On 10/18/2020 11:09 AM, John wrote:
According to the Service manual, page 5-38 (volume 1 if you have the version split across volume 1 and volume 2) the service ROM is fitted in place of U565.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.
Look at Q772 for example, the transistor is PNP, so the emitter should be more positive than the base (base more negative than emitter). It's fed through two resistors from the +15 volt supply. the current going *in* to the collector will be about the same as the current going into the emitter (ignoring transistor beta).
so about 2.3K gives roughly 6 to 7 ma. That same current will want to flow into the collector. Looking at the horizontal amplifier, the base of the lower differential amplifier is the source (as well as any resistors connected to it from a negative supply). What you see looking into the emitter is a very low impedance, so the waveform at point 65 generates a current that adds or subtracts from the emitter current on Q772, which then changes the collector current.
To turn OFF the sweep, Q781 is turned ON, which shorts the emitter to ground (VCE saturation on Q781 is less than VBE on Q772, so the transistor is off), killing the A ramp voltage/current into Q772, and effectively disconnecting the horizontal amplifier.
The first stage of the horizontal amplifier is a differential pair, which takes the input (which is effectively a current), mirrors it with the upper transistor, and runs a differential output to the plates. All the magic is in that first stage.
Note that the horizontal amplifier is playing with a current input, not really voltage.
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.
Notwithstanding, when U564 is plugged back in without Q760/Q772 being present, the voltage at the common point drops to -170mV so its presence/abscence does affect the balance of the circuit. Consequently, I am not sure whether the transistor array is leaky or whether the problem is at the earlier stage or whether both parts of the circuit are contributing to the problem but I suspect it to be related to Q760/Q772 and that part of the circuit. Swapping or substituting those transistors has no effect.