Beomaster 1100

28th April 2013

This receiver suffered from low sensitivity on the FM band, with only strong stations being able to activate the stereo decoder and weak ones vanishing altogether. 2IC1 (TBA460) can be responsible for these symptoms but a replacement made no difference, so low output from the front end unit was suspected. On investigation, 1TR1 and 1TR2 (TIS88A) proved to be defective. Since these transistors are both fragile and difficult to obtain the decision was made to replace both of them with a single more modern dual-gate device, in this case a BF998 (as used in the Beomaster 5500, Beocenter 9000 and derrived types). With 1TR1 and 1TR2 removed, the BF998 was mounted on the print-side of PCB1, elevated from the surface by attaching the source connection (the largest of the tabs) to the non-grounded end of 1C4. Short lengths of fine wire were then used to connect the drain to the junction of 1C5, 1C6 and 1L2, the first gate (G1) to the junction of 1C1, 1D1 and 1L1 and the second gate (G2) to the junction of 1C3 and 1R2. Since the gain of the BF998 is much greater than the pair of TIS88As it is also necessary to change the value of 1R3 from 100R to 2K2 to avoid instability.

The modification proved successful but having completed it, it is advantageous to trim 1L1, 1C1, 1L2 and 1C6. This can be done without an RF generator if a strong aerial signal from a properly matched source is availalbe. The procedure is to connect a DC voltmeter to the AGC terminal of the front end unit (e.g. at 0C3) and to make the following adjustments using an appropriate non-metallic trimming tool, do not adjust any other items:

1) Tune to a strong signal around 88 MHz and adjust 1L1 for a minimum reading on the meter

2) Tune to a strong signal around 104 MHz and adjust 1C1 for a minimum reading on the meter

3) Repeat these steps a few times for optimum results

4) Tune to a strong signal around 88 MHz and adjust 1L2 for a minimum reading on the meter

5) Tune to a strong signal around 104 MHz and adjust 1C1 for a minimum reading on the meter

6) Repeat these steps a few times for optimum results

If executed with care, this repair yields results which are at least as good as those obtainable from the original design, which was known to be of exceptional quality. Similar front end units can be found in many of the Beomaster and Beocenter models of the 1970s, typically those with pre-set stations on FM. Different component references will be found on the circuit diagrams for the other models, but otherwise this modification is applicable to all similar types. [Beomaster 1100]