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Thread: Tek 4051 startup sequence

  1. #11
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    Yes I think it is overcurrent. If I disconnect the black connector that has wires running to the video board and elsewhere, I see the normal voltages on the motherboard and can blindly enter a loop program and get BUSY to illuminate. But as soon as I plug in the black connector.. 5v disappears on that board and it will do nothing.

  2. #12
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    Come on Falter - we can do it !

    Can you be a bit more specific about the 'last connector on the motherboard'? Which board and which connector? I can then have a look at the schematics...

    Dave

  3. #13
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    Dave! I am game if you are!

    If you are looking at the motherboard with the computer open (going by memory - it is down at the shop still til I pick up) the connector furthest left is the problem. It is a black colour and appears to have cabling going to the video display board on the left. It also seems to have wiring disappearing back under the CRT somewhere... possibly to the power supply. The other two connectors, I think are for keyboard and tape drive. The motherboard powers up fine with normal voltages if either or both of those are connected.

  4. #14
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    I'm game!

    Let me have a look at the Technical Manual after lunch tomorrow. Rather late in the UK at the moment...

    Dave

  5. #15
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    OK, so I have had a look at the service manuals and this is how I would proceed (either when you get the machine back or - before it comes back - ask your repair guy to have a go...).

    First of all, download both service manuals from our favourite website - bitsavers...

    http://bitsavers.org/pdf/tektronix/4...Vol1_May77.pdf

    http://bitsavers.org/pdf/tektronix/4...Vol2_May77.pdf

    Volume 2 is more useful than volume 1 (but you may as well get them both whilst you are in 'download mode').

    I don't like high voltages either (bad for your health) so (as service manual volume 1 states on page 4-4) "Remove J50 [from the display board], which will disable the high voltage generator". This will prevent you from killing yourself if you power the unit up and accidentally touch something you shouldn't. Don't forget to put J50 back when we think we have found the fault...

    I am assuming they the following is a description of the fault: When J4 is disconnected from the CPU board - the +5V supply rail is OK. When J4 is connected to the CPU board - the +5V supply rail is not OK. I will base my fault finding on this assumption.

    I know you hate following schematics () but can I suggest you look at the ones in service manual Volume 2 starting at page 4-1 onwards. Note that there are a couple of different display boards documented. I am not sure what the differences are between them or which one you have in your machine. I suspect for this fault finding exercise it probably doesn't mater that much (but just bear this in mind). The schematics are annotated with the display board revision to which they apply (see the lower right-hand corner).

    Disconnect J54 from the display board (some of the cables from this connector go over to J4 on the CPU board - notably the -12V, +15V and +5V power supply rails). Disconnecting J54 isolates these supplies from the CPU board.

    Use a multimeter to measure the resistance between pins J54-9 (+5V) and J54-7 (GND) on the J54 connector on the display board. Have we got a low resistance / short circuit present between +5V and GND?

    If we have, I would initially suspect C650 (1uF).

    The other power supply rails (+15V and -12V) have low-valued (2.7 Ohm) series resistors present (R672 and R670 respectively) so it should be possible to measure the voltage across these resistors with a multimeter (and hence calculate the current and power dissipation) later if this proves necessary. [I = V/R and P=I*I*R]. If these power supply rails are OK however when the display board is connected and powered up - we don't need to perform this step.

    So, if we have a low resistance between +5V and GND - and C650 is not to blame - you need to remove the display board and use a bright light and a magnifying glass to see if you can find any short circuits anywhere on the printed circuit board. Highlight on the schematic diagram where +5V goes to on the display board. From what I can see, +5V is not really used that much on the display board itself. I can print the relevant schematics out tomorrow at work and do the same check myself. I would suspect that there is a TTL device that may have gone short circuit (or draws a heavy current when powered up). We just need to find it...

    Let me know what your initial investigations turn up.

    Regards,

    Dave
    Last edited by daver2; March 18th, 2018 at 09:29 AM.

  6. #16
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    When J4 is disconnected from the CPU board - the +5V supply rail is OK. When J4 is connected to the CPU board - the +5V supply rail is not OK.
    This definitely describes the problem. I mean, I think that connector was called J4. That's great.. now we have a target and a plan.

    So with J50 removed, are there any other precautions I must take when handling or testing the display board? Definitely all the 'risk of death!' stickers around my machine made me think twice about tangling with it.

    I know you hate following schematics ()
    I don't hate it, I just often don't understand what's going on. I can follow the - this pin to that pin - logic of it. I can understand what certain logic gates should be doing. But when it comes to going above that.. looking at a complex schematic and understanding what the different parts should be doing, what kind of circuit a bunch of ICs and caps form.. that's where I get lost.
    Last edited by falter; March 18th, 2018 at 09:50 AM.

  7. #17
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    All of my testing at the moment is with the machine powered down (we - well you) are only doing resistance checks.

    If you do power the machine up (with or without J50 removed) the "normal rules apply" - don't touch anything, don't short anything out and keep well away from any and all high voltages!!!

    Think "this machine is out to kill me at the first opportunity it gets" and treat it accordingly!

    When you power down - don't assume you can still touch anything - energy is stored in capacitors and on the CRT...

    I have had a very quick look at where +5V is used on the display board and can't find much at all...

    The TTL logic devices are powered directly from +5V - so they are candidates for drawing too much current and getting hot.

    The schematics imply that U255 (an LM311 operational amplifier) driving TARSIG-1 (indirectly via U566A) is powered from +5V.

    All other uses of +5V seem to be with fairly high-valued pull-up resistors.

    A visual check here to see if any components look burnt or damaged...

    >>> I know you hate following schematics...

    Sorry, tongue in cheek...

    Dave
    Last edited by daver2; March 18th, 2018 at 10:15 AM.

  8. #18
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    Just thought of another 'test' we can perform (if necessary later on).

    With the display board removed from the Tek, we can supply the +5V from an external source.

    If you have a variable voltage DC power supply - with a current limit - you can wire the +5V and GND up to J54-9 and J54-7 respectively. if you start off with a very low current limit, you can increase the voltage to +5V slowly and see if the current limiter operates. If not, we have to look further afield. If the current limiter does operate, you still have to find the problem - but we have a way of testing the card outside of the machine.

    If you don't have a current-limiting variable supply, you could use an ordinary variable voltage supply and a low-value series current limiting resistor. As you increase the voltage, any low resistance in any component on the display board will cause most of the voltage to be dropped across your external series resistor. I would start out at 10 Ohms. That would give a maximum current of 5 V/10 Ohms = 500 mA. The power dissipation would be 0.5 * 0.5 * 10 = 2.5W. You could increase the series resistance to reduce the maximum current consumption of you wish (this should reduce the maximum power dissipation in the resistor as well). 500 mA sounds on the high side now I have written it down for what the +5V supply is feeding on this card! 47 Ohms? 5V / 47 Ohms = 100 mA. 0.1 * 0.1 * 47 = 0.5W (give or take a little bit).

    Dave
    Last edited by daver2; March 18th, 2018 at 10:27 AM.

  9. #19
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    Hey Dave.. just wondering what page you found the bit about power to motherboard if J4 disconnected? Wanted to send it to the tech in Vancouver.. he's curious.

  10. #20
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    Sorry for the delay in replying. Just got back from a business trip...

    Service Manual Vol 2.

    Page 3-2 shows the general wiring diagram between boards - but it is not fully comprehensive.

    Page 3-7 shows the J4 pinouts for the CPU board. In particular, it shows the function and destination for certain power supplies to J54.

    Page 3-16 shows the corresponding J54 connector on the display board. In particular the source for the power supply lines.

    Page 4-1 onwards shows the schematics for the display board. In particular, look for the power supply connections sourced from J54.

    I hope this answers your question?

    Dave

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