and it did not help.

I'll keep studying the service materials to try and understand the system better. It may also make sense to check all clocks, or to try simply lifting the \IRQA and \IRQB pins of the three PIAs we're investigating instead of removing whole chips altogether.

In case it's necessary: do you know if 6821s can be used in the machine instead of 6820s?

 

[daver2]

Ah...

The 4051 does not operate correctly without a functioning tape drive (from memory).

I would be inclined to hold the 4051 in a permanent reset and then measure the voltage on CPU pin 4 (IRQ-0). Check for a HIGH signal. If it is not HIGH then, you definitely have a faulty device driving it, or there is a short circuit somewhere grounding IRQ-0. Check with a multimeter with the power OFF.

Dave

 

[stepleton]

I see! I wonder if I can fake it somehow.

 

[daver2]

This takes me back to when I did the 4051 emulator. I had to 'persuade' the tape I/O to behave correctly. It would then boot up. The problem is, I can't quite remember which signal(s) is/are involved now.

Looks like something to do with the TAPE-LOAD signal...

Dave

 

[stepleton]

Through sheer diagnostic cunning I have discovered that the previous owner had not correctly aligned the tape drive ribbon cable to the header on the tape drive: the bottom row of pins was not in the connector. Correcting this problem eliminated the unending tape motor exercise and the stuck \IRQ line. The computer now boots to a working BASIC prompt.

The tape drive appears to require further work in order to read tapes dependably, however.

I find also that this 4051 refuses to MOVE or DRAW, saying that these commands require the "EXTENDED FUNCTION ROM" or similar. I was surprised by this given this that @nikola-wan mentioned last year:

All the internal 4050 BASIC ROMs have DRAW and MOVE commands for DRAWing on the screen or DRAWing on external GPIB or Serial interface to plotters like the Tektronix 4662 and 4663.

Not these ROMs, it would seem. Is there a way to find out the ROM version from BASIC?

And as long as I'm asking questions:

- Is there a good, convenient way to test the 4051's RAM? I'd be curious to know whether all of these MCM6605ALs are working. (Assume I have the MaxiROM pack from VintageTEK and the tape drive emulator.)

- Does anyone have a spare keyswitch and/or a 2/" keycap for a 4051 keyboard?

 

[nikola-wan]

Great progress!

Have you tried the Flash Drive in that 4051 yet? There is a 4051 ROM Checksum program in the main menu.

There is a 4051 RAM test assembly program here:
https://github.com/mmcgraw74/Tek405xEmulator/blob/master/Programs/RAMTEST.txt

Something is wrong with the ROMs if MOVE and DRAW don't work.

Try the 4050 Tutorial in the main menu. I was designed to work on the 4051 with only 8KB of memory.

You may need to clear the Flash Drive configuration file - if the Flash Drive was used in a 4052 or 4054:

Power on the Flash Drive then:

FIND@5:123
PRINT@5:0
FIND@5:1
OLD@5:

This should start the First Time Setup which will detect the 4051, RAM size and option ROMs and then run the main menu.

This should enable running the 4051 Checksum Program.

 

[daver2]

Nice spot about the tape drive connector. That could have done some real damage...

The offset connector must have 'hard wired' one of the signals to a PIA so it was always generating an interrupt when the PIA was programmed.

Something is ringing a bell regarding the graphics ROM. Let me think and check later...

I think they ran out of ROM space, and this ROM really was an 'option ROM' even though it was always fitted. This could indicate either someone removed it, the ROM is faulty, or the circuitry driving it is faulty.

Dave

 

[stepleton]

Something is ringing a bell regarding the graphics ROM. Let me think and check later...

I think they ran out of ROM space, and this ROM really was an 'option ROM' even though it was always fitted. This could indicate either someone removed it, the ROM is faulty, or the circuitry driving it is faulty.

Very interesting. This got me thinking...

I mentioned that I've got the 4051 in the "service configuration" shown on page 3-27 of Service Manual vol. 1. Here's the figure on that page:



As you can see, it's not so practical to fit the communications backpack in this setup. But if the ROM is really an "option ROM", it suggests that the Tektronix engineers placed it somewhere... optional. A peer into the depths of the backpack through the cartridge slots also reveals a suspicious excess of ROM-like chips. Well, there's the right way and then there's the expedient way to test whether the backpack has the EXTENDED FUNCTION ROM inside:



Sure enough, that's where the missing ROM lives. With the backpack fitted, I can MOVE and DRAW as I please.

This should start the First Time Setup which will detect the 4051, RAM size and option ROMs and then run the main menu.

This should enable running the 4051 Checksum Program.

This works --- provided you have the EXTENDED FUNCTION ROM
Both the checksum checker and the RAM tester make use of the string append BASIC feature (A$=A$&"more stuff") which is an EXTENDED thing to do.

Things are looking pretty good on both 4051s. ROM tests report correct checksums, as you can see here in the "working" machine:

New item by Tom Stepleton

photos.app.goo.gl

You can see that the MaxiROM cartridge is fitted. It's also apparent that there is an issue with the screen not storing well in the upper right-hand corner, which must mean more calibration in my future.

The "working" machine has 32K of RAM fitted. The RAM test program on this machine prints 7FFF at top left of the screen, which I assume is the memory size. It then appears to do fairly little, very occasionally clicking the speaker and toggling the state of the BUSY, BREAK, and I/O lamps. This goes on for a few minutes after which the machine beeps and appears to reset itself (or to exhibit the same behaviour as executing DEL ALL). I assume that no news is good news in this case.

On the (formerly?) "broken" machine, which has only 24K of RAM fitted, the behaviour is the same except the machine prints 5FFF at top left. This is consistent.

Incidentally, the Flash Drive menu detects 32K of RAM on my 24K machine (and on the 32K machine of course).

The Flash Drive and these collected utilities certainly do make all this kind of testing much easier than it would have been otherwise!


Things are making good progress now. Remaining work items that I know of:
"Broken" 4051: tape drive needs attention; keyboard restoration; yoke could probably be trued slightly; cleaning
"Working" 4051: display calibration; repair the X-axis vector delay filter; yoke could probably be trued slightly; cleaning

 

[nikola-wan]

Excellent work Tom!

Happy to see you like my 4051 checksum program output on a single screen showing up to two 4050E01 ROM Expanders with sixteen option ROM Packs!

I see in my main menu program line 309:

1540 M9=MEMORY>33000

So my First Time setup says if MEMORY>33000 then declare it is 64KB - otherwise M9=32KB. I little high for a 4051 with 24KB

I made the M9 variable a binary number because the 4052 and 4054 only had two choices - 32KB or 64KB.

I think I will leave that alone - because the only program I've found that is sensitive to 4051 8K/16K/24K/32K is the System Tutorial Tape that makes heavy use of APPEND to allow the original 4051 with 8KB of memory to run the tutorial to work by using the detected memory size to select the next tutorial program to run.

BTW - I love your photo of all the 4051 boards spread out for testing like the service manual photo! I've had to do the same thing on my 4052 repairs.

I do see a missing key on one of the 4051 chassis on the right side of the photo. Check with @jdreesen as I believe he may have some spare key switches and key caps.

 

[daver2]

You beat me to it!

I was going to say exactly what you found...

The 'bell' works again...

Dave

 

[stepleton]

Thanks for your help, @nikola-wan and @daver2 !

I think I will leave that alone - because the only program I've found that is sensitive to 4051 8K/16K/24K/32K is the System Tutorial Tape that makes heavy use of APPEND to allow the original 4051 with 8KB of memory to run the tutorial to work by using the detected memory size to select the next tutorial program to run.

I find that the Flash Drive's "picture menu" option (can't recall the exact name; it's under "Favorites") runs out of memory on my 24K machine, unfortunately.

Check with @jdreesen as I believe he may have some spare key switches and key caps.

Thanks for the suggestion --- we are in touch! I don't think 4052 keycaps are the same as 4051 keycaps, but this is based on a guess that 4054 and 4052 keycaps are the same. 4051 and 4054 keycaps are certainly different. Here is a side-by-side comparison (4051 on the left):

New item by Tom Stepleton

photos.app.goo.gl

Still, although it looks out of place, a 4054(2?) keycap looks better and is much more useful than the hole that's there now! Here's what it looks like:

New item by Tom Stepleton

photos.app.goo.gl

The 'bell' works again...

PRINT "G"

 

[nikola-wan]

I find that the Flash Drive's "picture menu" option (can't recall the exact name; it's under "Favorites") runs out of memory on my 24K machine, unfortunately.

Thanks for the suggestion --- we are in touch! I don't think 4052 keycaps are the same as 4051 keycaps, but this is based on a guess that 4054 and 4052 keycaps are the same. 4051 and 4054 keycaps are certainly different. Here is a side-by-side comparison (4051 on the left):

I didn't have a 4051 to test the programs - other than @WaveyDipole and his had 32KB of memory.

If the tape works in that 4051, I think you could 'fix' the Picture Menu program by deleting the need for the DIM A$(26582) variable. This is from the original R12 Demo Tape - as all the original pictures fit in a 26582 byte string and were drawn with a single command.
My Flash Drive has the binary picture file chunked in 257 byte blocks - which I mistakenly though was a multiple of 3 byte R12 vectors - but it is one byte short.
If you change the program to read and concatenate three chunks and RDRAW the resulting string of 771 bytes the program will definitely fit in your 24KB 4051.

For the keycap - maybe you could make a 3D model and print one? Or make a mold to make a copy of a keycap?

 

[WaveyDipole]

I was just trying out that reset button on the MaxiRom cartridge (mine does already seem to have one) and it does indeed work. Unfortunately while trying to reach it, I pushed forward the stull that was on top of the 4051 and something heavy fell on the keyboard. To my annoyance (with self), I now have 3 broken 'holes' on the keyboard where keys should be....

The key tops themselves are fine, but the shafts are broken. Will these superglue OK, or can some other adhesive be used? Or will I need replacement switches?

Jon, good to see you got both 4051's repaired. Did you have to re-align the tape drives? I haven't done anything with the tape drive in mine yet except clean the heads, but not getting anything out of it. From what I understand this could be the tapes but just wondering whether and how you got yours going.

 

[nikola-wan]

4051 service manual vol2 pdf page 163 in http://www.bitsavers.org/pdf/tektronix/405x/070-2286-00_4051_Service_Vol2_May77.pdf - indicates Cherry M6-0120.

I believe I tried superglue on the broken keyboard switch shaft - but it didn't work for long.

This website says 900 in stock for $1.89 each - min order 11 switches:
https://federalconnectors.com/M61-0120-CHERRY

I think both @stepleton and @daver2 are trying to get switches and keycaps from Jos. The 4052/4054 service manual shows a slightly different M6-0100 part number but I put in M6-0120 without any issue.

 

[WaveyDipole]

Ooh, well spotted! Thank you.

I did contact Jos and he says he has a 4052 keyboard that he is using for spares but wasn't sure whether the keys are the same so I just replied that I would send photos and measurements is he wanted. However, if you have been able to put in an M6-0100 in place of a M6-0120 then theoretically reverse should also work. Jos reckons the key-tops are different, but mine or undamaged. Its just the shafts on the switches that were broken. Interesting to note that superglue did not work for very long. One other option I was thinking was to use two part epoxy, but that might be tricky to do neatly, so it looks like replacements will be necessary. I notice in Jon Stepleton's picture that his replacement key stands a bit higher. I am considering ordering from the source you mention but I see that the minimum order quantity is 11 and minimum shipping cost is 63$. I don't mind buying 11 to have plenty of spares and maybe to share the odd one or three, but the postage cost is a bit steep. Will have a think about it while I wait to see what Jos comes back with.

 

[nikola-wan]

Hmm, I have a broken Left Shift key switch in my 4052.
I checked on price to ship to me in Texas and it was only $13.70.
If I bought 11 and kept two and mailed the remaining nine key switches to one of you - the USPS International Flat Rate Priority Mail would only cost $44.35 for a small flat rate box. It is only $16.50 for a First Class International package less than 8 oz to the UK. Assuming you, Tom and Dave live relatively close to each other. Then each of you would have three key switches.

 

[jdreesen]

Well, since the discussion is public anyhow : I do indeed have a spare, but incomplete, 4052 keyboard. Keycaps differ between 4051 and 4052/54, as per Tom's pictures.
And while rummaging through my parts boxes yesterday I found around 25 (used) switches, without keycaps, that are a perfect match. I do not remember which keyboard I took them from, not a Tek for sure.
So there is plenty to go around.
I intend to make a single package to Tom, and hope he can distribute within the UK, as I hate to do the paperwork trice....
Rest per PM.

 

[WaveyDipole]

Jos, thank you for your PM, taking the time to have a look and willingness to help us out. Monty, thank you also for checking out pricing and being willing to help out with shipping to the UK. I have now replied to the PM from Jos and indicated that I will take him up on his kind offer. I am, of course willing to cover Tom's onward postage to me or to distribute should the need arise.