#### Re: 7603 weirdo inverted Z axis

On 24 Jan 2016 13:45:16 -0800, you wrote:

Typo in "I think you took 130 V across R1120 while it is about 128.4 V". Should read 130 V - 11.6 V = 118.4 V, not 128.4 V.

Albert
I have been going through your post and figured that was a typo. I
used 11.6 volts at the top of VR1148.

On 24 Jan 2016 13:40:48 -0800, you wrote:

Hi David,

It seems that you forgot about R1155?
Yep, I missed it. I might have designed it with an AC coupling
capacitor in series with R1155 but technically that is not needed. I
thought R1155 controlled the gain of Q1156 but the effect would be
very small. Who designed this thing?

Hmm, the later design used in the 7623A and 7633 lacks R1155 among
other minor changes and I had all of the schematics open when I did my
first analysis although I only scribbled on the correct schematic.

4.8 V across R1155 gives 0.48 mA. That 0.48 mA reduces the zener current and reduces the current via R1159 by the same amount.
I think you took 130 V across R1120 while it is about 128.4 V, this gives 0.21 mA less via R1120 and 0.21 mA more via R1159.
Together this yields 0.27 mA less via R1159 or 4.7 V lower output voltage. Then our results are the same.

Albert
4.8V / 10k [R1155] = +0.48mA
-15.6V / 9.1k [R1141] = -1.7mA
(130V - 11.6V) / 56k [R1120] = +2.1mA
-11.6V / 10.1k [R1121 and R1122] = -1.15mA
2.48mA - 2.85mA = -0.27mA
0.27mA * 17.4k [R1159] = 4.7V
4.7V + 11V = 15.7V

With Q1107 (a slow 2N3904???) removed on my 7603, the output at point
B is 14.8 volts which closely agrees with our current calculation so I
wonder if Marian's <public@...> measurement of 20 volts at the
output of the z-axis amplifier is significant and indicates a problem.
I get the same 14.8 volt output with Q1107 installed, the readout off,
and the intensity control turned to minimum.

I also measured the output at point B and as the intensity control is
turned fully clockwise, the output increases to about 28 volts
monotonically.

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