Kenwood TKR-850 repair

On/around 28 July 2012 it was reported that the 70cm W9YT/R repeater was having some sensitivity issues. It was pretty deaf, although not completely. Strong signals could still open the repeater but anything less than 50W in direct sight of the repeater antenna may not work.

After ruling out the easy suspects (antenna, duplexer, feedline, lightning arrestor) it became obvious that the repeater was at fault.

The Kenwood service manual for this repeater shows "surge supression" diodes and a preamp transistor are the first active components in the receive path prior to the first mixer.

Top view of the repeater. Underneath the top door is the PA board. This really could not be easier to work on.

And door number two. The parts I need to access are underneath this board, between the coils. Removal is easy enough, two rocket connectors, one flex cable, two multi-pin connectors and ten screws.

Board removed, lower side. Upper-right is where the parts in question are.

On this model, the surge supression diodes, D9, is a HSM88AS and the first preamp, Q1 is a 2SC3357.

The HSM88AS cross-references to a common BAT17 Schottky diode, $0.44 each in single quantities from DigiKey.

The 2SC3357 cross-references to an NEC NE85634, $1.25 in single quantities from DigiKey.

(That's not my solder flux residue.)

10x enlargement of a replaced Q1. This is an SOT-89 part, the actual size of the case is roughly 2.5mm x 4.5mm

10x enlargement of a replaced D9.

After replacing these two parts, the receive sensitivity appears to be normal and the W9YT 70cm repeater is back on the air.

update - 9/2012

The diodes didn't fix it. Yes, the first set was bad, but after that they were OK. The repeater would go deaf within 12 hours (or sooner) of power-on.

N9BDR very generously offered to take a look -- he tweaked the repeater duplexer and measured the system sensitivity. I think he said that 700 microvolts were required to break squelch? Wow!

He also brought a can of spray component cooler.

Cooling a certain area of the TX/RX board brought the sensitivity back. The area was generalized to be the FM demodulator chip and the associated crystal. I ordered both the chip and crystal from Kenwood parts.

Fun fact: I ordered parts from Shenzhen China for an unrelated project the same day as the order from Kenwood parts. The parts from China arrived one business day later than the parts dispatched stateside from Kenwood.

Parts arrived in the mail on Friday 7Sep2012. I took the repeater to my home workshop on the 8th.

Fortunately the repeater was acting up on the bench at home. On first power-up, the unsquelched audio was the familiar FM white noise. That slowly changed over the course of 30 seconds a muffled hiss. It sounded similar to an extremly weak FM signal that just isn't strong enough.

Because it was easy, I replaced the crystal first. Crap. Same thing.

Then I replaced the TK14489 NBFM IF demodulator chip.

A decided improvement on power-up and through the rest of the weekend.

The repeater is back on the air and time will tell if that fixes things.

This page last modified Mon Sep 10 22:30:29 CDT 2012 by timc!

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