During the Heelweg meeting in 2019, A friend asked me if I could try to repair his HP8672a , it gave no output and indicating error no phase lock
My first idea was that probably the YIG was failing so I followed the service manual and tried to isolate the error.
I tried to follow the instructions for checking the YTO loop, this did not get me a good indication what was wrong.
I then measured at the YIG oscillator RF output, it was zero output so definitely something wrong in this part.
As I did not find an error in the YTO loop procedure I concluded that the YIG was defect.
The side mounting of the YIG mount, the YIG removed and standing beside the instrument
I removed the YIG from the HP8672a and opened it to check if I could find something wrong, it did not help as all looked fine, no visual damage found.
On Ebay I found a replacement YIG and decided to order this, when it arrived I mounted it and measured RF output, ........... also zero !!!!!
My conclusion: the replacement YIG was also defect.
Then I had a long period with no progress, we had COVID and there was no big need to proceed with this repair.
In the end of 2022 it was decided that in January 2023 there will again be a Heelweg meeting, this was a new trigger to start working on the repair.
After I opened also the replacement YIG and found no problems I became
suspicious that perhaps the YIG's were not the problem.
For this test I needed several voltages +20, -10 and then a current generator to
be used as coil current.
In the picture below you can see two wires from the YIG connecting to the
external feeding of the coil current.
I was using a regulated power supply adjustable from 0 to 40 Volt and a resistor
of 4.7 ohm in series with the coil to be able to adjust the current through the
This was a confirmation that the YIG was not defect and that the problem should be within the HP8672a YTO driving circuit
In this way I also tested my other YIG's and all were functioning fine.
From this YIG I found that the YIG sphere was not in the middle of the
Now being obvious that the YIG from the HP8672a is working, I had to continue the search for error inside the HP8672a YTO driver circuit
I again tried to follow the service manual to isolate the error but found no success using this way.
I then tried to measured the YTO loop steering voltage as coming from the
digital to analogue converter, this was working like it should.
Then by visual checking the YTO driver board I found a burned place with a
In this picture you now can clearly see the burned resistor, the broken capacitor is already removed giving a much clearer view on the problem
After replacement of the resistor the capacitor and the somewhat damaged
resistor direct next to the burned resistor,