RCI 2985DX - No AM transmit audio

[ZuMBLe]

I have a 2985dx which stopped transmitting audio on AM. The radio still works fine on SSB. The radio will throw a carrier on AM but no audio. Anyone have any ideas? Thanks!

 

[sp5it]

Replace TR51
Mike

 

[ZuMBLe]

It's kinda scary that you just knew that.

Scary awesome!


EDIT: I don't see TR51 listed in the service manual. =(

EDIT AGAIN: I'm assuming you thought the radio was the same as the 2950dx (which I assumed also) but it seems the part number matches the first gen 2950 but translates to Q66 2SB754 on the 2985dx (AM Mod/Regulator)? Where would one pick up this part? Is there a place online which I can order it? I have a old Uniden Grant spare parts radio, would this part be found in there?


More searching came up with this:
http://www.worldwidedx.com/tech-repair/23357-super-star-3900-hp.html

This is exactly what I'm seeing. My wattage control no longer works either! I think between what you've told me and the research it lead me to, I'll be okay!

Replace TR51
Mike

 

[sp5it]

AM mod regulator failure is very common especially with power mics and high power in AM.
I'm using BD250C as replacement. It will last forever.
Sorry for wrong TR number. I could not find schematic for your radio, but I see you figure it out.
Good luck, Mike

 

[ZuMBLe]

From the research I did, I found that the 2SB817 is a replacement for the 2SB754 and will last longer. Do you agree? This seems like a pretty straight forward job.

Unsolder and unbolt old transistor, apply new heatsink compound, bolt on new transistor, solder... Enjoy? Is that all there is to it?

AM mod regulator failure is very common especially with power mics and high power in AM.
I'm using BD250C as replacement. It will last forever.
Sorry for wrong TR number. I could not find schematic for your radio, but I see you figure it out.
Good luck, Mike

 

[ExitThirteen]

A 2SB827 would actually be better than a 2SB817 for the 2985DX, if you can find one, they should be readily available thru RFParts and/or other similar websites. The 2SB827 is slightly higher gain, if memory serves me right.


And yes, it's a direct drop-in. remove the old one, put heat sink compound on the new one, install, solder in, and you might have to re-adjust your AM carrier on your high side down a little. Other than that, it's plug 'n' play.


Cheers!

 

[ZuMBLe]

Oops, too late. I ordered the 2SB817 already. I can always find the 827 later on. =)

Thanks for everyone's help.

A 2SB827 would actually be better than a 2SB817 for the 2985DX, if you can find one, they should be readily available thru RFParts and/or other similar websites. The 2SB827 is slightly higher gain, if memory serves me right.


And yes, it's a direct drop-in. remove the old one, put heat sink compound on the new one, install, solder in, and you might have to re-adjust your AM carrier on your high side down a little. Other than that, it's plug 'n' play.


Cheers!

 

[ExitThirteen]

A 2SB817 will be just fine, you'll do well with it. Good luck!

 

[ZuMBLe]

I got the 817 in today. I should be attempting this tonight or tomorrow morning. =)

 

[ZuMBLe]

SUCCESS! After about 30 min, I had the 2SB817 installed. I think failure was caused by the transistor working loose from the heatsink. The washer was crooked and there was a air gap between the washer and the heatsink. I replaced the transistor, put the screw in, then soldered the screw to the heatsink to make sure it won't back out again.

My AM audio is back and so is my variable power! Thanks everyone!

Now I just noticed my signal meter doesn't work on SSB. I'm hoping there is a pot inside that I can turn to fix this. I'm done opening the radio up tonight.

=)