I wonder if any one can help me, my FT847 stopped transmitting on LSB, USB and AM, FM is fine with Full 100 Watts on HF and 50 Watts on VHF. I traced the audio going in and it gets to Q1043 at TP 1035, there in no audio I removed Q 1063, and the audio is back, Q1063 is is a voltage controlled attenuator, as it has an OP Amp on its input, this could result in a "virtual earth" on pin2 and the signal is getting dropped across the preamp transistor, however there is nothing coming out of Pin 4 which goes off to the Balanced Modulator eventually. There is a good 455 KHz if signal present at the Balanced modulator.
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?The mic audio enters the radio via Transistor Q1043, then trough a cap into Q1063, on TP 1035 three was no audio! Rather odd. No Q1063 is an electronic controlled attenuator controlled by a voltage on pin 3, this voltage come from IC1093 pin 3. Now IC1093 is a strange device it received digital data from Q1105 the main Yaesu 100 pin microcontroller, as you turn the mic gain control up and down it sends a series of data to Q1053, and this turn send a voltage out to control the attenuator. To make matters more interesting the data is only a few bytes sent in less than a milli-second ?and only when you change the mic gain or select AM SSB, CW, of FM, the data is not continuous. My 2GB/S digital scope did capture the data, without it I would have been snuck!
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I do not have a decent copy of the FT847 service manual, all the ones on line are photocopied and low resolution and when you blow them up they are unreadable, however, Roger came to my rescue, as he had a genuine FT874 service supplement.
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Anyway this took 5 days to sus out, as I had no block diagram and no component over lay that I could read, so it took me up to 4 hours to trace each part and make measurement, some components were on the reverse side of the AF main board. To make matter worse, there are some undocumented mods on the back of the board close to the audio input attenuator, This digital transistor shorts out Pin 3 and 5 on IC1063, which increases the attenuator to Max, i.e. no AF signal leave pin 4 to the balanced modulator for SSB and FM.
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However, there should be a healthy signal on Pin 4 and there is none, as the voltage goes up and down on Pin 3 in sympathy with the Mic gain control, the inhibit is not active. ( I have no idea when the inhibit kicks in) I have enclosed the data sheet for IC 1093, by no means a simple device.
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Removing IC 1063 restores the audio to TP1035, :?
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I have ordered a new attenuator chip, and will solder it when it arrives fingers crossed! ( I am not holding my breath on this one)
any pointers in the right direction?would be nice?
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73
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Andrew ?
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Andrew Lenton