Marconi Instruments 2019A RF Signal Generator

Problem

A Marconi Instruments 2019A RF signal generator needed repair. It displayed an error code “—24—” instead of the frequency setting on the first LCD.

Although it looked like the unit was non-functional, we found it was possible to use the controls to set the output level and modulation settings also appeared to work.

Since the controls were semi-working, we attached it to a spectrum analyser and tried to set the frequency and output level.

We found that it was partially working, even with the error code displayed in place of the frequency.

Diagnosis

Finding that the instrument was partially functional, we suspected the EEPROM was bad and removed it.

The unit still behaved exactly the same with the EEPROM removed. To check the EEPROM we tried to read it several times and compare the results.

Every time different data was returned, appearing to confirm the EEPROM had failed. We sourced a new EEPROM and discovered the issues were exactly the same with a completely blank eeprom.

We found several versions of the service manual existed online, but none were an exact match for the PCB in this unit.

After further investigation, the EEPROM socket was wired slightly differently to the pinout of the IC that was in there. It appears we were not first to try to repair this unit.

Solution

We changed the connections of the IC socket to match the new EEPROM, then the unit worked correctly but required a full calibration.

Calibrating the unit revealed some further issues with the output level being much lower when the frequency was set to greater than 520 MHz, the issue was traced to a cracked solder joint in the frequency doubler section of the RF generation circuits.

Once re-soldered, the level is much more consistent across the whole frequency range and calibrating the output level has restored the instrument to close to the original factory specifications.

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