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  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.



Sudden loss of power


Jim Sheldon, W0EB, while prepping a V3 uBITX, he had already built up to give to a friend, all of a sudden observed the RF Power output went to ZERO.

He thought for sure that he’d somehow blown one or both of the IRF510’s, but  nothing is always as it seems.

He checked voltage to the drains of the IRF510’s — ZERO (yes it was hooked up and turned on). Tracing the voltage back, he had nothing on the supply ends of L8 OR L9.  He checked continuity between those same points of L8 and L9 and had good connections there.

He found that he had +12V on the PA Power pin of P1 but nothing on the supply end of L8 which gets connected by a short trace to a very small VIA through the board and a short trace to L8 on the underside.  Looking at the underside, the trace was not burned but it turns out that tiny little VIA was open — possibly from poor plating through the hole.

He put a piece of 30 gauge wire wrap wire through the via and soldered it on both sides. This brought the +12v back to the supply side of L8, L9 and subsequently the drains of the IRF-510’s . After putting it all back together again he now had full power restored.

A few people have observed similar problems with loss of power. This might be the cause in these cases as well, and is definitely worth checking.  There will be no visible indication of anything bad between the PA Power pin on P1 and the rest of the PA circuit.


LED power out indicator

Lee  “I Void Warranties” N9LO ran across this circuit when looking for a transmit indicator for his Small Wonder Lab PSK31.

[Note the circuit was originally drawn incorrectly, but has now been updated]

Lee threw it together (there are not many parts) and it lights up bright and solid for CW and you an watch your modulation on SSB.   I can really see the difference when I switch in my SSM2167 module.

Jerry KE7ER recommends placing the circuit before the transmit LPF, not at the antenna port.   The diodes potentially create a bit of harmonic content that the LPF’s would remove.