Re: Tektronix 475
Chuck Harris <cfharris@...>
Ok, you are almost there.toggle quoted messageShow quoted text
There are two things at play at the same time with the intensity
grid function. The first is the DC restorer, which you have checked.
It translates the voltage output from the intensity control to a
voltage range at the control grid of the CRT.
The second thing is the grid bias control. It biases the grid so that
it is in the operating range of the CRT.
I forget, have you verified that the grid bias control has an effect?
Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.
I tested all the transistors in the Z-axis amplifier and all were OK. The amplifier puts out a square wave which varies from 0 to 35 V with the intensity control, but the beam was still too bright at all intensity settings. I started to suspect the voltage multiplier circuit (C1372, CR1377, CR1379) as the input was at least 40 V p-p. The components tested OK with a multimeter, but I replaced the diodes in case they were breaking down at circuit voltages. No change, but all controls operate as described in the manual with the scope probe on the cold side of C1372. The circuit diagram in my copy of the manual does not correspond exactly to the circuit description as it shows a separate winding on the HT transformer for grid bias, which I cannot locate (transformer on other side of the board). However, a 40-50kHz signal is coming through the 390 kΩ resistor (R1326) from somewhere. I will just live with a bright trace for the moment and concentrate on the Y output amplifier, which looks as thought it can be removed and tested on the bench.