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No, it's my mistake, it is "board A2": C1274 and C1291. It seems to me that Tek "2465" does not have C1292.

Your observations in the video look OK for B sweep:
1) There is no change in B sweep in 1n/div and 2n/div settings. 2x is
handled by the horizontal amp for 2n -> 1n setting.
2) Delay changing B gate position in INT setting is OK. As far as I
remember B internal trigger still waits for delay before waiting for
trigger. In "B runs after delay" it will run when delay expires without
waiting for trigger.

This shows your B sweep is working in 5/2/1n setting as you mentioned
before. Now we need to figure out why A sweep doesn't pick B sweep properly
in 5/2/1n mode.

There I could line up A B gate and A B sweeps. The problem is what comes
out.

Which point are you measuring the output? All the nodes from emitter of
Q1318 and collector of Q1312 until you hit TP1364 are low impedance points
(this section is a transimpedance amp). TP1364 is the earliest node that
shows muxed sweeps.

I recommend this setup:
1) Scope is in 5ns/div and A sweep
2) Ch1 at left side of R1322 (B sweep)
3) Ch2 is at base of Q1318
4) Ch3 is at TP1364

We should see B sweep going through Q1318 and Q1358. In this state Q1356,
Q1312 and Q1338 should be off.
Ozan

Tom is right. Negative feedback can only reduce the noise introduced by the amplifier itself. If the system works properly, the input will be perfectly amplified - with no noise added by the amplifier. But if noise is part of the input, then that noise is amplified by exactly the same amount as the intended input signal.

Hey Tom,
I once heard the argument that negative feedback could eliminate noise and it was supported by live measurements. Actually, more than once from more than one source. It usually went something like this...

Here's a noisy input signal (shown by live measurement - very fuzzy) and here's the amplified output with a larger amplitude, but less noise content (also shown on the scope - not fuzzy at all). It was clear that the output had *less* noise than the input. Therefore, the negative feedback must be reducing the noise! Many attendees just accepted the result...and continued to propagate the idea. The problem of course was the input noise signal was high frequency, beyond the bandwidth of the system. It was just being filtered out by the inherently lowpass universe. Have you seen any talks like this?

I think there are a lot of similar situations out there that contribute to common misconceptions like this one. And this one is really common.

I will jump in with my explanation. Negative feedback does nothing for noise on the input. That input noise looks just like a valid signal to the circuit input so it gets amplified at the same rate as the input, their ratio stays the same. However, noise that "sneaks in" to the amplifier is magnified by a very large number because that is what amplifiers do. Then negative feedback subtracts a constant part of the output from the original input, But there was no "sneak in noise" at the input so the full amplitude of the sneak in noise is "fed back" and subtracted to get near zero total noise at the output. Even the math says so but it gets messy to do.
On Monday, March 22, 2021, 05:35:14 PM CDT, Keith <coolblueglow@...> wrote:

Here’s my explanation of negative feedback.

1. A group of happy cheerful children are playing noisily as they enter the house through the door (input)
2. The beleaguered father is just leaving the other door (output) to go to work and he’s extremely annoyed by these children’s sudden and noisy but happy appearance, spoiling as it does his perfect departure.
3. The irritated and angry father goes back around to the front door (input) and yells at the children to stop making so much noise and go clean up their rooms (inverted signal fed into input signal).
4. The children, now somewhat subdued but still basically happy, continue their play but at a lower volume level, partially subdued by their father’s chastisement.

?
Cheers,
CBG

As Dave wrote this could be because of missing load at W991. Does adding a 470-ohm 0.5W to ground from W991 reduce it to ~ 5.2V?

Ozan

On 22/03/2021 17:55, David Slipper wrote:

EDIT: U9028 should be U902A/B
Dave


On 22/03/2021 17:18, David Slipper wrote:

OK I replaced Q913 and Q923 and I'm using an external supply of 45V

I have isolated W984, W987, W989, W972, and W991

W972 reads 35V

W984 reads 95V

W987 reads 8.5V

W989 reads -8.5

but W991 is showing 11V

I suspected Q921 but it seems to be OK.

I guess the next suspect is the LM358 in U9028 ???

The inverter stage seems to be working OK but I'm mystified by the 5V
rail at 11V !!

Any suggestions welcome.
Dave


On 17/03/2021 16:09, Ozan wrote:

C914 reads as 12uF with an ESR of 1.2 - is that reasonable ???

It is a bit low in capacitance but it is not the reason for the failure.

However, I would replace it anyway since you removed it and it was stressed by
high voltage.

Ozan