Re: Is my Tek 468 beyond repair?


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.




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