Hello guys,
Finding an M90 for 200 bucks is nice, but of course nothing works perfectly
If I switch on the boombox on FM, I have a little bit of reception, but that disappears slowly.
I have some noise, and therefore some signal, but not on the whole FM band. Half of the FM band is indeed silent.
Any "back and forth" on another band makes me lose reception on FM.
Touching a capacitor or any other ground point generates noise.
The first long job was to disassemble the switch strip and clean them one by one.
I did some measurements on the ICs and transistors.
Everything seems OK on transistors
But it's the ICs measurements that are the most interesting:
If everything is globally OK on the IC1 voltages, (except pin 15)), I noticed a surprising phenomenon: when I measure pin 8 of IC1, the reception is very partially restored, at low volume and mixed quality. This is followed by a slight increase in volume and then a slow decay to nothing.
At the end of less than 2 minutes, we return to silence until the next tickling of pin 8.
At the output of IC1, i.e. on pin 9, we keep a stable voltage around 2.4v. (This also corresponds to TP2)
However, just after C91, we attack the MPX on its pin 2 and there it is the disaster.
The VCC of the chip is stable but pins 2, 4, 5, 6,, 13, 14, 15 never have their nominal voltage and this one varies according to the low volume of sound in reception, the one consecutive to the miraculous measurement of pin 8 of the IC1.
The voltages therefore decrease irremediably, at the same time as the reception.
Thanks in advance for your help
Dan
Finding an M90 for 200 bucks is nice, but of course nothing works perfectly
If I switch on the boombox on FM, I have a little bit of reception, but that disappears slowly.
I have some noise, and therefore some signal, but not on the whole FM band. Half of the FM band is indeed silent.
Any "back and forth" on another band makes me lose reception on FM.
Touching a capacitor or any other ground point generates noise.
The first long job was to disassemble the switch strip and clean them one by one.
I did some measurements on the ICs and transistors.
Everything seems OK on transistors
Mesures Tuner M90
Feuille 1 IC1 FM,IC2 FM,E FM,C FM,B FM,IC1 AM,IC2 AM,E AM,C AM,B AM 1,1,84,5,2,1,66,5,02,2,35,1,1,25,0,02,0,0,02,0,02 2,1,84,1,7,0,6,5,1,34,2,1,25,0,3,0,0,02,0 3,5,2,1,13,4,5,0,3,8,3,0,02,0,0,02,0,0 4,1,86,1,13,0,0,04,0,4,2,0,0,-1,0 5,0,1,13,0,5,37,0,22,5,0,0,0,78,5,11,1,4 6,1,3,0,58,0,5,27,0,6,...
docs.google.com
But it's the ICs measurements that are the most interesting:
If everything is globally OK on the IC1 voltages, (except pin 15)), I noticed a surprising phenomenon: when I measure pin 8 of IC1, the reception is very partially restored, at low volume and mixed quality. This is followed by a slight increase in volume and then a slow decay to nothing.
At the end of less than 2 minutes, we return to silence until the next tickling of pin 8.
At the output of IC1, i.e. on pin 9, we keep a stable voltage around 2.4v. (This also corresponds to TP2)
However, just after C91, we attack the MPX on its pin 2 and there it is the disaster.
The VCC of the chip is stable but pins 2, 4, 5, 6,, 13, 14, 15 never have their nominal voltage and this one varies according to the low volume of sound in reception, the one consecutive to the miraculous measurement of pin 8 of the IC1.
The voltages therefore decrease irremediably, at the same time as the reception.
Thanks in advance for your help
Dan