Hi Albert,
W.r.t. sensitivity for DC input (did you try?) I forgot that the setting
of S901 "Correction Memory Switch", see page 2-5, is important. For now the
best way is to set the switch in the rear (OFF) position (as long as you
don't have a pulse generator head).
Explanation of S901 purpose is quite vague in 7S12 manual, I did try all 3
positions and there is no change. Btw I also fed the head with very low
freq sine wave thus simulating DC input, and still no go.
With a DMM attached to the front panel Vertical Out connector you can see
that Memory is slowly drifting.... the DMM reading will immediately jump
back to the normal value. The trace should reappear immediately at the
previous vertical position.
Yes, the drift is very very slow. I'd say it would tale a whole minute for
it to cross 1 div, with the settings I used. When I switch free running
back on, it immediately jumps back to its old position.
The heads have different connectors, SMA and BNC, and your signal
generator perhaps one of these. You need a power divider or a splitter or a
T and at least an adapter to from one type connector to another. All these
things could introduce faults. Are you 100% sure that the sine wave signal
arrives "inside" the S-4?
Yes, I'm using BNC-SMA adapters and BNC T adapters. I can definitely
measure 50Ohm (52 to be exact) at the point where I'm inserting the signal,
so I guess that's the best I can tell that I'm feeding the signal to S-4.
Even though this feedback-offset loop seems to be working, I decided to
check the head internally, namely the gate bias. It is possible to adjust
R32 according to manual, and its position falls around midpoint. However I
found the gate bias to be somewhat asymmetric, it's all part of a feedback
loop so it's hard to tell what causes it. The voltages on the S-4 internal
preamp board points are:
- C (feedback): -337mV (500mV/div), -146mV (200mV/div), -79mV
(100mV/div)...and reducing geometrically while increasing mV/div
- M=1.93V, H=-2.29V, V (at 100mV/div - I kept this setting because of the
relatively small feedback voltage on C, also applies to measurements below)
- Voltage across D20=405mV, D23=412mV, D25=424mV, D28=422mV
- on Q36 Vg= approx. -1.80V, Vs=-1.76V, so Vgs seems to be slightly
positive (can't measure directly between gate and ground as this point is
very sensitive)
I can't say if this slight asymmetry can be a problem. Preamp output (point
E) shows sampling pulses, but zero reaction to my low freq sine wave input
signal. I still cannot say whether the snap-diode circuit works, is there
any test I could do to verify if it's working? (E.g. are the sampling
spikes observed on point E due to snap-off working, or are these coming
through the feedback line?)
Best Regards, Nenad
