The TD280 was sort of end of the era turntable for Thorens. While there are service manuals online for the MK2-4 I have not found after extensive searching the manual for MK1. This is problematic for trouble shooting a defective turntable as Thorens changed the test point voltages and circuit board layout - with several design changes - for the MK2 and above versions. This is a informational post for future AK'ers that may have to trouble shoot this turntable.
So, for a friend I started to work on a 280 MK1 that would not run at 33 - just "stutter". Adjusting the P2 33.3 pot had no effect. I then recaped, replaced the three IC's, and the eight pair gain transistors. This got the turntable running at 35.5 RPM at one extreme of the adjustment pot - anything else caused the "stuttering" to start again.
The problem (after a lot more trouble shooting) turned out to be the speed adjustment 100K pot had failed so that at one extreme the resistance was zero and the slightest turn caused it to jump to 1 mega ohm! Incidentally I did (multiple times) de-oxit the pot.
Not fun to figure this out with no schematic or service guide. In the end it has been worth the effort. My friend will be getting back a fully functional Thorens owned by his father. The lesson learned is pots can fail.
So, for a friend I started to work on a 280 MK1 that would not run at 33 - just "stutter". Adjusting the P2 33.3 pot had no effect. I then recaped, replaced the three IC's, and the eight pair gain transistors. This got the turntable running at 35.5 RPM at one extreme of the adjustment pot - anything else caused the "stuttering" to start again.
The problem (after a lot more trouble shooting) turned out to be the speed adjustment 100K pot had failed so that at one extreme the resistance was zero and the slightest turn caused it to jump to 1 mega ohm! Incidentally I did (multiple times) de-oxit the pot.
Not fun to figure this out with no schematic or service guide. In the end it has been worth the effort. My friend will be getting back a fully functional Thorens owned by his father. The lesson learned is pots can fail.
