Where would the "bright-up" circuit be on a 2445A schematic?

[glossywhite]

Hello

I have been told by a Tek service engineer to look for the "bright-up" circuit, to possibly replace some resistors - (apparently they can go high sometimes with age) to improve the trace visibility on fast (nS) sweeps.

Could someone tell me the proper term for the "bright-up" circuit... or IS that it?

Many thanks

Matt.

 

[crutschow]

I assume it's a circuit that increases the trace brightness in response to high sweep speed settings. If that's what he called it then I would look for that or similar wording. It likely has connections to the high sweep speed settings on the sweep speed switch.

 

[Externet]

"Bright-up" ? That is not a standard term... Is it about the graticule illumination or the trace brilliance ?
The circuit for the graticule/scale illumination is on page "diamond4" or page 225, U130, attached.
The trace intensity is on page 229 of the .pdf
Download the manual at :
----> http://www.ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_2445_Oscilloscope

 

[glossywhite]

"Bright-up" ? That is not a standard term... Is it about the graticule illumination or the trace brilliance ?
The circuit for the graticule/scale illumination is on page "diamond4" or page 225, U130, attached.
The trace intensity is on page 229 of the .pdf
Download the manual at :
----> http://www.ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_2445_Oscilloscope

View attachment 84038

Thanks for your post. I'm actually referring to trace brightness

 

[rumpfy]

I think he is referring to the 'trace brightening'.
From your video, when you were using the second timebase, you will note that the first timebase had a short length of it brightened. This brightened section tells you which part of the second timebase (fast timebase) will be seen. I think the trace brightening will be a signal generated as an output from the first timebase sweep and will feed into the intensity control system.
However, I would be inclined NOT to poke around with something as good as you have. I can look at the circuit for you if you wish, but again, I suggest you take the manual to bed and read it there. I did remark somewhere, that jitter on these kinds of dual trace measurements can be adjusted out by understanding the triggering controls. I cant stress too much, how important it is for you to get your drivers licence first before you start imagining problems that dont exist.

 

[glossywhite]

I think he is referring to the 'trace brightening'.
From your video, when you were using the second timebase, you will note that the first timebase had a short length of it brightened. This brightened section tells you which part of the second timebase (fast timebase) will be seen. I think the trace brightening will be a signal generated as an output from the first timebase sweep and will feed into the intensity control system.
However, I would be inclined NOT to poke around with something as good as you have. I can look at the circuit for you if you wish, but again, I suggest you take the manual to bed and read it there. I did remark somewhere, that jitter on these kinds of dual trace measurements can be adjusted out by understanding the triggering controls. I cant stress too much, how important it is for you to get your drivers licence first before you start imagining problems that dont exist.

Thank you for your comments

Yes, I understand the workings of the dual timebase, and the bright area being the "selected" portion which you are zooming into; I don't think the guy I was talking to was referring to this - he was talking about 1M resistor(s) going high with age to about 1.3M etc, and how some capacitors went a bit resistive, and how, when they were replaced, the visibility of very fast signal traces was improved somewhat. The bloke I am talking is, I'd say, around 65-70 years old and is the manager of a NATO approved test equipment repair and recycling company, so I'd say he knows his stuff - he told me in INTRICATE detail how to repair the tiny plastic "doors" that let the flourescent green plastic indicator through on the power switch, and was on the phone for ages explaining many, many more in-depth intricacies of which Tek scopes were best and worst, and the various repairs they come in for.

I agree that I'd not "fix" something that works perfectly okay, but I am asking because I want to know, not necessarily because I am going to attempt the repair. I want to "feel like a king", as someone said earlier in the thread - if I chose to mess up my scope, that would be my problem... but I've no intention of doing so - I like learning things, and I am highly competent and learn rapidly, so maybe one day I will refresh the "bright-up" circuit he is referring to, once I am more adept and have a feel for this stuff

As regards needing to get to know the scope thoroughly... that's exactly what I am doing, and these kind of questions are part of that learning curve. Remember, we all learn in a different way, and I ask the specific questions that I ask, because that certain aspect is pertinent to me and to the way I use deductive reasoning in my thought processes and understanding. I really, really value your insight and cautionary comments toward me, though - I thank you for warning me; you don't know me, and I could be a complete klutz who is a butter-fingered solder splashing enthused amateur, eh! ^_^

Thank you

 

[alec_t]

The trace brightness control on my old analogue scope is labelled 'Intensity'.

 

[rumpfy]

Crutshow might have been close to the mark.
On page 275 of the manual (2645A) the block diagram shows part of block 5 as a 'display sequencer'. Two of the outputs are 'sweep rates' and 'delay select', and these outputs find their way into the Z axis amplifier which is also part of block 5. The function of the Z axis amplifier is to affect/control/monitor the display trace brightness. My guess is that, if what your guy said is along the lines of what crutschy reckons, then your trace brightening will found in this part of the box.
U650 has 2 outputs HSA1 and HSA2. these are found on page 306 at 7F, and come from page 297 point 2G. These logic functions go to U950 and then become VZout at 8M on page 306. then on page 231 at 6A, you find VZout. This line goes to the control grid/grid no1/intensity grid of the CRT. This starts to get mixed up with the DC restorer and there is voltage multiplier shown, and there is a 22 megohm resistor attached to the grid. The voltage here is possibly up to say 100 -200 volt and the high value resistor is probably what your friend was referring to. There may be other high voltage resistors around the CRT circuit which cause trouble too. These High voltage resistors can be troublesome. They are generally long in length and the carbon track is very resistive, and all this conspires to give unreliable resistors. Resistors have a voltage rating when you get high voltages across resistors and this point is not often appreciated. Sometimes a designer will use two normal resistor in series to improve the voltage rating.