Re: Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44
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.
2016-11-04 4:14 GMT-02:00 David @DWH [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
We have discussed it in the past and not reached any conclusion about
The major issue appears to be that if the transistor Ft is too high,Wow... I didn't know the fT for the older 2N3055 to be so low... Never
that they improved it over the years keeping the same part identifier...
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
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
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 thatAck 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"
changed Intensity control, but my first thoughts (and fears) was that
could have blown in the H.V. feedback divider (the large custom film
I was relieved when I noticed that feedback summing point was negative.
* Cathode voltage was too high and out of regulationWell, 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
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. WhyI 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
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
collector pulling the transformer down, base connected to the feedback
of the transformer which is biased, from the other side, by the feedback
Only difference is that it has a 100R base stopper resistor, probably
to tame H.F. spurious oscillation.
At the point that I am right now, I am in position of doing some
I don't pretend to ever reach mathematical conclusions but I can look at
as it stands now and, if it's presenting unwanted H.F. artifacts, I can
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
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
to its base, indeed leaves a lot to desire...
At some expense of gain (which is not plentiful in the 2N3055 but much
modern devices), I think an emitter degeneration resistor could improve the
circuit's susceptibility to change of device's beta (not just from device
but also from temperature change).
As soon as I get done with the CRT rejuvenation, which is my next chapter
point, I can dedicate some time to research on those improvements.
Increasing the value of R1483 is a fine solution if the problem isI 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
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
pretty well (with the H.V. cages removed) so...Can I call it an H.V. scope
And I can measure the signal at the H.V. transformer's primary... which
To my understanding whatever H.F. artifacts that may be arising, must be
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
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 emitterI 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
easy to understand while under "regime"... but how they startup is often
(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!