Re: Another scope 7854

 

On Thu, 4 May 2017 17:38:45 -0500 (CDT), you wrote:

yOn Thu, 4 May 2017, David @DWH [TekScopes] wrote:

A shorted C375 shown on schematic 4 would cause about that much high
gain. A shorted C445 shown on schematic 3 could cause it if R445 is
adjusted to a low value.
Wow! I want to be you when I grow up! C375 was indeed shorted. I
understand why this affects the gain, and would be able to follow it
better if it were a modern OpAmp rather than discreet transistors (I can't
visualize those as well yet).
It helps that I am somewhat familiar with the details of the inside of
U270. Pins 8 and 9 expose the emitters of the channel 2 input
differential amplifier which has about 50 ohms of resistance to set
the gain. With C375 shorted, the 10 ohms of R375 is placed in
parallel with the internal 50 ohm resistance raising the gain by about
5 times.

Your observation that the gain was about 5 times high was a major
clue. There just was not that many things which could cause that
without other symptoms.

I got a suitable replacement from my local store and will swap it out
later today.
Recalibrating the transient response is not trivial on these; it
requires a reference level pulse generator like a PG506. What you
could do instead is apply the same fast pulse to both channels and
adjust C375 until the pulse response of channel 2 is as close as
possible to channel 1. If you do not have anything better, then the
sync or trigger output from a function generator is probably fast
enough.

The occasional triggering glitch I think must by the cam finger switches
on the AC/DC/GND/DC Offset switch. I got them to go away on channel 1
through exercising the switch, but channel 2 still has this glitch. I need
to figure out the best way to physically access these to clean them with
as little disassembling as possible. Will have a look at that too.
This problem might also be caused by the gold soldering issue that I
mention in my other post. Remove the aluminum cover over channel 2
and do a close inspection of the connections to the input circuits.
Mine had a problem with the AC coupling capacitor.

Also, in addition to the averaging the 7854 does have the cursors and
measurements built-in over the 7904. They are actually relatively easy to
use, about the same as my 2232, though having a knob for the cursors is
nicer than the buttons.
That is true but the 2230 and 2232 operate in real time and have a
much better interface. The complexity of using the 7854 limits most
users to its basic functions.

Did you know that the 2230/2232 can make ground referenced absolute DC
measurements? When the coupling switch is set to ground in storage
mode, the level is recorded and marked on the display as the first
couple of points of the waveform record. When one of the cursors is
placed on that spot, the voltage readout changes to show a ground
symbol indicating that the other cursor is now measuring relative to
ground.

The 7854 has a similar function which sets the ground reference level
with a special aquisition cycle.

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