Date   

Re: 485 super weak brightness control

Ozan
 

Hi Ondrej,
Videos are very useful.

https://youtu.be/IPcyfDF65oU
I was surprised sweep and gate signals are not aligned but I think you are using different probes for each. Using a BNC cable for all of them will eliminate any mismatch. Just for reference here is what I had observed:

https://groups.io/g/TekScopes/photo/262059/0?p=Created,,,20,2,0,0
Setup: Inputs were set to 50-ohm (used BNC-T plus termination for CH3). No signal input, free running.

Raymond's question reminded me my earlier message:

Are you sure B sweep shaft is not rotated out of alignment? You can look at J1 (relay signal) and check it switches at .1us/.2us boundary.
I read your reply about timebases being locked. You may already know that but you can pull the time/div knob and rotate clockwise to speed up B timebase compared to A timebase. In that position B is not locked to A, and you can check if B can be rotated past the 1ns/div setting. There is a lock mechanism that won't allow B slower than A, so don't force in that direction.

I will look at the video again an reply later, I wanted to catch you while it is still earlier in the evening.
Ozan


Re: 485 super weak brightness control

 

On Tue, Mar 23, 2021 at 02:48 PM, Ondrej Pavelka wrote:


Wouldn't it correct itself at max and min setting?
Probably, didn't check that. You probably run against end points.


There is a locking mechanism in the switch itself
I didn't check any further. You obviously know what you're doing...

Raymond


Re: 485 super weak brightness control

Ondrej Pavelka
 

Wouldn't it correct itself at max and min setting? Both my shafts A and B
can move from 0.5second to 1nS and no further and neither can rotate freely
all the way around. There is a locking mechanism in the switch itself

On Tue, 23 Mar 2021, 14:21 Raymond Domp Frank, <hewpatek@gmail.com> wrote:

On Tue, Mar 23, 2021 at 02:04 PM, Ondrej Pavelka wrote:


The time base knob cannot be inserted in any other way. It's all locked
Sorry to insist and no idea whether it matches any of your other symptoms
but on my 485, it's no problem repositioning the knob on its shaft so B
runs 10x as fast as A while A and B knobs are interlocked.

Raymond






Re: How to explain how negative feedback lowers noise?

Mike Merigliano
 

As  a non-EE, maybe my take on this could be useful, because the OP was looking for a non-technical analogy or explanation. But it should still be accurate.

The kids going into the house example does not work because the kids are the input - they are already noisy, so negative feedback that compares input to output would not eliminate it. Another way to say it is: nice quiet kids come into the house, other kids already in there yell at them and makes the once-quiet house noisy. The dad asks the noisy kids to stop and be just like the quiet kids, and the house stays quiet.

I don't think this is such a simple, easy analogy, but it seems more accurate (the feedback is not truly instantaneous, for one thing). The key is that negative feedback makes a comparison between input and output, and subtracts what is not in the input.

Unteaching is very difficult - as a researcher dealing with managers, I deal with that almost everyday.

I hope I was gentle enough.

On 3/23/2021 7:04 AM, Keith wrote:
re: Negative Feedback example, etc.

Folks tell me that a gentle answer turns away wrath, so I hope this is a gentle reply. I mean it in that spirit.

Tom, with respect to your criticism of my example, I would suggest that you do a quick review of the original post #1. Here is the pertinent part of it, for your convenience:

Hi all,
I'm trying to explain to people at my company (none of whom are EEs or
statisticians) how negative feedback works in a system. That's one
thing that I'm trying to get across, and I can't come up with an
explanation of it in every day terms. All the examples I find in
biology etc seem kind of dubious and not very straightforward -
there's a lot of "trust me on this" as to why it's actually negative
feedback and not some form of other regulation. What's a simple
/physical/ negative feedback?

This is question 1 in the OP...period. The core of it is simple. "...how negative feedback works in a system..." Only in the next sentence (not quoted above) does the OP then use the word "another", and only then does he bring up a second question about feedback and noise. So, the original post is really two questions. Question 1 is the issue of a real world example of negative feedback in a system. Question 2 is the interaction of feedback and noise.

Nothing in my example is intended or stated to address that second question. I had nothing to add to that discussion, and so attempted to provide the OP with his example for question one. Now I admit that the use of the word "noise" in my example does unintentionally blur the line - since I say "noisy happy children". I see how that might cause confusion, so I will attempt to edit my post to remove that word. Thanks for that.

But, to be clear here, my example was only intended to apply to question #1 in the OP. Question #1 was the only part for which I felt I had an example that met his requirements, specifically that it be;

1. "non technical" - (which I admit I assumed would mean for persons who have no electronic background)
2. use "everyday terms" (everyday means things that average people from all walks of life could grasp)
3. provide a "simple / physical / negative feedback" example.

Of course every analogy breaks down at some point, but in learning and teaching, it is quite common to go from the simple to the complex in a series of stepped examples - first simple and familiar, and therefore necessarily incomplete at some level. Then more subtle, complex, and therefore more narrow and demanding in proofs and adherence to reality.

Thanks for reading my reply in a mild spirit. I mean no disrespect, but at the moment I stand by my example (modified to remove the word "noisy" of course) as meeting the requirements of OP's question one only. Of course, if you see it differently, then perhaps we'll just have to agree to disagree on this one? In any case, thanks for your contributions to the forum. You're a valuable resource here.

Warmly,

Keith




Re: 485 super weak brightness control

 

On Tue, Mar 23, 2021 at 02:04 PM, Ondrej Pavelka wrote:


The time base knob cannot be inserted in any other way. It's all locked
Sorry to insist and no idea whether it matches any of your other symptoms but on my 485, it's no problem repositioning the knob on its shaft so B runs 10x as fast as A while A and B knobs are interlocked.

Raymond


Re: How to explain how negative feedback lowers noise?

Keith
 

re: Negative Feedback example, etc.

Folks tell me that a gentle answer turns away wrath, so I hope this is a gentle reply. I mean it in that spirit.

Tom, with respect to your criticism of my example, I would suggest that you do a quick review of the original post #1. Here is the pertinent part of it, for your convenience:

Hi all,
I'm trying to explain to people at my company (none of whom are EEs or
statisticians) how negative feedback works in a system. That's one
thing that I'm trying to get across, and I can't come up with an
explanation of it in every day terms. All the examples I find in
biology etc seem kind of dubious and not very straightforward -
there's a lot of "trust me on this" as to why it's actually negative
feedback and not some form of other regulation. What's a simple
/physical/ negative feedback?

This is question 1 in the OP...period. The core of it is simple. "...how negative feedback works in a system..." Only in the next sentence (not quoted above) does the OP then use the word "another", and only then does he bring up a second question about feedback and noise. So, the original post is really two questions. Question 1 is the issue of a real world example of negative feedback in a system. Question 2 is the interaction of feedback and noise.

Nothing in my example is intended or stated to address that second question. I had nothing to add to that discussion, and so attempted to provide the OP with his example for question one. Now I admit that the use of the word "noise" in my example does unintentionally blur the line - since I say "noisy happy children". I see how that might cause confusion, so I will attempt to edit my post to remove that word. Thanks for that.

But, to be clear here, my example was only intended to apply to question #1 in the OP. Question #1 was the only part for which I felt I had an example that met his requirements, specifically that it be;

1. "non technical" - (which I admit I assumed would mean for persons who have no electronic background)
2. use "everyday terms" (everyday means things that average people from all walks of life could grasp)
3. provide a "simple / physical / negative feedback" example.

Of course every analogy breaks down at some point, but in learning and teaching, it is quite common to go from the simple to the complex in a series of stepped examples - first simple and familiar, and therefore necessarily incomplete at some level. Then more subtle, complex, and therefore more narrow and demanding in proofs and adherence to reality.

Thanks for reading my reply in a mild spirit. I mean no disrespect, but at the moment I stand by my example (modified to remove the word "noisy" of course) as meeting the requirements of OP's question one only. Of course, if you see it differently, then perhaps we'll just have to agree to disagree on this one? In any case, thanks for your contributions to the forum. You're a valuable resource here.

Warmly,

Keith


Re: 485 super weak brightness control

Ondrej Pavelka
 

Hi,

It good you asked but I made sure it's in defined known position.
The time base knob cannot be inserted in any other way. It's all locked

On Tue, 23 Mar 2021, 12:55 Raymond Domp Frank, <hewpatek@gmail.com> wrote:

On Tue, Mar 23, 2021 at 12:20 PM, Ondrej Pavelka wrote:


Here are the observations on the A sweep.

Note the difference between free running and triggered
Ignorant question:
On the video, I see you are missing some knobs, like the one for Holdoff.
Are you sure that you have set that to minimum and others correctly as
well and what's more: Are you sure that the registration between the A- and
B-timebase is correct, i.e. that the B-timebase knob is mounted correctly?

Raymond






Re: FG502 doesn't start at some specific settings

durechenew@...
 

Both marked 820 ohms (grey-red-brown); initial measure in circuit R155 real value 1053 ohms (same value when removed), R290 real value 832 ohms and they look like 0.5W (if not more). Now the two have places switched, but, as I said, no difference in behavior of the stage
TT


Re: How to explain how negative feedback lowers noise?

 

Thanks everyone for the very interesting contributions. I have one
more question about the feedback/noise debacle. Without feedback we
have less linearity, therefore the original signal comes with a bunch
of distortion products. Meanwhile the noise which is much lower in
amplitude and therefore remains in the linear part of the curve can
keep raising in power longer before it starts clipping. So in that
case, once you amplify your signal enough that you get to the
non-linear segment, if you turn up amplification even more, you're
actually increasing noise, and increasing distortion products, while
the signal doesn't increase just as much as the noise does. Therefore
you can get more noise with the same amount of original signal. So
more linearity means less noise in this case. Is this logic correct?

On Tue, Mar 23, 2021 at 6:36 AM Chris Wilkson via groups.io
<cwilkson=yahoo.com@groups.io> wrote:

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.





Re: Recapping Tektronix 2465

iv3ddm
 

I thank everyone for their help.

M Yachad: That is an error in my very old list.
No, it's my mistake, it's "A2 board", not A3: C1274 and C1291.
Tek "2465" should not have C1292.


Re: Recapping Tektronix 2465

VK1GVC
 

I'm also planning to recap my 2465 (plain vanilla, not -A or -B) as some RIFA's have already exploded, so if Giorgio or someone else puts together an up-to-date list could it please be posted here in the files section?  Ideally it would be an update of Menachem Yachad's list including board/component identification and Mouser or Digikey part numbers.

Thanks in anticipation,

Graham
(Down under)

On 23/03/2021 10:48 pm, iv3ddm wrote:
No, it's my mistake, it is "board A2": C1274 and C1291. It seems to me that Tek "2465" does not have C1292.




Re: 485 super weak brightness control

 

On Tue, Mar 23, 2021 at 12:20 PM, Ondrej Pavelka wrote:


Here are the observations on the A sweep.

Note the difference between free running and triggered
Ignorant question:
On the video, I see you are missing some knobs, like the one for Holdoff.
Are you sure that you have set that to minimum and others correctly as well and what's more: Are you sure that the registration between the A- and B-timebase is correct, i.e. that the B-timebase knob is mounted correctly?

Raymond


Re: 485 super weak brightness control

Ondrej Pavelka
 

https://youtu.be/IPcyfDF65oU

Here are the observations on the A sweep.

Note the difference between free running and triggered

On Tue, 23 Mar 2021, 07:20 Ozan, <ozan_g@erdogan.us> wrote:



https://youtu.be/A4IGmQKUV2w
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







Re: Recapping Tektronix 2465

Alain Mionnet
 

Giorgio,

I may have TDK/Epcos B57238S809M 16 mm 8R 5,5A , solded 3 euro,
or B57153S0150M000, 15R, 1.8 A, possible 2 in // for the current, my supplier gives 2 to 3 weeks (to be realy verified...) solded 1 euro each.

Regards

Alain
(France)


Re: 485 super weak brightness control

Ozan
 


https://youtu.be/A4IGmQKUV2w
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


Re: How to explain how negative feedback lowers noise?

Tom Lee
 

Hi Chris,

Yes, there are many "proofs" that reveal various misconceptions about negative feedback. Many of them start with a kernel of truth, but then veer off into the weeds. Unfortunately, it is often the case that the misconceptions can be traced to textbooks. Teaching is hard enough, but unteaching is nigh impossible.

-- Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/22/2021 22:35, Chris Wilkson via groups.io wrote:
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.




Re: FG502 doesn't start at some specific settings

Ozan
 

to be normal, with a note: R155 value is ~1k, R290 almost normal (both should
be 820 ohms). I switched the two, thinking I may be able an unbalance the
stage, but no difference.
R155 should be 820-ohm, R290 changed from 1k-ohm to 820-ohm in later revisions. They were both upgraded to 0.5W from 0.25W. What value do you have for R155 and R290 now? It makes a difference in figuring out what the circuit does.

Ozan


Re: How to explain how negative feedback lowers noise?

Chris Wilkson
 

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.


Re: Recapping Tektronix 2465

 

A3 1 uF 50v nonplzd 505-MKS2C041001FJC00 Film capacitor 1uF 63 Volt 5%

That is an error in my very old list.

It should be 4.7uF


Re: 151-0367-00 & 151-0402-00 leakage tests

Mark Vincent
 

I had made a mistake on the round leaded ones. They are leaky the same way as the square leaded ones. The bend in the leads is opposite, Base is to the back instead of the front. I apologize for any confusion.

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