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Friday, December 26, 2014

Beomaster 8000: Signals at IR Remote Receiver Test Points

After the remote receiver came alive again in the Beomaster 8000 that I am currently restoring, I decided to take some measurements at the test points for future reference. I thought this memo might come in handy the next time there are some problems in this area.

Below is the relevant section of the circuit diagram. The IR sensitive diode 6D27 changes its impedance when IR light impinges. This drives the damped 41kHz resonator formed by L1/C2/R1. IC1 drives the signal into the input of IC2 (TP5), where it is amplified. The signal emerges from the amplifier at pin 3 where it has a 12V amplitude (TP7). The 41kHz modulation is removed in the filter formed by R16/C27 and a cleaned up digital signal containing only the bits of the remote codes is forwarded by TR25. TR26 finally changes the signal to a 5V compatible signal via a pull-up on the processor board (PCB#9)

It is interesting to note that this entire circuit is nowadays integrated into the receiver package (example IRM3638), which has pretty much the same form factor as the simple IR diode D27 that is used in the Beomaster 8000. Amazing progress in a couple decades!

This is the table of remote codes from the service manual:

It is evident that all codes start on a 1, which represents the start signal for the transmission and its evaluation in the microcontroller. It is interesting to note here that the signal of the remote is fed into pin 24 of the 6500/1 microcontroller, which is on its port B. Only Port A is interrupt capable on this processor (S. A. Money: Microprocessor Data Book) and it is completely used for the keyboard readout. This suggests that the remote function is fully software implemented. Anyway, after this trip down the memory lane (I am a proud Commodore 64 veteran...;-), here are the oscilloscope signals that I measured while pressing continuously 'Volume Up' (10100111) on the Terminal:

TP5 (input to amplifier) - pretty strong 700 mV signal, which is no surprise, since the Terminal was about 20 cm away from the IR diode (note the 20ms timebase...the other two pics below were measured at 10ms):

TP7: Output of Amplifier (signal is still modulated, but amplitude is now ~12V):

TP8 (after removing the modulation and translation to 5V a clean digital signal emerges):

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