[Kenwood] TS-850 low sensitivity

2003年11月08日 11:14:11 -0800

Henry
Could this be it:
"Low Receiver Sensitivity (from K7FR): The only failure that I had on
either of my 850's was that one of the switching transistors that muted the
receiver during xmit went sour and wouldn't go all the way to cut-off in
receive. This had the effect of putting about 20dB of pad into the
receiver. This happened on both. The first one went out to be fixed the
second one I did myself. 
Low receiver sensitivity could also be related to the first item on this
list. Also, I noticed an interesting problem while at W5WMU last time. One
of the 850s that lives there was down in sensitivity from the one I took
with me. I am not sure what the problem is, yet. It could be this switching
transistor problem."
bob
At 11:42 AM 11/8/2003 -0500, you wrote:
>Hello, all-
>I have been fighting a problem with my high serial # 850 for several weeks
>now. After combing thru all the archives and a couple emails to Tree
>concerning things on his website, here's where I am now- I'm hoping
>someone out there has some more suggestions.
>After using the rig in WW in a M/S environment, I noticed that sigs were
>down about 15-20db from an adjacent 850. Especially on the higher bands-
>(10 was about dead.)Here's what I've done;
>1. Checked RXb and TXB- rcv voltage is 7.95, and goes to 0 on TX- same
>with TXB.
>2. Checked voltages on the SN74LS145 band decoder chip- had some squirrely
>voltages, decided to replace it, putting in a 16 pin socket for future
>repairs. Signals on 80 and 40 seemed louder, but still down from the other
>850. (also did the NB mod by clipping out the caps on the NB board- a
>dramatic improvement.)
>3. Tree recommended checking the attenuator circuit- I found that the
>150ohm R2 on the RF board was open- After replacement, the attenuators
>work properly. But still low on all bands, thopugh there was some
>improvement.
>4. Checked various voltages and diodes, all seemed within tolerance,
>expect the D31 test point on the rf board, which shows in the service
>manual that it should be 6.7v on rcvc, 4.8v on TX. Those voltages measure
>out to 7.4v on RX, 5.6v on TX (both high).
>5. All 6631 chips are now 66312's (that was done months ago.)I dont think
>the problem is there.
>6. I did check the AIP IC on the RF board (the 3900 board)- I found 7.95
>volts there on pin 4(RXB), but the 12v line is only at 11.5.
>>The big questions- is there a way to boost the RXB/TXB voltage with an
>adjustment? Does this sound like an RF switch issue? Source for that 3900
>board? Never had a DC-DC board changed- Maybe the problem lies there? Any
>other suggestions welcome. The rig is working much better now, but still 
>a problem on all bands- still seems to be some sort of attenuation on RX.
>Still, TX is perfect; full power out, good audio reports.
>>Another week and Clif gets work, but I feel like I'm really close- 73,
>Henry, W2DZO
>Winston-Salem, NC
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