Re: uBitx v.4 build information

Bill Cromwell


Did you reset the bias pots for the finals at the new voltage? You would have to set them to minimum (full clockwise) and then apply power. Go through the whole procedure for setting the bias with the 20 volts applied instead of the previous 12 volts.

Can your buck/boost gizmo supply enough current. Twenty volts with no load drops to what voltage when transmitting (measured at the radio)?


Bill KU8H

On 4/16/20 10:40 AM, Rubens Kamimura wrote:
Step Up (boost converter) arrived, so I get 20Vdc output from 13.8Vdc input, I used 20Vdc to power PA Power of uBITX v4, when adjusting the Ibias, the current went up at once reaching 15A , obviously damaged IRF510, PA Power plug, Ammeter ... I replaced the damaged components, so this time I installed a 0.34 Ohms power resistor (5W) in series with the step up output (20Vdc), but the output power is low, even with RV1 at maximum ... and C81 capacitor with 100nF, I only got about 8W.
What can I do now?
Rubens Kamimura .'.
Cel. (18) 9-9819-2225 WhatsApp ou (18) 9-9819-2255
Em ter., 7 de abr. de 2020 às 15:44, Rubens Kamimura via <> < <>> escreveu:
Ok, tks.
73 de Rubens py2pvb
Saudações fraternais, 73/51 PY2PVB
Rubens Kamimura
Tel. (18) 3746-8680 ramal 116
Cel. (18) 9-9819-2225 WhatsApp ou (18) 9-9819-2255
Em ter., 7 de abr. de 2020 às 14:16, Evan Hand <elhandjr@...
<mailto:elhandjr@...>> escreveu:
Most likely it was heat that fried your finals.  Digital modes
have been reported to have this issue on all of the uBiTx
models, as they keep the transmitter on for 15 seconds at a time
at full power.  Before going back on the air I would suggest
that you add a fan, larger heatsinks, or both.  Remember that
the stock radio does not have electrical isolation of the
heatsinks.  Either keep the new heatsinks isolated, or add the
insulators for the IRF510s.  The alternative is to reduce the
output power with the RV1 adjustment.  I would adjust RV1 using
an RF power meter and keep it under 10 watts for 40 meters. Monitor your finals temperature to verify that they are not
getting too hot.  As long as you can keep your finger on them
right after the end of transmission, then they should be OK.  I
would send a continuous tune signal for at least 15 seconds into
a dummy load, turn off the power and immediately check the
finals.  I did learn that if you do not turn off the power,
there is a large risk that you will move the heatsink and short
it out, usually just blowing the fuse.
As to how to set up the bias values for the new finals, there
are instructions on the HF Signals and
<> sights.  I have used HF Signals for the bias
You should not need to adjust the BFO or frequency calibration.
The above are just suggestions.  There may be better ways to do
the testing, however, the ones that I know of would require an
IR measurement device.
bark less - wag more

Join to automatically receive all group messages.