Calibrating X68000 CRT geometry through service mode

The Sharp X68000 is unusually demanding when it comes to CRT setup. Its software can switch between 15 kHz, 24 kHz and 31 kHz display modes, often within the same program, while Japanese monitors such as the CZ-600, CZ-601 and later multiscan models may use different geometry controls and service procedures. A picture that looks acceptable in one mode can be too wide, shifted, bowed or vertically compressed in another.

Geometry calibration is therefore more than centring an image with the front-panel knobs. It involves identifying the monitor chassis, entering its service adjustment mode safely, displaying a reliable test pattern, and recording changes as you work. The aim is a stable raster with correct proportions, enough overscan to hide the unused edge of the tube, and no loss of information around the active picture.

What the service mode actually controls

A CRT monitor has several layers of adjustment. The user controls usually provide brightness, contrast, horizontal position, vertical position, width and height. Service mode exposes the electronic parameters behind those controls, including horizontal amplitude, vertical amplitude, pincushion correction, trapezoid correction, vertical linearity and sometimes separate settings for each scan frequency.

The names vary between monitors. “H size” may appear as “H amplitude” or “horizontal width”, while “V size” can be called “vertical amplitude”. East-west pincushion correction changes the inward or outward curve of the left and right edges. Trapezoid changes whether the top and bottom edges are parallel. Vertical linearity affects whether squares remain the same height throughout the screen rather than stretching near one end.

Many X68000 monitors store separate geometry values for 15 kHz, 24 kHz and 31 kHz. Some chassis share part of the adjustment set, and some save changes immediately in non-volatile memory. Others require a separate save command or a service switch to be returned to its normal position. Never assume that leaving the menu will preserve the setting; check the service documentation for the exact model.

A useful distinction is between geometry and convergence. Geometry determines the shape and position of the raster. Convergence determines whether the red, green and blue electron beams meet correctly. If white text has coloured shadows, geometry adjustments will not fix it. Likewise, purity errors, colour blotches and a dark corner usually point to the tube, degaussing circuit or magnetic influence rather than a service-menu setting.

Prepare the monitor before changing anything

Start with the monitor identification label, not with a button combination found in a forum post. Record the model, chassis number and visible control layout. Japanese X68000 displays were sold in several generations, and an undocumented service shortcut that works on one CZ-series monitor may activate a completely different function on another. Keep the service manual, a photographed original setting screen, or a written baseline beside the machine.

The monitor must be warmed up before final adjustment. Allow at least 20 to 30 minutes with a stable picture, because CRT geometry changes as the deflection coils, power supply and chassis temperature settle. If the image slowly expands, contracts or shifts during warm-up, repair that fault first. A failing electrolytic capacitor, tired horizontal regulation circuit or unstable power supply can make calibration appear impossible.

Use a proper X68000-compatible source and a known test image. A simple grid with a border, centre cross, corner markers and square blocks is more useful than a busy game screen. The border shows overscan, the grid exposes pincushion and linearity errors, and the square blocks reveal whether the horizontal and vertical scales match. Test at every scan rate that you intend to use rather than calibrating only the desktop.

Electrical safety is essential. CRT monitors contain lethal high voltage, and the picture tube can retain a dangerous charge after the mains lead is removed. Service mode should normally be operated through the external controls or an insulated service switch, with the rear cover fitted. Do not touch the chassis, flyback transformer, anode cap or exposed adjustment points unless you are trained to work on CRT equipment. In Australia, the nominal mains supply is around 230–240 volts, so improvised isolation arrangements are especially poor practice.

A repeatable geometry adjustment sequence

Enter service mode only after noting the original values. If the monitor uses a hidden button combination, write down the sequence and take a photograph of each page before making changes. Change one parameter at a time, in small increments, and wait for the picture to settle. Large jumps make it difficult to tell which adjustment introduced a new defect.

The usual order begins with vertical and horizontal centring, followed by overall size. Position controls move the active raster; size controls change its amplitude. Set the picture so the test border is visible on all sides, then increase the size until the useful image nearly fills the tube without cutting off menu text or status information. CRT edges naturally have some overscan, but excessive cropping can hide a game’s score panel or a computer desktop icon.

Once position and size are close, correct the shape. Adjust pincushion until vertical edges are straight, then use trapezoid if the top and bottom edges lean in opposite directions. Some monitors include separate top and bottom corner or parallelogram controls. Use the smallest correction necessary. Heavy digital correction can reduce the usable adjustment range and may make the picture less uniform.

Vertical linearity comes next. Display a grid or repeated square blocks and compare the upper, middle and lower areas. If circles become taller near the top or bottom, adjust linearity rather than simply changing vertical height. Horizontal linearity is less commonly exposed, but a chassis may provide it through a service parameter. Finish with a careful check of the centre, corners and all three scan rates.

Adjustment	What to watch on the test pattern	Typical symptom when incorrect
Horizontal position	Equal left and right border space	Picture is shifted or clipped on one side
Horizontal size	Full width without losing edge markers	Vertical borders disappear or excessive blank space remains
Vertical position	Equal top and bottom border space	Image sits high or low
Vertical size	Square blocks retain useful proportions	Picture is squashed or cropped vertically
Pincushion	Straight vertical edges	Sides bow inward or outward
Trapezoid	Parallel top and bottom edges	One edge is wider than the other
Vertical linearity	Similar square height from top to bottom	Upper and lower areas look stretched
Convergence	White lines remain white	Coloured fringes appear around text and grids

Matching geometry to X68000 display modes

The X68000’s scan-rate changes are the reason a single “perfect” setting is often unrealistic. A game running at 15 kHz may occupy a different portion of the tube from a 24 kHz utility or a 31 kHz high-resolution desktop. The monitor may recall individual values, or it may apply one set of controls across several modes. Find out which behaviour applies before trying to make every screen identical.

Begin calibration with the mode you use most. For a games-focused system, that is often 15 kHz, where arcade-style software and many console conversions run. Then check 24 kHz and 31 kHz using a suitable test program. If one mode cannot be filled without cropping while another has excessive margins, compromise carefully rather than forcing a setting that loses information.

The border should be treated as a reference, not an invitation to maximise image size. Japanese software may place important graphics close to the edge, and a monitor that overscans too aggressively can conceal text or menu prompts. Conversely, a visible black band of a few millimetres is usually preferable to clipping. The phosphor area is not perfectly rectangular, and the corners often need slightly more tolerance than the centre.

A stable test workflow can include an application that repeatedly loads a known screen, switches modes or displays a fixed pattern. This is particularly useful after hardware work. For an example of preserving software in a convenient form, the RAM-disk game conversion approach can reduce loading interruptions while you compare several screens, although it should not be used as a substitute for a dedicated geometry pattern.

Faults that calibration cannot repair

Geometry adjustment cannot compensate for a weak horizontal output stage, incorrect B+ voltage or a power supply that changes under load. If the image width pulses, the raster collapses briefly, or the monitor loses sync when switching modes, stop adjusting and investigate the electrical fault. Repeatedly correcting a drifting image can leave the stored values at an extreme and disguise the underlying problem.

A picture that is permanently too narrow or too wide may indicate a failed capacitor, altered resistor, bad solder joint or incorrect replacement part. Vertical foldover, a bright horizontal line, severe pincushion asymmetry and intermittent shutdown are repair symptoms, not calibration tasks. The same applies when one side responds to adjustment but the other remains distorted.

Australia adds practical complications for restoration work. Genuine Japanese replacement parts can be slow to obtain, and postage from Japan can cost more than the component itself. Local electronics suppliers in Melbourne, Sydney and Brisbane may stock modern equivalents, but voltage ratings, temperature ratings, lead spacing and capacitance must be matched properly. A part that is electrically close on paper may behave differently in a vintage deflection circuit.

Power quality and storage conditions matter as well. A monitor kept in a hot shed in Perth or a damp coastal room near Newcastle may develop corrosion, degraded insulation or accelerated capacitor ageing. Use a sound power board, keep ventilation openings clear and avoid repeated rapid power cycling. A step-down transformer is generally unnecessary for a monitor designed for Australian mains only when its label confirms the required input; Japanese 100-volt equipment must never be connected directly to 230–240 volts.

Recording a reliable final setup

When the geometry is correct, record every service value, the selected scan rate and the monitor’s visible user-control positions. Photograph the test pattern and save a small note with the model and date. This makes future repair work far easier, especially when a replacement monitor or chassis has a different service-menu layout.

Check real software after the grid. Test a 15 kHz game, a 24 kHz title or utility if available, and a 31 kHz desktop application. Look at Japanese text, status bars, circular sprites and screens with edge-hugging artwork. A grid can look excellent while a practical display reveals that the top line is clipped or that a menu sits too close to the bezel.

Do not chase mathematical perfection at the expense of a comfortable picture. CRTs have manufacturing tolerances, and the visible raster changes slightly with brightness, contrast and temperature. The best calibration is stable, readable and conservative: straight edges, sensible margins, matched proportions and no service parameter pushed to its limit.

Preserving these settings is part of preserving the X68000 itself. Record the monitor model, share verified service information with the enthusiast community, and keep a copy of your test patterns and adjustment notes with the machine. If you work on a local monitor, document its results for other Australian owners who may be dealing with the same scarce parts and unfamiliar Japanese controls. Use the procedure carefully, keep the CRT safety rules in view, and return to the test pattern whenever a new geometry problem appears.