Fix vertical hold on a monochrome monitor - Page 1
Forum section: electronics / repair

author: rea5245
author_rank: Frequent Contributor
timestamp: June 10, 2021, 07:12:57 pm
content: Hi,I have a monochrome monitor with a beautiful amber display, but I can't get the vertical hold to stay put.It's a Magnavox 7BM623 074T. I can carefully find a sweet spot for the V Hold to stop the screen from rolling, but if I turn the computer and monitor off then on, it loses the hold and starts rolling slowly.I've replaced all the electrolytic caps and the V Hold potentiometer. The problem remains.Anything else I can try?Thank you,Bob

author: bob91343
author_rank: Super Contributor
timestamp: June 11, 2021, 05:48:00 am
content: You would get better results if you had a schematic. The synchronizing pulse for the vertical oscillator is apparently weak or nonexistent. You need to find that circuit and troubleshoot it.

author: PKTKS
author_rank: Super Contributor
timestamp: June 11, 2021, 11:15:54 am
content: Quote from: bob91343 on June 11, 2021, 05:48:00 amThat is true..When you *CAN* pause the rolling even for a second...it means the local oscillator is perfectbut you have problem w/the sync integrator..Just try to find hard that part of the circuit...The fault should be at the integrator.Paul

author: rea5245
author_rank: Frequent Contributor
timestamp: June 11, 2021, 12:00:43 pm
content: Quote from: bob91343 on June 11, 2021, 05:48:00 amYeah. Alas, I do not, and Google can't find one either.

author: themadhippy
author_rank: Super Contributor
timestamp: June 11, 2021, 12:13:24 pm
content: I thought the correct fix for a rolling crt display was a sharp thump on case,or does that only apply to televisions?

author: rea5245
author_rank: Frequent Contributor
timestamp: June 11, 2021, 04:32:46 pm
content: Quote from: PKTKS on June 11, 2021, 11:15:54 amPardon my ignorance, but what is the integrator? Is it related to phase-locking with the input video signal? Is it likely to be an integrated circuit, or just some discrete components? (This monitor was manufactured in 1990. I only see one IC in it.)- Bob

author: PKTKS
author_rank: Super Contributor
timestamp: June 11, 2021, 04:36:43 pm
content: Quote from: rea5245 on June 11, 2021, 04:32:46 pmthe integrator is the modern term used to a LPF (low pass filter)The synch pulse train is generally combined and the verticalsync is mixed with the horizontal.. so you need to LPF the verticaland HPF the horizontal pulses..When they are combined in a usual stream..Which is the likely in more old stuff CRT..Digitally generated pulses are not like thatThe integrator is just a LPF which will separatethe V synch in the SYNCH train.. (always H based freq,)Paul

author: rea5245
author_rank: Frequent Contributor
timestamp: June 11, 2021, 04:43:58 pm
content: This monochrome computer monitor is driven from IBM's Monochrome Display Adapter standard, which has separate Horizontal Sync and Vertical Sync pins. So wouldn't that mean there's no need for a LPF?

author: PKTKS
author_rank: Super Contributor
timestamp: June 11, 2021, 04:53:40 pm
content: Quote from: rea5245 on June 11, 2021, 04:43:58 pmThat depends a lot on the circuitryDo you have the CHIP number? if it is a chip ..All discrete ?You need to carve that detail .. if you have the CHIPcontroller number .. search for a usable datasheet..old stuff is likely to be scanned in PDFs these days..If it is really OLD (amber stuff is old) it may be all discreteand likely to have a mixed tech of analog and digital..hard to say from here by guessing..but just ONE CERTAIN: the is a problem w/SYNCH PULSE somewherenot tripping the local vertical oscillator properlyPaul

author: rea5245
author_rank: Frequent Contributor
timestamp: June 11, 2021, 04:59:26 pm
content: The only IC is a 74LS86 - a quad XOR gate. Everything else is discrete.

author: PKTKS
author_rank: Super Contributor
timestamp: June 11, 2021, 05:04:27 pm
content: Quote from: rea5245 on June 11, 2021, 04:59:26 pmFor that shit you need a 4 chan scope and a lot of patience.Paul

author: rea5245
author_rank: Frequent Contributor
timestamp: June 11, 2021, 08:06:47 pm
content: Quote from: PKTKS on June 11, 2021, 05:04:27 pmI'm more worried that I need more knowledge of analog electronics than I have.- Bob

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 12, 2021, 07:21:41 pm
content: Without a schematic for the forum to look at and guess the trouble point you're pretty much reduced to tracing the V sync signal and sketching out that partial schematic, and figuring out where it gets weak to pinpoint the faulty area.Another thought, are you certain it's the monitor and not the equally old display adapter driving it? A scope of any sort will be useful here.

author: rea5245
author_rank: Frequent Contributor
timestamp: June 12, 2021, 07:31:50 pm
content: Quote from: BrokenYugo on June 12, 2021, 07:21:41 pmWell... no... now that you mention it, I guess I'm not certain. I don't have another monochrome monitor to test with the PC.I have a scope though. What would you suggest I look at? The vertical signal, and whether it has a rock-solid frequency?

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 12, 2021, 08:07:17 pm
content: Without looking up the standard my assumption (a dangerous word) would be there should be a 5V TTL signal on the port and it shouldn't be distorted much when the monitor is plugged in.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 13, 2021, 03:20:36 am
content: I had the same problem with an old computer and monitor. It turned out to be the sync signal in the computer. I had a bad CMOS logic chip. Have you tried connecting a different device to the monitor?

author: james_s
author_rank: Super Contributor
timestamp: June 13, 2021, 04:42:27 am
content: I'm not sure why you replaced the pot, if adjusting the pot affects the rolling then the pot is obviously not the problem. I do not advocate shotgun replacement of electrolytic capacitors either, much too often I've seen people replace good quality parts with the cheapest no-name Chinese junk around and then mess something else up in the process.Are you sure the video card is producing a valid sync signal? Is the signal getting all the way to the board in the monitor? The symptom you describe is exactly what you will see if there's a bad connection somewhere and the sync signal is not getting through.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 13, 2021, 05:16:01 am
content: Quote from: BrokenYugo on June 12, 2021, 08:07:17 pmIf you have a scope, have a look at the back of the sync input socket, or the equivalent pin of a multiway socket.If there is a signal, & it looks correct, the connector, cable, & display adaptor is ok.You should be able to trace either a cable, or if it directly connects to the PCB, the track which runs from that point.(often, if a cable is used, it connects to the PCB, so you will end up following a track, anyway).There may be a dry joint, or other disconnection--------if so, the signal will either disappear, or become reduced in amplitude.If that hasn't happened, you can then probe around the device it ends up connected to.If this is an unidentified IC, take note of the type number, then Google for it, as these often show typical circuits using the device.Many of these monitors are fairly generic, so it also may be possible to find the schematic of a similar one.

author: rea5245
author_rank: Frequent Contributor
timestamp: June 13, 2021, 01:16:10 pm
content: I don't have another monitor or another PC to try it with.On my scope, the V Sync signal seems fairly steady, but there are two odd things, and I don't know if they're significant. First, the scope (a Rigol DS1054Z) calculates the V Sync signal alternately as 50Hz and 49.5Hz. Second, if I watch the trace for several seconds, I can notice that it's slowly crawling the left. But is that really a problem with the signal, or just with how the scope is calculating and drawing it?At the same time the scope's trace is slowly crawling, the monitor's V Hold is stable enough that the screen is not rolling.As I mentioned before, if I turn the monitor off and on, it loses the vertical hold. If the problem is in the signal from the PC, would you expect that I could adjust the V Hold knob and stabilize the screen, then lose it by turning the monitor off and on?

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 13, 2021, 01:57:15 pm
content: Did you happen to get a picture of the scope trace? Probably a monitor side problem if it looked something like this.http://minuszerodegrees.net/mda_cga_ega/mda_sync_ver.bmp

author: rea5245
author_rank: Frequent Contributor
timestamp: June 13, 2021, 02:02:49 pm
content: Here's a pic of the vertical sync.

author: james_s
author_rank: Super Contributor
timestamp: June 13, 2021, 07:02:03 pm
content: That sync signal looks fine to me, if it's crawling it's due to the trigger settings on the scope, you'll have to adjust those to get it to display properly there. The frequency readout changing is also totally normal, scopes are not precision frequency counters. The sync at whatever point you are measuring is ok, but it must be disappearing somewhere before it gets to the vertical oscillator.

author: PKTKS
author_rank: Super Contributor
timestamp: June 14, 2021, 10:38:37 am
content: Quote from: rea5245 on June 13, 2021, 02:02:49 pmPlace BOTH waves using the OSC as primary trigger...The SYNC is failing to adjust the OSC properly..if it is deviating due to aging components *you may* see that..Paul

author: vk6zgo
author_rank: Super Contributor
timestamp: June 14, 2021, 01:39:26 pm
content: Quote from: rea5245 on June 13, 2021, 02:02:49 pmThe 1054 should have no trouble triggering, but the very low displayed signal level may be a bit problematical.It would probably be a good idea to reduce the volts/div so the display is say, 5 divisions in amplitude, then try retriggering.Another idea is to reduce the time/div setting, as you don't really need to look at so many cycles.

author: rea5245
author_rank: Frequent Contributor
timestamp: June 14, 2021, 01:52:30 pm
content: Quote from: vk6zgo on June 14, 2021, 01:39:26 pmAccording to the MDA spec, high voltage levels are 2.4V to 3.5V. Seehttp://minuszerodegrees.net/oa/OA%20-%20IBM%20Monochrome%20Display%20and%20Printer%20Adapter.pdf,page 9.I'm surprised. I would've guessed 5V. But apparently not.

author: Renate
author_rank: Super Contributor
timestamp: June 14, 2021, 02:05:40 pm
content: Yes, the MDA signals are TTL which never goes to 5V, only > 2.8VQuote from: rea5245 on June 11, 2021, 04:59:26 pmThis is probably doing sync detection for H and maybe V.You should be able to find two input pins that have nice V signals on them (also two with H).Using two channels on the scope should show you what's what as you play with V hold control.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 15, 2021, 12:40:12 am
content: Quote from: rea5245 on June 14, 2021, 01:52:30 pmIt still shouldn't bother your 'scope, or the monitor, PAL syncs are 400mv, & even with noticeably lower levels monitors stay locked.My reasoning with dropping the volts/div on the 'scope (giving more vertical divisions of the display for the same input signal) was not to do with concern about the actual sync level, but because, at least, in all the Oscilloscopes I have used, the triggering is taken after the input gain & attenuator stages.If a larger signal appears at the trigger circuitry, it will almost universally be easier to trigger on.'Scope triggering is also more likely to present problems at quite long time/div settings, like 10ms/div, than at, say, 2ms/div.Have you compared the video signal out of the source with the corresponding syncs, using two 'scope channels?Obviously, they should be in "sync".Does the monitor pix roll vertically "as a whole", or does the vertical blanking drift w.r.t. the picture?

author: vk6zgo
author_rank: Super Contributor
timestamp: June 15, 2021, 12:45:24 am
content: Quote from: Renate on June 14, 2021, 02:05:40 pmIn any case, checking the pins will show you what the IC is used for.It could be something to do with the front controls, for instance, & all the sync stuff done using discrete devices.

author: ambrosia heart
author_rank: Regular Contributor
timestamp: June 15, 2021, 02:53:27 am
content: Quote from: rea5245 on June 10, 2021, 07:12:57 pmSearch for word黑白電視故障檢修 and垂直不同步 athttps://translate.google.com/translate them to English. Choose left hand side third one . Right hand sidesecond one. Copy 黑白電視故障檢修 and垂直不同步 to left hand side third one to translate.This is an old book bought in 80s. Schematics are National TV.In those day called National NOT panasonic.I know you are unable to read chinese characters. I translate itto you. In your cases, there are 2 situations I or II. Read it carefully.In the top right hand corner written 黑白電視故障檢修Comparing those waveformsprinted in this book. Despite differentbrands of black and white TVs, the working principle inovlved isbasically same.1. Even rotate V.Hold, screen is still unable to be still. Horizontalsync. is of course normal.2.Reason:(a) integration circuit is faulty, making vertical sync.signal unable to trigger oscillator. (b)vertical oscillation circuitand the oscillation freq. are abnormal. (c)sync. circuit is faulty.In spite of getting sync. the sync. signal is very weak.3.prepare to adjust.Rotate V. HOLD to see if(a)after adjust., the screen is still rotating upwards or downwards.(b)Still unable to make the screen be still at one point. This impliesthe oscillation freq. is of about 60Hz, but no sync. signal inputted.This explains why screen is rotating.(a)Screen is rotating either upwards or downwards in one direction only.This implies the oscillation freq is highly distorted. In spite of thepresence of sync. signal, rotation still occurs.4.SolutionForWhen vertical oscillation circuit without sync. signal, this is due toeither integration circuit faulty or sync. pulse not inputted.In fig. 218 either R421 open or (積分電路) resistors of integrationciruit open or cap. shorted. (垂直振盪):vertical oscillationIn fig. 219 is integration circuit consisting of R421-423, integraitoncaps. C421-422 and coupling diode D421. They are fautly.They all connectto vertical oscillation transformer. If transformer,T421, primary side opens,problem appears. Finding these words in schematics 積分電路 = integrationcircuit, 垂直振盪 vertical oscillationFor This is a great distortion in vertical oscillation.(a)screen rotates downwards, vertical osc freq. > 60Hz.When freq. > 60Hz.In fig. 219(y)feedback resistor R425 and VR471 either open or greater in value.If open, adjust V HOLD, no any change.(z)time constant cap. C423, sawtooth charging cap C425 capacitancedecrease.(b)screen rotates upwards, vertical osc freq. < 60Hz.In fig.219(v)bias feedback resistor R424 value becomes greater. Emitter timeconstant resistor R426 value becomes greater. The transistor 2N3461is bad due to increase in Icbo.In fig.220(w)vertical osc freq. is high due to R312 open. C301-303 capacitancedecrease. R314 and VR31 in high value or C321 capacitance decrease.(x)vertical osc freq. is low due to R311 is greater or R305, R312,R321,R323-324 also high in value.(y)vertical oscillation transistor 2SC828A is bad.

author: rippenburn
author_rank: Newbie
timestamp: August 03, 2024, 01:29:46 pm
content: Just an aside to the topic but can someone explain ... does the vertical hold on old TVs synchronize the set with the incoming signal OR increase/decrease the duration of the frame? For example, if my set expects frames at 1/60th intervals but the circuit is off by (say) +/-0.01%, is the circuit able to compensate for that error or do I have to adjust it up or down to get the exact frame rate? If so, by what percentage could the circuit be adjusted? Thanks.

author: wasedadoc
author_rank: Super Contributor
timestamp: August 03, 2024, 05:44:58 pm
content: The pot adjusts the free running frequency of the oscillator to be slightly slower than the frame rate of the video signal. The vertical sync pulse "kicks" the oscillator to complete its cycle and start a new one slightly sooner than it would without the kick.In other words the period of the oscillator becomes exactly the same as the time between vertical sync pulses. There is typically at least a few percent leeway on how slow the free running frequency can be before the kick fails to be effective. It is not uncommon to see the picture height change slightly within the lock range.

author: rippenburn
author_rank: Newbie
timestamp: August 05, 2024, 09:04:42 am
content: Thank you! I have been asking this question all over the Internet in relation to the Doppler effect on signals broadcast from the Apollo spacecraft. NASA used back-to-back VTRs to compensate for the effect. But I can't see that travelling at 3,000 mph would have much effect on the timing of the frames when EM travels at the speed of light. I calculated it would be around +/- 0.024% difference, which I would have thought was within the range of the vertical hold on a TV. So why not just stabilize the incoming picture and rebroadcast it with a TV camera, like the old Kinescope?