HP 8590A Wrong Frequency and Amplitude Measurements below 17MHz

[Johnny10]

I just purchased a problem unit HP8590A Spectrum Analyzer it has measurement problems below 17MHz.

I have read the service manual and done the software tests but before opening the unit I wondered if there was any advice from those of you who have worked on unit.

No errors listed or in memory
Manual says problems are most likely in the FM coil circuitry.

So what I have done so far.

Done a Cal Frequency and Amplitude cal and store.
Run the FM Coil Drive Routine
Pictured

Done the ADC routines.
Seem normal switches at listed frequencies.

Checked LO for Frequency
0                      2.050901 GHz
16.92MHz         2.07947 GHz
1.5 GHz.           3.548310 GHz

Measurements and Amplitudes


Seems I get wrong Frequency measurements from 17MHz down.
I get wrong Amplitude from 2MHz down.
Drifting signal (Where peak walks across screen) from 200K down.

Amplitude drops pretty consistent -60 dBm

I also noticed as I worked the Amplitude on the signal generator the Amplitude measurements in 8 to 2MHz intermittently were correct.

[Johnny10]

Been reading through all the forum information and watching feedbackloop's youTube videos.

Will order the new diodes for the mixer and look over components in the FM Coil Driver circuitry on the Analog board.

[Johnny10]

Well I had a chance this week to finish the HP8590A repair.
Parts came in and I made a new work area to do the soldering.

Before repair I could only get non-sensical readings below 17MHz in both Frequency and Amplitude.
Sometimes readings would be off 1-3MHz, Amplitude would be accurate or drop -60dBm.

I used the Avago RF Mixer/Detector Diode HSMS-2822. The original diode pair were not shorted or open.
I pulled the A7 board and checked all Capacitors, checked the Voltages and the Reference voltage at the Reference IC output pin. Re-soldered any joint that looked odd, reseated connectors.

The manual showed these problems could be traced to the FM Coil Drive circuitry, The First LO or the Mixer Diodes.
Checked over FM Coil Driver no problems. Used example traces from the troubleshooting section of Supplementary Manual.

Pic showing 1MHz signal after repair.

[bdunham7]

The original diode pair were not shorted or open.

Thanks for posting the repair.  I wonder if you could characterize the presumably faulty diodes to see what the issue is?  Capacitance, leakage, forward voltage,

[Johnny10]

I do have a question for all you HP SA owners out there.

In the manual there is a description of the Cal Data which shows RL-VENR
ranging from 255-30. I believe these are the DAC correction figures, even after my repair my last figures are still in the 116-138 range. Does this imply the DAC's are aging? And should be replaced.

[Johnny10]

Checking the diodes on the curve tracer... the first diode fell apart when I connected the probes.
The diode must have had cracked lead inside.

[Johnny10]

I believe my unit has an early A7 Board. Wondering if any know of Schematics?
08950-60001

There are lots of different schematics available but haven't found more than portions of this one.

[Johnny10]

10KHz Lower frequency limit, 100KHz shorter Span