"There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy"
Hamlet
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--- In TekScopes@..., "Albert" <aodiversen@...> wrote:
I swapped the CRT of this 564 with the CRT of a known to be good 564. Lo and behold, both 'scopes worked fine now! Then I moved the CRTs back to their original positions and again both 'scopes worked fine.
Spontaneous self-healing of a loose contact inside the CRT?
Albert
--- In TekScopes@..., "Steve" <ditter2@> wrote:
--- In TekScopes@..., "Albert" <aodiversen@> wrote:
Thanks Bob,
I still don't understand it. Today I measured DC developed between the blanking plates (across the 570k resistor) with no plugins installed. There is DC, rising to about 0.4 V after some warming up. So there seems to be at least "some" connection to pins 5 and 7 internally. But there was no response in intensity whatsoever when I supplied 20 V DC between those plates, using a floating DC supply.
In a good 564 there was also some 0.3-0.4 V DC between the plates, and 20 V was sufficient to make the dot disappear.
Albert
I would be careful about making assumptions on potential difference between the plates, generated by a floating supply. While it is true that the field difference between the plates bends the beam, there is a component that needs to be referenced to the relative charge of the electrons at the point in time they pass the plates – in other words a DC bias on both plates relative to ground.
What I don't understand is why the 56x series uses BOTH deflection unblanking AND a DC coupled grid control. I have seen deflection blanking used when the grid is AC coupled to the intensity blanking / control, and DC biased directly from the cathode. In these cases, deflection blanking is needed for very long blanking intervals, as the time constant in an capacitor coupled system can only remain off for the time constant. But the 56x uses the more complicated DC system, which uses dedicated HV windings for both the cathode and grid bias, eliminating any time constants and the need for another blanking system.
So why did Tek design deflection based blanking into the 56x series, and why do the time bases use it, with the DC grid control only used for intensify in dual time bases?
- Steve
