Two things about this one. I would like to replace the batteries with something more modern to get more time and quicker charging. Not sure about what it out there to do it though. The originals look like sub D cells and they are in a sort of holder but wired together as far as I can tell. But that is not the current issue, that is more of a backburner thing, including possible modifications to the charging circuitry and maybe even making it so it can charge and run at the same time. Limitations of the AC power supply might preclude that though.
The issue right now it the intensity. This is the second 422 I've had and neither one of them goes all that bright, This one is a bit worse, to the point where it is hard to see the trace at higher sweep speeds. The other one that was a bit better was not an AC/DC model but that should make no difference. If it had to do with the power supply something else noticeable would happen, deflection variations etc.
Looking at the circuit, the negative side of the HV is shunt regulated with a GV4-S1400 which obviously means 1,400 volts. It feed G1, with the cathode dropped resistively to get a relative negative voltage from GI to K.
The cathode current is limited by the resistance, 1,400 volts across 6.2 megohms, which should be 225uA+, this by the entire accelerating voltage, provided none of the current is diverted should result in 1.3 mW at the phosphor. Would that be enough ? I should think so, Tektronix would not be prone to make a mistake in that regard.
In the other 422 the intensity control worked up to about halfway and then stopped. That was all she had and it was independent of sweep speed, darker at higher speeds. This one is not only darker than that, the intensity control seems to work in reverse when it is much past halfway. Since the intensity control is a rheostat, I am having trouble fathoming just how this can happen.
Could some of the beam current be diverted to the blanking grid ? And, since it is fed by relatively low impedances, how would I tell ?
I don't believe it likely that any significant current is diverted to the next grid as it is the focus grid and that should cause some sort of abberation there.
What are the odds of a weak CRT here ? What do you think ?
If it is a weak CRT I have a way to boost the filament voltage. It entails caps in series with resistors across the bridge, CR 849, to cause some limited voltage doubler action. The issues with that are that I absolutely don't want to do it if the CRT is not weak, and the obvious isolation required. (and space of course, I have no idea how cramped it is in there)
I have the CRT from the other 422, but it is shorter. I am not sure it will work, and I am not sure it is compatible completely when it comes to geometry etc. Actually I really don't want to replace the CRT anyway and if necessary will find other options. Perhaps only use it at slower speeds indoors. But it would be nice to have a bright enough trace to use it outdoors on batteries.
So the intensity problem comes first. Weak CRT ?
Jeff Urban