Re: Again a 7854 power supply beast to be repaired...


Harvey White
 

On Sat, 04 May 2019 11:52:56 -0700, you wrote:

OK Harvey,

I did as you suggested, putting a V and H plugin. Here the results:

1) Trace is moveable in both horizontal and vertical position. So I put the trace off-screen to avoid CRT damage.
2) Readout is visible.
3) All the display is highly blurred
4) Focus and astigmatism works but in a barely manner because the the display is at very very high intensity.
5) With another scope I checked the output of the Z-axys board (board A21, diagram 13) test point TP183: the output is stuck at approx. +120V. No pulses or other artifacts are visible. Moving the A, B or readout intensity has no effect.
Ok. This is good. I'm not the expert here, but....

the H and V amplifiers work, the readout circuitry works. Assuming
that focus and astigmatism aren't related directly to intensity (I
don't know the circuit, so I can't tell). then the problem is mostly
the intensity.

Now this next is a guess....

Looking at how the test point is stuck at 120 volts, then I'm
suspecting that there is a transistor that has to pull things down to
blank. If so, then this transistor is normally on (blanked) and you
unblank the CRT by turning off the transistor.

Even if that isn't true, I'd look at the circuitry feeding that test
point. Don't have the schematics myself, but I'm sure others do.

Looking at the test point, is it pulled up by a transistor, or pulled
down?

Which direction turns on the CRT?

The description of how the Z axis board works ought to tell you a lot.
Reading that ought to show you what waveforms are needed to blank the
display. You need (with another scope, and be careful, since I
*don't* know what voltages are on that board) to look and see if there
are unblanking pulses (go for an H sweep plugin, put it on auto
trigger).

You should see pulses somewhere to unblank the scope at roughly 50 to
100 Hz. If you want them line locked at whatever your line frequency
is, then set the trigger to line and adjust the trigger voltage until
the triggered light is on. You'll likely see a line on the scope
screen.

Now see if those unblanking pulses are getting where they need to be.

What I'd do....

Harvey




Max

Max


Join TekScopes@groups.io to automatically receive all group messages.