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Author Topic: Need receiver troubleshooting advice  (Read 186 times)
W4XKE
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Posts: 67




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« on: May 16, 2003, 07:19:32 PM »

I need some advice on troubleshooting a receive problem in a Kenwood TM 261A amateur radio 2 meter transceiver.  I believe it was overwhelmed by the close proximity of an adjacent 40 meter transmitting antenna (mobile) which has greatly diminished the receive ability of the radio.  
 With the volume full up, stations can be heard among a lot of white noise.  
 With the squelch on, few stations are strong enough to break in.
 With the squelch off, turning up the volume increases the “rushing” white noise sound very well, so the AF is working fine.  No problem with transmit.
 The incoming signal to the first amplifier stage is very wide and noisy, as though the band pass filters have opened up.  (Using a Tektronix 465 scope)  When I looked at the incoming signal of an identical 261A (working), the incoming signal was a very narrow and clean trace on the scope.  
 All measured DC levels, bias on transistors etc., all appear to be normal.
 All diodes seem to be okay through all amplifier stages up to the AF IC.
 From the antenna connection, the incoming signal has multiple circuits that branch off into a myriad of other circuits.  I have become frustrated trying to determine what components might have what effect on the incoming signal.  The Kenwood service manual has little to say, mostly just a schematic layout with a few DC level indications.
I am in need of advice from an experienced tech.  (I am still in school, studying algebra and trig and haven’t started on the experience end of things yet.)  TNX, Johnny
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AK5K
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Posts: 11




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« Reply #1 on: May 16, 2003, 09:21:58 PM »

It sounds like you blew a component in the front end of the receiver; unfortunately I have no specifics for you. I did a similar thing a few years ago with the TM-221A, one of the earlier models in this series.

I have a question for you. I, too, have a TM-261A and find that the DTSS doesn't appear to work. I'm using the feature for activating the receiver when the code is received to activate our local RACES group, and even though it's claimed the code is sent, the receiver never comes on. Have you successfully used this feature?

Good luck with your receiver problem.

Regards,

Bill, AK5K
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K8LEA
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« Reply #2 on: May 16, 2003, 09:36:36 PM »

Johnny:

Just off the top of my head one, of the coils in the receiver's very early stages may have gotten hot enough to melt it's form.  That'd take it way off resonance, and probably way beyond screwdrivering.

All that extra RF should have gone to ground, but if it did that through a coil for long enough time it may be enough to overheat something.  Could have shorted or opened a cap, too.

Another possibility is that the first transistor in the input chain (probably labeled as a preamp or rf amp) may be toasted - you'd see some loss in that stage instead of some gain.  Ought to be a mixer just past that which might be fried too - probably not the transistor, but a coil or popped cap....

Every stage ought to have _some_ gain, except maybe the mixer, and it shouldn't introduce too much loss.  

Anyway, I'd consider about anything from (and including)  the first mixer back  to the antenna, including the passive components, as suspect.

Stu.
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