Kenwood TS-690S poor sensitivity on AM Broadcast Band - Page 1
Forum section: electronics / rf-microwave

author: CaptDon
author_rank: Super Contributor
timestamp: April 09, 2025, 12:15:16 am
content: Does anyone here own a 690S that could confirm if I have a problem. From 500KHz to 1704.99KHz the sensitivity drops by right around 20dB. I don't hear any relays clicking but the change is sudden and at those frequencies. I verified the proper operation of I.C.2 on the RF board. Apprx 7.5vdc deselected and .39vdc when a band filter is selected. There is a .5Mhz to 1.6Mhz filter band which on my radio is selected from .5MHz to 1,704.99MHz. I have tested all of the components in that filter which include D8, L8, L9, D18 and some capacitors and resistors. All are in spec. The filter diagram of the RF board is on page 135 of the service manual. I am stumped why the gain is down 20dB when in that frequency range. The 20dB attenuator relay is NOT being activated and proven by the fact that I can engage the attenuator and drop another additional 20dB. Does your 690S act the same way? Am I chasing a problem or is this part of the design?

author: CaptDon
author_rank: Super Contributor
timestamp: April 09, 2025, 12:45:42 am
content: Oh shit, it is in the specs, 32uv AM broadcast band Vs. 2uv for other bands. WOW!!! That is real unhandy for AM BCB DXing!!! An old 6 transistor pocket radio probably works better!!

author: pienari
author_rank: Regular Contributor
timestamp: April 10, 2025, 05:19:03 am
content: I think i have seen there is resistor modification on the web.

author: CaptDon
author_rank: Super Contributor
timestamp: April 10, 2025, 01:10:36 pm
content: I found that mod on the web. It is true they really swamp out the broadcast band with two series resistors and a 10 ohm swamping resistor which is connected to the 'filter select' voltage. Sadly, this very expensive somewhat top of the line transceiver was never intended for BCB DXing. It appears that the filters are selected through diode bias and if you jump out the series resistors you create a situation where if living close to a BCB station there may be a strong enough signal voltage to make it through the filter selector diodes even in their 'filter deselected' condition. Probably would have been a better design to use a relay to deselect that particular 500KHz to 1705KHz filter. The manual states 1600KHz but I think they made a firmware change to include the extended BCB's of today. I don't really DX the BCB but it would be nice to hear a station I used to work for which is about 300 miles away but can be heard on skywave during the night time hours. I have a 2 X 2 advanced class U.S. callsign and I heard a rumor when I renew they will issue extra class privileges. I wonder if my call will change?

author: CaptDon
author_rank: Super Contributor
timestamp: April 10, 2025, 05:30:57 pm
content: I jumped out the pair of 39 ohm resistors and picked up 10dB on the broadcast band without the other possible associated problems. IMPORTANT NOTE I am glad I was tearing into this radio!!! The memory battery was apparently original and was leaking and beginning to cause damage to the circuit board. The battery is easily accessible but is soldered in place. I clipped mine out using end cutters and soldered in a retrofit battery on wires which I tucked into the lower portion of the front panel. If you own one of these 450/690 rigs it is time to check the battery!!!