I had struggled to achieve a balanced TX audio frequency response (that I liked) due to the limitations of the TS940 and my higher pitched and somewhat nasally natural voice. As of now, after the mod's below I am very satisfied with my station sound!
The stock 940 will pass some very low frequency response with a compromise in the mid to high end. You can adjust the carrier points to remove some lows and increase the mid to high response but I have not been happy with just the carrier point adjustments. The stock filter set (8830K & 455K) provides about 2.4K total bandwidth and this is not really sufficient to pass or receive the high quality audio I desired. The mod's below along with my processing equipment have helped me achieve a more balanced smooth sound.
BALANCED MODULATOR MODIFICATION
I made this modification to the audio circuit in the TS 940 (thanks to Terry, W6RU) and it has improved the TX audio response dramatically! Before the modification I ran the output from my DBX gear into the phone patch input on the back of the 940. In the past I also had run the audio gear directly into the mic jack in front of rig. Both of these configurations achieved the exact same result (not a lot of clean low end and lacking mid to high response). It was much better than just running the mic without the processing gear but it still was not what I wanted so ... With Terry's help I located the balanced modulator diode on the IF PC board and applied the audio from my outboard processing equipment directly ... well almost directly.
I installed a small SPDT switch that I use to select the audio (stock mic vs outboard audio) to apply to the balanced modulator. The outboard audio source is appled to the - side of C206 on the IF board. I use a 22Mfd capacitor in the audio + to provide proper coupling. Click here to see a schematic of the hook up. This configuration completely bypasses two small audio amplifiers that are stock in the 940. It also bypasses the mic gain control. It appears to me that one or all of the bypassed components were restricting the TX audio response. Now... this modification is not for everyone ! If you use VOX it won't work without additional tinkering. The VOX circuit requires audio to be applied well before C206. For my application I was able to apply audio from the unused right output of my MicroVerb4 to the mic jack on front of rig. This audio allows the VOX circuit to work. Click here to see my audio connection map
The balanced modulator modification detailed above did improve my sound (in my opinion anyway ;-) but it still lacked sibilance ! The next modification I performed was to replace the stock 940 filters (both 8830 & 455) with stock TS930 filters. The TS940 uses somewhat narrow 2.4 kHz filters and this was affecting my TX & RX audio response. The stock 930 filter set is a bit wider (2.7 or 3.0 as I understand it). The audio is applied to the 455 filter just after the balanced modulator. From the 455K filter the audio is applied to the 8830 filter. Replacing the filters provides a bit wider TX & RX audio response. Please see the details of this change below!
As stated above, the stock TS-940 bandwidth for transmit & receive is 2.4kHz and this does not provide the frequency response (TX or RX) that I desired. So... I found (thanks to Zimbabwe, N4MXZ) that the stock filters used in the TS-930 were wider (each filter provides around 3kHz bandpass) than the stock TS-940 filter set. After some study of the frequency configuration for my rig I determined that both filters were affecting the TX & RX frequency response. I decided to replace both of the stock filters with the wider TS-930 filter set.
I completed the installation on 9/09/99 ! Luckily, one of the audio nuts from 14.178 (Jim, W6FZC) was on the air and he was the first station I had the opportunity to hear. I immediately noticed a huge improvement in the RX audio. I had never heard Jim sounding so good ! I made a quick contact and confirmed that things sounded different. My audio was a bit to heavy on the low end during the initial testing of the new filter set configuration. After I confirmed everything on the rig was still working properly I returned to 20M to do more testing with Terry (W6RU) & Jim (W6FZC). We tinkered with the carrier point but I decided that shifting it up decreased my very high quality low end RX (and TX) audio response. I ended up keeping the carrier balance pretty much the same as stock and reducing the low frequency EQ settings to remove the heavy bass TX response. Jim (W6FZC) analyzed my station after all the adjusting and reported a fairly flat response with total bandwidth of 2.874.
INRAD #475 & ICOM FL-257 FILTER SET INSTALLATION
11/29/2002: The stock 930 filterset installation described above does provide very nice audio response but it is still a tadd narrow. To get a bit wider bandwidth (around 3.6 + if you push it) the INRAD # 475 (8830kHZ 4000Hz) and the ICOM FL-257 (455kHz 3.300Hz) filters are the ticket. These filters are not drop in, you will need to modify the IF board as shown below but it is an easy "adjustment". Please note that you will need to adjust the carrier set points for both LSB and USB, this is easy if you have a signal generator. Click this link to see the procedure I used to adjust the carrier set points
My TX and RX frequency response results = around 11Hz to 3600kHz (50W out at 1.800 Hz, 12.5W out at 11Hz and 3600Hz). I can push it to around 3800kHz with the EQ. See more detail regarding this by following the link above (or click here: carrier point adjustments)
I worked with John (NU9N) on 160M and confirmed the response detailed above. Additionally, John listened for carrier and opposite sideband surpression problems. John's comment is that the 940 is performing at a level he considers to be the best possible considering it's analog. Very minimal opposite sideband bleed over and no carrier.
I mounted the Inrad 475 in the space on the IF board that the optional AM filter would go. In order to achieve this, I removed the 4pin connectors from the IF PC board. The filter then "screwed in". I used short shielded cable for the the "jumpers" to the IF board.
The ICOM FL-257 was mounted in the space on the IF board that the optional CW (455KHz) would go. To achieve this, I made a mounting plate out of cardboard. The connections are made via shielded jumper wires to the IF board. The grey wire above the filter is attached to C206. This is the point on the IF board where I apply the audio to the balanced modulator
Above is a photo of the finished IF board ready to install
The TS-940 receiver in stock form is not acceptable for ESSB or AM operation. The modifications below will dramatically improve the response and flatness of the receiver.
Many thanks to John, KI4NR and Mike WZ5Q for the help and guidance with these mods.
IF Board Mods:
Change C101 from .047uf to .001uf
Change C102 from .022uf to .01uf
Change C103 from 1uf to 4.7uf
Change C147 from .027uf to .001uf
Change C148 from .01uf to .1uf
Control Board Mods:
Change C157 from 200uf to 1000uf
Change C170 from .022uf to .001uf
Change C167 from .01uf to 4.7uf
Change C112 from 47uf to 100uf
Change C105 from 10uf to 47uf
Remove C106 (.001uf)
Change C123 from .1uf to 1uf
The spectral plots below were made after modifications:
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