Re: Tek 475 intensity control inoperative

Fabio Trevisan
 

Hello Don,
I don't know the 475 CRT grid driving circuit in particular, (I know a few,
and they're all very similar).
Intensity control not functioning on the "on" side seems to be quite
recurrent and often related to the DC restorer circuitry.
First of all, get a copy of the 475 manual (my PDF copy I think I got from
the TekWiki website) and try to perform the adjustment of the CRT grid bias
that's described there in the calibration section. It's the 3rd part of
that chapter.
Don't worry, changing the CRT grid bias doesn't affect the calibration of
the oscilloscope so, if you don't mess with anything else, you won't loose
any of the oscilloscope's calibration.
From the defect that you described, chances are that you won't be able to
make the CRT dot to dim enough to "just" show (as the procedure asks for),
but it doesn't hurt to try.
The CRT grid bias trimpot can be just flaky and the problem may be just
there.
At that "CRT Grid Bias" calibration, the result of the steps e, f, g and h
should give you a good measure if your Z-axis amplifier is in good shape or
not.
If you can't get the TP1364 to read 15V (required for the CRT grid bias
cal, at step b), or get it to something between +20 ~ +30V at step f, or
something about +25 at step h, then your problem should be in the Z-axis
amplifier or the signals that feeds into it.
If, however, those voltages are sound, then your Z axis is probably OK and
you probably have a problem on the DC restorer circuitry.
The Z-axis amplifier is the circuit producing the voltage at TP1364.
The DC restorer, is the circuit just after that.

Unfortunately, to measure / diagnose the Hi-Voltage section of the DC
restorer circuitry (everything on the anode side of CR1377, or the cathode
side of CR1379) requires a hi voltage probe and usually doesn't help too
much.
Diagnosing the DC restorer circuitry usually is about to check all the
voltages that are "inputs" to it.
That means checking:
1. The Z-axis amplifier output at TP1364 as seen above,
2. The grid bias adjustment voltage at cathode of CR1373, which should be
able to vary between 82 and 130V as you rotate the trimpot. Too low a
voltage may cause the problem and a look at the zener VR1374 is due.
3. The DC restorer AC feed voltage, which I guess is something around
300Vpp coming into R1326. The frequency at this point is the same frequency
as the Hi-Voltage inverter, around 50KHz and not all volt-meters will deal
well with it,
4. And of course, the CRT cathode voltage itself at the -2450V test point.
Measuring this will of course requires a Hi-Voltage probe, and won't reveal
much, since your scope does show a healthy trace and therefore, that
voltage must be probably sound.
This voltage of about 300Vac, I've seen in older posts that it's quite
sensitive to loading, so, even an oscilloscope with a 10x probe (at 50KHz)
already affects this measurement... so some guessing and calculations must
be made.

If all those "inputs" are OK, usually there's not much that can be done to
further troubleshoot the DC restorer besides taking each component out and
replacing them.
One by one or all at once, it depends on your desire to point down the
culprit or just get rid of the problem.
From my experience and from reading this forum, the usual suspects are the
diodes CR1371, 1373, 1377 and 1379. A leaky C1371 can also cause the
problem but those film capacitors are usually very reliable, and a drifted
(down) R1378 can drain the CRT grid flimsy voltage.

Hope this helps,

Rgrds,

Fabio



2017-11-17 6:07 GMT-02:00 mcc131266@... [TekScopes] <
TekScopes@...>:



Well, it does work a bit : The intensity goes from just a bright trace to
very bright, and slight blooming. Focus and ASTIG work OK. I assume the
correctly functioning intensity control should adjust from no trace to
bright trace in this model. Anyone struck this, or have any suggestions?
P.S. : Cathode measures -2450 Vdc at the test point, as it should.
Cheers...................................................................Don
Collie jnr ZL4GX

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