Kenwood TS-850 Noise Blanker Mod


There was a lot of interest about the noise blanker mod posted here a week or so ago so I'm passing along my results. After performing the mod (copied below) the noise blanker in my TS-850 reduces line noise from an S8 on 10 meter AM to S3 or less. This is much better than before the mod. On SSB, the noise blanker reduces noise several S units, but not as dramatically as on AM. The mod is "simple": Remove C618 and C619 from the circuit. But finding the chip caps is next to impossible without using the Service Manual. List member Rick, KN3C, located the chip caps - they are on the solder side of the board - then removed them for me. Again, don't try the mod without the Service Manual because the two caps are unmarked. 73, Tom, WA3REY

---Mod follows---------

Hi Dave When I took delivery of my TS850 and used it along side of my TS940...the 850 was noisy and turning the noise blanker on had no effect...some of the noise was radio noise and some was power line noise ...the noise blanker in the 850 had no effect on the power line noise...I sent the radio back to Kenwood...they said it operated as they wanted it to...i.e. no trouble found... I compared the noise blanker of the 940 (works very good) schematic to the 850...electrically they were the same except for a bypass capacitor in the output of the noise pulse detector...I guessed this bypass capacitor was slowing the noise gate down and it was not blanking the leading edge of the power line noise (very fast rise time noise)...I cut the bypass capacitors) out of the circuit (surface mount capacitors) and now my 850 noise blanker works as good as the 940 noise blanker.... On the schematic you got in the operators the RF UNIT at the bottom of the RF unit schematic is the noise blanker schematic...the circuit symbols are in the 600 range i.e. R601...C601 etc...the bypass capacitors to cut out are C618 and C619 (0.01 mfd)...the noise blanker board is located in the upper compartment on the left hand side (approximately under the speaker) is about 2inches by 3 inches and can be removed for the cutting of the traces to the capacitors.... The service manual has a much better readable schematic and also a drawing of the board...the drawing will not copy and remain if you have any trouble locating these capacitors, try to locate a service manual. Hope this helps Jim KI6MZ