D104 Unamplifed With DX60B

[ko4nrbs]

When using the unamplified version of the D104 with my DX60B it is difficult to get more than 50ma of plate current.  I am driving a Swan Mark 1 linear with it.  If I use my old Heathkit hand mic I get about 75 or 80ma.  I suspect the element in the D104 is weak??

I have the mic gain pot in the radio maxed out.  The 12AX7 and the modulator have been replaced with spare tubes  with the same results.  CW works as it should.

When using a D104 unamplified microphone it should produce more drive than this one, correct?

Bill KO4NR

[DMOD]

When using the unamplified version of the D104 with my DX60B it is difficult to get more than 50ma of plate current.  I am driving a Swan Mark 1 linear with it.  If I use my old Heathkit hand mic I get about 75 or 80ma.  I suspect the element in the D104 is weak??

I have the mic gain pot in the radio maxed out.  The 12AX7 and the modulator have been replaced with spare tubes  with the same results.  CW works as it should.

When using a D104 unamplified microphone it should produce more drive than this one, correct?

Bill KO4NR

Give us some history. Did you just acquire this DX-60 recently?

Not necessarily a mic problem. Do you have a schematic and have you made any voltage measurements according to the manual?  Have you upgraded any power supply components?

Unless someone has made changes in the audio circuitry, a D-104 should drive it very well.

Just remember, screen grid modulation on an unmodified sixty is only about 40% efficient.

How does it sound in a receiver?

[ko4nrbs]

Have had it about 2 years.  No change to audio circuits.  Voltage checks done when I replace several out of tolerance carbon resistors last summer.  Will do that again to rule it out.  Power supply filter capacitors, diodes and resistors replaced.
Bill KO4NR

[DMOD]

Have had it about 2 years.  No change to audio circuits.  Voltage checks done when I replace several out of tolerance carbon resistors last summer.  Will do that again to rule it out.  Power supply filter capacitors, diodes and resistors replaced.
Bill KO4NR

Since you're going to recheck voltages, could you post them here please, especially those for V5 and V4?

I have found that a leaky C33 can bring down the V5 plate voltages.

If you have a scope or a modern DMM, it would be helpful to determine the actual output voltage of the D-104 about 1" from the mike by saying 'Ahhh" for about 5 seconds.

I have found quite a bit of output variability in my 3 D-104's.

  

[w7fox]

My unamplified D104 gives me 50 to 100 millivolts, the highest output of all my mics.  It is easily damaged by moisture or temperature extremes.  Some years back someone on Ebay found a treasure chest of new D104 elements and I bought one.  I've been careful with it cause I think they are gone forever.

[DMOD]

My unamplified D104 gives me 50 to 100 millivolts, the highest output of all my mics.  It is easily damaged by moisture or temperature extremes.  Some years back someone on Ebay found a treasure chest of new D104 elements and I bought one.  I've been careful with it cause I think they are gone forever.

I have seen my best one go as high as 400mV peak to peak, "unloaded.'

Did you load it with 3.3M to 10Mohms resistance when you measured it?

In my experience, a good frequency response of an unamplified D-104 is when it was loaded between from between 3.3Mohm to 10Mohm. I usually replace R18 2.2M with a 3.3M grid resistor, and I modify the speech amplifier stages V5A and B so they won't oscillate.

R18 is only 1Mohm and really loads down a D-104 severely, so the total input resistance as seen by a D-104 is actually 690k ohms, way too low a load for any D-104. R18 is there to actually reduce gain so the stage won't oscillate. A very poor design. All they had to do was reduce the gain of first stage gain slightly and the speech amplifier would not have oscillated.

C31 and the mic pot R19 also need to be replaced with a .01uF cap and a 500kohm pot, respectively.

[ko4nrbs]

Have had it about 2 years.  No change to audio circuits.  Voltage checks done when I replace several out of tolerance carbon resistors last summer.  Will do that again to rule it out.  Power supply filter capacitors, diodes and resistors replaced.
Bill KO4NR

Since you're going to recheck voltages, could you post them here please, especially those for V5 and V4?

I have found that a leaky C33 can bring down the V5 plate voltages.

If you have a scope or a modern DMM, it would be helpful to determine the actual output voltage of the D-104 about 1" from the mike by saying 'Ahhh" for about 5 seconds.

I have found quite a bit of output variability in my 3 D-104's.

  

I tried an old Heathkit mic and the DX-60B worked as it should.  Evidently the element in the D104 is bad.
Bill

[DMOD]

"I tried an old Heathkit mic and the DX-60B worked as it should.  Evidently the element in the D104 is bad.
Bill"

What kind of mic was that? Did it have a ceramic element or was it a dynamic mic?

Different mic element types have to work into different load resistances.

I think the D-104 is being overloaded from the low input resistance of the DX-60 first audio stage as I mentioned before.

Did you measure the voltage output of the D-104? Without a measurement one cannot know for sure if the D-104 element is faulty.

  

[w7fox]

Sorry to hear your mic is bad.
Dmod, I was testing my mic on a scope input, probably one megohm.  Too low for good bass response eh?  I have read that one of the selling points of the D104 was to restrict the freq response for better "punch" by loading down the element.  We have other priorities.

[DMOD]

Sorry to hear your mic is bad.
Dmod, I was testing my mic on a scope input, probably one megohm.  Too low for good bass response eh?  I have read that one of the selling points of the D104 was to restrict the freq response for better "punch" by loading down the element.  We have other priorities.

Most scope probes are 10:1 scope probes with approx. 10Mohm impedance

https://www.allaboutcircuits.com/technical-articles/an-introduction-to-oscilloscope-probes/

In AM we usually want more than a "tinny" sounding audio. Tinny and restricted audio is usually relegated to SSB.

In my experience with upgrading these transmitters, a 3.3Mohm grid resistor in the first stage of the speech amplifiers seems to be a good compromise, but the rest of the speech amplifier stage needs minor modifications to get good sounding audio.