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--- radios:ft-2980r [2025/05/12 21:08] – [Tap points] m0mzf
+++ radios:ft-2980r [2025/05/12 21:37] (current) – [WIP: Modifying a Yaesu FT-2980R 2m transciever for packet usage] m0mzf
@@ Line 5: / Line 5: @@
 The FT-2980R is a beast of a transceiver putting out up to 80W FM on 2m. From spending a little time using it for 2m simplex nets and whatnot it is an excellent radio; it's the best 2m FM receiver I own and (anecdotally) an S-point "better" when compared to my other radios (FT-8800R, FT-857D, FTM-300D etc). S1 stations which were unreadable on my other sets come through clearly on the 2980R as it "seems" more sensitive and selective; impact from local QRM is also much reduced. When running full power the case gets extremely hot very quickly and on a 30W rag-chew the case still becomes maximum-touch-hot - but for long runs below 20W it's very happy.
-The design is, all told, extremely simple and elegant. On the TX chain We have an analogue amplifier and filter for the microphone audio, and mixing in the CTCSS tone is a simple resistor network. This is then fed to a "clever DAC chip" Q1043 (which also generates various analogue control voltages for mic gain, tuning varactors etc, more on that chip later!) which controls the deviation, then out to a VCO. On RX we have a pair of crystal filters after the first IF to strip out unwanted mixer products, and two crystal filters on the second IF for wide (-6dB 12KHz) and narrow (-6dB 9KHz), an RC filter for de-emphasis and all that is fed directly into an audio amplifier to the speaker. There is plenty of other voodoo for DCS, decoding WX broadcasts and such but the bits we're interested in are just a lesson in how to design a nice radio.
+The design is, all told, extremely simple and elegant. On the TX chain We have an analogue amplifier and filter for the microphone audio, and mixing in the CTCSS tone is a simple resistor network. This is then fed to a "clever DAC chip" Q1043 (which also generates various analogue control voltages for mic gain, tuning varactors etc, more on that chip later!) which controls the deviation, then out to a VCO. On RX we have a pair of crystal filters after the first IF to strip out unwanted mixer products, and two crystal filters on the second IF for wide (-6dB 12KHz) and narrow (-6dB 9KHz), an RC filter for de-emphasis and all that, through various gain stages, is fed into an audio amplifier to the speaker. There is plenty of other voodoo for DCS, decoding WX broadcasts and such but the bits we're interested in are just a lesson in how to design a nice radio. Far as I can tell anyway, I am not an EE :)
 The service manual is excellent; alignment and power / deviation adjustments are made through the front panel in service mode; setting the power levels to 6dB increments (e.g. ~1/5/20/80W) or adjusting the power to give 25W ERP is simple. Using an IC-705 as a reference receiver the transmitted signal has no horrendous key-up transients (a la FT-1500M) and the wideband PLL phase noise is extremely low.
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   * Pin 9 of Q1028 is the output from the discriminator
   * Junction between C1122 and C1123 is de-emphasised audio before the RX-MUTE circuit
-  * Junction between C1276 and Q1003 is the DC-coupled input to the modulator (this doesn't work)
+  * Junction between C1276 and Q1043 is the DC-coupled input to the modulator (this doesn't work)
   * Pin 12 of Q1043 is the input to the multifunction DAC (this does work!!)
   * Tap on the MIC socket to then be processed / emphasised by Q1049 / Q1043 etc
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 =====Tap points=====
-This radio had me somewhat perplexed for a while when attempting to inject audio into the transmit path. My first effort was to tap into the output of our DAC chip, Q1043, in an attempt to drive the modulator directly, but this effort was fruitless; for some reason the transmitted audio had lost much of its high frequency components and was completely undecodable by the reference receiver! I can only assume there are some oddities to the output impedance of the DAC chip, and it was loading down the audio injected after it. Perhaps removing the AC coupling capacitor in between the DAC and the VCO would work, but I wanted to retain the ability to use the radio as intended.
+This radio had me somewhat perplexed for a while when attempting to inject audio into the transmit path. My first effort was to tap into the output of our DAC chip, Q1043, in an attempt to drive the modulator directly, but this effort was fruitless; for some reason the transmitted audio had lost much of its high frequency component and was completely undecodable by the reference receiver. I can only assume the output impedance of the of the DAC chip was loading down the audio injected after it. Perhaps removing the AC coupling capacitor in between the DAC and the VCO would work in an attempt to isolate the modulator circuit, but I wanted to retain the ability to use the radio as intended.
 Attempt 2 at injecting audio was completely successful; though I don't fully understand the role of the DAC chip it appears that it produces a variety of control voltages to operate various parts of the radio. It also appears to contain various VCA / buffer amplifiers; one of these is used to take the mixed audio / CTCSS signal and drive the VCO via a VCA / buffer in order to control the deviation. Tapping directly to the input of the DAC chip, after the point the pre-emphasised speech and CTCSS tone have been mixed, produced a lovely transmit signal. Success!
-The Yaesu schematic notes 1.5Vpp audio into this point in the circuit and it appears that the NinoTNC is capable of producing this, just, when the deviation pot is turned to maximum. With the pot at maximum a Bessel null appears with the 999Hz input tone for +/-2k4 deviation. I suspect that this input voltage could be set lower if the radio was changed from narrow to wide, but this action also selects the wide input filter - if more drive is required then a modification to the NinoTNC might be in order, but as it stands this is perfect.
+The Yaesu schematic notes 1.5Vpp audio into this point in the circuit and it appears that the NinoTNC is capable of producing this, just, when the deviation pot is turned to maximum. With the pot at maximum a Bessel null appears with the 999Hz input tone for +/-2k4 deviation. I suspect that this input voltage could be set lower if the radio was changed from narrow to wide, but this action also selects the wide input filter - if more drive is required then a modification to the NinoTNC might be in order but as it stands this is perfect.
-As there's potentially an awful lot of RF floating around inside the case I decided to use RG174 to tap both the audio inputs. It seems that Yaesu have gone to great lengths to shield and earth-bond this radio internally so it seems prudent to do the same. Receive audio is probably just fine but it may be a good idea to include a few pF from output to ground at the rear connector.
+The radio appears to take ~150msec to go into transmit from the time the PTT line is asserted; I'll update this time with an appropriate TXDELAY once I've had a more scientific fiddle.
+As there's potentially an awful lot of RF floating around inside the case I decided to use RG174 to tap both the audio inputs. It seems that Yaesu have gone to great lengths to shield and earth-bond this radio internally so it seems prudent to do the same. Receive audio seems just fine but it may be a good idea to include a few pF from output to ground at the rear connector.
 ====Main PCB underside=====