New 148gtl : No AM Audio

[Robb]

I would track from the mode switch and see where the voltage fails.
Check the TR that switches between AM and SSB.
Just some thoughts . . .

*EDIT*
The schematic is for the NW/ST version; but it uses the same voltage regulator as the radio you have. It should still work for your radio.

 

[sdmahr]

Robb: You are correct, that schematic IS the correct one. What has changed is some of the part numbers of the components.....which has me confused for a few. I'm back on track and will be working my way back to where the voltage is incorrect. As that point I hope I find the culprit.

Thanks again....I'll let you guys know what I find out.

 

[sdmahr]

I'm having trouble understanding what is going on in the switching and distribution of the voltage for AM TX. Looks like TR41 and TR42 are involved, but when I'm looking around there are a BUNCH of transistors that are related to those two. Do I just go through and check them all out one by one? It appears I have double the voltage I should on the pins that should have around 7 volts on them. But, when keyed I get snake eyes, (Zero's) where I should have voltage on the finals.

Can anyone explain how these circuits interact so I have a clue as to how I decifer this rascal ??

Thanks for your expertise.

 

[sdmahr]

Got it. I kept looking at that schematic thinking "it has to be TR41", but I had just replaced it with a new one. Out of frustration I opened the little baggie and fished out another one and installed it and PRESTO, I have full transmit on AM.

Moral of the story: Not all new components, (out of their little baggies), are fit for use...........and can result in large frustrations assuming they are NOT the problem.

Whew......thanks for all your help guys ! !

 

[sdmahr]

UPDATE

So I know these new 148 GTL's catch a lot of flack for not performing as well as "vintage" 148's. I'm certainly NOT going to argue about the build quality and the fact that my vintage 148's just ROCK IT in performance when compared to the new ones.

I can say that the two I inherited for "FREE" have tuned up quite nicely and are performing well. Both were new in the box and were CRAP upon power up. Totally and completely worthless out of the box. They both required a complete alignment, (I coudn't believe how out of frequency they were). One had no AM and required a new AM Power Transistor, the other's PLL chip went crazy shortly after power up and required replacement.

I did replace the whimpy resistor going into the PLL chip from the little funky afterthought board mounted to the side of the radio per the COBRA service bulletin......unlocked clarifier per Robb's post, (lifting one side of D58 - brilliant mod by the way), and did a few "touch-ups" with the golden screwdriver.

The receive on both of these is really very good. Low noise level, and very crisp. XMT audio on both is acceptable also some pretty good power output, (6W AM Dead Key, 16W Swing SSB), although I didn't leave them there for fear the thing couldn't take it.....so I settled on more "legal" power output.

Had I bought these new I'd be pissed. Since they were free, I took some time and made them the best they could be......and that didn't turn out too bad.

So, if you can pick one of these up at a bargain price, (or free ), and you can take some time with it, they really don't perform to bad.