Q90 blown by strong RF input on antenna line

Further to an earlier post about the vulnerability of Q90 in the µBITx, Gary
W6RAG wrote to uBITx.net today noting that the potential danger to Q90 from nearby strong RF signals is well founded.

Recently Gary was attempting an FT8 contact on 80 meters with his son about 400 miles distant.  Being unsuccessful with the µBITx, he turned off the rig and turned on his TS430 and made contact at about 60 watts output power. The ubitx remained connected to an end fed wire about 10 ft from his dipole that was in use with the TS430.

A few minutes later he tried the ubitx again and it had no output. He traced the failure to Q90. The emitter was shorted to the base just as reported by others.
The diodes in the balanced mixer will not save Q90.

Gary recommends that µBITx users disconnect the antenna if they are going to operate other transmitters with nearby antennas.

Toroid winding hints

Mark N7EKU/VE3 has  found the calculator on toroids.info to be very accurate — so much that he rarely measure after winding anymore. Toroids.info quotes a quite generous length of wire to use (even considering the 1″ leads) so he usually subtracts an inch.

No need to re-wind if you loose count. As long as you have a cell phone or other digital camera, just take a picture and then zoom in on it. It becomes very easy to count if you do that.

How to wind: first put the wire through the center, gather the ends, and pull them taut. Now wind one half of the windings, and then the other. This way you only have to deal with one half the length of wire at a time (instead of threading the entire length through for the beginning turns).


Microphony in your uBITx

Peter G8FWY had a problem with his µBITx.  It picked up sound from touching the case and pushed this through his headphones.

If you have a similar problem it is most likely to be ceramic capacitors in the audio stages of your rig.

In this case it turned out to be C50 in the audio pre amp.  Peter replaced it with a leaded electrolytic capacitor and the problem went away.


Tool tips – making a rectangular hole

John KZ1G notes a recent post by Fred (K3TXW):

“…I have problems making a rectangular hole in a piece of aluminium of exactly the right size, with the edges exactly parallel to the edges of the case. I inevitably make the opening too big or slightly wavy. The result screams “homebrew carelessness” though I’m not careless. Maybe there is a rectangular punch or something to do this right, but I don’t have such a tool; I use a set of files.”

In addition to files for work on front panel fabrication, I recommend an electronics shop include:

1.  A drill press with a chuck that will accept 0.50-in.-diameter bits.  Harbor Freight sells a couple of bench-top units for under $100.  Put a magnet on the base as a place to keep the chuck wrench.  (It’s usually best to clamp work to the drill press table.)  Once you have a drill press you’ll find many other jobs for it.

2.  A good set of sharp drill bits.  Bits with a titanium nitride coating remain sharp for a long time.

3.  A step drill bit.  I use an Irwin Tools Unibit 3/16-Inch to 7/8-Inch Step-Drill Bit with a 3/8-Inch shank.  Great tool when you need to make larger holes for controls or to start a rectangular cutout.

4.  An Adel-brand metal nibbler.  They show up on Ebay.  Or buy a new one at https://www.adelnibbler.com.  I’ve used one since I was a teenager and couldn’t work on chassis or panels without it.

Lay out your hole with masking tape around the outside.  To make a rectangular hole (see attached image) I use a step bit and smaller bits to make round holes that remove a lot of metal.  Just don’t get too close to the rectangle’s edges.  Next I use the nibbler to remove remaining metal close to, but not at, the rectangle edges.  Finally I clamp the panel or chassis in a bench vise so an edge of the hole aligns with the top of the vice jaws.  File away any remaining metal until the edge is parallel with the vice jaws.

Kevin KU8H hides imperfections around a rectangular hole in the front panel with a bezel. A bezel will hide a lot of sins. They are also easier to fabricate with straight, clean edges. Rough edges that might show a little on internal panels of chassis – who cares.

Your use of the bench vice to keep the edge straight during filing is good but will eventually damage the vice. I use a sacrificial piece of steel like a piece of angle iron clamped up along with the workpiece. In woodworking we call that a “fence”.

Kevin VK3DAP / ZL2DAP clamps a short length of angle iron along the line of the opening, and files to the edge of angle iron. This gives a nice straight line. Rectangular chassis punches are available but are costly, and the larger ones require lots of force, and may need an hydraulic press.


Q90 failure and prevention measures


Lightning took out Q90 on Mike K5ESS µBitx.  It also fried the switching power supply he was using.   Others have reported similar frying of Q90 due to lightning.   

Doug WA8UWV suspects any strong signal or static pulse on the antenna during receive will damage Q90.  He has another ham 2 houses away and DX Engineering is also located near by.  He contemplated a reverse connected diode across the base and emitter of Q90  to see if it eliminated failures.

Gordon KX4Z has speculated before that the failure mode is a REVERSE voltage on the base-emitter, as the series capacitor charges up due to rectification by the base-emitter junction.  This document on pp18-19:


seems to suggest that indeed, overloaded reverse voltage bipolar junctions fail SHORTED, while forward current would be expected to OPEN them.  The reverse diode would eliminate the charging of the series capacitor and protect the junction against reverse voltages, and someone tested this for ill effects and found none.

Gordon has put the reverse diode on his µBITx and similar radios and has had no failures as yet.


Some mods to improve RX sensitivity

George UR4CRG/RX3ARG  suggests some mods to improve RX sensitivity:

  1. Add a parallel capacitor to R12.  Try C=470 pF.  George found in his signal analyser that this gave a 3 dB rise in gain at 30 MHz.  He placed an 0805 SMD (or your could use a 1208 SMD part) over R12.
  2. Add two 100 Ohm resistors to  the collectors of Q11 and Q12.  You will need to cut the PCB tracks to add in the resistors.   The emitter-followers do not like HF signals and a small collector resistor gives light feed back

After these modifications, reception was noticeably more sensitive/



Crystal filter experimentation

Rahul VU3WJM has been scratch building a µBitx and experimenting with the 12 MHz QER filter.  He notes the following:

1) With the set of xtals that I have with 100pF caps and 200Z I/O filter the bandwidth is around 1.85Khz for HC49S crystals and 3.3Khz for HC49U crystals.

2) With a filter based on HC49U crystals, a lower bandwidth is achieved using 150pF caps but impedance drops down to 150-160 ohms. Lowering the
bandwidth also degrades the shape factor to around 1:2. This can be well understood considering that HC49U crystals have a lower motional inductance Lm.

3) As suggested by Allison KB1GMX, 82pF is working just fine in the filter and achieves a bandwidth of around 2.2Khz. Capacitor value is bit touchy between 82 to 100pf.

4) The Q of capacitors used has a major impact on the filter response. Parallelling up two values to arrive at a desired overall capacitance value results in a better response.

To see sweeps of a few different filter configurations select the reference on the list.   Rahul would love to hear from others of their experiences on the 12MHz crystal filter.