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I have cleaned thoroughly the entire area with IPA and now I have steady
-2950. This also resolved the problems at the Obama where now I have

Please no politics in the group :)

basically zeroes at pins 2 and 3 and 0.6V at pin 6, the grid bias seating
is changing the U1665 but has no visible impact on the trace.

It looks like your HV supply is in good shape now.

The Z out DC is now 16 to 165V when I regulate this but there is no

impact

on the trace

With these new measurements observations are pointing to faulty diodes.

Regarding the timing

I can see triangle wave and pulse on respective outputs and they change
shape /frequency as I change the timing basically making them narrower

but

this trend stops and they don't change shape in last few settings but
remain exactly the same.

Flat at what voltage level? Compared to good waveforms are these waveforms
at the max/min of the good waveforms are at a different DC voltage?

Ozan

Behavior of your 485 is changing over time so some of these comments may
not apply to current state of the scope but here is my feedback:

If you look at pins 2 and 3 of U1624 what voltage do you measure

(should

be essentially the same and close to 0V).

Pin 2 is zero pin 3 is 1.9V

This means the regulation loop (U1624) thinks HV voltage is too low and
working as hard as it can to increase the voltage but can't increase
anymore (railed). When you look at pin 6 do you see it railed (>10V)? In
this condition HV supply is not regulated anymore.

I'm not sure what's going on today I turned it on and I measure 2963V on
the HV spot.

However, I also saw this post that now voltage is OK. Did pin 3 also
changed to ~ 0V?

Does the HV voltage change when you turn intensity knob? Maybe there is
some interaction and when you change some settings HV voltage is changing.
Or something might be baking and recovering ...

The Z OUT DC can be controlled from -70 through -170 and finishes around

-50

This should not be possible. If I am not missing a supply connection,
minimum supply of that block is -5V. In normal operation "Z OUT DC" starts
from ~ 14V and goes higher for lower intensity. What voltage do you see at
the left side of R1792 (collector of Q1772)?

Only possibility I can see is a DC path from the HV line, which would also
explain why HV supply is low. Suspect components are C1684, C1664, C1663.
You can also try cleaning the HV area more thoroughly in case there is some
conductive residue.

In almost all the "beam too bright" cases HV diodes were bad so it is a
good idea to replace them all as a good measure. I am not an expert in HV
DC restorers but if you look at past discussions UF4004 was recommended by
Tom as a replacement. A random diode won't work.

For time base issues, in the problematic 3 fastest settings how does the
"A Sweep" and "A gate" signals look? They are easily available at the

back

of the scope with a BNC connection.

Both if the signals don't change past the 10nS setting they stay the same

for 5 2 and 1ns

I assume slower than 5ns you see a pulse at "A gate" and a triangle wave
at "A sweep". In 5/2/1ns are they both flat? Flat at ground or some other
voltage? Is the flat voltage same as the lowest point or highest point of
proper signals (when sweep slower than 5ns)?

Ozan

The grid bias changes the voltage at TP1665 but does not have any visible
impact on the trace

What voltage range do you see at TP1665 when you change grid bias. No need
to re-measure, just ballpark numbers you remember. Especially the sign,
positive or negative voltage.
Ozan

Behavior of your 485 is changing over time so some of these comments may
not apply to current state of the scope but here is my feedback:

If you look at pins 2 and 3 of U1624 what voltage do you measure

(should

be essentially the same and close to 0V).

Pin 2 is zero pin 3 is 1.9V

This means the regulation loop (U1624) thinks HV voltage is too low and
working as hard as it can to increase the voltage but can't increase
anymore (railed). When you look at pin 6 do you see it railed (>10V)? In
this condition HV supply is not regulated anymore.

I'm not sure what's going on today I turned it on and I measure 2963V on
the HV spot.

However, I also saw this post that now voltage is OK. Did pin 3 also
changed to ~ 0V?

Does the HV voltage change when you turn intensity knob? Maybe there is
some interaction and when you change some settings HV voltage is changing.
Or something might be baking and recovering ...

The Z OUT DC can be controlled from -70 through -170 and finishes around

-50

This should not be possible. If I am not missing a supply connection,
minimum supply of that block is -5V. In normal operation "Z OUT DC" starts
from ~ 14V and goes higher for lower intensity. What voltage do you see at
the left side of R1792 (collector of Q1772)?

Only possibility I can see is a DC path from the HV line, which would also
explain why HV supply is low. Suspect components are C1684, C1664, C1663.
You can also try cleaning the HV area more thoroughly in case there is some
conductive residue.

In almost all the "beam too bright" cases HV diodes were bad so it is a
good idea to replace them all as a good measure. I am not an expert in HV
DC restorers but if you look at past discussions UF4004 was recommended by
Tom as a replacement. A random diode won't work.

For time base issues, in the problematic 3 fastest settings how does the
"A Sweep" and "A gate" signals look? They are easily available at the

back

of the scope with a BNC connection.

Both if the signals don't change past the 10nS setting they stay the same

for 5 2 and 1ns

I assume slower than 5ns you see a pulse at "A gate" and a triangle wave
at "A sweep". In 5/2/1ns are they both flat? Flat at ground or some other
voltage? Is the flat voltage same as the lowest point or highest point of
proper signals (when sweep slower than 5ns)?

Ozan

On Wed, Mar 17, 2021 at 08:45 AM, Ondrej Pavelka wrote:

On Tue, 16 Mar 2021, 22:42 Ozan, <ozan_g@...> wrote:

I checked the DC recovery diodes in the HV supply as listed in other

thread

(all under the plastic cover and they are ok.
Any other suggestions?

Note that checking the leakage of those diodes is important, just a DVM
diode check doesn't discover problems.

I would second this part of the post. I had a similar failure on a 475
and traced it to a bad restorer diode. The only way that I was able to
locate the problem was using a 5 On Tue, 16 Mar 2021, 22:42 Ozan, <
ozan_g@...> wrote:


I checked the DC recovery diodes in the HV supply as listed in other
thread
(all under the plastic cover and they are ok.
Any other suggestions?
Note that checking the leakage of those diodes is important, just a DVM
diode check doesn't discover problems.

I would second this part of the comment. I had a 475 with a similar
failure and symptoms. A simple DMM diode test showed the diode as “good”.
When I removed it and tested it on the 576 curve tracer the problem
became apparent, as soon as I checked it on the CT, it exhibited a very
strange breakdown at higher voltages. So replacing the diode and
associated capacitor with a known good diode and capacitor repaired the
problem.

--
Michael Lynch
Dardanelle, AR

I can only speculate why the 741s didn't like the unfiltered rectifier output as +Vs, but suspect the capacitor at the (+) input may have caused excessive input current as the device was violently cycled. As both 741s failed, there seems to be a weakness. A 741 Spice model might provide insight.

Now to investigate the DM502 and DM504 I haven't used in years and find a source for replacement capacitors.

Bruce, KG6OJI
-----------------
Why did the open filter capacitor destroy the 741s? Surely open filter capacitors is a problem the tester should identify and not itself fail from.

Best wishes,

Larry McDavid W6FUB
Anaheim, California (SE of Los Angeles, near Disneyland)Larry