I loved the TS-870 when I first got it. Size is ideal for expedition radio,
light weight, fits in carry on luggage. Features and computer controls are
excellent, until I used it in the contest on 160m. Bloody thing got
overloaded with signals 4 kHz away. Everybody was QRMing me! What a piece of
blinking junk! Liking the radio otherwise I inquired at Kenwood about the
problem of overload. They came up with modification to remedy the gain
distribution in the RF and mixer stages, which was implemented in later
production models. After I received the service manual, I started to dig into
the radio and came up with few more improvements.
Earlier production units before S/N 801xxxxx had problem with front end
overload and consequent distortion and crossmodulation in the later stages,
including DSP, especially noticeable on low bands. To remedy that on those
earlier units see the Overload Modification.
There is an absence of 2nd RX antenna switching (Beverages, etc.). When I
inquired at Kenwood, they supplied modification instructions that required
some drastic modifications, including cutting the traces. When I obtained
service manual, I figured very elegant modification using just plug in diode.
See 2nd RX Antenna Switching modification.
Examining the circuits and filtering arrangement, I have found that Kenwood
engineers decided to rely on obtaining the selectivity mainly by using the
DSP. Theoretically it is great, except when strong signals blow by the
mediocre crystal filters used in stock TS-870, they modulate the following
stages, including DSP circuits. The biggest improvement that puts this radio
one class higher, is to replace the stock 2nd IF crystal filter and 3rd IF
monolithic ceramic filter with high quality International Radio or other
filters. Any better shaped filters make tremendous improvement to the
selectivity, overload and DSP performance. Depending on intended use, you can
use 2.7, 2.4 or 2.1 kHz bandwidth filters. I chose 2.1 mainly for contesting
purposes. It is possible to use narrower filters for CW operation, but
selectivity selection scheme via microprocessor controlled switching makes it
complicated and throws the controls off their markings. I found that 2.1
filters work very well even for CW and are significant improvement over stock
arrangement. See Filter Modification.
NOTE: the changes do not apply to radios with S/N 801xxxxx and later.
If there is any strong signal in the proximity on the low bands, it will cause crossmodulation and create a "ghost" signal. It may occur in the contest operation or during the pile-up reception.
A larger power input signal causes the second mixer output, the NB amplifier input, the third IF amplifier input, and the third IF amplifier output to be saturated.
Change the gain allocation of the IF stage to raise the limit of saturation. This modification will cause almost no deterioration of receiver sensitivity.
Circuits from the antenna terminal onward:
Antenna, amplifier, mixer, 73.05MHz filter
Amplifier, mixer, 8.83 MHz filter - noted gain is -10dB (minus 10)
Amplifier, mixer, 455kHz filter
Two Amplifiers - noted gain is +10dB (plus 10)
Replace the following components with (Kenwood part numbers)
Caution: they are all SMT chips, you need to be familiar with SMT soldering procedures.
Changes to circuit board XX4-3210 (A/9) RF UNIT
R1157 : replace 1.8k with 560 (RK73FB2A561J)
R160, R161: 2.2k with 1k (RK73FB2A102J)
R164 : 1.5k with 560 (RK73FB2A561J)
L76, L79 : 3.3uH with 0(zero) (R92-0670-05)
R169 : 1k with 330 (RK73FB2A331J)
X57-4620 TX-RX UNIT
R332 : replace 2.2k with 6.8k (RK73FB2A682J)
Do not forget to perform the re-adjustment of the S-meter sensitivity after making the modifications.
Refer to "8. S-meter adj." on P.95 of the Service Manual for how to do it specifically.
If you cannot adjust the display to "05", even after you follow the steps described in "8. S-meter adj.", change the resistor as instructed below and make the re-adjustment.
X57-4620 TX-RX UNIT
VR-1 : 470 ---> 3.3k (R12-6737-05)
2nd RX Receive Antenna switching modification
Kenwood TS870 has two antenna jacks ANT1 and ANT2 and external receiver
antenna jack (RCA type) that can accommodate external receiver when activated
through the program menu. This adds some attenuation to TS870 receiver
signals. There is no provision for external receive antenna switching
(Beverages, preamp., etc.) When I inquired Kenwood about this option, they
sent me modification instructions that required cutting the traces and
rewiring the ext. RX connector. When I received my Service manual and had a
look at the circuit, I came up with this simple modification:
Parts needed: 1 diode (any type) I used rectifier diode, generic NTE125
Step by step instructions:
- (In the following references the front panel of the rig is facing you.)
Remove the top cover.
- Remove 7 screws holding the cover over the FILTER and RELAY unit (in the middle) and remove the cover. (Where the ANT1 and ANT2 connectors go.) This exposes two PC boards.
- Locate the connector CN301 on the RELAY unit (adjacent to ANT1 and ANT2).
It is about 1 inch back and 1 inch left from the ANT1 connector and has three wires going to it - two gray, left and one black, right.
- Take the diode, clip the leads to about 0.5 inch length, bend them down and form them so they are spaced about 1/8" and parallel (kinda forming letter Y).
- Plug the diode into CN301 connector from the top, pushing the diode wires between the plastic housing and the metal contacts of the connector. Cathode (the end with the band marking) of the diode should go to the middle contact
(gray wire), the other end to the left contact (gray wire). Make sure that diode makes good contact with terminals of the connector. Black wire should be free.
- That completes the modification. Replace the covers, do not pinch the wires going to the antenna tuner!
Connect your transmit or main antenna to the ANT2 connector. Connect your
receive antenna or preamp output to ANT1 connector. Now you can transmit only
through ANT2 connector.
Front panel button ANT now allows you to select ANT2 (receive/transmit on
ANT2) or ANT1 (receive on ANT1, transmit on ANT2). Antenna tuner would always
see the impedance on ANT2. The way the switching works, whenever you
transmit, you will switch ANT2 on transmit, regardless of ANT1 or ANT2
selection. Much better use of connectors than originally devised by Kenwood
Notes on TS870 usage: Normally there is enough gain on the receiver for
beverages. In case that you need to use the preamp or preselector, than turn
the AIP on TS870 on, and if needed add the attenuators. Try to keep the noise
level peaks close to low end of the Smeter scale. Helps with intermods. If
using DSP, play with different settings and different levels of AGC setting,
it has effect on overall behavior.
Major improvement in selectivity and DSP performance can be achieved by
replacement of 2nd and 3rd IF filters with sharp crystal filters, like from
International Radio. The 2nd IF 8.83 kHz filter used in TS870 is the stock
Kenwood crystal filter, while the 3rd IF 455 kHz filter is little ceramic
I have ordered set of 2.1 kHz filters (8.83 and 455) from IR. The
modification involves removal of printed circuit boards where the filters are
located, unsoldering the old filters. In their place solder short piece of
miniature coax to connect new IR filters. IR filters can be mounted between
the circuit boards and the metal chassis using double sided sticky tape. Coax
from old filter connections is then connected to new filters. Tweaking the
matching IF transformers next to the filters ensures proper match and gain.
Other filters with wider bandwidth can be used also. For contesting I found
2.1 filters to be the best and the mod works very well on CW also.
Selectivity improves as well as functions of DSP. Now it is a decent radio,
matching those costing thousand(s) more.