Re: Another scope 7854

Cliff Carrie
 

I had a 7A26 with intermittent vertical instability in channel 2 only. I found that alongside the attenuator plug-in modules, there are discrete 1/8 watt resistors. They are also plugged in and one had developed high contact resistance. I replugged it about 4 times after using a q-tip to swab its leads with contact cleaner, being careful not to get it anywhere except on the ends of the leads. The problem never came back.


Cliff Carrie

________________________________
From: TekScopes@... <TekScopes@...> on behalf of David @DWH [TekScopes] <TekScopes@...>
Sent: Thursday, May 4, 2017 7:22:26 AM
To: TekScopes@...
Subject: Re: [TekScopes] Another scope 7854



On Thu, 4 May 2017 02:10:45 -0500, you wrote:

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?
Those are them. They are mounted just like big 4 pin ICs.

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.
5 times too much gain at all volt/div settings? No, that is some
other problem and an odd one. The frequency response should be
screwed up also.

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.

Comparing the DC levels at different points between channel 1 and
channel 2 should help find the problem.

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.
The averaging capability is very nice in some applications. Except
for that, the 7854 is a slightly slower 7904 to most users.




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