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This is a genuine ETA 7750-255 hour wheel (Article number 53), an original Swiss-made component for the ETA/Valjoux 7750 automatic chronograph movement. The hour wheel is responsible for driving the 12-hour totalizer subdial at the 6 o‘clock position, a critical part of the chronograph’s elapsed time recording function .
This brand new original part is essential for watchmakers restoring proper chronograph hour counter function when the original wheel becomes worn or damaged.
The hour wheel is located on the dial side of the movement and is part of the chronograph recording mechanism.
Signs of Hour Wheel Wear or Damage:
| Symptom | Possible Cause |
|---|---|
| Hour totalizer runs continuously even when chronograph is stopped | Worn hour counter lock (8690) or disengaged hammer spring |
| Hour hand does not reset to zero correctly | Worn hour wheel or damaged hammer/cam engagement |
| Hour hand resets to different positions each time | Burr on plate or loose hand rivet |
| Hour totalizer does not advance | Worn hour wheel teeth or slipping clutch failure |
| Unusual noise during chronograph reset | Damaged hour wheel or hammer components |
Proper Handling Tips:
Use non-metallic tweezers when handling the hour wheel
Keep pivot ends clean and free of old oil
The hour counter lock spring is known to be a weak point – handle with care
When servicing, inspect the hour hammer spring for proper tension and positioning
Known Issue: Hour Totalizer Runs Continuously
A common problem with the 7750 is the hour totalizer continuing to run even after the chronograph is stopped. This is typically caused by the hour counter lock (8690) not engaging properly, often due to the hour hammer spring being out of position or having weak tension . In some cases, the spring may have flipped over or under one of the levers and needs to be repositioned .
CRITICAL WARNING: The ETA 7750 is a complex chronograph movement. Hour wheel replacement requires complete disassembly of the chronograph module. Professional installation is strongly recommended.
Tools Required:
Non-metallic tweezers
Screwdrivers (1.20mm, 1.40mm, 1.60mm)
Hand removal tools
Movement holder
Loupe or microscope (10x recommended)
Rodico cleaning putty
Appropriate watch oils (Moebius HP 1300, 9010)
Step-by-Step Installation (Professional Reference):
Step 1: Movement Preparation
Remove the movement from the watch case
Remove the hands (use extreme care – chronograph hands are often pressed on very tightly)
Remove the dial
Remove the calendar plate to access the chronograph mechanism
Step 2: Remove the Old Hour Wheel
Locate the hour wheel in the chronograph module (typically under a bridge near the 6 o‘clock position)
Remove the hour hammer spring and hour counter lock
Using non-metallic tweezers, carefully lift the old hour wheel from its pivot
Note the orientation for reinstallation
Step 3: Inspect and Clean
Inspect the hour counter lock (8690) for wear – this component is critical for stopping the hour wheel
Inspect the hour hammer (8689) and its spring for damage or loss of tension
Clean the pivot area using Rodico to remove old oil and debris
Check the plate hole for burrs that could affect reset positioning
Step 4: Install the New Hour Wheel
Position the new genuine ETA 7750-255 hour wheel onto its pivot
Ensure the wheel sits flush and engages properly with the drive mechanism
The wheel should rotate freely but with minimal play
The slipping clutch should allow the gear to turn while the hand post remains stationary when locked
Step 5: Reinstall Locking Components
Reinstall the hour counter lock (8690) – ensure it engages correctly with the hour wheel
Reinstall the hour hammer (8689) and hour hammer spring
Ensure the spring is correctly seated – this is a common point of failure
Step 6: Lubrication
Apply a very light amount of Moebius HP 1300 to the cam/hammer engagement surfaces
Avoid over-lubrication – excess oil can cause the hammer to stick
The slipping clutch requires no additional lubrication
Step 7: Reassembly and Testing
Reinstall the calendar plate, dial, and hands
Test chronograph start: start the chronograph and let it run for 10 minutes
Verify the hour totalizer advances correctly
Test chronograph stop: stop the chronograph – the hour totalizer should stop immediately
Test reset: press the reset pusher – the hour hand should return to zero cleanly
Repeat the test cycle multiple times to verify consistent reset positioning
Important Notes:
The hour wheel works in conjunction with the hour counter lock (8690) and hour hammer (8689)
The hour hammer spring is a known weak point – inspect carefully
Do not over-tighten the hour hand – this can bend the hour wheel pivot
Professional installation is strongly recommended for those without chronograph service experience
Common Symptoms of Hour Wheel Issues:
| Symptom | Possible Cause | Solution |
|---|---|---|
| Hour totalizer runs continuously when chronograph is stopped | Hour counter lock (8690) not engaging or hammer spring out of position | Reposition spring or replace lock |
| Hour hand does not reset to zero | Worn hour wheel or damaged hammer/cam | Replace hour wheel; inspect hammer |
| Hour hand resets to different positions | Burr on plate or loose hand rivet | Deburr plate or tighten/replace hand |
| Hour totalizer does not advance | Worn hour wheel teeth or slipping clutch failure | Replace hour wheel |
| Hour hand moves during normal running | Lock not engaging correctly | Inspect lock and hammer spring |
Diagnostic Process:
Visual Inspection: Under magnification, examine the hour wheel teeth for wear, chipping, or deformation. Check the slipping clutch for proper function.
Lock Engagement Test: Manually engage the hour counter lock to verify it stops the hour wheel correctly.
Reset Test: With the dial removed, operate the reset pusher while observing the hour hammer action. The hammer should strike the cam cleanly and return to its resting position.
Consistency Test: Reset the chronograph multiple times and note whether the hour hand returns to the same zero position each time .
When Replacement is Necessary:
Visible tooth damage or wear on the hour wheel
The slipping clutch no longer functions correctly
The wheel does not engage properly with adjacent components
The movement has been in service for 5+ years and chronograph function has degraded
About the 7750 Hour Recording System | 关于 7750 小时记录系统
The 7750‘s hour recording mechanism is known to have some design sensitivities :
The hour hammer spring can flip over or come out of position during assembly or operation
The hour counter lock must engage precisely; wear can cause incomplete stopping
The slipping clutch in the hour wheel allows the gear train to continue running while the hand is stopped
Preventive Maintenance:
Service the movement every 3-5 years
During service, inspect the hour hammer spring for proper tension and positioning
Clean the chronograph module components carefully – do not clean plastic parts with solvents
Use proper lubrication (HP 1300 on cam/hammer surfaces, not on the slipping clutch)
Replace the hour wheel and lock as a set if both show wear
When to Seek Professional Help:
You lack experience with chronograph module disassembly
Multiple components show signs of wear
The movement has other timing issues
You do not have proper tools for chronograph assembly
The hour hand reset position is inconsistent after reassembly
Q1: Is this a genuine ETA part?
A: Yes. This is a genuine ETA 7750-255 hour wheel, corresponding to article number 53. ETA parts are manufactured to original specifications for reliable fit and performance.
Q2: Why does my hour totalizer keep running even when the chronograph is stopped?
A: This is a known issue with the 7750 movement. It is typically caused by the hour counter lock (8690) not engaging properly, often because the hour hammer spring is out of position or has weak tension . In many cases, the spring may have flipped over or under one of the levers and needs to be repositioned . This requires disassembly of the chronograph module to access and correct.
Q3: What other parts should I inspect when replacing the hour wheel?
A: When replacing the hour wheel, inspect the hour counter lock (8690) and hour hammer (8689) for wear. Also check the hour hammer spring for proper tension and positioning – this is a known weak point . The plate hole where the hour wheel sits should be checked for burrs that could affect reset positioning .
Q4: Can I install this part myself?
A: This part requires complete disassembly of the chronograph module. The ETA 7750 is a complex movement with many interacting components. The chronograph module contains small springs that can easily fly away during disassembly . Professional installation by a qualified watchmaker is strongly recommended. Improper installation can damage the chronograph mechanism or affect timing accuracy.
Q5: What is the slipping clutch in the hour wheel?
A: The hour wheel has a built-in slipping clutch (friction mechanism). This allows the gear that is driven by the barrel to continue turning while the hour hand post remains stationary when the chronograph is stopped. The hour counter lock engages to stop the hand post, but the gear continues to turn via the slipping clutch. This design prevents stress on the gear train when the chronograph is not running .
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