Digital Synthesizer Tuner
1981
The RT 300 is the largest and most luxurious tuner of Telefunken's 1981 lineup. It offers almost all features one would have expected for a good tuner at that, with the notable exception of the timer functionality found in the RT 200, and the analog tuning wheel found in the TT 750. It can however still be regarded as a direct successor of the TT 750, since the digital tuning system is almost the same, built around three chips custom-made for Telefunken.
The RT 300 was part of Telefunken's T 300 high-end component system, and it has a remote control input that allows the drive functions to be controlled via the RP 300 pre-amplifier.
FM/MW/LW, 40 program memory places, manual/automatic tuning, digital frequency input, LED field strength indicator, exact tuning indicator, fluorescent display, automatic muting via RP 300
(currently no documentation available)
Selen-based rectifiers were still commonplace in Telefunken devices at that time. Though it is generally known that such rectifiers have a finite lifetime, the B30C400 in the RT 300 is an especially sad story; from the five RT 300s I own, three had a broken rectifier when I received them, and on another one, the rectifier blew within the first hour of operation - immediately detectable by the disgusting smell of selen and sulfur. It is therefore my advise that whenever you get an RT 300, immediately replace the rectifier with a newer silicon-based one, regardless whether the original one still works or not! I have a device where a fried rectifier led to a failure in the power supply section and killed the custom-specific ICs in the digital tuning section. And opposed to a simple rectifier, these ICs are irreplacable these days :-(
The rectifier is a red, rectangular-shaped component between the regulator's copper heatsink and the large filter capacitors. The photo shows a sample of the rectifier left to the board, the rectifier has already been replaced in this unit. Finding a silicom-based replacement shouldn't be a problem, a rating of 40 volts and 300 milliamps is not very high for a silicon rectifier. The pinout will probably not fit 100%, but there is enough space to bend the wires appropriately.
Another problem is shared with the TT 750: dried-out capacitors in the digital tuning section. It consists of two boards held together by a metal frame, and there is few chance for the heat to escape from the section. Furthermore, a small light bulb is used as a current limiter, and this bulb burns from time to time...
(taken from Telefunken's 1981-1991 service handbook)
| ICs, Transistors, Diodes | ||
|---|---|---|
| Position | Order No. | Designation |
| 309 368 312 | microprocessor DFC103 | |
| IC 170 | 309 368 133 | IC TDA 1046 |
| IC 704 | 309 368 283 | IC LM 324 |
| IC 707 | 309 368 311 | microprocessor DFC103A |
| IC 709 | 309 368 240 | IC ER 1400, DIL case |
| IC 703 | 309 368 284 | IC CA 3080 E |
| T 700/701 | 309 001 933 | transistor BF 241 |
| GR 501 | 309 320 928 | rectifier B 30 C 400 A |
| D 717 | 309 325 114 | diode BZX 85 C 6 V 8 |
| Miscellaneous Parts | ||
| Position | Order No. | Designation |
| 309 395 040 | LED display, 10-fold |
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