Re: 7A26 Ch 1 gain low

Albert Otten
 

Hi Colin,

Remarks only concerning the schematics:

On Wed, Jul 24, 2019 at 05:57 PM, Colin Herbert wrote:


I have done a bit more investigating on this and have found the following:

-Some of the voltages on the first CH1 IC (U1350) are different from those on
a known good 7A26, so I am inclined to think the problem is associated with
that circuit. Tek made some errors in Service Manual in that the polarities
some DC voltages here are wrong (pins 2 & 3 are given as +0.8V but are
actually -0.8V).
As far as I can see you can use there the proper values shown for CH2.


-Swapping the first ICs between CH1 and CH2 doesn't change anything.
-Putting the U1350 from CH1 into a known good 7A26 doesn't take the problem
with the IC, so it is probably good.
-When the IC from the problematic CH1 was in place in the known good 7A26, the
voltages around it were similar to when a known good IC was in place, again
suggesting that there is nothing wrong with the IC.
-The +15V and -15V rails (where I have probed them) seem to be in spec.
-I have tried some of the initial calibration routines (with the correct
equipment) and find odd behaviour (zero shift) when operating the BANDWIDTH
and INVERT switches during some of this.
-I have also tried checking the waveforms at the various points shown in the
Service Manual, but I am not too sure that Tektronix didn't make some mistakes
here too, as I see large DC voltages which move the trace off the screen when
using the stipulated DC input on the investigating scope.
I am at a bit of an impasse, partly because the circuits are differential and
I don't understand them too well.
The "Voltages and waveforms" page states that "if the 7A16A amplifier plugin is used [for measuring waveforms], the input should be AC coupled". That holds for any ordinary plugin. But if you use the 7A13 (also mentioned) then you can measure differentially, compared with a suitable internal 7A13 DC level.

Albert

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