An improved reverse voltage protection circuit

With four components, and the on:off switch, Bill K9HZ provides the ultimate reverse voltage and over-current protection system for your µBITx.  The fuse protects the circuit from excessive current draw – so you won’t blow your finals when you wind up the bias too far and they try to go into thermal runaway.  The relay must be powered on to power your µBITx (and the switch must be turned on).  With the series diode in place the relay cannot turn on unless the power is wired up correctly.

uBITx gifts to St Lucia YLs

Bill Schmidt, K9HZ / J68HZ  writes, “Thanks to my friend Frans J69DS, freshly built uBITx radios were given to two well deserving young ladies on the island of St. Lucia yesterday.

“They are part of the Youth On The Air (YOTA) movement on J6 and now you can listen for them on their own radios.  Getting the radios to them had its challenges but they are very happy to have them!   We have more radios to give away too!”



Adding a tune control that has reduced power output

Bill K9HZ has added a tune control to his circuit to even up output power across the bands.  This will be featured in his ATU that is expected to b be documented shortly.

With the addition of a transistor driver for an additional relay and one additional potentiometer you can now have a tune button that will tune your rig with lower power output.    Pushing a tune button while keying the rig will result in lower power output.  Alternatively, you can wire up a DPDT switch for tune with the second throw on the switch wired to PTT.

Drive control – 2 methods

Bill K9HZ who we have already reported had developed a relay control system driven by the Low Pass Filter I/O lines.  In addition, Glenn VK3PE has also come up with a circuit that could be adapted using a digital potentiometer module to be driven by the Raduino using a digital I/O line.

Relay drive control

Bill K9HZ has now drawn up the circuit he used to allow for relay switching of drive levels by band grouping, driven by the LPF band grouping I/O lines of the Raduino.  No firmware mod is required to deliver even drive power across all of the bands.

RV1 in the yellow circle is the existing drive pot in the uBITx.  Remove it and connect the wires from the relay as shown.  Q17C, Q18C, and Q19C references connect to the transistor Q17, Q18, and Q19 collectors.  Then set RV1A, RV1B,  RV1C  and RV1D on the basis of the appropriate band grouping selected to keep the power out flat.


Digital drive control

Glenn VK3PE has come up with an old circuit that could be used to work with a digital potentiometer module to use as adrive control set by the NANO  firmware on a band by band basis.

The original drive control, RV1, would need to be removed and a few parts added, along with a digital pot (I2C control) in place of R3 below to form an attenuator in the RF path.

With some careful work it should be possible to design a small PCB that fits into the holes vacated by RV1, either vertically or horizontally.

The schematic is from page 62 of the book by Randy L. Henerson on designing a Transveiver. Its a very old book (1997) ISBN 0-07-028263-3


K9HZ Full Rig Power Control Unit

Bill Schmidt, K9HZ has  designed a fool-proof control circuit for the uBITx for power control.  This circuit prevents bad things from happening by shutting down the radio before any damage is done. 

It faults on:

  1. reverse voltage
  2. over power
  3. High SWR
  4. High PA Current 
  5. High voltage. 

It provides a visual indication of WHAT fault occurred, and the individual fault LEDs begin to flicker BEFORE the trip so you can fix the problem before you hit a hard trip. 

The fault conditions listed above can be expanded to any number by adding more SCR Trip components (they are set to trip at 1.8V whatever the fault is).

 When initially turned on, the transistorised RS Flipflop circuit comes up in the “Operate” mode.  If a trip occurs, it flips into “FAULT” and shuts down the PA. 

The circuit is reset with the “RESET” button, but ONLY if the fault has been resolved.  Turning the power off and on resets the circuit too.

Bill bread-boarded the circuit last week and has been using it on his radio for a while and found that it works flawlessly (yes transmitting and yanking the coax off the back of the radio shuts down the PA nicely!).  The circuit and a build list can be found in the BITX20 list’s files section.

Parts List for the uBITx Power Control Circuit

Capacitors Value Voltage
C1 10uf 16VDC
C2 0.01uf 50V
C3 0.01uf 50V
C4 0.1uf 50V
C5 0.1uf 50V
C6 0.01uf 50V
C7 0.01uf 50V
C8 0.01uf 50V
C9,C10, C11 0.01uf 50V
D1 Green LED
D2 1N4148/ 1N4001
D3 1N4148/ 1N4001
D4 Red LED
D5 1N4148/ 1N4001
D6 12V 0.5W zener 1N759, or 1N5242, or 1N6002
D10 BT149G SCR
D11 1N4148/ 1N4001
D12 1N4148/ 1N4002
D13 1N4148/ 1N4003
D14 1N4148/ 1N4004
D15 Red LED
D16 Red LED
D17 Red LED
D18 Red LED
D19 SB530
Q1 2N3904
Q2 2N3904
Q3 2N3904
Q4 2N3904
Q5 2N2222
Resistors Value Watts
R1 1K 0.125
R2 1K 0.125
R3 10K 0.125
R4 10K 0.125
R5 10K 0.125
R6 10K 0.125
R7 10K 0.125
R8 100K 0.125
R9 2.2K 0.125
R10 47K 0.125
R11 1K 0.125
R12 2.2K 0.125
R13 1K 0.125
R14 1K 10-turn POT
R15 10K 10-turn POT
R16 88K 0.125 Can just use a 100K POT set appropriately
R17 12K 0.125 Can just use a 100K POT set appropriately
R18 10K 0.125
R19 100K 0.125
R20 1K 0.125
R21 1K 0.125
R22 1K 0.125
R23 1K 0.125
R24 10K 0.125
R25 10K 0.125
R26 10K 0.125
R27 10K 0.125
R28 62 OHM 2
R29 1K 0.125
R30 1K 10-turn POT 0.125
R31 10K 0.125
R32 100K 0.125
SPST Momentary contact
Integrated Circuits
U1 LM339 (Make sure to connect Vdd and ground!!!!).
U2 BTS660P
Fuses Value Voltage
F1 1A Poly Fuse 50V
F2 4A Poly Fuse 50V
Your choice

How to even out uBITx power out

Bill Schmidt K9HZ has come up with a way of evening out the power output from the uBITx.

He says, “I had a couple of hours of spare time today so I started off my measuring the gain of the pre-drivers in the uBITx.. and sure enough, there is a lot of variation from 1.8-54 MHz (where I want mine to work).  Substituting the RD15HVF1 (my choice of RF PA) into the circuit with no other changes gives results similar to what John saw.. but it is because of the pre-driver stages.  I contemplated ways of compensating the drive with frequency but there isn’t a really good solution using reactive components because it causes some other non-desirable behavior (like a peak in drive at 21 MHz that is too high for the PA).

“I did land on a rather bruit force method that does work well… I removed RV1 (drive control) from the circuit board and replaced it with a tiny board with three small relays and four 100 ohm 10 turn pots.  The relays are controlled by the KT1, KT1, and KT3 drivers…  The short story is that now I have gain that is adjustable for essentially each of the bands (at least sets of bands that follow the LP filters).  I’ve adjusted the drive so that the PA puts out the same power (+/- about 2 watts) across the entire frequency spectrum.  I suppose this should have been an obvious answer, but I’m not fond of using relays for stuff like this even if they only draw milliwatts…”