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Reversing the KL audio delay bypass mod

18 April, 2023 - Reading time: 2 minutes

Some of the KLs I've got have had the audio delay circuit in the receiver bypassed.  This audio delay is what makes the KL have the nice silent mute action.  I believe the delay is bypassed so that the receiver works better in data reception applications rather than voice.

I like the repeater to have the silent mute action, so if the delay has been bypassed, I restore the circuit to original.  The process is very simple.

The mod is evident by the resistor soldered to the bottom of the board near the black coax.  R168 has been removed to disconnect the delay path, and this resistor completes the connection into the rest of the audio processing circuitry.  Its value is 8.2k.

Here you can see R168s location north-west of the blue trimmer.

To reverse the mod, simply remove the resistor from the bottom of the board, and put 1.5kOhm into R168. I used a 1k and a 560r taken from a KL main board wreck.


2m KL150 PA repair

18 April, 2023 - Reading time: 4 minutes

The KL150 I took from my stock for the new VK7RVP 2m repeater seemed to be in good condition, until I tested it.

As usual, the low power mods were performed, channel frequency programmed, controller installed, etc.  Next the alignment was done, and it was then that it became apparent that the PA had issues.  The PA struggled to produce 47W, and power level wouldnt regulate properly.

Careful visual inspection revealed what seemed to be some missing parts around the base of the final device.  Next we refer to the schematic diagram for the PA.

According to the schematic there should be 4 capacitors on the base, I can only count 2.  Hmm...

Seems someone has decided to remove some for some reason.  Checking a known healthy and working KL150 confirms this.

The rest of the PA checks out fine.

I went searching all of the usual suppliers for what seems to be unobtainable mica SMD capacitors. Either they weren't listed, weren't stocked, or had minimum order quantities.  So I settled for next best, some AVX SQCB, expensive, but seemingly worth it.

No sooner had the package from Digi-Key arrived, I had the appropriate parts installed.

I had a bit of a clean up around the base. Some desoldering braid to mop up, acetone and cotton tips to dissolve and remove the excess flux gives it that nice clean factory look.  I installed the new capacitors as pictured, and prepared the unit for test.

Results are pleasing with the transmitter making an easy 70+ watts, good power regulation at 50w, and nice spectral purity.

Thats it, fixed.  Almost ready for installation, needs soak testing, and I might give it a 5DJ voice board to make it a bit nicer on air.

Converting the Hamtronics R303 to frequency agile

19 July, 2016 - Reading time: 3 minutes

The Hamtronics R303 can be ordered with up to 4 frequencies programmed, which are selectable with jumpers.

When I ordered my R303, I had ordered CCARC frequencies to be programmed.  Now that I no longer affiliate with the afore mentioned I needed to add new channels to the receiver.  Two options are available.  Order a new IC with new channels, or mod the receiver to accept the older style Hamtronics frequency programming via DIP switch.  I chose to mod the receiver to make it completely frequency agile - this will keep it going well into the future if Hamtronics cant update the micro for whatever reason, and I can reprogram it at will.

The process is relatively simple.

The R303 uses the LMX1501 PLL, which is a serially programmed device.  A microprocessor (U1) is programmed at order time with the channels required.

As chance would have it, the older R302 runs the same PLL, reference and IF frequencies, but has a different micro with DIP switch inputs allowing it to be fully frequency agile within the band.

A MC68HC705J1A was ordered from Hamtronics configured for the R302 in the 2m band.

The circuit around R302's U1 was duplicated onto some prototype board, and the connection to the R303 was made with wires attached to a 8 pin chip socket that plugs into the original U1 chip socket, making the mod fully reversible.

Thats it, done.  Now the receiver can operate anywhere in the 2m band, and in particular not on CCARC channels :)

This receiver is in 'MY' high performance home made repeater.  The exciter uses DIP switch frequency programming already, so no mod necessary.

In future I will present an article on the complete repeater, and its associated 120W PA.

Using the Sanwu SW-HF07 USB sound adaptor for VKLink nodes

3 July, 2016 - Reading time: 6 minutes

A short pictorial demonstrating modifications to the Sanwu SW-HF07 USB sound adapter for use with VKLink nodes.

A search on eBay finds what appears to be a suitable CM108 based USB sound adapter.  The eBay pictures suggest the chip is genuine, but theres only one way to find out.  I ordered 3 and they arrived fairly quickly from Hong Kong, and all for $17AU.  Cheap.


Visual inspection checks out OK.  Chip is marked CM108 - bit hard to see in the photo.  Good start.

lsusb identifies it as 0d8c:000c

Which looks up as

At this point its safe to say the adaptor runs a genuine chip.

Plugging the adaptor into working VKLink Pi produces the expected results.

Let the mods begin. Firstly to provide strain relief to the soldered joint to the IC pins, take some wrap wire and feed under then pins of the chip to solder to pins 13 and 48, thusly.  Use caution to not bridge the adjacent pins on the IC with solder.  If in doubt, check with multimeter.

That done, lift the capacitor nearest the edge of the board, nearest the green socket.  This breaks the path between the ring contact and the rest of the circuit.  We use the ring contact to provide PTT signalling.  Install a 1N4148 or similar diode to the ring contact of the pink socket.

Next, break the track between tip and ring contacts on the pink socket.  This allows the use of the ring contact for COS input from the receiver.  RX audio is brought in through the tip contact.

Use a BC549 or similar and a base resistor and attach suchlike.  I used 4k7ohm.  This is the PTT output. Collector to ring contact and emitter to ground contact.  In this photo the COS input wire has been attached to the diode.  This can wait until later if it makes things easier.

Make the connection of the wire to pin 13 and the base resistor.

Thats it, the modification is complete.

Its the readers responsibility to provide correct interfacing to the nodes transceiver.  The COS input logic direction can be defined in software, but the input is expecting a pull to ground.  In my particular case I needed to install a pull up to 5V resistor to the ring contact to prevent the input floating.  The software would not detect change of state on the input with received signal.  Level adjustment can also be done in software, but I prefer to run RX level at 0 and trim the audio level externally.  I run the TX audio level at 500 in the tune menu, and trim the level into the transmitter.  YMMV.


It would seem there are some variations between boards of the same manufacture and model.  Where the track gets cut between tip and ring contacts on the pink socket on the outboard side as described above, some boards have a track between contacts on the inboard side of the socket also.  Two of the three boards that I ordered had this feature, but the original board I modified did not.  Simply take a sharpened small flat blade screw driver or similar and break the track so that tip and ring are disconnected on both sides.

VK7RVP Gets Its Six Metre Repeater

24 December, 2015 - Reading time: 4 minutes

On Tuesday 22/12/15 4 persons made the trip to St Valentines Peak.  Those persons being Graeme and John Rand, the helicopter pilot David, and myself.

The purpose of the trip was to investigate the fault with the 70MHz Forestry system.  I was invited along on the trip to assist with that, and also fly my equipment to site.

My equipment consisted the VK7RVP 6 metre repeater, the 2 metre antenna array, power divider, duplexer and some Heliax, and the 70cm dipole array.

Initial tests were made on the Forestry system, and it was found that a significant voltage drop was in the power supply cabling.  The cause was a high resistance joint which was quickly and easily repaired.

While Graeme was doing some tests and alignments on the system, John offered to help with anything that needed to be done outside.  I suggested that he could help mount the 6m dipole, and he gladly accepted.

While John assembled the boom clamps onto the tower, I assembled the dipole, with the help of the pilot.  Timing was good, and just as I finished the dipole, John was ready to accept it.

I assisted with the installation of the dipole from the ground.

Shortly after, the antenna was ready to be tested.  Graeme used his Anritsu Site Master to check it, and it was found to be excellent.  John proceeded to seal the connector and tie the cable off.

The home made rack frame was slightly too narrow to accept the converted Unilab repeater, so some modifications were necessary to let it fit. Luckily Graeme had his cordless angle grinder and offered to do the modifications for me.

With the mods done, the repeater and duplexer could be mounted, cabled and powered up.  I proceeded to do that work, with some help from the pilot.

With everything in place, it was time to power it up..  It works!

Some photos,

We were lucky enough to be graced with the presence of a Wedge-tailed Eagle.  I think he was a little envious that our wings were bigger than his. :)

Special thanks must go to Graeme and John.  I could have done it myself, but it would have meant another trip back to site to do the installation at a later date, and that means at least a 2km walk, carrying tools and other gear.