philips: SBC 521; AF/RF Oscillator

ID: 360723
This article refers to the model: AF/RF Oscillator SBC 521 (Philips Belgium (Belgien))

philips: SBC 521; AF/RF Oscillator 
23.Oct.14 13:23
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John Sykes (GB)
Articles: 13
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John Sykes

Recently fixed one of these; unable to find any support documentation or schematic. 

Internally there are two single-sided pcb's: the top one is mainly the analogue circuitry+ PSU regulators, the lower digital + reference crystal + back-up memory battery.

The VCO is based around a Motorola chip type MC1648P, tuned using a BB212 varactor diode. The frequency ranges (9) are selected by a bank of inductors, individually switched by reed-relays. Note that one end of every reed relay and each inductor, ( including the inductor metal screening can) is permanently connected to a reference bias voltage pin on the MC1648, (circa +1.6V).

I found one electrolytic capacitor (1u5 @ 63V)  inserted with the wrong polarity (manufacturing error). One reed had leaked air and was not switching correctly.  

Main regulated DC power-supplies are +/-15V and +5V. The mains transformer is permanently powered, the front panel ON/OFF is a four-way toggle which switches the low-voltage ac secondaries of the transformer.  

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