Yaesu FT736 internal mic gain adj possible?

[GSBX1220]

Hi folks

I have obtained very cheaply a Racal Acoustics Jetgard headset. It has the dynamic mic insert.

I have been getting favourable unsolicited audio reports on HF with it when used with my FT890.

However, the mic does not seem to have enough oomph to drive my 736 on SSB. I need the mic gain up full and even at that point I don't think it's driving the radio correctly. On FM it seems ok with the mic gain around 3/4 from max.

Does anyone know of a way of making the SSB audio stage just a little more sensitive?

The 890 just need a little more mic gain with this headset to drive correctly so no issues there.

Cheers.

[Steve G4WCS]

some audio related mods here . nice radio

http://www.g4ztd.info/pdfs/Modifications.pdf

[GSBX1220]

Hi

Thanks for your reply. Yes, I done the C14 mod a while back, but that really only affects FM.

Cheers.

[orbanp1]

Hi Richard,

I do not have a unit like that, but looking at the "FT-736R_Tech_supp.pdf" manual (from the Fox Tango International web-site), the block diagram (pg. 52-53) shows Q4002 and Q4003 as the mic amp stages for the FM mode.
Looking at the schematics, Q02 is an NPN transistor based amp stage, Q03 is an LM324 based amplifier stage for the mic circuit.
There is an adjusting pot from the output of Q02 to the input of Q03, it is part of the "MIC/DRIVE" pot assembly?, 10kA, on the "VR A UNIT".
It is marked VR03, though it is not clear if that pot is part of those other ganged pots. Is it part of an operating control?
It can be seen on pg. 50 of the Tech_support manual, in the "Connection Diagram".
There is also another pot, at the output of Q03(b), VR01, that sets the audio level for the VCO.
I would try to adjust those pots.
Just in case, mark the original positions of those pots, if the above note is off base.

Please note that this is just by looking at the manual.

Regards, Peter

[GSBX1220]

Hi Peter

I 'think' VR03 may be the mic gain on the front panel. I think the pot VR01 is the deviation pot on the TX board. I have tried this, but as it is deviation, only has any effect on FM.

Thanks for the info. I think I'll pull out the diagrams and have another look.

I was hoping fir a simple fix, but I think it may be a case of changing components to alter the gain of the mic amp. I was reluctant to do that for the sake of a headset!

[orbanp1]

Hi Richard,

I guess I did not read carefully enough your note and missed the point that you need more drive only in the SSB mode!

Looking at the diagrams again, the SSB MIC AMP is the circuit made by Q4011 in the block diagram, Q11 in the schematics. It is an LM358 based dual opamp circuit.
It is the first stage, the (a) opamp that gives about 26.6dB gain, the second stage, the (b) circuit is basically a voltage follower.
If anywhere, the first stage is where you would increase the mic gain.
If you need more gain, you could increase R51, from 470k, that would leave the input impedance of the stage the same, which is basically the value of R50 (22k).

I would first verify if you indeed need more gain there or something else is amiss.

Regards, Peter

[SiriusHardware]

I have to confess I am no fan of modifying radios to work with non-original microphones, I always set them up with the stock / standard microphone and if anyone wants to use some other microphone, especially an active one, the microphone, not the radio, gets to be altered.

I'm assuming the standard microphone is to hand - if the audio levels on all modes are OK on that microphone then I would tend to try to dissuade you from tinkering with the radio and instead try to work out why the alternative microphone is giving insufficient drive.

Is it really only deficient on SSB? SSB is the only mode where audio drive is proportionately converted into RF output indication, so maybe you only notice it more on SSB.

[Jon_G4MDC]

FM Pye Westminster had this issue even with the standard miccrophone.
Usual trick was to add more emitter decoupling to first mic amp transistor.

In this case it has already been said it is an opamp. There is no harm boosting that 3-6dB as any excess when using the stock microphone can be taken out using the mic.drive control.

[GSBX1220]

Hi
Thanks for the replies. I have had a hectic week, so no chance to escape and play radio or look at this in any more detail.

For clarification, the stock mic drives the radio no problem.

Cheers.

[GSBX1220]

Hi folks.

Just an update. A few days back I carried out this mod. I increased R51 from 470K to 1M and this has done the trick.

Funnily enough, the stock microphone doesn't need the mic gain dropped any after carrying out this mod, so all good there too.

Thanks for the help.