Cobra 148 GTL AM s-meter issue

[Unit 75]

I have a 148GTL that receives low on AM only. SSB reads fine. When I inject a S-9 signal into the antenna jack the radio's s-meter reads 9 s-units on SSB. When it is injected on the AM side it reads about 1.5 s-units. All the cans in the rx chain seem to peak ok and none of the slugs are bottomed out or near the rim. Any ideas why sensitivity would be low on AM only and fine on both sidebands?

 

[nomadradio]

If the 7.345 MHz crystal becomes weak, that will affect AM only. We check for that by taking a good crystal and holding the two pins up to the foil pads where the factory crystal is soldered. If the AM signal comes up, that crystal is weak. Don't have an exact signal level from that crystal to measure on a 'scope.

C62 can go bad and affect AM but not sideband.

Those are the two places to look that first come to mind.

73

 

[Unit 75]

Thanks, Nomad. Will take a look.

 

[Dmans]

Unit 75,
I had a similar problem several years on a 148DX. Turns out the L4 coil was bad or shorted. The thread is linked below.

https://www.worldwidedx.com/threads/148gtl-dx-very-weak-am-receive.241939/page-6

Good Luck
73
David

 

[Unit 75]

Well guy's this was a tough one. I did manage to find the problem. If you look between your 455KHz filter and the big 7.8MHz Filter there is a resistor and a diode.(D3, I believe) The factory sure used a lot of that glue on one side of that little black 455KHz filter. I removed the glue and cleaned it off to find underneath it had eaten D3's anode clean in two. Replaced D3, re-aligned the receive and Bob's your Uncle. Thank you for your input guys. I checked your suggestions out but it still had not rectified my problem.
'73's

 

[nomadradio]

Rock on! Congratulations for digging down to it, so to speak. Not a suggestion that was on my radar.

73

 

[Handy Andy]

A heads up for the rest of us...​

 

[Unit 75]

That's the one, Andy. Another victim of glue gone corrosive.