Re: Another scope 7854

 

On Wed, 3 May 2017 19:50:55 -0500 (CDT), you wrote:

The 7A18 has some issues. Channel 1 seems fine, but Channel 2 has a lot of
noise on it (need to check the exact value, but the line is about 4 times
thicker than Channel 1). It also seem to have all of its attenuator values
off by around one click of the switch (not exactly). All waveforms
have much greater vertical amplitude then they should. The readout display
is correct based on the switch position.
Sounds like some of the cam switches are not making complete contact
but also check the attenuator modules. They are socketted and
sometimes need to be reseated. In some cases, the connections between
the exposed pins and the hybrid inside can go open; if this is the
case, they can be resoldered with silver solder after removing the
plastic top from the module which just pops off.

I tried cleaning all the basic switches and pots, but I have not
disassembled the cam based switches on the attenuator controls. I was
wondering what special tricks there were to dealing with those. I am
guessing one of the contacts is dirty/bad.
Clean the cam switches by drawing isopropyl alcohol soaked clay free
paper or card stock through the closed switch; I like to use business
cards or 3x5 card stock. Vellum paper would also work well. It is
important to avoid common paper which has clay because the clay will
abrade the gold plating on the switches.

It is possible that some of the switches are not making good contact.
Tweezers can be used to bend the switch element where it meets the
leaf spring so that harder contact is made.

Do not lubricate these switch contacts.

Check the attenuator modules (under the aluminum covers) before
messing with the cam switches.

The scope also seems to have trouble triggering cleanly off Channel 2. I
will occasionally see a glitch waveform. I do not see this on Channel 1.
Also the glitches go away if I toggle the Channel 2 polarity switch to
invert mode. They also go away if I use DC Offset (Option 06) mode on the
7A18.

I am guessing I should start with the cam switches, but do those trigger
glitch criteria ring any bells with anyone?
The socketted transistors and ICs inside the 7A18 should be reseated.
The trigger source switching is done electronically but maybe the
mechanical wafer switch is dirty and needs to be cleaned.

Note that all of the above applies to the 7A26 as well.

Also, what is the fastest setup I can get with this 7854? The 7A24 with
its 50 Ohm 400MHz inputs? And the 7B85 ? Just trying to figure out the
ideal plugins to look out for. I'd like this to be my fast scope, possibly
supplanting the 2465 (300MHz).
It is not quite that simple. The 200MHz 7A26 is only 180MHz in the
7854 and almost all of the faster high input impedance vertical
amplifiers are 50 ohms only. So in a practical sense, your 2465 is
usually 300MHz while the 7854 is limited to 180MHz when used with
common x10 high impedance passive probes.

On the other hand, high impedance passive probes are anything but high
impedance at high frequencies where low-z probes or FET probes should
be used. But the dual trace 7A24 is only 300MHz in the 7854 which is
no faster than your 2465.

The recommended timebases are the 7B85 and 7B87 and operate up to
1ns/div. The 7B92A dual timebase can operate at up to 500ps/div. In
theory the 7B15 could replace the 7B85 to get 500ps/div capability
while keeping the 7B87.

The 7B87 timebase allows low sample rate single shot pretrigger
capability. It replaces the 7B80 which would normally be used but the
7B80 works fine also.

What happens if you try to run the 1GHz plugins in the 7854? Are they just
limited to 400MHz or what?
The 1984 Tektronix catalog has a table showing the vertical bandwidth
for different combinations of vertical amplifier and mainframe. For
the 7854:

7A11 200MHz Low Capacitance FET Single Trace
7A13 95MHz 1M Differential Comparator
7A16A 200MHz 1M Single Trace 20MHz Bandwidth Limit
7A18A 75MHz 1M Dual Trace
7A22 1MHz 1M Differential Amplifier 10uV/div
7A26 180MHz 1M Dual Trace 20MHz Bandwidth Limit
7A42 275MHz 1M Quad Trace Double Wide Logic Triggering

7A24 300MHz 50 ohms Dual Trace
7A19 400MHz 50 ohms Single Trace
7A29 400MHz 50 ohms Single Trace

The best general purpose vertical amplifier is the 7A26 because of its
dual channels, high bandwidth, high impedance inputs, and 20 MHz
bandwidth limit. The 7A29 is preferred to the 7A19 because it has
better input protection and is a better design.

This thing is pretty amazing considering the date of manufacture. Of
course it was over $30k in today's dollars...
Add in the cost of the vertical amplifiers and timebases and it is
more like $40k in today's dollars.

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