Warning: Some uBitx kits shipping with incorrect female connectors

KF2510 headers in uBITx

There are two 8 pin wiring male headers in the kit – one on the main uBITx board and the other on the Raduino.  Today, Arvo KD9HLC started assembling the wiring on his kit.  He found that the female plugs didn’t match the headers on the board.  After a bit of head scratching and further investigation he found that the female header plugs with flying leads only had 7 pins!

It is likely that other kit purchasers may have the same issue.

These headers are of KF2510 type, widely used in China and are very affordable.  Sets of 50 male/female pairs of 2, 3, 4 and 5 pins are available for about US$3 ex China if you can wait.  And 8 pin females can be found on the various Chinese merchant sites as well.   However, these headers are a bit harder to source in the US.   In NZ and Australia they can be sourced from Jaycar.

THe uBITx kits come with  2 x 8 pin, 1 x 3 pin, 1 x 2 pin wiring harnesses with KF2510 female plugs.   The Raduino unit connects to the main board with a 16 point pre-soldered header.

HINT:   You can substitute ordinary Dupont header strips or single female pins.  They won’t be polarised, so you will need to be careful making connections.

Reference

Speaker Grill templates

Peter Cousins G4NJJ suggests that to get a tidy speaker grill on your EF01 or other enclosure you should have a rummage in the kitchen utensil drawer.

You may find the perfect template.  Peter used a plastic sifter lid as the holes were the size he wanted.

He drew a center line where he wanted it positioned, lined up the template, drilled one hole on the outer circle and inserted a nut and bolt, drilled a hole at the opposite end of the template, inserted a nut and bolt, and then drilled out the rest of holes in perfect place.  Hot glue allowed the speaker to be placed on the underside of the lid of the enclosure.  Done!

Oh and in case you wanted another idea for the template …  And just make sure the XYL is not aware of your minor “alterations” to essential kitchen equipment!

And if you want a variant on this theme of making a speaker grill look real classy, check out Glenn VK3YY’s website for  a ham shack alternative that won’t get you in so much trouble with the XYL:   https://vk3yy.wordpress.com/2017/01/22/bitx40-v3-and-raduino/

 

Tuning up

End whistling into your mic with this suggestion from David GW4AKZ:

When using an antenna tuner on SSB, we could easily add a tone generator and switch to do that.

I have fitted a switchable -6db pad between the output of the pcb and the antenna socket. With the circuit switched in the worst case SWR is about 1.65 to 1 with an open or short circuit.  This prevents the finals from ever seeing a poor SWR.

Tune is easily achived with the key down and reduced radiated power when tuning up.

I use 2 watt carbon resistors 33 ohms in a t attenuator circuit. 2 in parallel to give 16.5 ohms which is close enough to the calculated value.  A relay is s used to switch this attenuator in and out.

Chris suggests replacing the 51 ohm resistors with 2x 100 ohm 3watt resistors.
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IRF510 failure avoidance

Arv K7HKL notes that the problem with IRF510 RF amplifiers failing seems to be a recurring one for those who are not quite careful with antenna matching, bias level, and drive level.

As a way to start looking into this situation Arv performed some on-line searches to see how others were working around this problem.

There are a number of potentially useful ideas contained in those articles and discussions, but nothing that obviously applies directly to the problem of blowing IRF510 devices at only a few watts of power if the antenna is mis-matched.

Reference

 

Marco – KG5PRT notes from one of the stories reviewed …

… In the middle of the page:
“I destroyed many IRF510 FETs during testing. In fact I blew a small hole in one and another into several pieces. It was quite a shock when the first one was destroyed because it made a loud noise like a rifle being fired.

Once I got tired of replacing the FETs, I built a current sense circuit, which shuts off the bias once the amplifier draws more than about 3 amps from the PSU. I think this circuit is essential. You can build it into the Power Supply or into the Amplifier. I built it into the Amplifier because the power supply, which is also homemade, does not limit until 7 amps. With the current limit circuit the amplifier now survives transmitting into any SWR from an open circuit to a short.”

He notes that the  current limiting circuit has only 7 parts.

Reference

 

And  Arv K7HKL suggests implementing this bias control circuit used by AC2CZ in his 50 watt IRF510 linear.  He suggests you could probably do this using 2N3906 transistors.  Arv likes the fact that it includes a warning
light/LED.

Inline image 1
from: http://www.g0kla.com/scpa/SimpleCheapPA.php

Or this one:

Conclusions about protecting the IRF510 devices

Arv’s conclusions to date are:

  • Power supply current limiting (2.5 to 3A) seems to work.
  • Bias voltage shutdown on high current (3A) seems to work.
  • Driver impedance to IRF510 gate should be quite “stiff”.  Use a 50 ohm pad.
  • Use a low resistance (4.7 ohms) in series with each IRF510 gate lead.
  • Keep IRF510 leads short.
  • Impedance presented to the IRF510 drain should be 12.5 ohms.
  • RF output transformer should be 1:2 windings for 1:4 impedance transition. A Binocular core may work better…?
  • Avoid the problem by using some other MOSFET.
  • Use plastic fuses at 3A for self-healing action.
  • Fuse rig power at 2 to 3A to avoid IRF510 failures.
REFERENCE