Hi David,
It takes some time to remember what all those circuits in the 7T11 are doing!
The shift in output of U512A in a 5-2-1 sequence corresponds nicely (as should be) to the increase in feedback resistors 10k-25k-50k, ratio 2:3. At 1 ns/div the shift is about 0.5 V and that same shift will appear at the input R647 to the Dot Position Memory. This memory should detect that systematic error and send a correction to the Slow Ramp Inverter. That in turn would (finally) result in a shift of the THT output range to correct the error. This doesn't seem to happen, or not not enough. At the faster sweep speeds U512B and D get switched in and introduce their own offsets. Again the Dot Pos Mem should correct for this.
Maybe you can check its operation/calibration. I remember it's not an easy calibration and a lot of compromise is needed.
Albert
(BTW your packet calculator needs some attention as well...)
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In equivalent time, sequential mode, the trace is shifting left as I rotate the time/div knob.
All measurements were made on the 50nS sweep range with the timebase triggered from Pulse Out.
Looking at the signal at TP286 which is the output of the TTH circuit, I see:
Time/Div Start V Stop V Delta V
5nS -2.22V -7.82V -5.6V
2nS -2.22V -4.62V -2.3V
1nS -2.22V -3.5V -1.2V
.5nS -2.22V -2.76V -560mV
.2nS -2.22V -2.43mV -230mV
.1nS -2.22V -2.34mV -120mV
50pS -2.22V -2.62mV -60mV (approx values)
20pS -2.22V -2.25V -30mV
10pS -2.22V -2.23V -10mV
Looking at the output of U512A (TP512):
Time/Div Start V Stop V Delta V
5nS -0.1V +5.5V 5.6V
2nS -0.3V +5.3V 5.6V
1nS -0.6V +5.0V 5.6V
.5nS -900mV +470mV 560mV
.2nS -210mV +350mV 560mV
.1nS -420mV +140mV 560mV
50pS -70mV -10mV 60mV (approx values)
20pS -170mV -110mV 60mV
10pS -340mV -280mV 60mV
I don't think the ramp start voltage should be moving down like that.
Can anyone confirm my suspicions and suggest what the fault might be?
Thanks
David Partridge
