So, last week I experimented with modifying the audio input circuits on my TS
-850 to improve the frequency response. The results were pretty good, so I'm posting what I did here.
Note: the end result is similar to what can be accomplished with the mods shown here: Kenwood TS-850S AM Modifications
That mod recommends drilling a hole in the back of the rig to mount an extra audio input jack though, and it bypasses the VOX circuit. My mod doesn't require you to drill a hole and preserves all of the radio functionality.
For reference, here is a copy of the TS
-850 service manual (which you'll need to identify the capacitors that need to be changed on the IF unit): http://people.freebsd.org/~wpaul/ts-...nd%20notes.pdf
Improving high end frequency response
Part 1: program the radio so you can change filter settings during TX
Normally, the radio only lets you change filter settings when receiving. During transmit, the computer selects some preset filtering configurations and doesn't let you change them. However there is an option in the extended service menu that allows you to change the filter set during transmit too. The following instructions show how to turn it on.
1) Turn the radio's power supply on, but keep the radio off
2) Press and hold the SCAN key and the TX
M.CH key, while turning the radio
on (there are two M.CH keys, one for RX
and one for TX
; only the one for TX
will allow you to access the extended option menu)
3) You should see a 4-digit hexadecimal value in the frequency display -- this
is the ROM checksum. You will also see the number 00 to the left of the
display, in the memory channel selection window. This number indicates
the configuration option.
4) Rotate the small encoder knob to select option 01. The word "off" should
become visible in the display.
5) Press the 1Mhz step up [^] key -- the radio should beep and the display
should now say "on"
6) Press the CLR key on the number keypad to save the settings and exit the
menu. The radio should now be back to normal operation.
The setting will be saved. You don't need to repeat it again, unless you take out the back-up battery or perform a full CPU reset.
Part 2: install a jumper wire in the slot for the 500Hz CW filter (455Khz IF)
1) Turn the radio over
2) Remove the two screws from the filter access panel, and open it
3) Find the slot for the optional 455Khz IF CW filter
4) Place a jumper wire from pin 2 on one end of the filter connector to pin 2
on the other
5) Replace the access panel and screws
6) Turn the radio right-side up again
7) Open the little door at the left rear corner of the radio
8) Find the small DIP switch block
9) Set switch 1 to ON -- this will enable the option 500Hz filter selection for
the 455Khz IF
9) Close the door
Click below for an image that shows the jumper wire installed: http://www.cleanrf.com/ka0ka/Images/silver_wire_mod.jpg
To obtain the widest frequency response, key the radio up and press the button for the 8.83Mhz filter selection until all filter option lights are off (this is the bypass setting), and press the button for the 455Khz filter selection until it the '500' indicator is lit.
Improving low end frequency response
There are 8 capacitors that need to be replaced/increased:
IF unit (X48-3080-00)
1) Change C144 from .1uF to 10uF (+ side to pin 1 of the AN612)
2) Change C197 from 1uF to 10uF (+ side to base of Q32)
3) Change C198 from 1uF to 10uF (+ side to collector of Q32)
4) Change C145 from 47uF to 470uF
mic amp unit (mounted on IF unit) (X48-3850-00)
1) Change C4 from 1uF to 10uF (+ side to base of Q2)
2) Change C6 from 1uF to 10uF (+ side to collector of Q2)
mic switch unit (mounted on IF unit) (X48-3840-00)
1) Change C1 from 1uF to 10uF (- side to pin 3)
2) Change C3 from 1uF to 10uF (- side to pin 8)
All of these are on the IF unit, which is on the bottom of the radio. You will need to pull the IF unit out in order to do these mods. This can take you a while as there are a lot of cable harnesses and coaxes plugged into it. I suggest taking a photo of the board before you take it apart so that you can see where to put everything back when you're done. A pair of needle nose pliers will likely be needed to get some of the connectors out. Go slowly and carefully so as not to damage any of the cabling.
The "mic switch" and "mic amp" circuits are on separate little boards mounted on the IF unit. They're at right angle of the IF unit board, which can make soldering on them tricky. If you want to remove them from the IF unit, you can, but this will take longer. C144, C197 and C198 are on the bottom of the IF unit, so you need to find capacitors small enough to fit there.
Feeding audio into the rig
I feed the output of my audio equipment into the radio via the ACC2 port. The above mod changes the coupling caps on the ACC2 input amp (C197 and C198) so that it should have very good response. The ACC2 port requires a 13-pin DIN connector. You can get those here: CONN CIRCULAR DIN 13POS MALE - SD-130
The audio line (tip) goes to pin 11 and the shield (ring) goes to pin 12.
Lastly, I use a Radial Engineering J-ISO for isolation, impedance matching and balanced to unbalanced conversion. This unit is designed to allow connecting low impedance pro audio gear to the line level inputs on consumer equipment like iPods or computers. The J-ISO has the same Jensen transformers recommended in the original AM
mod as shown on the KA0KA site. (These are also the same transformers used in "voodoo modified" Kenwood TS
I got mine here: Radial Engineering J-ISO Jensen Transformer Equipped Stereo Isolator +4dB to -10dB Converter Standard and more Direct Boxes at GuitarCenter.com.
The instruction manual/spec sheet for it is here: http://www.radialeng.com/pdfs/manual-j-iso-pro-iso.pdf
Unfortunately, the J-ISO is a bit expensive. There is a cheaper version called the Pro-ISO which is the same except it uses transformers made by Radial Engineering. The Jensen ones have better response and lower distortion specs, but the Radial Engineering ones aren't bad. You can get that one here: Radial Engineering PRO ISO Stereo Line Isolator +4dB to -10dB with Radial Transformer Standard and more Direct Boxes at GuitarCenter.com.
The J-ISO/Pro-ISO has several different output connectors. I had some spare 1/4" unbalanced audio patch cables lying around, so I just chopped the end off one and soldered the 13-pin DIN connector to it, as described above.
Operating the radio
Once you have your audio gear hooked up, engage the transmitter and set your carrier for 25 watts. While transmitting, press the filter selection buttons so that the 8.83Mhz filter is bypassed (all lights off) and the 455Khz filter is set to '500' to select the slot with the jumper wire. (Note that normally the radio sets both filters to '6k.') Adjust the mic gain so that you have forward modulation with little or no indication on the ALC meter. If you see too much ALC meter activity, this means you have the mic gain up too high and are over-modulating. This can be difficult to see without a scope, so it's recommended that you use one. The optional Kenwood SM-230 station monitor is ideal for this. (If you're lucky enough to have one.)
The resulting frequency response is from 20Hz to 10000Hz. This means you'll be transmitting a 20Khz wide AM
signal. Note that this is *without* the optional DSP-100 unit. (It's possible to get the radio to transmit in HiFi mode with the DSP-100, but the response will be about 6Khz. With this mod, the response is actually better.)
NOTE: Unless you have the optional Kenwood DSP-100 with your radio, you should *NOT* operate the radio in SSB
mode when you have the 8.83Mhz and 455Khz TX
filters set to bypass. You won't break the radio, but Kenwood uses the crystal filters to suppress the alternate sideband when operating in SSB
mode. This means if you transmit with all the filters off, you'll actually be sending in double-sideband mode (i.e. transmitting on USB and LSB at the same time). For SSB
mode, make sure you have both the 8.83Mhz and 455Khz IF filters set to "2.7" for proper operation.
If you do have the DSP-100 unit, then the alternate sideband suppression is done inside the DSP unit itself, so it's okay to run with the crystal filters bypassed. If you set up the DSP-100 correctly, this will give you 6Khz wide eSSB audio.