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Re: How to explain how negative feedback lowers noise?
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 |
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Re: Tek 2225 that wont power up after a knock :-(
The resistor did the trick and dropped the 5V rail to the expected value, so I worked through the cut links reconnecting each in turn and checking the voltages and total current.
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The little beauty seems to be all OK running from the external supply for a range of simple tests. More testing required and I have yet to go back to the line transformer/rectifier but looking hopeful. I am still concerned that I haven't found a definite cause, I guess it might be the line board ... Thanks for all the help guys. Dave On 22/03/2021 20:44, Ozan wrote:
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? |
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Re: How to explain how negative feedback lowers noise?
A noisy amplifier or a noisy oscillator connected to a reference source will do whatever it takes to follow the reference source signal. This applies for slow reference noise only which means noise is within the loop bandwidth. Every amplifier or oscillator circuitry has a defined bandwidth. Now if the source is very low noise then the noise from the amplifier/oscillator will be reduced by the negative feedback action. But noise above the loop bandwidth will be unaffected.
Loop bandwidth! If you are driving your car a sunny Sunday morning in a rural country side at 30 mph you can easily follow the highway, this is within the loop bandwidth. On the other hand if you would try to drive your car through the desert at 100 mph this would be outside the loop bandwidth. And you will have troubles the rest of the day. G?ran |
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Re: Type 106
walter shawlee
That is brass, not gold. the top plating (nickel or something) is on top of
a brass blank.? not as exotic as you might think. brass is very easy to engrave, which makes it popular for this task. when you see engraved or stamped serial numbers that are black, they have just been filled it with black enamel or paint. all the best, walter -- Walter Shawlee 2 Sphere Research Corp. 3394 Sunnyside Rd. West Kelowna, BC, V1Z 2V4 CANADA Phone: +1 (250-769-1834 -:- +We're all in one boat, no matter how it looks to you. (WS2) +All you need is love. (John Lennon) +But, that doesn't mean other things don't come in handy. (WS2) +Nature is trying very hard to make us succeed, but nature does not depend on us. We are not the only experiment. (R. Buckminster Fuller) |
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Re: 485 super weak brightness control
When I had it in the A mode I measured approximately 20nS delay between
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gate A and gate B but my sweep out was just plain rubbish noisy nowhere near triangle and at wrong place. I found better place to tap to sweep which are the inputs on sheet <11> There I could line up A B gate and A B sweeps. The problem is what comes out. It basically looks like instead of choosing sweep A or sweep B the output is a one minus the other and because they have opposite polarity it's scewed and horizontal adjustment has different effects on A and B resulting in very often shapes coming out in the 125 setting. On Mon, 22 Mar 2021, 21:37 Ozan, <ozan_g@...> wrote:
had |
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Re: Selenium rectifiers in Tek equipment
On Fri, Feb 5, 2021 at 02:29 PM, Morris Odell wrote:
I have a 127 too and had no compunction in replacing the Se rectifiers withAn update on my 127. I got the power supplies running (one 6080 and a 12AX7/ECC83 were down to air) and the rails were all within spec, though +225V was a whisker low at 221V. While I was testing the output amplifiers, the rectifier for the -150V rail started cracking and sparking and there was, to quote my wife, a "strange smell". I switched off quickly! This was after about an hour of running, in total. I had a handful of 800V/4A bridge rectifiers in stock so have replaced all except the heater rectifier. I'll check the DC heater supply voltage. All rails are now even closer to their correct voltages, so the +225V rectifier was clearly a bit marginal. I haven't added any additional series resistances. There is a few ohms in series with all the rectifiers anyway, and I figure the 6080s can take the strain! Chris |
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Re: FG502 doesn't start at some specific settings
You're right, Eric!
But, by now, I believe everyone reading this post knows that. And it is what I said right from the beginning, triangle generator is not working (I mean, I didn't say there is no square, sin, that is obvious... pulse and ramp signal work based on the same stage but a little different). And It's starting and works normal on some frequency ranges (meaning - should we consider that there is no defective component in that stage? Nor cold soldering or switch mechanical functionality?). And I believe you meant "If it works like the FG504", as discussion here is about an FG502... Ozan, you were right and it was my fault when I wrote voltages for Q230A, B at the base(s): I misread the normal working voltages with those for 'happy state'. Here's the last and, hopefully, correct list of readings; note, last digit might be different depending on temperature, position of sun in the sky or god knows what else Q200- G: 1.122 V Q204-B: 1.105 V Q204-E:1.777 V Q210-E: 1.025 V Q290-C: -0.866 V (of no relevance) Q230B-B: 0.503 V Q230A-B: 0.504 V Q230A&B-C both: -8.57 V Q292-C: 1.118 V Q175-B: -6.56 V Q175-E: -7.25 V Q175-C: -6.54 V Q140-B: 6.60 V Q140-E: 7.37 V Q140-C: 4.165 V CR140 and CR145 junction: 1.513 V CR170A&CR175 junction: 0.720 V CR248&CR250 junction: 0.313 V CR245&CR246 junction: 1.317 V All diodes reading like normal diodes when measured cold, and (as seen from voltages list) normal voltage drop for those in conduction; all resistors seem 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. TT |
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Re: How to explain how negative feedback lowers noise?
Negative feedback reduces the amplitude of signals that do not appear at the input point of the feedback circuit.sorry, I don't really follow - would you mind rephrasing? thanks! On Mon, Mar 22, 2021 at 8:21 PM ebrucehunter via groups.io <Brucekareen@...> wrote:
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Re: Tek 2225 that wont power up after a knock :-(
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?
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Ozan On 22/03/2021 17:55, David Slipper wrote:EDIT: U9028 should be U902A/BHowever, I would replace it anyway since you removed it and it was stressed by |
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Re: Type 106
Cleaning it up revealed something else I had never seen before. Not that I¡¯m an expert or anything, but this is new to me.
The lettering of serial number is gold. I¡¯ve always seen it black... Has anyone come across a ¡°gold¡± serial number before?? /g/TekScopes/photo/261997/3?p=Created,,,50,2,0,0 Here¡¯s the hole on the right hand side when facing the unit. It doesn¡¯t lead to anywhere, though. The trim pot isn¡¯t centered. The case is also very well ventilated everywhere. /g/TekScopes/photo/261997/0?p=Created,,,50,2,0,0 More pictures: /g/TekScopes/photo/261997/2?p=Created,,,50,2,0,0 /g/TekScopes/photo/261997/1?p=Created,,,50,2,0,0 |
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Re: 485 super weak brightness control
Hi Ondrej, Less than perfect videos are OK. My main concern is misunderstanding the state of the 485 and suggesting the wrong path. It is important to look at B gate during 5/2/1n setting of A because some of what you are seeing could be related B gate timing. Ozan |
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Re: 485 super weak brightness control
I started a video about 5 times always ended up interrupted or doing some
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mistake I can send you link to those attempts because if you forgive the complete blabbering you may see something but my biggest mistake was I had b sweep selected not A and I had switch in the b after delay position. On Mon, 22 Mar 2021, 20:36 Ozan, <ozan_g@...> wrote:
back |
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Re: 485 super weak brightness control
What is the time/div setting? Your gate pulses are about 2x the length of what I measure in 1ns/div setting. Polarity of the sweep is opposite of what it should be (note that internally it has a negative slope but BNC amplifier Q942/Q944 has an inverting gain). Given that you are seeing a much smaller signal and opposite polarity probably you are just seeing feedthrough and A SWP OUT amp is not working. Is the top waveform B gate and bottom waveform A gate? Delay between them is too much for 5/2/1n setting. I measured 27.6ns, you are looking at 200ns+. Ozan |
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Re: How to explain how negative feedback lowers noise?
Negative feedback reduces the amplitude of signals that do not appear at the input point of the feedback circuit. Thus, it will reduce noise generated within the feedback loop through degeneration, but it cannot reduce noise already present at the feedback input point.
Bruce, KG6OJI |
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Re: How to explain how negative feedback lowers noise?
An employee does something wrong.? Management tells them about it.? They try to do better.? Management isn't satisfied, so they have another talk.? This cycle continues until management is satisfied.? The talks are the negative feedback
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Other examples:? Think governor on a gasoline (or steam) engine. Especially the old whirling ball one. Think adjusting the heat in the house until you feel comfortable. Harvey On 3/22/2021 1:40 PM, cheater cheater wrote:
Hi all, |
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Re: 485 super weak brightness control
This is what I get, the a sweep signal is very weak, under 20mV at the back of the scope but it looks fairly close to what you get On Mon, 22 Mar 2021, 18:57 Ozan, <ozan_g@...> wrote:
One more question, the A sweep is triangular but the B sweep has a flattopand becoming almost square, that doesn't look right?I can't access B sweep without taking the case off so I don't know the |
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Re: Recapping Tektronix 2465
Giorgio,
80-R82DC4100DQ60J for 1mfd 80-R82CC4470DQ30K for the three 4,7mfd on the power supply board 647-ULD1C221MPD1TD for 220mfd 647-ULD2C100MPD1TD for 10mfd. These are 160V. 647-UHE1C121MED for replacing the two 100mfd to the TO-220 rectifier. The additional capacitance of 20mfd will not hurt. I used 150mfd ULD in my B version with no problems. 527-CL60 for the NTC This is 10 ohms which will not hurt. 647-LGR2D391MELA30 for replacing the 290mfd. These I used in mine. 647-UHE1E470MDD1TD for the 47mfd. If you are willing to wait for the ULD, they will be in later this year. 594-MBE04140C1503FC1, 279-H4P180KFDA and 279-H4P180KFDA for the three resistors on the high voltage board that are voltage stressed and can open. They got back to the -300V supply. They are the 150,000, 180,000 and 430,000 ohm resistors. See if Mouser has the ULD types for ones you want, if not, go to the UHE type. The two 390,000 ohm 1/8W bleeders across the 290 mfd condensers can be lowered to say 100,000 ohms at 1w if you want the voltage to bleed off faster. Mark |
G?ran Krusell
ebrucehunter
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