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Post by Deleted on May 8, 2015 12:13:26 GMT -5
Ok. Explains a lot. I get now that the upper half is a push pull by itself. That explains that with the lower half not working there was still half gain or so.
Clear story on CB miller capacitance so these are compensation inductors. Does it make sense that they where designed to function on lower frequencies than our present 1080p resolutions? I see the vallues modified on my neckboards.
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Post by gjaky on May 8, 2015 13:24:34 GMT -5
Theoretically the cancellation of the parasitic effects should not depend on what resolution you use. A perfectly tuned circuit should produce a flat response up to its final roll off, a poorly tuned circuit might have fluctuations in the frequency response...
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Post by Deleted on May 9, 2015 3:38:58 GMT -5
I wonder if this stage would also compensate for the roll off in earlier stages? If Q19 is replaced with a high frequency version than perhaps less peaking compensation is needed with the inductors?
The way to test this would that be to put a square wave on the input and scope on different places than try to get the square flat? I have a 250 MHz probe and 150 MHz scope. What scope and probe specs are needed to do a proper tuning? In real 1080p video the highest frequency is not that high. Was it 75MHz?
edit: you mentioned a 300 MHz scope. I wonder if I could make a testsetup with a +-5 V and +-85 V power supply that should work. Perhaps setting the 85 V to 20 V to use a normal 10:1 probe? Or use 90 MOhm in series?
edit2: my probe still has 12 pF or so parallel and that is 136 ohm at 100MHz. So that will disturb the responce to much. A FET probe should have 1 pF that meens 1,6 kOhm.
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Post by gjaky on May 9, 2015 5:53:29 GMT -5
If Q19 is changed most probably C5/R95 and C53 has to be changed. Remember the law for bandwidth of cascaded systems also applying to oscilloscopes as well. With a 150MHz scope and with the 250MHz probe your (-3dB) performance will be around 130MHz. With 1080p the base frequency is indeed around 75MHz for the squarewave, but as we already discussed the rising and falling edges of the square wave signals contain higher frequency components, with the CRT amplifiers we want to amplify these as faithfully as possible. With the 130MHz scope performance you will be able to detect the signal in "good shape", but the rising and falling edges will suffer from the lack of bandwidth and that is exactly the part in what we are interested in. I entered the game with a Hewlett Packard 1720A oscilloscope, with a rated bandwidth of 275MHz, and with a (Tek) 250MHz 10:1 passive probe. And I have to say with even those I could not measure accurate signals on the output of the CRT amplifiers. In fact the passive probes have little use here (whatever bandwidth they have). The reason for this is the passive probe's tip capacitance, that is around 10pF, with that 10pF you will detune the circuit wherever you probe with it. plus you most likely want to use the supplied ground clip of the probe that has again a few tens of nH of inductance adding to the circuit under testing. Later I could get a matching active FET probe (HP 1120A) for my HP oscilloscope, that probe has already 500MHz bandwidth, and a tip capacitance below 1pF when a 10:1 or 100:1 divider is used. With this and with the scope I could measure somewhat accurately at least. My oscilloscope's bandwidth is still somewhat bottleneck though. If you have a considerable money to spend on this 'fun' I'd suggest tolook for a Tektronix 2465A(or B) -or similar scope and a decent FET probe like a Tek 620x. Don't forget to look for the dividers for the FET probe too, because a FET probe can only measure a few volts alone. If you are on the budget side, a Tek 485, or even the HP172x isn't that bad  , or some other scope mainframe with 1GHz sampling plugins, but a FET probe is mandatory here too! |
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Post by Deleted on May 9, 2015 8:08:35 GMT -5
Yes we have been over this highest frequency before and it is not so important but I have to compare with audio. When you have a cd the sample rate is something like 44kHz. The highest frequency is 20kHz. With blu-ray the sampling is done over a surface and with 1080p we have 1080 pixels or samples. Calculating the time and the porches we get something like 75MHz highest frequency. Everything above that is filtered during recording or during post processing of the recorded material. It would make aliases in the movie if not filtered. So when you record a square wave at 75MHz (1:1 pixel) only a sine wave is recorded all the harmonics are filtered. This would relax the bandwidth requirements of scope and probe somewhat as long as we can establish that there is no roll off at 75MHz but I do need a FET probe as you say.
But aside from that I see you have all the right equipment. Do you feel like doing some measurements on some boards I could send you? To see how well they are tuned and perhaps we could improve?
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Post by gjaky on May 9, 2015 14:48:31 GMT -5
But aside from that I see you have all the right equipment. Do you feel like doing some measurements on some boards I could send you? To see how well they are tuned and perhaps we could improve? Not right now, but in a few months it is possible... I myself also have a set of VNBs that I want to taylor in my NEC. I am currently waiting on a special power supply to arrive, that was offered to me by tse, it was made at VDC to mate the AmPro projectors with the Marquee VNBs, it is essentially a +/-85V PSU from 220V. Once I have that I can start to play with my neckboards on bench, but first my concern is not its performance but to mate all the signals with the NEC, tube safety before all  |
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Post by Deleted on May 10, 2015 4:06:47 GMT -5
But aside from that I see you have all the right equipment. Do you feel like doing some measurements on some boards I could send you? To see how well they are tuned and perhaps we could improve? Not right now, but in a few months it is possible... I myself also have a set of VNBs that I want to taylor in my NEC. I am currently waiting on a special power supply to arrive, that was offered to me by tse, it was made at VDC to mate the AmPro projectors with the Marquee VNBs, it is essentially a +/-85V PSU from 220V. Once I have that I can start to play with my neckboards on bench, but first my concern is not its performance but to mate all the signals with the NEC, tube safety before all Very good! I am very interested in the results. I will try to get a HP to do some measurements. I once worked with a simple DMM and had a simple diode rectivier resistor and capacitor to do high frequent amplitude measurements. However turning the plus and minus changed the results so I do not much trust that. on the peaking/composation networks, the Barco has several settings to try for different frequencies. I guess they overcompensate a little to make the high frequencies a little to strong as perhaps it looks sharper but you also see the overshoot in the picture. So I would not be supprised if the standard correction networks are peaking to much. |
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Post by Deleted on May 10, 2015 9:46:26 GMT -5
Finally it seems I also solved the gamma problem. Replacing the first opamp U2 and some surrounding capacitors the gamma of all three now seems the same. I had to lower the lowend of red and blue after the replacement so somehow the old opamp was not linear?  |
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Post by Deleted on Jun 2, 2015 9:41:01 GMT -5
Ok I bought some new old style Marquee neckboards as I was planing to modify them. Now I was rather surprised that all three had the same bandwidth that was actually a bit higher than my repaired and modified boards. I looked at some movie material and prefer the unmodified neckboards as it looks better. I expect that the strange way the peaking was changed on the modified boards was not an improvement. Also I can conclude that all the changed caps add nothing to picture quality as I can see and I guess the changes where more in the subjective domain where the modifier hopes to see an improvement. I still hold the impression that the VIM is better than a standard VIM but the neckboards are better unmodified. Unexpected results I also have some very old MP modified neckboards and they are really bad in bandwidth and in picture. The newer modifications did improve on the older modifications. But to conclude from the 5 unmodified neckboards I bought this far 1 was broken and all 4 others where about the same in bandwidth. The only neckboards that where worse where the modified ones. The 3 new modified neckboards where a total mess but after repair 2 where a little worse than unmodified, the last was very worse. So good job modifier  |
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Post by gjaky on Jun 2, 2015 11:58:51 GMT -5
And what is the time code on the MRF transistors? On my VNBs there are 9627.
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Post by Deleted on Jun 2, 2015 12:29:36 GMT -5
And what is the time code on the MRF transistors? On my VNBs there are 9627. 9812 and 9813. I must say that it seems one of the neckboards is a very little better than the other two. Does hardly show in the testpicture but man it looks better than ever in real material. I had this bandwidth before with the low gain broken neckboard but repairing the gain made the bandwidth a little worse. It is now obvious that this standard neckboard is as good as the best I had this far. Shows in those star nebula from gravity |
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Post by mastertech on Jun 2, 2015 20:43:51 GMT -5
Ok I bought some new old style Marquee neckboards as I was planing to modify them. Now I was rather surprised that all three had the same bandwidth that was actually a bit higher than my repaired and modified boards. I looked at some movie material and prefer the unmodified neckboards as it looks better. I expect that the strange way the peaking was changed on the modified boards was not an improvement. Also I can conclude that all the changed caps add nothing to picture quality as I can see and I guess the changes where more in the subjective domain where the modifier hopes to see an improvement. I still hold the impression that the VIM is better than a standard VIM but the neckboards are better unmodified. Unexpected results I also have some very old MP modified neckboards and they are really bad in bandwidth and in picture. The newer modifications did improve on the older modifications. But to conclude from the 5 unmodified neckboards I bought this far 1 was broken and all 4 others where about the same in bandwidth. The only neckboards that where worse where the modified ones. The 3 new modified neckboards where a total mess but after repair 2 where a little worse than unmodified, the last was very worse. So good job modifier Well now, this is interesting, to say the least. |
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Post by Deleted on Jun 3, 2015 4:06:36 GMT -5
Yes it is! I watched a movie yesterday and for the first time I did not long for my Cine max. Another story I heared was that someone took all the modified parts of a vim to another vim and the new vim did not perform as good....I am starting to think someone is taking credit for factory differences and I am thinking modifications might not improve as much as advertised. What I know for sure is I had a standard neckboard that seemed to have good bandwidth so I tried to improve with all the modified parts but after that it was a little worse. I talked to someone once and he said the peaking was done by looking at the picture. What I see is they let one part of the peaking untouched and the other part was inspired by the idea that there is something like real bandwidth and peaking. Now I hold on to what Gjaky put into words that peaking done right does not meen overshoot but is compensation for miller and cable capacitance like one can learn in any good textbook. My opinion is that the peaking in the motorrola parts should not be changed when you are only changing a driver opamp or a transistor, the miller and paracitic aspects of the motorolla is not changed by better input signal. |
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nashou
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Post by nashou on Jun 3, 2015 8:50:34 GMT -5
I have had some older version of his VNB of the said modifier and I agree they were worse than some stock ones that I changed out some parts to better quality. No drastic changes as those modified boards and they were sharper.
I did not look at noise level tho.
Nashou
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Post by jbmeyer13 on Jun 3, 2015 10:08:11 GMT -5
Yes it is! I watched a movie yesterday and for the first time I did not long for my Cine max. Another story I heared was that someone took all the modified parts of a vim to another vim and the new vim did not perform as good....I am starting to think someone is taking credit for factory differences and I am thinking modifications might not improve as much as advertised. What I know for sure is I had a standard neckboard that seemed to have good bandwidth so I tried to improve with all the modified parts but after that it was a little worse. I talked to someone once and he said the peaking was done by looking at the picture. What I see is they let one part of the peaking untouched and the other part was inspired by the idea that there is something like real bandwidth and peaking. Now I hold on to what Gjaky put into words that peaking done right does not meen overshoot but is compensation for miller and cable capacitance like one can learn in any good textbook. My opinion is that the peaking in the motorrola parts should not be changed when you are only changing a driver opamp or a transistor, the miller and paracitic aspects of the motorolla is not changed by better input signal. Mike told you to leave those boards alone but you couldn't help yourself...lol I believe he also offered you a full refund but you turned it down.
A few years back I was VERY skeptical of the performance of MP's video chain. Honestly, the only reason I tried them out was because Craig Rounds was really persistent about the results and he said I wouldn't be disappointed. I took a visit to Mike's and the rest they say is history.
I wouldn't suggest swapping parts from board to board and expecting identical results. For one thing, these are analog devices so differences in stock performance can even vary let alone on modified boards. But the most important point is that it just takes one component to be out of sync to throw the entire equation off. For example, a single resistor change can totally screw up the peaking.
The placebo effect is an interesting phenomena. Some people spend money and convince themselves that there are performance improvements while other people spend money and can't see the differences. Personally, I like to see obvious differences in performance to warrant the time and money involved to experiment with modified boards. I have had 4 of MP's VIM's and 2 sets of VNB's and in every case there were two obvious differences to a stock set up; light output/blooming and BW improvement. MP's board can deliver light output comparable to a contrast setting of 70 on a stock VIM with no blooming and also display significantly better performance using both the standard SMPTE chart and also Ecrabb's custom linearity SMPTE pattern. If you aren't seeing these two basic characteristics then you've likely messed something up while swapping out various parts.
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, or some other scope mainframe with 1GHz sampling plugins, but a FET probe is mandatory here too!
