VENTON EXL-S modification

I would like to document my single cable GPSDO feeding modification made to my VENTON EXL-S LNB.

While modifying this LNB for external GSPDO feeding is extremely simple (just remove original XTAL and feed GPSDO signal externally via a short SMA pigtail into the original pins where the XTAL was previously located from the bottom side of the PCB) and was working here well for a few weeks, I did not want to have a separate cable, so I was looking for a cleaner solution.

The VENTON EXL-S is using a 25 MHz PLL and due to it’s long circular waveguide I find it extremely well suited for use with the famous dual band POTY feeds. Also due to PCB and case layout it has quite some convenient space inside for adding modifications. This LNB type is also nice since it comes with a very efficient dielectric lens that can be removed and used for the POTY dualband feed also.

For feeding the GPSDO signal to the LNB via the main coax cable I am using the triplexer circuit described by OH2AUE here in the forum and on his website:

OH2AUE Document (see page 15)

Here are roughly the steps that I followed:

1.       Open LNB case and remove all screws and open the lid

2.       Unsolder center pin of F-Connector (removing the solder works well using solder wick) until the centre pin is noticeable loose inside the PCB hole

3.       Cut carefully with a very sharp thin cutting tool several times around the PCB edge into the silicon that is used to glue the PCB into the LNB case.

4.       Use a very tiny and thin watchmaker screwdriver to VERY GENTLY lever the PCB out of the case. Do not apply to much force as the PCB is very thin and may break.

5.       Once PCB is removed turn it upside down and remove the 25MHz XTAL. This works best with a hot air station (cheap one for 35 EUR works well for this purpose) or use two soldering irons applied left and right of the Xtal. It will be very difficult to remove with just one soldering iron.

6.       Now turn PCB to upper side and cut the trace leading to the meander line (see photo)

7.       Also scratch away some of the solder mask and tin it

8.       Now place a 11 ohms resistor in series from the F connector pin to the meander line (see photo)

This approach I just copied from OH2AUE modifications documented here in the forum.

9.       Now add a 120pF capacitor (0603) in on top of the line leading from the F connector pad towards the PLL chip. Following this trace if useful because there is a convenient space available inside the lid.

10.   Add a 330nH (0603) inductor in series to complete the series LC circuit.

11.   Now just use a short piece of thin enamelled wire to connect the just created LC circuit with one of the two inputs of the PLL chip. I simply connected it to one of the conveniently located load capacitors from the original XTAL inputs (see photo)

12.   Put the PCB back into the case and solder the F connector centre pin

13.   Check for any short circuits on the F connector J

14.   Close the lid – LNB mod complete

With this mod and the OH2AUE triplexer I can feed my Leo Bodnar GPSDO directly into the triplexer (approx. 6-7dBm into the LNB) and the LNB PLL locks without any problems.

However, I noticed that with approx. 0dBm at the LNB it does not lock anymore….so this mod needs quite some drive. But since the output of the Leo Bodnar GPSDO was adequate I did not bother at first to improve this any further.

I will try to improve this later by adding a parallel LC circuit in series of the meander line and document here is successful.

Also going to try a TCXO mod for this LNB type to be documented here hopefully soon.

73 de Oscar DJ0MY

NOTE:

This post has been edited to correct the wrongly posted 12pF into corrected 120pF values (I will also try to update the photos)

Excellent suggestion to just isolate the XTAL from the top side of PCB - much easier mod - thumbs up ! Thanks Johannes for sharing...

73 de Oscar DJ0MY

Here is Part 2 of my modification:

Even if this LNB type might be obsolete within very short future (I still have a spare one that I am going to use for the TCXO mod) perhaps at least this serves as learning material and inspiration for future modifications of other hams.

As mentioned above the original mod still requires quite some drive into the LNB to get the PLL chip to lock.

So here I used the PA3FYM approach and installed a parallel LC cirquit into the line going to the meander line.

I was too lazy to remove the 11 ohms resistor from my first original modification. So I've put the parallel LC circuit just behind it in series.

The mod is very simple – with lid open and PCB visible just cut the trace towards the meander line and insert a 120pF capacitor and 330nH inductor (both SMD 0603 size) and solder them in parallel. (see pictures of my mod attached)

Ideally if you follow my first mod after reading this, just do not install the 11 ohms resistor and solder in the same place the parallel 12pf+330nH. That is in my opinion the most convenient location, in order not to interfere with the lid.

Result:

Now 0dBm drive and even -6dBm drive (did not test to go lower) is fully sufficient to make the PLL chip lock. Full success !

73 de Oscar DJ0MY

NOTE:

This post has been edited to correct the wrongly posted 12pF into corrected 120pF values (I will also try to update the photos)

PA3FYM sorry Remco - seems I was too much in a hurry when entering the capacitor values. (Mr. Thomson still applies today)

Will edit it in my text....

You were actually cited - see my "Mod part 2" post above - I learned the parallel circuit trick from you (hence you were cited by me). But not the serial LC circuit one (which I picked up as an idea originally from OH2AUE's work).....so I hope one citation should be sufficient.

Thanks for all the good work Remco. LNB works like a charm now...

There are still some OM that prefer to feed the GPSDO signal via external cable.

This mod is very simple....just follow the disassembly steps as above until you hold the PCB in your hands.

Then unsolder the XTAL as described above.

Now there are two options:

1. Use a small coax pigtail

2. Use a short and round bulkhead (those types fixed with a nut) SMA jack

I opted for No. 1 since I had a short RG316 SMA pigtail still in my junkbox. (but I have seen an OM on twitter that placed a SMA jack exactly at the same position were I guided my cable through the case)

If you opt for the pigtail I suggest using one of the two round indentations in the LNB case to drill your hole and passing the cable. Do not foget to make also an aligned hole in the outer case, before soldering the coax to the PCB.

Now with the pigtail threaded through one half of the outer plastic case and the LNB case, just connect the two pins of your coax (or the SMA jack) to the two pins of the removed XTAL. I passed the cable through the two XTAL holes and soldered them from the bottom side of the PCB.

For the VENTON the polarity does not seem to matter as both XTAL inputs are "floating" against ground....both ways seem to work well.

With this MOD the PLL locks with less than 0dBm 25MHz signals from the GPSDO into the pigtail without any problems.

I had to use some scotch tape on the bottom side of the PCB and XTAL compartment to prevent shorts of the braid against ground. So you may want to measure for this after assembly.

73 de Oscar DJ0MY

Note:

TCXO mod to be published hopefully soon.

Almost right - you forgot the 120pF & 330nH in series to be added to the F connector pin, too. (and from there a short wire to the PLL chip input)

See the pictures attached to the first message in this thread.

For the rest it seems you understood it correctly.

73 de Oscar DJ0MY