Heathkit Dx-40 wont modulate?

[T23]

I have another dx 40 here that wont modulate. I have a unamplifed d104 wired to it. But nothing. Checked the speech amplifer tube and the modulation tube. Those seem fine. Any ideas?

 

[BJ radionut]

Modulation transformer....Continuity? ...That was a weak point in some of those...
I take it you have carrier?...just no audio?
Also the wiring from mic jack.... follow it back to the speech amp...it may never have been wired correctly...(maybe was only used on CW)
All the Best
BJ

 

[T23]

Yes I checked the wiring and the the mic is wired correctly from what I can see..
I get a carrier but no audio. I am thinking its the mic though
It is screen modulated i believe. No modulation transformer.

 

[BJ radionut]

T23: Forgive this OM...been awhile since I seen one of those old girls...

Check the 6DE7 (dual triode) make sure it's good....one section could be dead and still be be lite up

If it checks good then check the values of the resistor network between it and the 12AX7 speech amp(check this tube as well/or swap them out)...With everything shut down and un-plugged

Make sure the voltages are right when the function switch is in phone....

Did you try a different mic? might be that simple?

If all this checks out...check the contacts on the function switch... or maybe the screen voltage resistor on the 6146...
Be careful when you check those voltages...but you'll find the issue with the voltages listed in the manual or marked on the diagram...
GL
All the Best
BJ

PS: I have a partial manual(w/circuit description)/parts list/schematic with voltages...etc...mail you a copy if you need it...73

 

[T23]

I did some testing with an audio amplifier and it seems to be the mic, no audio out.


T23