Yesterday, I set up a separate antenna for the uBitX on my front porch, to test battery operations, potentially to use the rig in the field. The antenna I set up was not ideal, and my Elecraft T1 ATU would not tune it. But now I'm not so sure. I was powering the uBitX from a Stanley automotive jump and power box, the kind that has a lighter-type socket on the front, and automotive battery clamps on the back for jumping a car. The voltage is 13.7v, not a lot higher than what I was putting into it on the bench, which was 12.5v, but the available amperage was of course much higher -- this is a BIG jump box that I purchased for jump-starting my wheelchair van in an emergency.

I haven't measured the available amperage on the lighter socket, but I suspect it's much higher than the 1.5A I limited my bench power supply to giving. The bench supply has been set to 12.5v @ 1.5A, and with those settings I get almost exactly 5W out on both 40 and 20m into a big dummy load (the MFJ paint can model). But when I power it from the battery box, I get well over 5w, around 20 watts from whistling into the mike. The key gives a very high output power as well. It may be that it was too much output power for the tuner to handle, and it wouldn't such a strong signal.

My bench supply takes 19vdc in from a switching supply that gives 19v @ 3.5A, and allows the user to set a buck-boosted output between 0 and 18v, with commensurate amperage. The DC in socket I installed is the same size as the one on the uBitX, and most of my other projects, as I thought ahead to powering them. I just tested connecting the jump box to the bench supply, and the bench supply to the uBitX, and can keep the higher power from the battery from overpowering the radio. I'm going to order an extra bench supply module -- cheap on Ebay -- and install it permanently on the back of the uBitX to give me controllable flexibility in the output power. Running the uBitX at 12.5v @ 1A limit brings the output power into the dummy down to 2w.

Is there a point below which I might damage the radio from running it too low? Has anyone else had similar experience with too much amperage?

I'm going to rethink the speaker I have mounted in the case, as it doesn't have enough oomph to make it worthwhile. Or, I may add a small audio amp for the speaker. I have several in my junque box. Listening with a pair of headphones sounds great. I'm having no trouble with the audio on phones, just on speaker, and the incoming signals sound good. If nothing else, I have a 10w musical instrument amp from Monoprice on its way. I was going to use it for amplifying my harmonica, but I can use it for this without difficulty. It's a battery-powered model, so I'll be able to take it into the field as well.

I have some antenna kits on the way, including the multi-band antenna on the QRPGuys site and the parts for a "flimsy" W3EDP antenna on the way (I didn't have a 4:1 unun on hand; that's on order as well), and will report my results with them. I'm starting with a Sotabeams 40m end-fed with their "Pico Tuner", which I will be building today.

Gwen NG3P

Farhan,

It was too much! Q72 started to get hot but not Q73. Means they are not matched.
Iq went up almost 100mA. So I used a pot where 47 Ohms seems fine.

What may work that instead of these diodes use the same transistor BE junctions!
Then just two may work, need to experiment on this.

Raj

At 29-06-18, you wrote:

What if we use three diodes instead??

On Fri, 29 Jun 2018, 13:35 Raj vu2zap, < rajendrakumargg@...> wrote:

Got back to town and plugged in the uBitxr4 and found the audio distorted.

Poking around the finals I found that the two diodes D15 and D16 gave only a drop of 1.2V total

but the final transistors needed more like 1.5V. i.e. 0.75v each.

I simply cut the track between D5 and D16 and soldered a 100 ohms multiturn preset as shown

in picture below. (Preset leads used is center and one side)

Start with the two soldered connections of the preset set to ZERO ohms.

Power on and tune to a station at low volume and slowly increase the preset till the audio

becomes clear. In my board the RX power consumption increased by 25mA at the best sound.

Just check and see that the output transistors are not getting hot.

My board sounded great with 47 Ohms between D15 D16 and I run at 13.6V. If you run at the

same supply voltage then you can try a fixed resistor. For 12V supply you could try a higher value

like 56/62 or even 68 Ohms.

Cheers

--

Raj, vu2zap

Bengaluru, South India.

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Got back to town and plugged in the uBitxr4 and found the audio distorted.

Poking around the finals I found that the two diodes D15 and D16 gave only a drop of 1.2V total
but the final transistors needed more like 1.5V. i.e. 0.75v each.

I simply cut the track between D5 and D16 and soldered a 100 ohms multiturn preset as shown
in picture below. (Preset leads used is center and one side)

Start with the two soldered connections of the preset set to ZERO ohms.

Power on and tune to a station at low volume and slowly increase the preset till the audio
becomes clear. In my board the RX power consumption increased by 25mA at the best sound.
Just check and see that the output transistors are not getting hot.

My board sounded great with 47 Ohms between D15 D16 and I run at 13.6V. If you run at the
same supply voltage then you can try a fixed resistor. For 12V supply you could try a higher value
like 56/62 or even 68 Ohms.

Cheers

--
Raj, vu2zap
Bengaluru, South India.