HP8970b error18

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HP8970B


 

A friend of mine had a HP8970B with error 18, this is an error which is often found as error of this instrument

This 8970b was having option 20 with an extended frequency range of 2045 MHz maximum

 

After opening the instrument for a first initial check I found a massive amount of dust in the instrument, so I decided to give it a clean up.
I did not have the idea that this would solve the error 18 but I found it necessary.

 
 

After a clean up I stared again and tried if the error changed, it did not.

Then I started with measuring the different parameters.

I downloaded the different service sheet en searched for more info on the option 20 frequency extension.

I found the documentation service manual supplement of this frequency extension which helped a lot in error search.

I started adjusting the 1st LO offset but this did not change the error 18.

Then I started measuring according to the supplement manual.
I adjusted the 600 MHz SAW oscillator but it did not help for the error 18

Then I decided to measure the output from the YIG oscillator.

According to the service manual supplement it needs to be between 14 to 18.5 dBm
I measured a lot less a value between -12 to -13 dBm so it was clear that the YIG was the problem.


 

How to continue

We discussed on possible solutions but best would be a new YIG but buying a new YIG is much to expensive for hobby instruments

A search on Ebay did not bring me a YIG for sale as replacement.
Then someone else found a YIG on an Chinese website for sale for a very reasonable price so we decided to go for that

A few weeks later the replacement YIG arrived so connect it and measure the output power
What a disappointment output power was at -5 to -10 dBm so no good.

I decided to try and open the YIG with the lowest output to see if there was a chance to find the error.

Opening the YIG is easy the only difficulty is unsoldering the PCB on top of the YIG

    
 

Looking at the picture of the YIG oscillator I marked tree point in this picture.

The highest green arrow is the Fet from the oscillator,
I was almost certain this Fet was working fine as I found frequency from the YIG to be correct

The second arrow, red, is the Fet for amplifying the oscillator to the required power level,
my suspicion is that this FET is the weak part of this YIG and I expect it to be broken, still have to confirm this

The last green arrow is an output attenuator, don't know how much but I expect it not more than 3 dB.
This would mean an output level of 21 dBm at the FET from the red arrow, no wonder why it died

Now I have two ways to go, the first is to repair the YIG but an other way is using the second YIG and adding an external amplifier to get power level up to the required level, so a 20 dB amplifier with max 20 dBm over a freq range from 4 to 6 GHz.

On Ebay I found a nice and cheap amplifier and decided to order it.

I combined the amplifier with the defective YIG and measured an output level of about 9dBm

This is till too low level for the original use in the 8970b.

In the original 8970b the YIG output is first divided in two outputs so the level at the RF mixer should be between 11 and 15 dBm.

I decided to try if the instrument would work with this setup.

And starting up the instrument the error 18 was gone

 
    

I need to do some more checks if the instrument is working as it should but this sure looks good