hp8672a

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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.

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.
I got the idea to try the YIG to work outside of the synthesizer, using a separate power supply.

For this test I needed several voltages +20, -10 and then a current generator to be used as coil current.
I did not have that much power supplies available to get the different needed supply voltages,
so I decided to use the +20 and -10 Volt from the HP8672a, these could easily be checked to be OK,
 and using my LAB power supply as separate supply used as current source for the coil.

In the picture below you can see two wires from the YIG connecting to the external feeding of the coil current.
 I disconnected the two coil connections from the HP8672a, and connected them with the two wires to my separate power supply

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 coil.
I used a HP435 power meter as indicator of YIG power output. As I tried to adjust the current through the coil, I immediately saw RF output from the YIG !!

 
Using my spectrum analyzer I checked the frequency and it was clear that I was able to control the frequency of the YIG by adjusting the coil current.

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.
I even tested a YIG from my spectrum analyzer HP8566b which I know was malfunctioning and had successfully been
replaced for a new one in my spectrum analyzer; see hp8665b repair for this.
 This YIG was still working but not very well, at some frequencies it oscillated and at others not.

From this YIG I found that the YIG sphere was not in the middle of the surrounding striplines,
I need to try and readjust this YIG sphere into the middle to try and repair this YIG also.

 

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.
So it became clear that there was an error in the YTO driver board.

Then by visual checking the YTO driver board I found a burned place with a burned resistor.
I had seen this before but only from looking at it with my bare eyes, now with using extra light and some magnification glass
I saw two dark components from which one seemed to have burned.
This sure was a good clue and later proved to be the reason for the failure of the HP8672a.

In the service manual I found the value from the resistor at 10 ohm being between the -40 Volt from the mainframe
and the -40 Volt on the YTO driver board.
I checked the surrounding from this resistor, because it was strange that the resistor was burned.
I found a direct to the resistor connected decoupling capacitor, to be broken with short circuit, 0 ohm resistance.

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,
I tested the YTO driver and the YIG within the HP8672a circuit, it was working again with output and a locked indication.