Tek 464 - Confirmation on Z-Axis frequency response performance

Fabio Trevisan

Hello guys,
On my final attempts doing an overall wrap-up to my 464, to eventually put it for sale, it always annoyed me the disappointing performance (to my taste at least) of the Z axis intensity at the fastest sweep speeds.
When using "A Intensified by B" modes on the three or four last sweep speeds, the transition from normal intensity to intensified is rather sluggish.
When using "A sweep" mode only, especially disappointing on the last sweep speed (50ns/div), the trace fades-in very slowly, and takes almost 2 divisions to reach full intensity.
I took some pictures and upload them here:
Can someone of the group, who has a 464 like mine, or even others of the same family, like 465, or 466, take a look at the waveform coming out of the Z-Axis amplifier and coming into the Z-Axis DC restorer (on the 464 this is TP1443) and let me know if this rather slow rising edge is normal, or is it some problem with my 464's Z-Axis amplifier?
On the pictures, you will see the trace of the 464 itself, with A sweep at 200ns/div, intensified by B sweep at 50ns/div, and the Z-axis amplifier output, showing the waveforms, first without intensified and then with intensified portion.
The transition of the 25V step is about 80~90nS (from 10% to 90%) and the transition of the 8V of the intensified step is about 100ns.
1. The wave-forms shown are taken already with the best possible adjustment of the frequency compensation trimmer capacitor... but even if I try to push it harder, this trimmer doesn't really help to improve the rising edge of the waveform, it only makes the falling edge a little bit faster, and adds an undershoot, which creates a dip in trace intensity just after the end of the intensified portion (which is no good either).
2. I already tried to remove the upper transistor of the amplifier (the active load, or current source if you like) and performance degrades noticeably, giving insight that the active load is playing its role.
I also tested the transistor off-circuit (on an AVR transistor tester) and it measures well.
3. Looking at the signal incoming to the amplifier, at the emitter of Q1424 is not conclusive, as this is a current-driven input so, I wasn't able to tell if the slow rising transition is due to the input signal or low performance of the amplifier.
I appreciate any comment.

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