Bert Beukema
Well-Known Member
Hi guys,
This 750 has been in my possession for a long time, restored. This week I installed it in my wife's office and I realised that I had missed a feature. For all this time I had the muting feature off (mute button is pressed). When I disengage the button, complete silence greets me. The signal is coming in very strong, there is a signal tower near by. The signal meter is at it's highest point; the tuning meter doesn't work (not moving).
I checked all voltages in suspect parts of the circuit. The muting board (this is a separate board, it doesn't use the IC muting feature but bypasses it) is receiving the correct voltages. The HA1196 is not on point for pins 4/5/6/7 (roughly 2 volts too low). Schema doesn't say which voltages would be acceptable for the transistors on the muting board though. Diodes all check out fine, normal transistors also fine.
The fact that there is a complete audio dropout when the muting circuit is engaged is worrisome. The tuning meter could possible be an alignment issue and solved by working on the T6 coil. But the complete dropout makes me suspect either HA1201 (discriminator), HA1196 or something else in the circuit. I hope it isn't the filter on the muting board as that one is both unobtanium and difficult to check as the board is soldered in sideways.
Has anyone encountered this issue before working on the SX750 or it's higher level brethren?
Thanks,
Bert
Ps: I've added part of the schema.
This 750 has been in my possession for a long time, restored. This week I installed it in my wife's office and I realised that I had missed a feature. For all this time I had the muting feature off (mute button is pressed). When I disengage the button, complete silence greets me. The signal is coming in very strong, there is a signal tower near by. The signal meter is at it's highest point; the tuning meter doesn't work (not moving).
I checked all voltages in suspect parts of the circuit. The muting board (this is a separate board, it doesn't use the IC muting feature but bypasses it) is receiving the correct voltages. The HA1196 is not on point for pins 4/5/6/7 (roughly 2 volts too low). Schema doesn't say which voltages would be acceptable for the transistors on the muting board though. Diodes all check out fine, normal transistors also fine.
The fact that there is a complete audio dropout when the muting circuit is engaged is worrisome. The tuning meter could possible be an alignment issue and solved by working on the T6 coil. But the complete dropout makes me suspect either HA1201 (discriminator), HA1196 or something else in the circuit. I hope it isn't the filter on the muting board as that one is both unobtanium and difficult to check as the board is soldered in sideways.
Has anyone encountered this issue before working on the SX750 or it's higher level brethren?
Thanks,
Bert
Ps: I've added part of the schema.