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This the the rise time now at 5mV. I could probably get it better but but my
bnc cables and 50ohm terminator are not great quality. Should I keep working
at it?

/g/TekScopes/photo/59361/4?p=Name,,,20,1,0,0

On Thu, Jun 21, 2018 at 12:00 pm, Fabio Trevisan wrote:

Hello lop pol,

I've followed up this topic from the start, without having actually jumped

in,

hoping since the beginning that someone would ask or make remarks about two

or

three important things (to check, or to be aware of).

1. That's a question... What type of Vertical Output board does your 465

have.

the one discrete or the one with the custom Tek IC?
It's important because the High Freq. compensation networks are different

from

one type of board to the other and since your problem manifests in both
channels, there's a great likelihood the problem is located on the Vertical
Output Amplifier board, instead of on the pre-amplifier. Without that
information is difficult to advise further.

2. If your vertical amplifier is of the "Teck IC" kind, look also to the

BIAS

trimpot. If the wiper is open, the amplifier may be operating on the wrong
bias point and that is supposed to impact the high-frequency performance.
Since the procedure for calibrating the bias calls for applying a 100Mhz
signal and adjusting the bias for the highest p-p display, from there we
deduct it has a marked effect on the low nanosecond time constant, which is
exactly where your issue lies.

3. Besides what Raymond already mentioned, that is not uncommon for some of
those trimmer caps fail open (I had 3 of those failed on my 464), I want to
bring to your attention that ALL the trimpots that are used on the H.F.
compensation networks are connected as "rheostats" so, a false contact or

open

wiper will make any of them appear open, or in the best case, the full
trimpot's resistance, which will be just as bad for the step response.
Since many of them are in series with trimmer caps, it's easy to check them

in

circuit, without needing to lift any leg, or remove any component.

Last but not least, despite your problem clearly doesn't seem to be on the
pre-amplifiers, when you find out the culprit and eventually replace one
faulty trimmer cap or faulty trimmer resistor, you will eventually need to

go

through the Vertical High Frequency step response calibration, and for that,
it's important that you follow the procedure, and from your description, I
think you're already doing wrong...
While choosing for the right input level / input attenuator (pass-through)

and

vertical attenuator settings, it's important to do all the High-Frequency
calibration with the oscilloscope set to the most sensitive setting... i.e.:
5mV/div (not 10mV, not 100mV... and not 200mV/div).
In other words, you need to source 25mVpp to the oscilloscope's input (for a

5

division display).
This is what will assure you that none of the high impedance input

attenuators

are upsetting the subtle high frequency adjustments and artifacts.
While - in theory - the high-impedance input attenuators shouldn't degrade

the

step response of the osciloscope, they're not perfect and they do cause some
minor disturbances and aberrations to the step response (and they can also

be

misadjusted, something that you will only correct at a later stage) so, to
make a "clean" H.F. calibration, you need to make sure you got them (the

high

impedance input attenuators) out of the way.
The only compromise to that rule is the input probe itself... If you intend

to

use the oscilloscope with a particular probe (let's say, a 10:1), then you
need to connect that probe to the input, use the scope set to 5mV/div, and
apply, accordingly, a 250mVpp signal to the probe (if it's a 10:1 probe).

This

will assure you will compensate in the scope, for the probe's H.F.
inaccuracies or artifacts.
Yet on this topic, you need to be sure to "source" that signal (regardless

if

directly to the input or through your preferred probe), from a low impedance
source (50Ohm).
The latter is important because you don't want the frequency response of the
incoming signal to be affected by variations caused by the 'scope's input
capacitance, or your probe's input capacitance.
The lower the impedance of the source signal, the less the input capacitance
will matter and, of course, you will also want to assure the interconnect
cable is properly terminated.

Good luck with that,

Krgrds,

Fabio


On Sun, Jun 17, 2018 at 06:47 pm, lop pol wrote:

Hey guys. I have done a lot of work to this 465. I had initially planned

on

selling it to help me buy some plugins for my 7633. I decided I don't want

to

sell it and am keeping it. Today I checked its rise time with a pg506 fast
rise. Both channel 1 and 2 closely resemble each other and they are not

close

to right. Before I start trying to calibrate this thing, does it look like
there is some type of component issue left to fix? I am thinking since

both

channels are so alike there is something wrong besides just out of
calibration. Thanks guys.

here are some pictures

/g/TekScopes/photo/59361/0?p=Name,,,20,1,0,0

/g/TekScopes/photo/59361/1?p=Name,,,20,1,0,0