On Sun, Oct 18, 2020 at 05:10 PM, Roger Evans wrote:
Q760 and Q772 are PNP common emitter amplifiers, they have very close to unityThank you for that explanation. Much appreciated.
In analogue XY mode the X input comes via Q582/Q583 and the manual shows theWill check these out as well although I can confirm the presence of -8V at the input to R689, but also at the junction of CR679 and R679. There is also +15V at R380. I will do some more investigation and confirm the +5V input tomorrow.
You're looking at a configuration called common base (can be common base, common emitter, common collector). Common collector is an emitter follower, common emitter is what most people think of as a transistor amplifier.Harvey, thank you also for such a detailed explanation.
I had read through the Detailed Circuit Description section in the Theory of Operation chapter of the manual. Details of X_Y operation are throughout the chapter and I picked up bits and pieces but your description fills in the blanks which is very helpful.
Note that the horizontal amplifier is playing with a current input, not really voltage.Noted. I grasped the fact that there must be a variance in current between the two halves of the circuit when I found different voltage readings at the emitters of Q760 and Q772 but I wasn't quite sure what that meant.
Had you looked at Q781 and Q782? the trigger view offset voltage goes through a diode or so (as does the trigger view offset enable) and that contributes to the offset of the horizontal amplifier.Yes I did swap them around to see whether the problem would shift to the opposite side. It did not. I also substituted the two ICs (U690 and U789) and checked that signals at the gates were as indicated. All looked good. However, I completely omitted to investigate the right hand side of Q781/Q782 including the inputs and diodes you mention. I will check that out and re-read your comments tomorrow morning when when my mind is fresh when hopefully this will all fall into place.