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Repair: Nanao MS8 monitor

A Nanao colour CRT monitor chassis, fitted in Sega candy cabs (including the Sega Astro City). Dual-frequency 15/24 kHz. The #1 fault: tired capacitors → recap.

Identification

Field Value
Manufacturer Nanao (EIZO)
Type Colour CRT monitor chassis
Sizes MS8-26 (26"), MS8-29 (29")
Frequencies 15 / 24 kHz (auto-sync)
Cabinets Sega Astro City and other candy cabs

Level & risks
Difficulty Intermediate → Advanced
Risks CRT high voltage (lethal), fragile traces/joints
Time Recap: 2-4 h

High voltage — discharge the tube

A CRT holds very high voltage (~20-30 kV) even unplugged, for a long time. Discharge the anode (via a resistor to the chassis ground) before any work. Don't work alone if untrained.

Common faults

Symptom Likely cause Lead
Image shrinks / breathes / shakes Dried-out caps (power/deflection) Recap
No image (HV present) Caps, cold joints, flyback Recap + reflow, test flyback
Blurry image / lack of focus Focus adjustment, aging flyback Adjust focus; flyback if dead
Washed-out colours / shrinkage G2 (screen), caps Adjust G2; recap
Intermittent cut-outs Cold joints (flyback, deflection, connectors) Reflow the suspect points
Lines / fold Deflection stage, caps Recap; deflection diagnosis

Procedure — working through a chassis ✅

Discharge the anode before anything else

The tube holds its charge with the cabinet unplugged, and so does the main filter capacitor. Discharge it, keep one hand in your pocket, and do not work on it alone. → CRT safety

Hot chassis: the ground is not earth

Some arcade chassis are referenced to the mains. On those, chassis ground is not earth, and an earthed oscilloscope clipped to it makes a dead short through your probe. Work through an isolation transformer. → CRT safety

1. Cold, before switching on. Cracked solder joints are the fault on these chassis — around the connectors, the flyback and anything heavy enough to work its own joints loose. Also swollen capacitors, cooked resistors, the brown halo of something that has been running hot. Reflow what looks suspect before diagnosing: a good share of intermittent chassis faults end there.

2. Fuse and primary side. A fuse that goes again is a short downstream, not a fuse problem — find it before fitting another.

3. Is there EHT at all? The tick of the flyback at switch-on, the filament glowing in the neck. No EHT points at the supply or the line stage. EHT but no picture points at video or the G2 — two very different jobs, and this is the measurement that separates them.

4. Recap — before any adjustment. This page carries the capacitor list — take the one matching your exact chassis if it covers more than one. On a chassis of this age recap is not optional: the electrolytics are thirty years old and have been running hot since day one. → Recap

5. Adjustments, and only now. ⚠️ Never before the recap. Setting the geometry of a chassis whose capacitors are dying records the drift and makes you do the whole job twice. Write down where every pot was before you touch it. → CRT safety

6. Scan rate. This page gives: 15 / 24 kHz (auto-sync). A board whose rate is not one of these shows nothing, and nothing is faulty — check that before you suspect the chassis.

7. Verification. Let it run warm for a good while and check the geometry stops moving. A chassis that is right cold and wrong after twenty minutes is still a thermal fault — usually a joint, sometimes a capacitor you left in.

Recap (the main repair)

The remedy for most faults on this chassis.

  1. Unplug, discharge the anode, remove the chassis.
  2. Photograph before removal (polarity/orientation).
  3. Replace the electrolytic capacitors with quality 105 °C equivalents, low ESR for the switch-mode power. Respect µF, V ≥ original, polarity.
  4. Check / reflow the joints around the flyback, deflection and connectors (classic hot spots).
  5. Adjustments: focus, screen (G2), geometry via the chassis pots.

Capacitor list — MS8-26SU ✅

Source: arcademonitor.com. 30 capacitors (+ a separate 680 µF / 200 V filter, generally not replaced). Shopping list:

Value Voltage Qty
1 µF 50 V 6
1 µF 100 V 1
4.7 µF 100 V 1
10 µF 25 V 1
10 µF 50 V 1
10 µF 100 V 1
10 µF 250 V 1
22 µF 160 V 2
33 µF 16 V 1
33 µF 100 V 2
47 µF 16 V 2
47 µF 25 V 1
47 µF 50 V 1
100 µF 50 V 1
100 µF 200 V 1
120 µF 50 V 1
220 µF 35 V 2
220 µF 50 V 1
470 µF 16 V 2
470 µF 50 V 1

Check the sub-model

This list applies to the MS8-26SU (26"). The Astro City often uses an MS8-29 (29"); other sub-models (MS8-25…) differ. Confirm the exact sub-model of your chassis (seller kit / schematic).

Per-reference list (C332 → C913) — expand

NB = neck board, VB = vert board.

Ref µF V Ref µF V
C332 (NB) 10 250 C537 4.7 100
C401 1 50 C561 10 100
C420 (VB) 1 50 C563 1 50
C422 470 50 C570 22 160
C431 33 100 C572 47 16
C432 33 100 C573 470 16
C433 10 25 C574 10 50
C437 120 50 C576 470 16
C451 100 50 C579 1 50
C452 1 100 C906 100 200
C454 47 50 C908 47 25
C501 1 50 C909 22 160
C521 33 16 C910 220 50
C522 47 16 C912 220 35
C532 (VB) 1 50 C913 220 35

→ Alternative kits: Console5 · arcademonitor.com

Verification

Stable image, correct geometry, sharp focus, no hot cut-out, colours OK.

Pitfalls & common mistakes

  • Forgetting to discharge the tube (danger + hardware risk).
  • Reversing the polarity of an electrolytic.
  • Neglecting cold joints: a recap alone won't fix a mechanical break.

Sources & attribution

Review & corrections

How this space is used

Spotted a wrong value, an outdated procedure, a chip reference that does not match your board? Say so here, with what you observed (model, board revision, serial number, measurement). Every report is cross-checked against a source before anything changes — an unverifiable correction is published as “reported by …, not cross-checked” rather than silently applied.

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