Kenwood TK-690H Front-Panel Noise Blanker Control

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Front-Panel
Noise Blanker Control

By John Haserick W1GPO

Background:

The noise blanker is a very useful feature these days, however a lot of impulse noise is generated by solid-state devices such as traffic lights, where the noise blanker actually degrades reception, because the noise is rapid and constant. Usually the blanker will not reduce non-arcing noise because there are too many discrete pulses per unit of time, i.e. 30 or 60 pulses per second. While the blanker will very effectively reduce random ignition and pure arcing pulse noise at least 20 dB, often the blanker goes beyond just blanking the noise, and in these circumstances the noise pulses become more noticeable and distorted. You encounter this situation at many LED traffic light intersections. As far as the cause of noise blanker degradation, it appears to be intermod of the various discrete RF peaks, and often other discrete signals within the IF bandpass of the receiver occurring at the blanking transistors junctions when conducting, and landing on the receive frequency. Being able to turn the noise blanker off under such situations will actually improve reception greatly.

Kenwood LMR radios let you program whether the noise blanker is on or off on a channel-by-channel basis, with no front panel user-accessible button to control it. This article offers a solution to that problem.

How It Works:

The noise blanker is controlled by Q114. When it is turned on, its output pulls the NBS (Noise Blanker Switch) input pin of the noise blanker low (to ground) and activates the blanking action. All we're doing is supplying our own low (ground) signal to the NBS pin by using the programmable A03 output line.

Programming:

Program the Noise Blanker OFF on all channels. Under Function, Port Output, program Radio A03 to AUX B. Program a control head button to activate (ground) "AUX B", so that when this button is pressed, "B" appears on the display. When "B" is present and the output is activated (goes to ground), the noise blanker will be turned on.

Radio Modifications:

Remove the TX-RX board to access the underside. Solder a thin wire to the NBS terminal on the noise blanker. This is the center of three pins under the noise blanker, located near the middle of the TX-RX board, on the end closer to the side of the board. The center pin on the other three-pin connector is grounded, so this should help you make sure you're on the correct pin. Click on any image for a larger view.

Feed the wire over to the control board on the other side of the radio, and solder it to the extra feed-through hole for CN505 pin 16.

Here's a close-up of CN505 showing the wire inserted in the feed-through hole for pin 16.

And here's a partial X-ray view of the control board around CN505 showing the location of the feed-through hole.

Double-check that the wire to CN505 pin 16 connects to ACC 25 pin 21. See the partial wiring diagram below.

Secure the wire with dabs of RTV sealant. We used Ultraseal Ultra Blue for this.

Credits and Acknowledgements:

The wiring diagram came from the Kenwood TK-690H(B) service manual.

Thanks to Bob WA1MIK for turning this into a nice article.

[Note from Bob WA1MIK: Initially I felt that this idea might also work on the TK-6110 radio. John informed me that there are over two dozen screws holding the TX-RX board to the chassis, plus some soldered connections, and the parts are extremely small. Also, there are two board versions and the noise blanker is inside a "black box" on the schematic so we aren't sure it's even possible to get to a point to reliably disable the noise blanker on that model.]

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This page created on 22-Sep-2020.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.