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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 23rd Feb 2018, 9:31 pm   #1
Chris 1155
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Default Yaesu FT790r

Hi,
Does anyone have a good circuit diagram for the FT790r UHF transceiver?
I am trying to identify the deviation adjustment pot in order to inject a CTCSS signal.
This is a follow on from my successful modification of my FT290r.
Any help on this project would be appreciated.
Regards
Chris G4BYZ
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Old 24th Feb 2018, 12:56 am   #2
Julesomega
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Default Re: Yaesu FT790r

This seems to be the relevant part of the schematics. VR07 looks like the deviation setting, but I don't have any service information
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Old 24th Feb 2018, 11:39 am   #3
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Default Re: Yaesu FT790r

Yes, I think you have it right. Bear in mind however that your diagram is for an FT290R2, which is the later 'restyled' version. There may be some circuit and component numbering differences.

The output from Q24(c) looks to be deviating the frequency of X01 by varying the capacitance of varicap diode D14. The circuit to the right of VR07 looks like an audio amplifier / filter chain with a 'mute' transistor Q23, most likely used to make sure the crystal is only wobbled in FM TX mode and not at any other time.

To be sure, just mark or measure (with a resistance meter) the exact current position of the preset and then vary the setting while transmitting (on FM) to another radio. The transmitted audio should get louder (and eventually distorted) going one way, and quieter the other way. Then set the preset back to exactly where it was before.

As you intend to use this with a repeater, you may actually have to reduce the deviation setting eventually. The radio was made when the standard channel spacing was 25Khz, and the deviation was set accordingly. Since then we've gone to 12.5Khz channel spacing and many radio receivers have tighter filters in their receivers, including those used in repeater equipment.
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Old 24th Feb 2018, 12:08 pm   #4
Andrew2
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Default Re: Yaesu FT790r

I knew I had it somewhere! Here's the layout of the 790's main PCB. I've arrowed what seems to be VR1007. It's the one at the corner of the helical filter block. Hope it's good enough.
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Old 24th Feb 2018, 6:48 pm   #5
Chris 1155
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Default Re: Yaesu FT790r

Thanks for the replies and the information.
I have located the deviation pot and will proceed with caution. The FT790 that I have is unmolested and I do not want to do any damage.
The information on the deviation adjustment is very useful and I will also check this on my FT290 which I have fitted a CTCSS board to and seems to work very well.
Any more advice gratefully accepted and I will keep you all posted.
Chris G4BYZ
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Old 24th Feb 2018, 9:04 pm   #6
SiriusHardware
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Default Re: Yaesu FT790r

If the deviation on your FT290 is untouched since it was made then in theory the deviation is set too high for the receivers of repeaters conforming to the current 12.5Khz spaced standard.

If this was actually the case, your signal transmitted to the repeater's input (and retransmitted by the repeater) would sound very loud and / or distorted to other users of the repeater. If it is not the case then leave well alone: If it isn't broken, don't fix it.

Same goes for the 790 - if people say it sounds fine on-air, including when using it through repeaters, leave the deviation set as it is.
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Old 12th Mar 2018, 5:45 pm   #7
Chris 1155
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Default Re: Yaesu FT790r

Thanks to all your help, I have completed the installation of a CTCSS board on the FT790. I did not attempt to install the board within the case as there was very little room. Because of the varying offset frequencies for 70cm repeaters I am using the Memory Split Function to provide the required shift.
I managed to locate the wiper on the deviation pot and connected the code output to this.
It all seems to work very well and I have had good reports. As advised I have left the deviation setting well alone.
The board that I used is available from G8TMV colin@tuckley.org
Now all this is finished I have convinced myself that I would like a FT221, does anyone have one for disposal?
Regards to all,
Chris G4BYZ
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Old 12th Mar 2018, 7:51 pm   #8
Jon_G4MDC
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Default Re: Yaesu FT790r

Good news on that.
Watch for a good FT221R on Ebay, they do appear.

Beware there are several models. Some may have the MuTek front end fitted which is well regarded and worth extra.
The FT221 (no R) is the early one. Differences in the exciter (fewer tuned circuits) and not so flexible on repeater shifts - also older PA devices.

I like them very much but had to add a carrier squelch for AM & SSB. I couldn't be doing with it hissing away when no signal present. Original squelch was FM mode only.

They are worth a thread of their own!

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Old 12th Mar 2018, 8:20 pm   #9
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Default Re: Yaesu FT790r

The FT221(R) is a excellent piece of gear. If you do find the MuTek fitted as stated, so much the better.
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