Re: Tektronix 475


There are a couple things to check. First of all, turn the scope on in a dimly lit or dark room. Turn the intensity up all the way, and turn the focus up and down while watching the screen. If you get a glow or haze of light, most likely on the top, bottom, or either side, the CRT is probably good. The glow is likely coming from electrons bouncing off the side walls of the tube because the beam is being deflected totally off the screen. if the glow fills the entire screen, check the focus voltage. If it is drastically off, or missing entirely, it can cause a weak glow across the entire screen. Turn the intensity back to the middle of it's range, push the trace finder button, and check the voltages on the deflection plates. They should be fairly close, if one of them is drastically different, find out why. In addition to a drastic difference in the voltages on the plates, a faulty connection to a plate pin or one with zero voltage on it can cause the beam to be deflected totally off screen.

Bruce Gentry, KA2IVY

On 8/9/20 9:23 AM, Harvey White wrote:
The beam finder generally does two things:

1) unblanks the CRT by changing the bias voltage

2) changes the gain of the deflection circuits so that the maximum extent of any deflection is within the visible area of the CRT.

Some problems happen when the beam finder switch does work partially. If, on the other hand, one plate is not being driven, I'm not sure that the beam finder would work properly.


On 8/9/2020 7:29 AM, tenareze32@... wrote:
I have about -20 V between the first grid and the cathode, so I cannot see why the beam finder does not display at least a spot. I only saw a very faint vertical stripe with no input on the Y amp. An absence of spot or trace generally indicates an HT fault, but all HT voltages are OK.
From your explanation a running time base is required for a display,
but I thought the beam finder would override that. I will check the time base circuit with another scope.

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