The counter used in the later FT-901 and in all 902 is acustommade LSI chip, made by OKI, the MSM-9520RS, whichgreatlysimplified the circuit, because the chip displayedautomatically theright carrier frequency, without recalibrating.The multiplexedoutput of the counter is displayed by 6 HP 5082-7623yellow LEDdisplays, the same as used in the later versions of theFT-101ZD,the FT-107M and others.
Custom Integrated Circuit (MSM9520RS) Replacement Module.pdf 1
Download: https://jinyurl.com/2vJtZi
The main problem of this circuit is, that by a failure of theMSM-9520RS chip, and that seems to happen rather often, there isnoreplacement chip available, and you can use the transceiveronlywith the (rather accurate) analog dial.You will not be happywith that,and fortunately there is a solution:A Japanese amateurTeruhiko Hayashi (JA2SVZ programmed a PICprocessor to do all thefunctions of the MSM-9520RS.He sells a drop-in kit with a smallboard that fits in the existingcounterboard, replacing the defectIC. In this Guide you find apage with a detailed description andthe address where the kit canbeordered: _e/index.html
"Pre-DSP/MPU era FT transceivers" fromYAESU often depend on acustom integratedcircuit (IC), MSM9520RS for their frequencydisplayfunction. For reason unknown,MSM9520RSs from early productionlotsdefinitively fail after certain period ofoperation. It has beenalmost 35 years sincethese FTs left the YAESU factory, manyofexisting FTs have to retire only because oftheir frequencydisplay failure.
This replacement module takes advantages ofthe flexibilities ofMPU (PIC 16F883) tosimulate the exact functionalities ofMSM9520RSand extends the life of FTs, greatmasterpieces of analog circuittechnology.
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