Junghans Caliber J645 85 Form C

Junghans Caliber J645.85

Junghans Caliber J645 85 Form C

Caliber NumberJ645.85
Base Caliber
Ronda 3520.D
Size10 1/2”’ x 11 1/2”’
Battery Cell Number384 (1.5 V)
Claimed Battery Life54 months
FunctionsHours, minutes, central chronograph seconds, 30 minute counter, 12 hour counter, small seconds, date at 3:00
Swiss Parts (Made in Germany?)
Known Models
Junghans Form C (add yours in the comments)

The Junghans caliber J645.85 is a quartz movement found in Junghans Form C chronograph watches.

Junghans Ronda

This movement is heavily branded by Junghans with their caliber designation J645.85, but removing the caseback reveals an unmodified Ronda caliber 3520.D. Junghans does not even etch J645.85 on the movement.

Swiss Made VS Swiss Parts:

According to the Ronda website, there are two versions of the 3520.D base movement used as the caliber J645.85: a gold plated Swiss Made version and nickel plated Swiss Parts version. The Junghans J645.85 movement is clearly labeled Swiss Parts. Jungahns makes no mention of this being a Swiss Made movement, so the Swiss Parts version is acceptable for their watches being labeled Made in Germany across the bottom of the dials.

Power Saving Feature

The battery is claimed to last about 54 months in normal conditions, but if you are not going to wear your watch for a while, the J645.85 has a power saving mechanism when the stem is pulled out stem (aka hacking function). Reduction of battery consumption approximately 70%. It is not advisable to leave the chronograph second hand running because it will drain the battery faster and put more wear and tear on the gears.


According to Ronda, the caliber 3520.D should keep time within -10/+20 seconds per month with an operating temperature of 0 to 50 degrees Celsius.

How to change the battery:

The photos below are from a Swiss Made Ronda 3520.D but will help you to change the battery in your Form C chrono.

There are two screws holding the battery cover down. Carefully remove the two screws (don’t lose them!) and lift the battery cover off. Replace the battery with a 384 cell. Put the battery cover back on, do not fully tighten the first screw until the second screw is in. Make sure the orange insulator is in place.

Resetting the Chronograph to Zero:

After changing the battery, your chronograph seconds hand or subdial totalizers may not be not lined up to zero. You can use the following sequence to get everything lined up:

  1. Pull the crown out two clicks to time setting position (position III).
  2. Press button A (top chrono button) and B (bottom chrono button) together for at least 2 seconds.
  3. The chronograph seconds hand will spin around one full turn, this is how to know you’re in correction mode.
  4. Press button A to advance the main chrono hand forward until it gets lined up. Long press to make it advance faster, short press for one second intervals.
  5. Press button B to go to the next hand.
  6. Press button A until the hand is lined up.
  7. Press button B to advance to the next hand.
  8. Press button A until the hand is lined up.
  9. Press the crown back in to position I.

Note: In the unlikely situation that the hands still won’t line up, or if they are moving around, there is a chance the hands are loose and need pressed back on.

Stem Removal

The photos below are from a Swiss Made Ronda 3520.D but will help you to remove the crown and stem on your Junghans Form C watch.

To remove the crown/stem, find the hole with the arrow. While the crown is in position 0, pressing this with your tweezers or other tool will allow the stem be unlocked so you can pull it out. Do not press too hard.

Ronda Caliber 3520 Stem Removal

Replacement Price:

If you need to replace the caliber J645.85 in your Junghans Form C watch, you can use the base Ronda 3520.D movement. At the time of this post, replacement prices were found online as low as $43.95 (Swiss Made version).

Watches With This Caliber:

Additional Resources:

Caliber submitted by James Hamilton (@jimhamilton631)

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