Repairing X68000 CRT Horizontal Hold with a Trimmer Pot

An unstable horizontal raster can make a perfectly serviceable X68000 look as though its video hardware has failed. Diagonal tearing, sideways rolling bands, repeated vertical bars, or a picture that will not lock are classic signs of poor horizontal synchronization. On many CRT monitors, the first adjustment for this is the H HOLD trimmer potentiometer on or near the horizontal deflection and sync board.

The trimmer sets the horizontal oscillator’s relationship to the incoming sync signal; it does not restore a missing raster or set picture width by itself. Before touching it, confirm that the computer is producing a valid signal, identify the monitor chassis, and use the appropriate service documentation. Careful diagnosis prevents a simple control fault from becoming a long search through the flyback and output stages.

Diagnosing a Horizontal Synchronisation Fault

Begin by separating signal problems from monitor faults. Reseat the X68000 video connector, inspect its cable and pins, and try a known-good cable or compatible CRT. A low-frequency scope can verify the common X68000 horizontal line frequency of about 15.734 kHz and the presence of horizontal sync. If the monitor has a sync-polarity switch, use the position specified for the computer rather than forcing a lock electrically.

Classify the symptom before touching the board. Sideways sliding or diagonal tearing usually points to the horizontal hold loop. Up-and-down rolling is a vertical hold problem, while a stable but narrow or stretched image calls for width or deflection yoke checks. A dark screen, collapsing raster or unstable brightness suggests power or video-amplifier faults instead. Check controls and connectors before removing the cabinet.

Locating the H Hold Adjustment

Use the monitor’s service manual to identify the chassis, board revisions and component designators. For model identification and links to further material, the TeKnisi technical archive is a useful starting point. A monitor with a front-panel H HOLD control should be adjusted through that control; an internal trimmer is usually marked H HOLD, H SYNC or something similar near the horizontal oscillator and sync separator.

Trace the adjustment rather than guessing from its physical position. In many designs, the trimmer biases the horizontal oscillator or phase comparator, allowing the AFC loop to match the incoming line frequency. H WIDTH or H SIZE controls scan amplitude, while V HOLD is a separate circuit. Photographing the board and noting the original trimmer position makes later diagnosis easier, especially if previous owners have moved several controls.

Working Safely Around CRT Mains Voltage

A CRT monitor can retain a dangerously high charge after it is unplugged. The anode supply, flyback transformer, mains filter capacitors and CRT neck circuitry require specialist handling. Do not use a screwdriver to short the anode cap, and do not work on the set while it is energised unless you have the training and equipment for that procedure. An isolation transformer is not a substitute for proper discharge and protective equipment.

Australian mains is 230 V at 50 Hz, while many Japanese X68000 monitors were built for 100 V. A plug adapter does not convert voltage. Check the original rating and internal selector; if the monitor is not multi-voltage, have a correctly rated transformer fitted or arrange conversion by a qualified electrician. State Electrical Safety Acts and Regulations can also impose competent testing and test-and-tag duties on equipment used by clubs, employees or laboratories.

Replacing or Reflowing the Trimmer Pot

If the fault follows the monitor and the hold control has no useful range, remove the cabinet only after the safety precautions are understood. Check for a cracked solder joint, flux contamination or a loose casing around the trimmer first. Measure the suspect variable resistor out of circuit: its wiper should move smoothly between the end terminals. An abrupt jump, open wiper or extreme resistance variation identifies a bad component more reliably than repeated adjustment.

Replace it with the same nominal resistance, electrical rating, taper and adjustment style. Multi-turn cermet types are common, but a modern 3296-style part may not match the original dimensions or solder pattern. Photograph the pins, remove the old part with controlled heat, clean the pads with solder wick and flux, and confirm there are no solder bridges before fitting the replacement. Reflowing the original joints can solve an intermittent fault without changing components.

Do not assume a new trimmer will cure every failure. Leaking electrolytic capacitors, a damaged oscillator transistor, an out-of-range timing resistor or excessive loading from the line-output stage can also pull the horizontal frequency beyond the adjustment range. Change one component at a time and compare its value with the schematic, using an ESR meter for suspected electrolytics where appropriate.

Setting the Horizontal Hold

Once the circuit is serviceable, close the monitor and allow the CRT and circuitry to warm up. Display a steady test image or a simple X68000 screen at a stable brightness. Adjust only through the manufacturer’s intended closed-case access point; never reach into an energised chassis. With a properly insulated small screwdriver, move the trimmer in small increments. A picture drifting diagonally will usually become least steep at one part of the range; centre the picture there, then make the smallest adjustment that removes all movement.

Fine adjustment matters. After horizontal lock is stable, set width or H SIZE separately, then check picture geometry and vertical hold. Recheck the image after several minutes because electrolytic temperature drift and a marginal oscillator can make an apparently correct setting wander. If a scope is used, measure only the service-manual test points around sync and the oscillator; probing the anode, cathode or flyback output with ordinary grounded equipment is unsafe.

Use real software as a second check. The Human68k system information project is useful for confirming that text and changing screens remain stable on original hardware. Watch for diagonal tearing, vertical bars, line-frequency jitter and loss of lock during redraws.

Testing the Restored Display

Verify the repair with more than one X68000 display mode and with animated output, not merely a static test screen. Repeat the test after a cold start, a warm restart and sustained use. The monitor should produce the same stable raster each time, without intermittent bars appearing when brightness increases or software switches between screens.

Storage does not affect the hold circuit, but it provides a realistic load for sustained testing. The SCSI2SD game loading guide can help when checking animated output after the repair. Confirm that the final setting survives several games, then record the trimmer position only after the picture has remained locked.

The following symptom comparison separates common faults before the cabinet is opened:

Symptom	Likely area	Checks before adjustment
Sideways sliding or diagonal tearing	Horizontal hold or AFC loop	Input signal, sync polarity, H HOLD range
Up-and-down rolling	Vertical hold	V HOLD range and vertical oscillator
Stable but narrow picture	Width and line drive	H SIZE, deflection yoke, output transistor
Jitter that changes with brightness	Power or horizontal output load	Supply rails, capacitors, flyback loading
Dark screen or missing raster	Video or power path	CRT bias, power supply, video amplifier

Treat a setting that works only after a long warm-up as an unresolved fault. Record the chassis number, board revision, original and final trimmer positions, and the symptoms that disappeared. These details distinguish a successful hold adjustment from a temporary improvement caused by heat.

Share Your X68000 Monitor Repair

Retrocomputing collectors in Sydney, Melbourne and Brisbane regularly compare notes at weekend swap meets, and a repaired monitor is often best validated with another X68000 in the room. Australian eBay sellers, Gumtree listings and boot sales can supply uncommon trimmer pots, but verify resistance, package and seller history before buying. Australian Consumer Law guarantees generally apply when purchasing from an Australian business, while private sales have different rules.

When finished, photograph the board and repaired joint, record the chassis number and final trimmer position, and note which symptoms disappeared. Retire a damaged CRT through an approved e-waste or recycling service rather than general waste. Post the repair on X68K.NET with the symptoms, part details and adjustment method so another X68000 owner can keep a useful machine out of a skip.