HP8970A

During the Heelweg microwave meeting a HP8970A noise gain analyzer got damaged by using a power supply feeding a DUT without a good ground connection.

At the moment the DUT was attached to the Noise Gain analyzer the display started blinking and giving an error message.

After some time I spoke to the owner of the instrument who was quite disappointed that this happened.

The person who wanted to measure his DUT asked me if he could do his measurement on my HP8970B instrument and I decided to do the measurement.

At the moment I wanted to connect his DUT to my HP8970B I felt some tinkeling on my fingers from voltage between the DUT and my analyzer and I did not connect the DUT and wanted to do further investigantion what was going on. We measured the voltage between the DUT and the analyzer showing 110 Volt.

We then took the DUT power supply and grounded it to earth and then we could perform the measurement.

The HP8970A was still broken and when we had time in spare we decided to open the instrument. We looked at the input circuit and it looked a bit complex so we decided not to continue.

The next year during the Heelweg meeting the defective HP8970A was again showing failure and the owner was still very disappointed.

We discussed what happened and that it was repaired by someone saying it had a software failure. But at this new Heelweg meeting it was again not working.

I offered the owner to try and repair the analyzer.

At home I opened the instrument and connected a spectrum analyzer to the IF output port and entered a frequency and feeded this frequency at the input of the analyzer using a signal generator. I found a signal from the generator but that was not very convincing and it was much weaker then expected.

I opened the input unit from the analyzer en with some small wire as antenna connected to the signal generator I found that the IF signal out of the analyzer was much stronger when I came close on the output of the first input amplifier.
I measured DC voltages on the first input amplifier showing a correct DC setting.

Anyhow I found that the pre amplifier transistor was broken and I decided to replace the transistor.

After that the calibration of the analyzer worked well and it no longer showed any error.

The input transistor must have been blown during the connection of the DUT which used a ungrounded DC power supply.