VP7631T stereocoder National

stereo coder

I have a stereo coder VP7631T

apart from classic FM stereo it can be used for VerkehrsWarnfunk (predecessor of RDS used in car rradio).

Works fine apart apart from the audio sine oscillator which delvers a clipped signal.
Is more tapezium shaped as sinewave output.
I think it has to do with a regulation system on board that uses a photocell-lamp combination. Is a Japanese type Moririca MCL-713B.
The oscillator itself uses a NEC C55A chip. Unable to find any data of both things.

Is there anybody around having a schematic of the VP7631T coder from National or from the Moririca / NEC devices mentioned above?

Kind regards

from Mike


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I encountered exactly this problem, and am missing the manual and schematics too.
The MCL-713B is a photoresistor-lamp combination. I just repaired my set by opening the lamp-LDR device at the lamp side, and removed the small broken lamp. Replaced it by a high brilliance LED Avogado HLMP-CM26 of green color. Put a resistor of 680 Ohms in series and check the polarity of the voltage to the lamp first. It is a DC voltage that varies between 0 and 20 volts. Their is a trimpotmeter on the AF signal board mentioning "adjust 7 volts": turn this potmeter till the generator produces a signal at 1 KHz for a modulation level of 100% all potmeters at the front panel turned to max..I have put the trimpotmeter "Output voltage adjust" at the AF board to max.. The voltage supplied to the LED-680 Ohm combination is 6 volts at 1 KHz , 8 volts at 10 KHz in my case. So there is still enough "headroom" for the circuit to provide the necessary negative feedback. Distortion of the AF generator measured around .12% at all frequencies( 400-1KHz-6.3 and 10KHz).