Midland 38 power tuning

[Cousinjack]

Hey all,

Bought a job lot of 4x midland 38’s a while back. I have been going through them all, cleaning them up and sorting mic’s etc.

Connected each one to the test set, and they were all putting out about 2.8w.

I’ve managed to get them all up to 3w, but can’t get any higher. The alignment procedure advises to turn vr5 up to maximum, tune each coil and then pull vr5 back to achieve 3.8 - 4w. I have gone through all the cans very carefully but 3 is the maximum, not to mention pulling vr5 back !!

They are all much the same, surely something must be amiss somewhere?

Any advise greatfully received as always

[The Collector]

Probably a stupid question and I won't try and teach you to suck eggs, but is your power supply definitely putting out 13.8v? At lower voltage levels, the TX power may drop slightly.

If everything's okay voltage-wise, there have been a few radios in the past that haven't been able to put out the full 4 watts, usually cheapo ones with cheapo boards where 3 - 3.5 watts was the maximum that could be acheived.

I wonder if anyone's got any later CB mag's that have a Midland 38 review in them, as they may mention any output low wattage issues???

Other than that, I haven't got a clue sorry as I'm a bit of a dunce when it comes to most things like that

[Cousinjack]

Yeah, not sure...
Thanks for the suggestions, I too am not the greatest at this stuff, but I am learning !!

In this vid, he goes through a kernow beta 3100, which has the exact same board, and manages to get 5w out at full power, then trims it back to just under 4. I was wondering if maybe one of the components was prone to failure, giving less than optimum output.

[ch25]

How do you measure power? Antenna/dummy load? Are you sure your meter shows correct measurement?

[Cousinjack]

Measured in two ways.. got my marconi 2955 plugged into the antenna socket... assuming this is going to give the most accurate reading.
It seems to read fine on other rigs.
Also checked it with a cheap power/swr meter that also gives similar readings..

Going to have a go at following the rf with my homemade probe this evening... see if I can find a point where it should be amplified but it isn’t .....

Exciting stuff ....

[ch25]

I would rather lower carrier to get more space for audio.
Have fun.

[Cousinjack]

I would rather lower carrier to get more space for audio.
Have fun.

Could I have that in amateur language please ...

[ch25]

Put 30mV, 1KHZ tone on mic input and tune for maximum output power and 98% modulation.

[Cousinjack]

Ok, thanks for explaining.
But kind of what I’m saying is that I can’t tune it any higher than 3w, there’s no more to be had ....
and it’s an FM rig so I’ve set the deviation at 2.2

I can see that adjusting the deviation (or modulation ) would have an effect on the carried audio, but surely these rigs ought to punch out more than 3w ?

[ch25]

There is no modulation transformer, so I doubt you could get more.

[Cousinjack]

So my conundrum with these rigs continues.....

I’ve got 2 that appear to be maybe an older version, and two of a newer version. The older version seem to put out more power than the new, mainly due to an extra pcb On the newer type which alters the incoming power. I think this is some kind of inbuilt dropper that will drop from 24v to 12v if required. The older ones don’t have this board.

I’ve managed to get one of the older ones up to 4.2 watts max. Thus proving my theory that they are at least capable. The ones with the dropper circuit are the worst, only achieving 2.8w. This is in part due to the dropper circuit, which seems to drop the 13.8 to 12.0v.
By bypassing the extra circuit and applying 13.8v direct (as per the older type) I can get up to 3.4w .... ( so The Collector was bang on the money there !)

Still struggling to get higher though..