Re: Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44

Fabio Trevisan
 

Hi there,

John.. thanks for the reminder. The new resistor is of the same power
rating as the original. In fact, although we're talking about the H.V.
inverter and a 2N3055 transistors, which is a power device, this resistor
itself is a small one, a quarter or half a watt if much.

David, see my comments next to your replies... thanks again... very helpful.

Rgrds,

Fabio

2016-11-04 4:14 GMT-02:00 David @DWH [TekScopes] <
TekScopes@...>:

Air is a pretty good high voltage insulator.
i can't argue to that. As a matter of fact I had quite some trouble getting
rid of black dust and contamination from the H.V. board of my Kenwood... To
make matters worse, phenolic Urgh! board...
Putting that into perspective, air mount seems to be a good idea after all.
What amazes me in the Tek is the quality and thickness of the fiberglass
board!

We have discussed it in the past and not reached any conclusion about
what Tektronix selected for on these transistors; I suspect it was
either current gain or Ft (current gain-bandwidth product). Tektronix
used at least three different 2N3055 variations and sometimes even
used all three in the same oscilloscope like the 76x3 series.
The major issue appears to be that if the transistor Ft is too high,
then the high voltage inverter will suffer from spurious oscillation
at a much higher frequency. A modern direct replacement which is
readily available is the 2N3771G (40V 30A 150W) or 2N3772G (60V 20A
150W) which both have an Ft of 200 kH which is very close to the 300
kHz Ft of the original 2N3055 that Tektronix used and they are tougher
also. The later 2N3055s had a 800 kHz Ft and modern ones are 1.5 MHz
which can definitely be a problem in this circuit.
Wow... I didn't know the fT for the older 2N3055 to be so low... Never
mention knowing
that they improved it over the years keeping the same part identifier...
That's confusing!

I almost bought a 2N3773 instead of the MJ15015 but, as usual, I was
looking at a part
that could match or exceed the 2N3055 absolute maximum values... It
wouldn't cross
my mind replacing it by the 3771, which is 40V (and not 60)... although I
admit that in this
circuit, 40V is enough (I actually measured the Vce on the oscilloscope and
it goes from
-10V to +29V)

Indeed, I noticed that the picked-up interference that I see on the screen
(while having the
H.V. cages opened, the ~45Khz signal is clearly visible without any probe),
is less clean
than it used to be... it may be due to spurious high frequency
oscillations...
I will try to look into it with more detail... If I find evidence of higher
harmonics, I will try to
get a 2N3772 (which is 60V)...

Having the horizontal and vertical deflection change indicates that
the cathode voltage is not being regulated. Lower cathode voltages
means lower electron velocity which gives the deflection plates more
time to deflect the beam yielding greater deflection.
Ack to that! Vertical sensitivity was low by some 50% while the cathode was
at about -1800V.
After I got regulation back, it came down to the correct levels.
I immediately thought of regulation when I noticed that it was "breathing"
as I
changed Intensity control, but my first thoughts (and fears) was that
something
could have blown in the H.V. feedback divider (the large custom film
resistor).
I was relieved when I noticed that feedback summing point was negative.

* Cathode voltage was too high and out of regulation
* Changed R1483 to lower it indicating too much drive to 1486.
* Gain too high?
Well, I reached to that conclusion from the theory of operation in the S.M.
in where it says that the feedback controls how much bias is fed into Q1486
bringing it closer or farther from the conduction point and therefore
making it
conduct for more or less time and, therefore, putting more or or less energy
into the transformer which translates into higher or lower voltage.

I would like to know why the circuit is designed the way it is. Why
didn't Tektronix make it so that it would not be as sensitive to
transistor characteristics? It would be nice to have a general
solution so modern high Ft 2N3055s could be used without issues.
I confess that transformer dependent oscillator theory has never
been "intuitive" to me... I understand the concepts involved, but not enough
that I could design one from the paper up...and be sure that practice would
meet theory at the fiberglass.
I can think of building one empirically, but that doesn't count to know or
understand
why folks at Tek did it that way and not otherwise.

In their defense I can say that my Kenwood's H.V. oscillator seems quite
similar. An NPN
driving transistor with emitter connected directly (no emitter resistor) to
-12V,
collector pulling the transformer down, base connected to the feedback
winding
of the transformer which is biased, from the other side, by the feedback
circuitry.
Only difference is that it has a 100R base stopper resistor, probably
inserted there
to tame H.F. spurious oscillation.

At the point that I am right now, I am in position of doing some
experimentation...
I don't pretend to ever reach mathematical conclusions but I can look at
the circuit
as it stands now and, if it's presenting unwanted H.F. artifacts, I can
play around
the transistor and search for a reliable and consistent way of taming it.
One that wouldn't compromise the phase margins (or the lack thereof) at the
nominal
frequency, which is mandatory to assure oscillation.
That part, to make it less dependent on transistor's fT.
About the dependency on the transistor's beta, I think that a common emitter
amplifier without any emitter degeneration as it is, and without any DC
negative feedback
to its base, indeed leaves a lot to desire...
At some expense of gain (which is not plentiful in the 2N3055 but much
better in
modern devices), I think an emitter degeneration resistor could improve the
circuit's susceptibility to change of device's beta (not just from device
to device,
but also from temperature change).

As soon as I get done with the CRT rejuvenation, which is my next chapter
at this
point, I can dedicate some time to research on those improvements.

Increasing the value of R1483 is a fine solution if the problem is
just too much current gain but can you measure the transformer voltage
using an oscilloscope to see if it is oscillating properly? Do you
have a high voltage oscilloscope probe? We know from a previous
discussion that spurious oscillation is possible.
I don't have an H.V. oscilloscope probe. I have a home-made H.V. probe made
of a string of about 30 x 3.3Mohms resistors, all insulated by plastic
spacers,
inserted on a 16mm dia acrylic knitting needle. I tried... but it's not
good for AC.

But, as I mentioned earlier, the vertical pre-amplifier picks-up the
oscillator frequency
pretty well (with the H.V. cages removed) so...Can I call it an H.V. scope
probe?

And I can measure the signal at the H.V. transformer's primary... which
must be
good enough.
To my understanding whatever H.F. artifacts that may be arising, must be
present
on the primary side, if we are to fix it from the primary side or, in other
words, if it can't
be probed on the primary, it can't be fixed from the primary.

I have pictures of how those primary waves were before I changed the
transformer
and transistor... I can look at them now and see if there are differences.

I'll get back to you with the results.

I wonder why R1483 was needed at all. The AC impedance at the emitter
of Q1484 is low because of C1483 so R1483 only provides a minimum
operating current. I would have expected R1483 to go to ground and
Q1484 to provide all of the base current but maybe R1483 has something
to do with the startup characteristics like preventing the output
voltage from overshooting which could damage something.
I don't think it's a matter of being REALLY needed... I think it's more
like a practical circuit's approach.
They probably departed from a working circuit (an oscillator without
voltage level feedback)
and added what was necessary so that it self regulated its output voltage.
As it stands, the control circuit is not responsible for making it work,
but just responsible
for adding a variable (controlled) amount of bias, enough to achieve just
the desired line
and load regulation.
And you can be right as well regarding the startup... oscillators are more
or less
easy to understand while under "regime"... but how they startup is often
guesswork
(especially before Spice simulation... but even though, circuits that
doesn't oscillate in Spice
do oscillate in real world and vice-versa).

We shall see... Or not!

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