Even a master  makes mistake sometimes. However to fix it could be complex.

Check and modification of a Kuhne 76 GHz transverter

In november 2025 it was found by CT1BYM that his Kuhne 76 GHz transverter was not working as it should.
 

After measuring the in and outputs of the transverter it was obvious that it was not working on 76032 MHz but that it was receiving on the image frequency .

For 76032 MHz the transverter uses an IF of 144 MHz.

With such a low IF it is difficult to filter the image frequency.

Solution in modern design is using an image rejection mixer which takes care of the rejection of the image frequency.

Due to a assembling error at the factory, the image rejection mixer was working on the wrong sideband.

It was receiving on LO-IF instead of LO+IF

After the report from Miguel I became suspicious and decided I needed to check out my own transverter.

Important: do not open up the transverter if you don't have enough experience with working on delicate equipment

From the sold 76 GHz transverters only very few have the error as mentioned,
so if you have a 76 GHz Kuhne transverter, there is a very big chance that yours is working fine

If you have a transverter the best way forward is to measure if it's working on the correct frequency before taking any other actions

First step: Measure the receiving of the wanted frequency and compare it to the image frequency

The wanted frequency (76032 MHz) should be significantly stronger (more than 20 dB) than the image frequency (75744 MHz)

For this mesurement I used my HP8672a synthesizer at 9504 MHz followed by a times four multiplier to 38 GHz.
The harmonics from the 38 GHz multiplier at 76032 MHz could easily be received by the transverter.

After tuning the synthesizer to 9468 MHz the harmonics of the 38 GHz multiplier at 75744 MHz could be copied and then it became obvious that the 75744 MHz signal was received much stronger than the 76032 MHz signal.

	Measuring at 76032 MHz the signal from the multiplier
	Measuring at 75744 MHz the signal from the multiplier

This proved that my transverter was not working as it should, and it needed to be modified to be able to receive weak signals on 76032 MHz.

If your transverter is not working as it should, then you should decide if you want to modify it yourself or send it back to Kuhne and let them do the modification
If you have any doubt then send it back to Kuhne. Some of the used components are not available anymore, blowing them would be the end of your transverter.
 

Step 2 modification

I asked Miguel what he did to modify the transverter to the wanted frequency.

He send me pictures of the location where two resistors needed replacement at the image rejection mixer.
Also he send me pictures how the transverter could be opened.

	The location of the image rejection receiver mixer and the two resistors needed to be replaced.
	Some screws to open up the transverter are hidden behind stickers, In total 13 small screws need to be removed to open the transverter.

After I opened my transverter I made a close up of the parts where the modification was needed.

At first I was a bit doubtful as the involved components were really very small.
Being tired I decided to postpone the work until the next day as it would need all my concentration
I prepared all tools needed to do the job.

For the job you will need a very good soldering iron with a very small tip. Use a fine tweezer for picking up and repositioning the loosened resistors.

Also you need to take all necessary precautions for static discharges, ESD protection.
Ground the transverter and connect your soldering iron with an extra wire to the transverter, and put on and Anti Static wrist strap to put yourself to ground.
If available use a ESD table mat.

The next day I took all my courage and started to dismount the two resistors and reposition them on the correct location.

After the de-soldering and repositioning and cleaning of the board it looked like this

After some waiting time to be sure that all alcohol remains from cleaning had evaporated I closed the transverter and started measuring if the modification was successful.

	Measuring at 76032 MHz the signal from the multiplier
	Measuring at 75744 MHz the signal from the multiplier

This showed that the modification was successful

 

After feedback from Michael DB6NT about what I did he send me the original drawing of the involved part of the components placement.

 

Today 5-11-2025 I tested my second 76 GHz transverter and it's also not on the correct frequency

Perhaps serial numbers can tell something

The following serial numbers were wrong from delivery

00014659 from CT1BYM

00014652 from DC1EHG

00014674 from DC1EHG

 

Special Thanks for the information for this modification go to

Miguel CT1BYM discovering the error of the transverter wrong frequency

Luis CT1DMK for the proposed solution in replacing the two small resistors

Michael DB6NT for more background information and the drawing of the components placement as intended.

Now we can use the transverter to receive the very weak moonbounce signals at 76032 MHz.