Looking for Tektronix 4051 or 4052 Display Board
bhilpert at shaw.ca
Tue Jul 3 19:06:32 CDT 2018
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
> 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:
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
On this version you reffed:
they get it right.
However, compare to:
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.
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