The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

Howdy folks, I recently got a good shape Cobra 2000 GTL in on a trade, but it has 2 problems.

The board is PC-497AB. This radio has low receive, both in meter and in audio. Stations that normally hit me S-9 barely move the meter. One of my friends suggested that something in the front end of the radio got overloaded, like a diode or a transistor, and shorted out, or something in the TX/RX switching circuit isn't right, causing TX to not fully turn off, or RX to not fully turn on. The radio transmits fine, good audio output on TX, both AM and SSB.

Also, on AM, I can hear myself talking through the speaker. This happens in both speaker jacks. It's not very loud, but it's decent volume. It gets louder as the mic gain is turned up. I cannot find any indication of talkback ever installed in the radio.

Any info on this would be great. If anyone needs more info I'll answer the best I can. Thanks!

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Check D12, D13, D14, D15, and D16, D12 and D13 are a special type of diodes and must be MC301, all the other diodes can be replaced with 1N4148.

The radio may have AGC fault, just try unsoldering one end of D68 and check to see if the RX is any better, DO NOT TX WITH D68 OUT OF CIRCUIT.

You could also try replacing TR14 (2SC1674L).

Let us know how you get on.

Thanks for the quick reply, Rick!

I replaced all the diodes in question, as well as TR-14 (2SC1674L). No change, still real low RX. I lifted D68, no change in RX when lifted.


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It is not a good idea just to change components without test them first as this can make fault finding harder.

Have you realigned the RX strip?

Ack! I'm sorry about that.

I saved all the components I pulled, so I went through and checked all the diodes, all check good. The transistor is good also.

As I do not have a signal generator, I can't perform a proper alignment of the RX strip. I can talk to a friend and see if I can borrow his signal generator in a day or two, and could perform an RX alignment then.

I appreciate your help so far!

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Update:

I reinstalled all the original components, no change.


At R47, I get 8V in RX, 0V in TX.
Anode side of D16, 0V in RX, 8V in TX.

Everything is getting switched properly, from the way it looks, might there be a bad capacitor somewhere? Where would I go looking for that?

Thanks for any info.

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This is not good guessing what it maybe or may not be, until you have realigned the RX strip, 90% of old radio RX faults are down to alignment.

makesure, your rf gain is turned right up.thx hamguy2 nsw asustralia

Case closed, ended up selling the radio. Thanks to all who responded!


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