(At the bottom,the "BASIC 1.1" is readable)

Open potentiometers (usually due to dirt on the wiper contact, not because of part failure but that also can happen) anywhere in the V-sync path can cause that. V-hold, V-size, V-position (this one is less likely), and some of them might be not user controls but adjustable on the PCB inside the monitor.

Then there's the usual culprits - electrolytic caps. Now, even the slow vertical would be using a stable film capacitor and not an electrolytic for the sawtooth generator but there might be issues in the power supply (like excessive ripple) that throw the sync circuit off. And these monochrome CRTs often have a cap in the yoke coil circuit rather than direct connection to GND (as a safety measure). If that one dries out then the feedback loop might not be able to work out the frequency difference. Though that would usually also result in partially collapsed vertical and/or pretty horrible sweep linearity.

And obviously it could be that your Amstrad machine is not outputting the V-synch pulses properly for some reason (usually also caps or bad transistors). Be sure you are not chasing ghosts, test the computer on TV or another monitor first. If you are sure it works properly then spray the control pots with suitable cleaner spray (there are also cleaner+lubricant combos). If that doesn't work you'll have to investigate the PCB pots and capacitors, and power supply output. These monitors are easy and fun to work with - if the tube is good and flyback transformer is not dead then everything else can be repaired or replaced.

APOPAP wrote on 2024-10-17, 19:40:

Hello, I recently got an amstrad GT64 monitor. The computer works ok,but the v-hold seems to have a problem.I tried ajusting the […]
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Hello,
I recently got an amstrad GT64 monitor.
The computer works ok,but the v-hold seems to have a problem.I tried ajusting the pot(something changes on the screen),but nothing

Any ideas?

nice! ive got one of these in my loft havent turned it on in about 20 years, also got a colour one i recently got back down, just made up a din to scart cable for my games consoles after finding out its a 15khz rgb monitor, the colour monitor has a v-hold knob sticking out the back, adjusting it will allow you to adjust screen roll for 50/60hz picture, the green screen is probably the same, obviously check the contrast/brightness knobs on the bottom and see if it has any effect, another problem is the coiled din cable that plugs into the keyboard, which are shit and easily damaged especially as it comes out under the front of the monitor, move the cable about and see if it does anything, even the one on my colour scrren goes funny if i knock the cable,

though it does look like you've got the v-hold set wrong, i can see something on the bottom of the screen turn it back untill that line at the bottom moves up and disappears to the top of the screen, also looks like the brightness/contrast is way too high, if the knobs dont turn it down enough, someone might have opened it up and turned up the sub-bright potentiometer way too much (its an internal brightness control)

heres my colour one, need to get an rgb cable for snes so i can use the greenscreen for super gameboy, for cpc games i use retroarch on the gamecube, and PS2 retroarch for MSX and sharpX1, looks great on a cpc crt

Thanks everybody,
I am going to open it today to see what it does.

Tried cleaning the pots,not helped.
When I ajusted the small pots on the pcb something changed,but I don't know exactly the pots I must ajust.

The lines now go higher

Deunan wrote on 2024-10-17, 21:31:

Then there's the usual culprits - electrolytic caps. Now, even the slow vertical would be using a stable film capacitor and not an electrolytic for the sawtooth generator but there might be issues in the power supply (like excessive ripple) that throw the sync circuit off. And these monochrome CRTs often have a cap in the yoke coil circuit rather than direct connection to GND (as a safety measure). If that one dries out then the feedback loop might not be able to work out the frequency difference. Though that would usually also result in partially collapsed vertical and/or pretty horrible sweep linearity.

Thanks I think you just described a problem I am seeing on a mini 5" TV

APOPAP wrote on 2024-10-18, 13:41:

Tried cleaning the pots,not helped.
When I ajusted the small pots on the pcb something changed,but I don't know exactly the pots I must ajust.

Don't just blindly turn the PCB pots - you will do more harm then good. Unless you only moved them just to rule out wiper issues and returned the pots to the original position.
There are service manuals for these monitors available for free, like here: https://www.grimware.org/lib/exe/fetch.php/do … ctm640.gt64.pdf

Check VR701 (V-HOLD) resistance, that it is not open (or shorted somehow). Check VR702 (V-SIZE) and VR709 (V-LINER) as well. And make sure you got the symptoms right - it's hold issue if the screen is rolling. If it's just too big and picture ends up beyond visible screen area then it's a size problem (and most likely that would be an open pot).

There really isn't much here to check. Start with the PSU voltages and in fact the schematic gives you typical voltages for all 10 pins of the vertical deflection IC (both on the drawing and in the corner). Since it's an IC and those typically either work properly or not at all you can focus on all the parts around it. Check the pots, diodes, resistances next if the voltages are fine (or, if one is badly off then it's an obvious hint). This is a super simple circuit, it picks up the combined signal from the collector of Q601 and the IC does all the rest internally. Unless it's a yoke issue (very rare on these) it has to be one external parts I mentioned, or perhaps a broken connection (cracked or cold solder for example) somewhere.

Don't start randomly tweaking and poking things, you'll only make things worse. Stick to the manual and schematic.

BitWrangler wrote on 2024-10-18, 14:05:

Thanks I think you just described a problem I am seeing on a mini 5" TV

Note that vertical often uses a higher voltage than what PSU provides (for better linearity). That voltage is generated by the flyback, it's more complicated but has some advantages like softer start and vertical not running at full power (or not at all) if the horizontal is not working properly. So there is a cap in the horizontal sweep that's connected to flyback to more or less double the voltage.

In the GT64 case (manual with schematic in post above) this is not the case, but the voltage doubler is there - just for the horizontal sweep. It's C727, C728 and diode D704 next to the flyback. Of these the C728 is usually very close to flyback or the HOT (and its heatsink) and tends to degrade faster than other caps. It's one of the very few (possibly the only one) caps in such a monitor that also needs a decent ESR and not just the capacitance. If in doubt, replace it.

You can find it by knowing that it's on the primary side of the flyback, it's (+) terminal will have a small resistance (through primary winding) to the HOT collector. Note that (-) is rarely conncted to GND, rather it will be connected to B+ because then it only needs half the voltage rating. But such connection makes it difficult to measure it in-circuit since it's effectively shorted by a diode and low-resistance part of the primary winding. Preferably replace it with 105C variant if it's near heat sources but it's not that important to go for low-ESR cap. Any new cap will have decent enough ESR to work here.