7904 readout 'jitter'


pe1fbo <sh331182@...>
 

Hi All,

I have a 7904 where the on-screen readout vertical position is
slightly depending on the vertical trace position.

On a steady beam this is not a problem but if you put the horizotal
time base in alt, the readout becomes bouncy. I think the problem is
in the unbalance of the vertical amp, but would like to hear some
other people about this 'problem'

Thanks,

Fred de Vries


Craig Sawyers <c.sawyers@...>
 

I have a 7904 where the on-screen readout vertical position is
slightly depending on the vertical trace position.

On a steady beam this is not a problem but if you put the horizotal
time base in alt, the readout becomes bouncy. I think the problem is
in the unbalance of the vertical amp, but would like to hear some
other people about this 'problem'
Well, Fred - my 7904 is the same. The same thing happens slightly with my
7704A, and not at all with the 7834 and 7104. I've done a complete recal on
the 7904, and all offsets in y and t are absolutely on the button.

I suspect that it may have to do with the horizontal amplifier - there were
(at least) three different circuits that were used (B21000 and up, B080000 -
B209999, B010100 - B079999). The very earliest range of S/No.s had a
thermal balance control that was set to reduce readout jitter to less than
0.1 div, but this control does not exist on later units.

FWIW mine is in the middle range, with S/No B113428.

Any other information greatfully received....

Craig


 

--- In TekScopes@yahoogroups.com, "pe1fbo" <sh331182@1...> wrote:
Hi All,

I have a 7904 where the on-screen readout vertical position is
slightly depending on the vertical trace position.

On a steady beam this is not a problem but if you put the horizotal
time base in alt, the readout becomes bouncy. I think the problem
is
in the unbalance of the vertical amp, but would like to hear some
other people about this 'problem'

Thanks,

Fred de Vries
-------
That could be due to a poor mid or lo frequency response. Check
different frequencies of squarewaves and you might see that it's not
perfect. It will also show up as a slight cross talk between
compartments when using chop and for example have a large signal on
Left and nothing on Right. In the 7904, like in most of 7k
mainframes, there are several selected RC combinations in the output
amp that, if reselected, most likely will cure this.
/Zenith