The failure prone parts in the TF-12 are the optical encoder volume control, 1Meg resistor in the mute time out (located on the power supply board), the HER602 diodes in the power supply, 470 µF capacitors associated with these diodes, socket for 7805 voltage regulator IC on digital board (soldered in in later versions), and foil connections under tube sockets on the Teflon circuit board.
The failure of the optical encoder involves oxidation of the solder connections due to improper assembly by US Digital, in that the leads were cut too short. These controls can be pried open and these connections resoldered. Alternatively, a new control can be ordered from US Digital. http://www.usdigital.com Go to products tab, and then incremental encoders, and then look for the SP-16.
It is recommended that the HER602 diodes be replaced with the MUR820 (was Motorola, now ON Semiconductors), and that the 470µF capacitors be upgraded to 1000µF high-frequency types.
Loud pops, particularly in the phono section when the tubes are moved indicates broken foil on the circuit board. Thin foil traces should be bridged with light-gauge bare wire, even though the breaks can not be seen. Audio noise is sometimes caused by noisy 15K, 2W plate resistors on the 6DJ8/6922 tubes.
