FT-901DM Mic amplifier circuit

[kb3ouk]

Last night Timtron and I were discussing this, I've done all his mods but we both came to the conclusion that there's something else in the transmit audio path that is causing problems, which we think is the mic amplifier.  

First of all, here's a link to the manual for the 901: http://bama.edebris.com/download/yaesu/ft901dm/FT-901DM_v6.pdf

Page 24 (PDF page 27) shows the carrier unit schematic (which contains the mic amplifier), page 27 (PDF page 30) describes the transmit audio path for SSB, and page 28 (PDF page 31) describes the transmit audio path for AM. Notice that they use two amplifiers, one for SSB, and one for AM. The circuit for the amplifiers themselves are the same, but the input for the AM amplifier is different. On SSB, the mic input goes into pin 2 of Q706 (TA7063P IC) through a .047uf cap (C727) and a 10k resistor (R731). On AM, we have a completely different circuit, which looks like some kind of filtering. The audio is fed into pin 2 of Q705 (TA7063P IC) but first it goes through R717, which is a 3.3k resistor. On either side of R717, there is a 240 pf cap to ground (C718 and C719). After this it goes through R718, which is a 1k resistor, and C720, which is a .047uf cap, after which it is at pin 2 of Q705. What is that extra stuff doing? The mic input to this thing is 600 ohm, so it seems to me like that 10K on the SSB side is too much. Should I make the AM side look more like the SSB side, by removing R717, C718, and C719, then switching around C720 and R718, so that the audio goes through the .047uf cap first then through the 1k resistor, which is the opposite of how it works now?

[kb3ouk]

My fix so far was to remove C718 and C719.

[WZ5Q]

Howdy,
Not knowing exactly the problems you are experiencing, I am assuming that you do not like the frequency response? There is allot of Frequency Limiting occurring in that circuit.

Without knowing what your target Frequency Response is and not being able to see what the actual frequency response of that stage is on an audio Spectrum analyzer, it is hard to know exactly what needs to be done. I can only guess at what component values need to be changed in order for you to gain some audio bandwidth.

The low frequencies are being limited by the low value of the .047uF coupling cap C720 which should be changed to .1uF to start. You might have to increase this later if there are not enough lows after the other mods.

Cap C739 and Resistor R713 are forming a High Pass Filter with a low frequency roll off of about 72cps, you could change this to a .1uF to lower the cut off to around 30cps.

The Q707 Bypass Cap of C747 should be increased to 10uF to improve the low frequency response.

The Q710 Emitter Bypass cap of C744 needs to be increased to 47uF. This should reduce the Low Frequency cut off to around 30cps or so.

To give the Mic Amp IC a little extra power reserve for the Low Frequency amplification, increase the 22uF Cap C722 to 100uF.

It looks like the High Frequency Response is being limited to around 5kc or so by the Q707 feedback circuit consisting of C723 and R723. Change C723 to 100pF for around a 10Kc roll off or 70pF for around a 15Kc roll off.

The components C718, C719, & R717 look like a Mic Input Pi Filter for RFI. This should not affect the audio bandwidth.

I wouldn't worry about the impedance of the mic to the input circuits. The mic is a low impedance feeding into a higher impedance which is just fine for audio input.

Hope this helps,