Browning Mark: 4 old dog ponders new trick.

[nomadradio]

I would just as soon keep it broadbanded. Especially to make it compatible to either the 21 MHz Mark 4A first lo, or the 31 MHz in the Mark 3 and older. Better than having to stock two separate versions.

Proof is in the results, once I see them.

PCBs are due to show Monday 5/11. Still can't shake that "kid the night before Christmas" impatience for them to arrive.

73

 

[nomadradio]

They say "measure twice cut once".

Measure once, cut three times, right?

That's how this prototype played out. First, I wanted to mount it with the right-angle header pins, like on the perf board.

First problem. I made the 12 holes too small for the header pins.

Well, no big deal. I'll just use wire jumpers. Only needs six for my application.

Just one problem. I decided that the USB socket would be blocked by the voltage regulator, so I turned it the other way. And screwed up which side gets which connections.

So the connections got kluged to where they should have gone.

Good news is that it works.

But the two-to-one output-voltage step-up doesn't. Core is T38-6. It has five turns twisted to make it a transmission-line transformer. RF voltage into the tube grid is slightly less than a direct connection.

At least it's not "cut four times".

Decided I like the jumper wires better than the header pins after all. They take up less space, so the next version can be smaller.

Some time betwen now and when the next iteration arrives I should try it in a Mark 3 receiver. And maybe figure out why the toroid didn't get me a drive-voltage step up.

The TO-220 voltage regulator looks like overkill, but the tiny TO-92 regulator I used at first was getting too warm for my tastes. Putting the covers on the radio would probably have pushed it from "warm" to "too hot". These days, the price is typically the same for either size regulator.

Bact to the drawing board.

73

 

[brandon7861]

When you first mentioned a step-up transformer, I thought I would try my type 61 mini binocular cores. It didn't work too well. I don't know much about ferrites, but this is the chart that made me give up:

u' is the inductive part of the permeability and u" is the loss part. At 22MHz, the loss becomes significant. I assume the same issue occured with your type 38 cores.

This is from the T38 datasheet. Not only does the loss become significant at a much lower frequency, but the inductive portion rolls off before it becomes useful.

I don't know what ferrite material would be best for 22MHz, but I think type 67 might be where its at.

edit: If you have any of those old TV coax to window line adapters, you might find a good VHF core inside one.