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Is my Tek 468 beyond repair?


robeughaas@...
 

Colin:

I see the IPROM's are still on the way to you. I've started a survey of the 13 468's in the Museum's boneyard and of the first seven, found two with shorted +5 Storage Supplies, as evidenced by blown F4007 and F4009, 5-amp picofuses at the back of the trigger board. I've dismantled one and found a shorted CR169, the catch diode in the supply. I'll go through the remainder tomorrow, and I suspect I'll find more with shorted supplies. Of the six I've been able to test ROM's on, I have found all the ROM's were good. I suspect that the power supply problem is causing the failure that is thought to be ROM rot.

--
Bob Haas


 

On Tue, Oct 6, 2020 at 07:17 AM, jrseattle2002 wrote:
Since there was a request for this in the above discussion, I uploaded an
image of the Tektronix 468 service rom to the files section. It's in a
directory called "Tektronix 468 Service Rom"
Hi,
Thanks for uploading the Service ROM. Does this go into socket U565 or U575, or somewhere else please?

Regards,
John


John
 

According to the Service manual, page 5-38 (volume 1 if you have the version split across volume 1 and volume 2) the service ROM is fitted in place of U565.

It has been 4 days and I seem to be no closer to solving the X_Y mode problem. Any help would be greatly appreciated.

For example, could someone please explain to me how Q760 and Q772 on the Horizontal Display Logic & A&B Sweep Generators diagram work. Both bases are connected to chassis/GND so how do they conduct?

With both traces centered both horizontally and vertically, the oscilloscope in analog mode and then switching to X_Y mode, test points 65 and 67 are both at +12.5V but Q772_e is at 160mV whereas Q760_e is at 45mV so there is a difference. The common point at Q740_c/Q772_c/U564_2 is at 180mV, rising to approximately 640mV when U564 (first stage of horizontal amp) is removed. I would have expected close to or at 0V at this point and as previously mentioned, grounding U564_2 brings the spot to dead centre which would tend to suggest that the horizontal amp, including U564 transistor array, ought to be fine.

If BOTH U564 and Q760/Q772 are removed, then this common point is at 0.02mA which demonstrates that other feeds into this point are having little or no effect. This is as expected because the position control is dead centre and no signal is being fed into the X or Y inputs.

This tends to suggest that the problem might be somewhere within the Horizontal Display Logic & A&B Sweep Generators sweep part of the circuit but I am not sure how to troubleshoot this. Notwithstanding, when U564 is plugged back in without Q760/Q772 being present, the voltage at the common point drops to -170mV so its presence/abscence does affect the balance of the circuit. Consequently, I am not sure whether the transistor array is leaky or whether the problem is at the earlier stage or whether both parts of the circuit are contributing to the problem but I suspect it to be related to Q760/Q772 and that part of the circuit. Swapping or substituting those transistors has no effect.

The scope otherwise operates perfectly fine in triggered modes.


Roger Evans
 

Q760 and Q772 are PNP common emitter amplifiers, they have very close to unity current gain and they conduct because their base is positive with respect to the emitter so the BE junction is forward biassed. In XY mode (see page 3-26 of the manual) the sweeps are disabled, the BE junction should be reverse biassed, there is almost no collector current and they just look like a high impedance from the output side.

If you look at the outline of the X amplifier on page 3-26 there are three inputs to the X amplifier all connected to the base of U564A. According to the operating mode two of the inputs should be 'off' ie high impedance and the X signal comes from whichever input is active. If the display is OK in normal Y vs time, sweep mode then the X amplifier from U564A/B onwards looks OK.

In analogue XY mode the X input comes via Q582/Q583 and the manual shows the DC switching signals (+5V and -8V) to turn on this input (and disable the sweep). So you need to check that the +5V, -8V switching signals are present and that Q582, Q583 are OK.

Roger


 

On Sun, Oct 18, 2020 at 06:10 PM, Roger Evans wrote:


Q760 and Q772 are PNP common emitter amplifiers, they have very close to unity
current gain and they conduct because their base is positive with respect to
the emitter so the BE junction is forward biassed.
That should be *negative* for PNP's.

Raymond


Harvey White
 

replies interleaved:

On 10/18/2020 11:09 AM, John wrote:
According to the Service manual, page 5-38 (volume 1 if you have the version split across volume 1 and volume 2) the service ROM is fitted in place of U565.

It has been 4 days and I seem to be no closer to solving the X_Y mode problem. Any help would be greatly appreciated.

For example, could someone please explain to me how Q760 and Q772 on the Horizontal Display Logic & A&B Sweep Generators diagram work. Both bases are connected to chassis/GND so how do they conduct?
You're looking at a configuration called common base (can be common base, common emitter, common collector).  Common collector is an emitter follower, common emitter is what most people think of as a transistor amplifier.

Look at Q772 for example, the transistor is PNP, so the emitter should be more positive than the base (base more negative than emitter).  It's fed through two resistors from the +15 volt supply.  the current going *in* to the collector will be about the same as the current going into the emitter (ignoring transistor beta).

so about 2.3K gives roughly 6 to 7 ma.  That same current will want to flow into the collector.  Looking at the horizontal amplifier, the base of the lower differential amplifier is the source (as well as any resistors connected to it from a negative supply).  What you see looking into the emitter is a very low impedance, so the waveform at point 65 generates a current that adds or subtracts from the emitter current on Q772, which then changes the collector current.

To turn OFF the sweep, Q781 is turned ON, which shorts the emitter to ground (VCE saturation on Q781 is less than VBE on Q772, so the transistor is off), killing the A ramp voltage/current into Q772, and effectively disconnecting the horizontal amplifier.

The first stage of the horizontal amplifier is a differential pair, which takes the input (which is effectively a current), mirrors it with the upper transistor, and runs a differential output to the plates.  All the magic is in that first stage.



With both traces centered both horizontally and vertically, the oscilloscope in analog mode and then switching to X_Y mode, test points 65 and 67 are both at +12.5V but Q772_e is at 160mV whereas Q760_e is at 45mV so there is a difference. The common point at Q740_c/Q772_c/U564_2 is at 180mV, rising to approximately 640mV when U564 (first stage of horizontal amp) is removed. I would have expected close to or at 0V at this point and as previously mentioned, grounding U564_2 brings the spot to dead centre which would tend to suggest that the horizontal amp, including U564 transistor array, ought to be fine.

If BOTH U564 and Q760/Q772 are removed, then this common point is at 0.02mA which demonstrates that other feeds into this point are having little or no effect. This is as expected because the position control is dead centre and no signal is being fed into the X or Y inputs.

This tends to suggest that the problem might be somewhere within the Horizontal Display Logic & A&B Sweep Generators sweep part of the circuit but I am not sure how to troubleshoot this.
Note that the horizontal amplifier is playing with a current input, not really voltage.

Had you looked at Q781 and Q782?  the trigger view offset voltage goes through a diode or so (as does the trigger view offset enable) and that contributes to the offset of the horizontal amplifier.

Harvey


Notwithstanding, when U564 is plugged back in without Q760/Q772 being present, the voltage at the common point drops to -170mV so its presence/abscence does affect the balance of the circuit. Consequently, I am not sure whether the transistor array is leaky or whether the problem is at the earlier stage or whether both parts of the circuit are contributing to the problem but I suspect it to be related to Q760/Q772 and that part of the circuit. Swapping or substituting those transistors has no effect.

The scope otherwise operates perfectly fine in triggered modes.





John
 

On Sun, Oct 18, 2020 at 05:10 PM, Roger Evans wrote:

Q760 and Q772 are PNP common emitter amplifiers, they have very close to unity
current gain and they conduct because their base is positive with respect to
the emitter so the BE junction is forward biassed. In XY mode (see page 3-26
of the manual) the sweeps are disabled, the BE junction should be reverse
biassed, there is almost no collector current and they just look like a high
impedance from the output side.
Thank you for that explanation. Much appreciated.

In analogue XY mode the X input comes via Q582/Q583 and the manual shows the
DC switching signals (+5V and -8V) to turn on this input (and disable the
sweep). So you need to check that the +5V, -8V switching signals are present
and that Q582, Q583 are OK.
Will check these out as well although I can confirm the presence of -8V at the input to R689, but also at the junction of CR679 and R679. There is also +15V at R380. I will do some more investigation and confirm the +5V input tomorrow.

You're looking at a configuration called common base (can be common base, common emitter, common collector). Common collector is an emitter follower, common emitter is what most people think of as a transistor amplifier.
Harvey, thank you also for such a detailed explanation.

I had read through the Detailed Circuit Description section in the Theory of Operation chapter of the manual. Details of X_Y operation are throughout the chapter and I picked up bits and pieces but your description fills in the blanks which is very helpful.

Note that the horizontal amplifier is playing with a current input, not really voltage.
Noted. I grasped the fact that there must be a variance in current between the two halves of the circuit when I found different voltage readings at the emitters of Q760 and Q772 but I wasn't quite sure what that meant.

Had you looked at Q781 and Q782? the trigger view offset voltage goes through a diode or so (as does the trigger view offset enable) and that contributes to the offset of the horizontal amplifier.
Yes I did swap them around to see whether the problem would shift to the opposite side. It did not. I also substituted the two ICs (U690 and U789) and checked that signals at the gates were as indicated. All looked good. However, I completely omitted to investigate the right hand side of Q781/Q782 including the inputs and diodes you mention. I will check that out and re-read your comments tomorrow morning when when my mind is fresh when hopefully this will all fall into place.


John
 

On Sun, Oct 18, 2020 at 10:31 PM, John wrote:



In analogue XY mode the X input comes via Q582/Q583 and the manual shows
the
DC switching signals (+5V and -8V) to turn on this input (and disable the
sweep). So you need to check that the +5V, -8V switching signals are
present
and that Q582, Q583 are OK.
Will check these out as well although I can confirm the presence of -8V at the
input to R689, but also at the junction of CR679 and R679. There is also +15V
at R380. I will do some more investigation and confirm the +5V input tomorrow.
I finally found it! :-)

After checking that -8V and +15V were present around the trigger offset inputs and voltages around the diodes were as indicated, and so ruling out this part of the circuit, I turned my attention to Q582/Q583. The problem turned out to be here.

Both +5V and -8V supplies were present but the voltages at Q582_b and Q583_b were considerably adrift from those indicated (-800mV/-100mV instead of -1.8 [-1.5V?] and -2.8V). This means that Q583 would fail to turn on affecting the output from between R480/R481 which is directly connected to U564_p2.

The transistors were fine and tracking back led me to J683, the connector providing the input from the X axis. On pulling this out I discovered that the centre pin was bent and evidently not making contact with the socket on the PCB. Since I had not touched this at any time, I plead not guilty! Once the pin was straightened and the connector plugged back in, the spot moved onto the screen. The X-gain was low and required a tweak of (V)R681 but otherwise X_Y mode now worked fine. When switching between X_Y and triggered modes an adjustment of the POSITION control of about half the width of the CRT display is still required but at least this is now within comfortable adjustment range.

Thanks Harvey for pointing me in this direction and both Harvery and Roger for their circuit descriptions, bit of which were immensely helpful as once that fell into place, it allowed me to better understand and rule out the sweep generator part of the circuit, which I had got side-tracked into, and allowed me to concentrate on other possible causes.

Everything on the scope now works fine although I have not yet tested B-delay yet as I need to research this. Once everything is properly re-assembled I should hopefully be able to move on to running through the calibration steps although from what I have seen so far, everything looks just about spot on.


John
 

On Sat, Oct 17, 2020 at 05:46 AM, <robeughaas@gmail.com> wrote:


I see the IPROM's are still on the way to you. I've started a survey of the 13
468's in the Museum's boneyard and of the first seven, found two with shorted
+5 Storage Supplies, as evidenced by blown F4007 and F4009, 5-amp picofuses at
the back of the trigger board. I've dismantled one and found a shorted CR169,
the catch diode in the supply. I'll go through the remainder tomorrow, and I
suspect I'll find more with shorted supplies. Of the six I've been able to
test ROM's on, I have found all the ROM's were good. I suspect that the power
supply problem is causing the failure that is thought to be ROM rot.
That's an interesting observation. Some time ago I came across a 465 that had an intermittent shorting -8V supply due to a faulty smoothing can but so far in the 468 all supplies are good and as it turns out, the ROMs are good as well. The problem turned out to be a non-running CPU. I don't know much about the history MOSTEK EEPROMs but can't help wondering how long these are likely to last although I am glad to have backup spares "just in case" courtesy of a helpful contributor on here. Still it is a testament to Teks' engineering that they can be brought back to life and still work 40 years later.... For sure the 468 is an interesting example of a vintage scope with early storage computing.


John
 

Just as a final note on this thread, I ran through the performance checks in the service manual and apart from a couple of minor adjustments all the features worked as expected and calibration was spot on. A couple of the push switches needed a squirt of switch cleaning lubricant to free them up and restore reliability and the case handle needed some repair, but aside from those relatively minor items the scope and is now fully re-assembled and working very well.