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

 

On Thu, 3 Nov 2016 16:13:33 -0200, you wrote:

...

Since I had dismantled almost the whole H.V. circuitry, to test all the
critical components "off-board" as I did and which led me to discover that
the H.V transformer was defective, it took me a while to put everything
back together, specially because the components in this area are laid-down
in a messy mixture of P.C.B., air-mount, piggy-back and ceramic-strip
techniques.

It amazes me how Tek managed to mass produce this line of 'scopes with this
kind of assembly technique.
Air is a pretty good high voltage insulator.

...

This circuit was still working when I took it apart to test the
components... And the 1A fuse blown so instantly... I don't believe that
such a short overload could have blown that massive 2N3055. The Unreg +15V
supply isn't even able to source the 15A that this beast is supposed to
withstand... And it is (was) an original TEK part!!!

I don't understand!!!

Understanding or not... Q1486 was blown... and it would be hard to source a
reliable (not fake) 2N3055 at short notice...
I managed to get an MJ15015 from a reliable supplier that I know... but I
was about the fact the the original Tek is listed as being a "SELECTED"
2N3055 device.
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.

...

Still... things were not perfect. Retrace was visible... but I could make
it disappear readjusting the CRT bias (all in all, I could have touched it
while I disassembled the components all around)... But worse than the
visible retrace... the trace size (horizontal and vertical) was expanding
as I increased the "Intensity" control...
Shoot!.. Poor H.V. Regulation (or none...) I was already afraid that
something else in the circuit could have blown!!!
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.


* Cathode voltage was too high and out of regulation
* Changed R1483 to lower it indicating too much drive to 1486.
* Gain too high?


What do you think? Should I leave it like that, or try something else (such
as getting another transistor or dumping some of its gain stealing some of
its base current to a drain resistor...maybe to a negative rail)?

BRgrds,

Fabio
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.

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 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.

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