CRT CCTV monitor can't sync to composite signal - Page 1
Forum section: electronics / repair

author: belzrebuth
author_rank: Frequent Contributor
timestamp: December 13, 2018, 08:29:30 am
content: Hi all,I've saved a 9" CCTV monitor from the trash.It's an RCA TC-1910 from 1984(?)I'm trying to feed it a composite video signal from an Atari ST as well as a Commodore 64 just for fun.The monitor looks really crisp but it doesn't seem to hold the image on screen.It's rolling kind of diagonally and I can get it to go slower but never to kind of "lock there"Messing with the V_HOLD and H_HOLD pots I think the V_HOLD is the one that messes the signal more because If I can get it to a certain point the image moves really slow and the H_HOLD has less effect.I played a bit with the trimmers next to the potentiometers with no progress whatsoever so I tried to leave them at their original positions..Any ideas?I'd really like to get this thing working

author: PKTKS
author_rank: Super Contributor
timestamp: December 13, 2018, 10:29:34 am
content: Actually you need a proper test signal to check the triggerof the local oscillatorsRandom imagery will not be easy to diagnose the fault...which may include the local oscillators being screwedPaul

author: belzrebuth
author_rank: Frequent Contributor
timestamp: December 13, 2018, 11:26:30 am
content: So I need specialized test gear like a video signal (pattern) generator or I can produce such signals with something else like a raspberry pi?I mention the raspberry because:a) I have oneb) It has composite outThe problem reminds me of oscilloscopes with no auto trigger level..Just trying to display a stable signal but it can't somehow "trigger"..

author: PKTKS
author_rank: Super Contributor
timestamp: December 13, 2018, 12:12:05 pm
content: Exactly you need a composite video signal generatorThat device not only outputs the proper stable patternbut also it has it's own dedicated SYNCH BNC outputso you can feed that in several ways to narrow thefault optionsthings like the scope trigger can use the SYNCHof the generator and you see more precisely your test pointsregardless what is the status of local stufflike this old fellow (figure)Paul

author: PKTKS
author_rank: Super Contributor
timestamp: December 13, 2018, 12:29:53 pm
content: It may looks like scopes but MONITORS are rather differentfrom scopes internally.While scopes are devoted to synch EXTERNAL signalsMONITORS are devoted to SYNCH the INTERNAL base oscillators(the CRT sweep base)I have came across a wide plethora of "digital" things tryingto replace REAL ANALOG generators... IMHO some may feeda sort of acceptable inputs ..But not even close the real analog thing. e.g. IT IS ANALOGThe TOP LIST in my tool box is A SPICE NTSC GENERATORwhich can feed that crap anywhere you wantSome funky examples ... they are ** PURE SPICE PLOTS **rather interesting these modern devices todaythey simulate anything... in pieces .. artificial dataPaul

author: TK
author_rank: Super Contributor
timestamp: December 13, 2018, 12:36:11 pm
content: Not all CRT CCTV monitors are NTSC composite signal compatible. It might have a different SYNC requirement. Are there any other inputs with separate SYNC signals?Here is the service manual:http://www.schematicsforfree.com/archive/file/Video/Monitors%20-%20CRT/Rca%20Tc1910%20Series%20Cctv%20Monitor.pdfInput requirement: 0.5 to 2 Vp-p sync-negative composite video. Hi Z / 75termination switch on back

author: CJay
author_rank: Super Contributor
timestamp: December 13, 2018, 12:40:47 pm
content: Quote from: belzrebuth on December 13, 2018, 08:29:30 amWhat would be useful would be to confirm the C64 and ST are working with another monitor (unlikely to both be faulty) and if they are PAL, SECAM or NTSC, you don't need a pattern generator, at least not yet.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: December 13, 2018, 01:42:27 pm
content: I've had great success with C64 and my 1040ST testing various monitors I find on flea markets and such so I doubt my signal source has an issue.I've tried both 75Ω and Hi-Z to no avail..and no, there aren't other inputs.There are two BNC connectors marked video in and video out on the back and that's it.I'll try other sources as well (I'm thinking the raspberry or an amiga which has a dedicated composite output) to be sure it's not the source.It's probably PAL not NTSC since I'm in EU and the monitor is 220V too so they should be compatible.What I haven't tried yet is hooking up an actual CCTV camera to it; I think I have some in storage but from what I remember they have a mini din connector so I have to check the pinout.The service manual (thanks TK) is actually pretty good with all the reference measurements and the initial setup procedure so I should start with that.I'm not sure if I'm using a proper 75Ω BNC cable at the moment; it's just a good quality rca cable but I don't think it matters that much because I've successfully tested similar monitors with the exact same setup.I think I can post some pictures when I get home.

author: CJay
author_rank: Super Contributor
timestamp: December 13, 2018, 03:57:13 pm
content: It won't be the cables, it probably won't work with your CCTV cameras either if they're PAL.The pots next to the H-Hold and V-Hold won't help you.Check C201, highlighted in the attached pic.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: December 13, 2018, 05:45:03 pm
content: I replaced capacitor C201 (which checked fine) and no change whatsoever..Those electrolytics must be mitsubishi branded btw..I bet that despite their age they still hold up pretty good.I think I should take the measurements mentioned in the service manual to ensure that everything's correct at least at some point..I could probe around the vertical area too.I initially suspected the vhold pot but I checked that too and it seems fine.I was hoping for a resistance jump or discontinuities of some sort but it seems to hold a stable ohmic reading when set to a certain position.I could also check the passives around that pot but the components in general seem fine visually, no discolorations or other signs of obvious failure..but you never know..

author: TK
author_rank: Super Contributor
timestamp: December 13, 2018, 05:53:02 pm
content: Follow all the safety recommendations for handling CRT monitors. There is dangerous High Voltage inside.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: December 13, 2018, 06:00:43 pm
content: Yeah I discharge the flyback anode to ground everytime it's going to the bench..you have to be alive in order to fix it anyways.

author: PKTKS
author_rank: Super Contributor
timestamp: December 14, 2018, 10:59:01 am
content: Checking them only may still rise unreliable conclusion.Old school techs from the all discrete TV set days used to havea very simple RULE OF THUMB in such cases:- **IF** you can pause the rolling even for a second.==> local oscillators are FINE--> suspect is the LPF or HPF synch separators (aka diffs or integrators today)-- **IF** the image can **NOT** be paused in any range of the pots.==> LOCAL OSCILLATORS are out of specIn general terms:- SYNCH separator is fed from the VIDEO AMP(in B&W very simple on the output of that amp in COLOR is BEFORE de RGB decoder)- 99% the amp is an INVERTER and SYNCH separator is "ABOVE BLANK LEVEL"- OSCILLATORS are plain VCO astables just phase trigger by synch pulsesCarefully check (MEASURE) freq of your STANDARD (PAL M/N or NTSC M/N)that will alter 60 or 50 Hz the Vosc and the required Hosc freq (PAL and NTSC aresubstantially different numbers for that OSC) but rather around the 15750 Hzbut differentA good freq meter will help thataging and problems on the parts of those astables may cause unreliable operationPaul

author: james_s
author_rank: Super Contributor
timestamp: December 15, 2018, 06:34:50 pm
content: I've seen a vaguely similar problem with a monitor that had switches on the back to select between overscan and underscan, and 75 ohm or high impedance, dirty switches caused wonky behavior with the sync. It's worth checking.Also as someone else mentioned, make sure the monitor is designed for the type of video signal you're sending it, many PAL monitors will sync to NTSC and vise versa but not all.

author: PKTKS
author_rank: Super Contributor
timestamp: December 16, 2018, 03:24:13 pm
content: Quote from: james_s on December 15, 2018, 06:34:50 pmYes that is crucial and assuming you ARE USING THE PROPER FEED.Here in SouthAm this is and was very common issue.We use a standard PAL-M system which operates at 60i**BUT* 3 or 4 countries around the borders operates at 50i (by mains imposition)So a sort of aberration system is used which mixes PAL-M and NTSC-M with PAL-N and NTSC-N(also aliased here as N-LINE)IN SUMMARY for the record:Code:**G** **M** **N**f(Hz) 15 625 15 734.264 15 625V(Hz) 50i 60i 50iLines 625 525 625fsc (MHz) 4.43 3.58 3.58Bw (MHz) 6 4.5 4.5Chan(MHz) 8 6 6So if you want to tune the local oscillators that will vary ifusing the G STANDARD (as in Europe w/625 lines)the M STANDARD or the hybrid N system.Even the XTAL is required to be changed while G uses a 4MHzthe others uses 3.58 XTALs (but different ones)I have a box of them yet.. way back decoding these beasts were frequentlySo make dead sure you are feeding the proper STANDARD ( M N or G )to your SYNCH separator which operates in 50 or 60 Hzor 15734.264/15625 Hzotherwise it will not synchPaul

author: coromonadalix
author_rank: Super Contributor
timestamp: December 16, 2018, 03:29:34 pm
content: Aaaaaaaaaaah thoses Pal Secam Ntsc standards frequencies bring back souvenirs.

author: PKTKS
author_rank: Super Contributor
timestamp: December 16, 2018, 03:38:21 pm
content: A lot of that crap still around here..And ... that is the main point using a proper ANALOG FUNCTION GEN.You have the proper feed for all that systems by changing a button.Let you narrow the tune process substantiallyPaul