Re: Another scope 7854

Joe Laffey

On May 3, 2017, at 10:43 PM, David @DWH [TekScopes] <TekScopes@...> wrote:

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.
So by attenuators modules you mean the little plastic boxes under the aluminum shielding? Each box has two trim caps?? I just pull those up and re-seat them? Any trick or special tool required other than patience and care?

I already cleaned and aligned the cam switches. They seem to be working better, but I still have an overall gain issue on ch2 (about 5X too much gain on all volts/div settings). I turned down the gain setting using the push in with a screwdriver method on the variable gain on the front of the unit. That did not go low enough. I checked the resistors around the gain pots and they check out in spec. That push in gain pot was going down to about 1 Ohm.

Perhaps this is caused one of those modules.

I will reseat the ICs. I don't think I see any socketed transistors.

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.

Interesting indeed. That 2465 family is a great bang for the buck. I was mainly trying to justify buying (albeit cheaply, locally, and with a real nice Tek cart included) the 7854. The GPIB might be interesting to mess with, and there are other features to justify it.

Thanks for the compatibility info as well. This is all great data.

Joe Laffey

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