Signal compatibility problem when replacing old CRT monitor  - Page 1
Forum section: electronics / repair

author: LesR
author_rank: Newbie
timestamp: November 10, 2019, 06:12:37 pm
content: Hi, I have come here in search of wisdom!I have been battling with a failed CRT monitor on a CNC machine that I have in my home workshop.The machine dates from 1985 and until last week, worked fine. The monitor packed up and now whilst the CRT tube lights, nothing happens.I have been searching the web for information about how to replace the monitor with an LCD one. It is possible but I can't find any idiot's guide, only companies selling new ones at stupid money.The old monitor runs at 15Khz and inputs via five wires - video, intensity,horizontal sync, vertical sync and ground. I have found reference to it being a digital TTL signal.I had hoped that a GBS8220 board might solve my problems but on its own, it doesn't.I have had very brief flashes of garbled display that I recognise but nothing else.It seems from what I have read that I have a digital signal that needs to be turned to analogue (RGB) and then fed into the GBS8220 to re-sample and increase the frequency to match modern monitors.I have basic electronics knowledge and can knock up a circuit board but this task has me in unknown territory.Can anyone point me in the right direction please?Thanks,Les

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 10, 2019, 10:57:57 pm
content: It can be quite confusing if you've never messed with video before, worse still without any oscilloscope or electronics basics.Assuming you have RTFM for the GBS-8220 (https://www.arcadeworlduk.com/content/CGA-VGA-Manual.pdf) and that your video source is close enough to Hercules/MDA.You may have understood that the connector closest to your needs is P11 as this one accepts separate H sync - V sync rather than a composite sync on P3. (Getting this wrong or ground in wrong place will produce a garbled display.)The GBS-8220 is intended for a colour TTL input, so if you want a B/W display you want to send your video input to all 3: R-G-BDepending on how many levels of grey you actually need to see (plain on/off on original display or also 2 intermediate forms of grey?) you'll want to send "Video" or "Intensity" or a mix of both through a couple of resistors to the RGB inputs. The variable resistors on the GBS-8220 may need adjusting to get the best display, for test purposes I'd set them each at approx 1/3, 1/2, 2/3 of their run.A couple of links that could be of interest:https://oldcrap.org/2018/03/11/cga-to-vga-scaling-with-gbs-8220-board/https://www.seasip.info/VintagePC/hercplus.htmlhttp://aknamunka.uw.hu/mdah/1st.phpDon't forget to report back if you do get your expensive equipment running again...

author: LesR
author_rank: Newbie
timestamp: November 11, 2019, 09:59:51 am
content: Thanks for that information, particularly the first link.I have ordered some components and will report back after I try them.Les

author: woodchips
author_rank: Frequent Contributor
timestamp: November 11, 2019, 08:21:32 pm
content: Almost any similar era monitor will work. It is not MDA/Hercules, that runs at 18.432KHz, but a CGA will even though it is colour.A BBC, Electron, Sinclair etc monitor will work but they are composite sync and video, you need a chip, an LM1881 from memory, to split the sync signals from the video. But you might find that feeding the composite signal into all three inputs might work, or will need a signal level boost for the syncs.Otherwise monitors used in many test equipment such as logic analysers, network analysers etc might also work. These were two a penny years ago, I still have several to bin. Where are you?

author: LesR
author_rank: Newbie
timestamp: November 11, 2019, 09:08:23 pm
content: Thanks, I'll ask around.I'm in Stoke area.

author: LesR
author_rank: Newbie
timestamp: November 12, 2019, 06:51:49 pm
content: Progress!I have now built the circuit shown on the first link (oldcraporg).That worked but gave wrong black areas. So I included the circuit described hereIt now works acceptably, just a bit of interference to shut out with some proper wiring and screening.Thanks very much for your help!

author: james_s
author_rank: Super Contributor
timestamp: November 12, 2019, 11:28:16 pm
content: I have a small monitor that came from a CNC controller which I use with my FPGA retro computers, it is monochrome but uses the CGA interface standard, just with one color instead of three. You might be able to get the LCD working simply by inverting one or both sync signals.I would just repair the CRT display though, they are not complicated and since you say the tube lights up you can be reasonably confident that the EHT, horizontal deflection and the CRT itself are good, the remaining problem is probably something simple.If you do end up replacing it I'd be happy to give the old CRT monitor a new home, small CRT monitors are getting harder to come by.

author: digsys
author_rank: Unknown
timestamp: November 12, 2019, 11:55:03 pm
content: Having had a run of CRT > LCD "upgrades" lately, I resorted to these type of solutions -https://www.ebay.com.au/itm/333230981163There are literally 100's of different converter pcbs out there - fortunately the games / console nerds are a huge force :-) and damn cheap for what you get, <$100Many even have open source code, so you can "tweak" to your hearts content :-)

author: LesR
author_rank: Newbie
timestamp: November 13, 2019, 08:38:14 am
content: Sorry, I didn't make myself clear. I used the separate circuits to firstly invert and condition the video/intensity signals and secondly to combine H-S and V-S before feeding those into one of the GBS8220 video converters.It actually cost me less than GBP£20 in total.To buy a commercial replacement was quoted at GBP£930....