Dentron MLA-2500B key up issues

[nomadradio]

a bypass cap directly across the RF diode.

We use a pair of diodes as a voltage doubler. Increases sensitivity. Adopted the habit of putting a .01 disc across the DC output because we saw stability issues without it. A double "quick key" would sometimes see it only key the first, but not the second time. Never figured out why, but that cap cured it. Took a bath in it, with another disc from collector to emitter Wasn't thinking about spectral issues, only making it work smoothly.

Then again, we use a fairly-large resistor between the radio and the keying circuit's blocking cap. Gotta figure its a decent attenuator of any RF schmoo that gets generated in the sensing circuit.

73

 

[Shockwave]

We use a pair of diodes as a voltage doubler. Increases sensitivity. Adopted the habit of putting a .01 disc across the DC output because we saw stability issues without it. A double "quick key" would sometimes see it only key the first, but not the second time. Never figured out why, but that cap cured it. Took a bath in it, with another disc from collector to emitter Wasn't thinking about spectral issues, only making it work smoothly.

Then again, we use a fairly-large resistor between the radio and the keying circuit's blocking cap. Gotta figure its a decent attenuator of any RF schmoo that gets generated in the sensing circuit.

73

I thought the same thing. For mono band amps I was using a five picofarad cap driving a Darlington pair. That reactance works out to be approximately 1,200 ohms. Not too much smaller than the 2.2 k resistor used on the barefoot example.

Just curious, are you using the voltage drop across the diodes to bias the transistor? In my case the diode was shunting the negative alternation of the waveform, only leaving the positive side intact to bias the transistor. I'm not sure how much of a difference this made however, I suspect it made things worse.

To make a long story short, I found the cleanest way to rectify RF into DC, was to use R25, D17 and C54 from the Heathkit SB-220 schematic. With the right coupling resistor or capacitor, that drives a transistor well. I also made a mistake in my previous description, as the bypass capacitor is not directly across the diode. That wouldn't work.

 

[nomadradio]

Here's the one we sell, with the input resistor used to bridge from the radio jack's center pin to the pc board.

To use it at frequencies much lower than 11 meters, the input cap probably has to be bigger. Never had a reason to find out how much.

73

 

[Shockwave]

Here's the one we sell, with the input resistor used to bridge from the radio jack's center pin to the pc board.

To use it at frequencies much lower than 11 meters, the input cap probably has to be bigger. Never had a reason to find out how much.

73

This is a good circuit. I see you have the base-emitter junction bypassed with a .01 capacitor, which effectively RF bypasses the diodes as well. I would not expect this circuit to have the ability to introduce any harmonics. Nice job. Heavy duty too.

 

[nomadradio]

Heavy duty too.

That was the idea. Doesn't matter how much power you tell them it needs, they'll always pump in more. Thanks for the feedback.

73