The line oscillator has to run close to the line frequency and is then phase-locked to it by the horizontal hold mechanism. If the oscillator is tuned for 19kHz it's a big stretch to get it to sync to a 15kHz input signal.
Yes, you might be able to modify some components to slow it down, or even just change the horizontal preset frequency to get it closer but that line oscillator is the most complex thing in as monochrome monitor. It generates a linear ramp for the horizontal coils and is also wound up with line output transformer which generates the high voltage for the tube. I guess this makes sense as during retrace the line coils are getting a big pulse to shove the spot to the other side of the screen. But drastically changing the line frequency is likely to have an effect on the line linearity and the EHT voltage. I'm not saying don't try it - it would be very educational, though best if you can get hold of a schematic. But a lot easier to get hold of an NTSC monitor, I think. The people who maintain arcade machines have been scraping up all the large colour monitors recently but I doubt they'd have much use for a small monochrome. If the stored energy in the line coils can be dumped into the line output transformer at the right phase of the cycle, it's saving a lot of power that would otherwise require a separate circuit. In the old days. the more use you could get out of any one vacuum tube the better, and the style persisted until multisync monitors needed to be more flexible (and even the simple earlier ones of those had more complex LOPTs to deal with the variation). On Mon, Apr 12, 2021 at 3:22 AM jb-electronics <[email protected]> wrote: > Still new to the whole composite video thing---could you clarify what is > the stretch? Do you think the driving electronics might not work at the > lower NTSC frequency? Or is there a mechanical impediment, like the > windings of the CRT's yoke? Thank you! > > Jens > > On 2021-04-11 7:19 p.m., Adrian Godwin wrote: > > It sounds a bit of a stretch - 15kHz to 19kHz. You could try running the > monitor without an input signal and adjusting horizontal frequency (if > there is one) to see what range it can manage. > > Another possibility is to fix it in software by creating a custom monitor > signal. Some information is at > > https://github.com/raspberrypi/documentation/blob/master/configuration/config-txt/video.md > > > > On Sun, Apr 11, 2021 at 10:51 PM jb-electronics < > [email protected]> wrote: > >> Thank you! I think it should be possible to modify the horizontal and >> vertical deflection circuitry on the board to make it a proper NTSC >> frequency, yes? >> >> Jens >> >> On 2021-04-10 10:52 p.m., 5-ht wrote: >> >> Jens, >> The horizontal sync requirement of this monitor (52 uS / 19.2Khz) is >> somewhat higher than a standard NTSC signal which is about 63 uS / 15.7 Khz. >> >> Mark >> >> On Saturday, April 10, 2021 at 8:04:15 AM UTC-5 Jens Boos wrote: >> >>> Thank you! I did some more digging and found the service manual for the >>> display (Panasonic TR-60S1A, see here: >>> https://www.opweb.de/english/company/Panasonic/TR-60S1A) >>> >>> There is is a timing chart (see below). It looks an awful lot alike NTSC >>> to me, can somebody confirm? >>> >>> Best wishes >>> Jens >>> >>> >>> >>> >>> On 2021-04-10 12:28 a.m., Adrian Godwin wrote: >>> >>> It's easy enough to extract the sync signals, as you've seen. It may not >>> be particularly difficult to modify the TTL input to analog, or to create a >>> TTL level signal if you don't need a grey-scale. On many general-purpose >>> monitors like that they could often be built for either standard - the >>> microvitec Cub popular with the BBC Microcomputer had that option. >>> >>> But the critical thing is that it runs at the right speed. CRT circuits >>> are built around the line oscillator which generates the horizontal scan >>> AND the EHT voltage. It was only when multisync monitors came along that >>> the optimisation was split to reduce the dependency. And if the frame >>> frequency is wrong, you will often have problems getting a full frame scan >>> when you force it into sync. >>> >>> So the first thing is that you need to make sure the 8920 monitor ran at >>> either PAL or NTSC rates so you can choose a raspberry pi format to match. >>> There's a good chance it's NTSC but later monitors like the ones in the 54 >>> series of digital scopes were more like the IBM standards MDA and EGA (CGA >>> was NTSC). >>> >>> On Sat, Apr 10, 2021 at 4:28 AM jb-electronics < >>> [email protected]> wrote: >>> >>>> Hi all, >>>> >>>> This is a bit of an off-topic question, but I hope there will be >>>> somebody here that can help. I have bought a new-old stock monochrome >>>> CRT for a HP Agilent 8920A, basically this unit here: >>>> https://www.ebay.com/itm/273930914548 . >>>> >>>> It looks to me that it is just a rebranded OmniVision 6" display: >>>> >>>> http://www.omnivisionusa.com/Industrial-LCD-CRT-Monitors/replacement-crt/crt-monitors/6-inch-kit-.html >>>> >>>> Now unfortunately it takes TTL video as input. But I have a composite >>>> source (a Raspberry Pi). How can I convert composite into TTL? I >>>> basically need to extract Hsync and Vsync and feed it separately to the >>>> unit, okay. There is an old circuit here that does just that: >>>> https://www.elektormagazine.com/magazine/elektor-198812/47485 >>>> >>>> But I know there are also dedicated chips, like the GS1881: >>>> >>>> https://www.mouser.com/datasheet/2/761/GS1881_GS4881_GS4981_Datasheet-769183.pdf >>>> >>>> So basically here is my question: before I dive into this any further, >>>> is there any chance of success? What is the optimal choice? Or is it >>>> quite unlikely that I will be able to convert the signals? I mean, even >>>> if I manage to extract Hsync and Vsync, is it likely to work? >>>> >>>> I am sorry for the naive question, but I am no expert on video signals, >>>> and it would be nice if anybody more experienced could chime in with a >>>> few words of caution/experience. >>>> >>>> Best wishes >>>> Jens >>>> >>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "neonixie-l" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to [email protected]. >>>> To view this discussion on the web, visit >>>> https://groups.google.com/d/msgid/neonixie-l/e0f398e8-24b1-952c-c998-0ddfbed6849e%40jb-electronics.de >>>> . >>>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "neonixie-l" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> >>> To view this discussion on the web, visit >>> https://groups.google.com/d/msgid/neonixie-l/CALiMYrvHCiXd3QSDmEJ%2Bpe3XgUutOC_C3yLFGxeOEo2bjtEAWw%40mail.gmail.com >>> <https://groups.google.com/d/msgid/neonixie-l/CALiMYrvHCiXd3QSDmEJ%2Bpe3XgUutOC_C3yLFGxeOEo2bjtEAWw%40mail.gmail.com?utm_medium=email&utm_source=footer> >>> . >>> >>> >>> -- >> You received this message because you are subscribed to the Google Groups >> "neonixie-l" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To view this discussion on the web, visit >> https://groups.google.com/d/msgid/neonixie-l/a3fe30c7-c778-444a-a745-b6f6b1910bb1n%40googlegroups.com >> <https://groups.google.com/d/msgid/neonixie-l/a3fe30c7-c778-444a-a745-b6f6b1910bb1n%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> >> >> -- >> You received this message because you are subscribed to the Google Groups >> "neonixie-l" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To view this discussion on the web, visit >> https://groups.google.com/d/msgid/neonixie-l/37bf74f3-842f-2d17-c5b7-0606f09d9d4e%40jb-electronics.de >> <https://groups.google.com/d/msgid/neonixie-l/37bf74f3-842f-2d17-c5b7-0606f09d9d4e%40jb-electronics.de?utm_medium=email&utm_source=footer> >> . >> > -- > You received this message because you are subscribed to the Google Groups > "neonixie-l" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web, visit > https://groups.google.com/d/msgid/neonixie-l/CALiMYru71Senz_sdSjFuWJMenZCgkRLyX%3DM%3D73kzP2TCveg6%2BA%40mail.gmail.com > <https://groups.google.com/d/msgid/neonixie-l/CALiMYru71Senz_sdSjFuWJMenZCgkRLyX%3DM%3D73kzP2TCveg6%2BA%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > > > -- > You received this message because you are subscribed to the Google Groups > "neonixie-l" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web, visit > https://groups.google.com/d/msgid/neonixie-l/e49bca6f-fc9e-e4ee-9003-ccd950ce1d82%40jb-electronics.de > <https://groups.google.com/d/msgid/neonixie-l/e49bca6f-fc9e-e4ee-9003-ccd950ce1d82%40jb-electronics.de?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. 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