Does anyone have a readable copy of a 1966 Letter from Hammerlund regarding slippage on the SP-600?
I found one on the web but its not readable.
http://www.hammarlund.info/info/sp-600/slippage/letter.jpg

Tuning Mechanism - It's common to find the tuning mechanism dirty. Remove the front panel along with the main tuning dial, the logging dial and the band indicator dial. Spray the brass gears with WD-40 and work the dirt out with an acid brush. WD-40 is cheap and a good de-greaser. When clean, lubricate only the bearings - sparingly. The brass gears really don't need lubrication and too much oil or grease are just "dust traps" that end up causing wear and other problems. Reinstall the dials and check the tuning action. It should be super-smooth. After cleaning, if the tuning slips, which is not uncommon, the problem is usually the "S" tensioning spring that loads the drive wheel against the rim of the logging dial. The "S" spring sometimes looses its "springiness" over time but you can remove the "S" spring and bend it outwards to increase the load tension when it is re-installed. Sometimes the "S" spring has just "had it" and it needs to be replaced with a new one (or a good one from a parts set.) Additionally, the contacts areas where the drive wheel engages the reduction wheel and where it engages the logging dial rim must be super clean. When everything is "right" with the tuning mechanism its operation is better than "velvet smooth" and there will be no slipping of the dial drive.

This is from another site, but seems to be the cure.

Removing the front panel and playing with the S spring are last resorts only. There may be an easier solution. I had two SP-600s with slipping dials and the method outlined below solved the problem both times.

Clean the two small brass tuning wheels behind the front panel. You can use Deoxit, rubbing alcohol or the cleaning agent/solvent of your choice. A Q-tip is helpful. Concentrate on the areas where those two wheels mate. Make sure the surfaces are super clean. Even if they look clean, they may still have dirt on them. Examine them with a magnifying glass to check for any dirt that you missed. Make sure you clean the groove on the top wheel. Clean these wheels until a clean Q tip no longer picks up any dirt. Most of the time, that will solve your slippage.

You can also buy rosin from a music store. Apply it to the mating surfaces of the two wheels. It will add some traction, at least for a while.

Don't mess with that S spring unless absolutely necessary. It is a pain to remove and an even greater pain to re-install. It tends to shoot across the room and hide in the damnedest places. If you lose it, you're screwed.

If you're going to lubricate the gears, make sure you position the chassis so the lubricant doesn't hit the two drive wheels. A stray drop of lubricant can, by itself, cause dial slippage. In fact, a prior owner's careless use of lubricant could be the cause of your current slippage.

Here is the site that Warren referred to: http://www.radioblvd.com/sp600_rebuild.htm It's an unbelievably good resource for the restoration of SP-600s.

I hope this helps.

Joe Connor