On 2018-Jul-03, at 1:21 PM, Monty McGraw via cctalk wrote:
> I've been repairing my Tektronix 4052.
>
> I've got the digital logic working - but the text and graphics are messed
> up.
>
> I posted photos of the screens in my Tektronix 4052 troubleshooting thread
> on vcfed.
>
> With a scope on the final X amplifier stage - it is oscillating - so I see
> weird horizontal strokes instead of dots for text. I know from the service
> manual that this circuit includes a feedback loop, and with the scope I see
> oscillation all around the loop - so I haven't found the source.
>
> Does anyone have a spare Tektronix 4052 (or 4051) Display Board that I
> could buy?
I take it you are referring to discussion and info from your post on may 27th
at:
http://www.vcfed.org/forum/showthread.php?63656-Tektronix-4052-Troubleshooting/page2&highlight=4051
Re a comment in that discussion:
The feedback loop in the op-amp circuitry is the negative-feedback gain-control
loop.
If you open that loop the gain will shoot off to -->infinity (in the real
world, the highest the amplifier can do, the output would go binary
rail-to-rail).
If you noticing and wondering why the "negative" feedback loops around to the +
input on the op-amp,
it's because the output drivers provide a stage of inversion (specifically
Q84/95).
Note there are some mistakes in those Tek schematics around U184A/B. They are
mixing up the pin numbering and +/- input labelling.
There seems to be half-a-dozen variations on the deflection circuitry (was
looking also at the 4051 service manual from bitsavers)
and the error seems to morph around between the versions, like someone tried to
fix it at one point but misunderstood and reintroduced
at another.
On this version you reffed:
https://drive.google.com/file/d/1FQxqfVyOeA1BJjUS7b7X-FzL7Wb8g7q5/view
they get it right.
However, compare to:
https://drive.google.com/file/d/1TJjeZqszca1C7XILsnDyO9D_r64ZMwoo/view
the 2 & 3 pin labels on U184A should be swapped.
On another version on page 96 of the manual they have the +/- flipped on U184B.
Suggestion re the problem:
As you are seeing the oscillation at C196 on the common X/Y bias supply, I
would try looking at Q186, in particular for a B-C short or leakage.
A B-C short/leakage there would be adding C196 to the X output and could be
providing the time constant for the ramp/oscillation.
You could try scoping the B & C of Q186, if the signal and voltage levels there
are similar, consider removing Q186 to resistance check it OOC.
